Table of contents

Table of contents
Table of contents
1
Table of contents
TABLE OF CONTENTS ....................................................................................................................... 1
1. PRINCIPLES OF OPERATION ...................................................................................................... 5
1.1 PREFACE .............................................................................................................................................. 5
1.2 THE CONCEPT ...................................................................................................................................... 6
2. GETTING STARTED WITH GPES ................................................................................................ 9
2.1 RECORDING A CYCLIC VOLTAMMOGRAM WITH THE DUMMY CELL .................................................... 10
2.2 THE USE OF THE MANUAL CONTROL WINDOW ................................................................................... 13
2.3 DATA MANIPULATION OF A CYCLIC VOLTAMMOGRAM. ..................................................................... 14
2.4 CALCULATION OF A CORROSION RATE. .............................................................................................. 17
2.5 NOISE REDUCTION ............................................................................................................................. 19
2.6 DATA ANALYSIS WITH CHRONO-AMPEROMETRY............................................................................... 20
2.7 DATA ANALYSIS WITH DIFFERENTIAL PULSE VOLTAMMETRY. ........................................................... 21
2.8 ANALYSIS OF ELECTRO CHEMICAL NOISE ......................................................................................... 22
2.9 IR-COMPENSATION ............................................................................................................................ 23
2.10 DETECTION OF NOISE PROBLEMS ..................................................................................................... 24
3. THE GPES WINDOWS................................................................................................................... 27
3.1 GPES MANAGER WINDOW ................................................................................................................ 27
File menu ............................................................................................................................................ 27
Method................................................................................................................................................ 32
Utilities ............................................................................................................................................... 32
Project ................................................................................................................................................ 43
Options ............................................................................................................................................... 51
Window ............................................................................................................................................... 52
Help .................................................................................................................................................... 52
Tool bar .............................................................................................................................................. 52
3.2 STATUS BAR ...................................................................................................................................... 53
3.3 MANUAL CONTROL WINDOW ............................................................................................................. 53
Current range ..................................................................................................................................... 54
Settings ............................................................................................................................................... 54
Potential ............................................................................................................................................. 55
Noise meters ....................................................................................................................................... 55
iR-compensation ................................................................................................................................. 55
Integrator............................................................................................................................................ 56
Filter panel ......................................................................................................................................... 56
3.4 DATA PRESENTATION WINDOW.......................................................................................................... 56
File...................................................................................................................................................... 57
Copy.................................................................................................................................................... 58
Plot ..................................................................................................................................................... 58
Analysis............................................................................................................................................... 60
Edit data ............................................................................................................................................. 60
Work scan ........................................................................................................................................... 60
Work potential .................................................................................................................................... 60
Editing graphical items and viewing data .......................................................................................... 60
3.5 EDIT PROCEDURE WINDOW ................................................................................................................ 63
3.6 ANALYSIS RESULTS WINDOW............................................................................................................. 64
4. ANALYSIS OF MEASURED DATA ............................................................................................. 65
4.1 PEAK SEARCH .................................................................................................................................... 65
4.2 CHRONOAMPEROMETRIC PLOT .......................................................................................................... 69
4.3 CHRONOCOULOMETRIC PLOT............................................................................................................. 69
4.4 LINEAR REGRESSION .......................................................................................................................... 69
4.5 INTEGRATE BETWEEN MARKERS ........................................................................................................ 70
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4.6 WAVE LOG ANALYSIS ........................................................................................................................ 70
4.7 TAFEL SLOPE ANALYSIS ..................................................................................................................... 71
4.8 CORROSION RATE .............................................................................................................................. 71
4.9 SPECTRAL NOISE ANALYSIS ............................................................................................................... 73
4.10 FIND MINIMUM AND MAXIMUM ....................................................................................................... 74
4.11 INTERPOLATE .................................................................................................................................. 74
4.12 TRANSITION TIME ANALYSIS............................................................................................................ 74
4.13 FIT AND SIMULATION ....................................................................................................................... 74
The simulation method........................................................................................................................ 75
The fitting method............................................................................................................................... 75
Elements of the Fit and Simulation Window....................................................................................... 76
Fitting and simulation step by step ..................................................................................................... 76
Fitting in more detail .......................................................................................................................... 81
Fit and simulation error messages ..................................................................................................... 84
Descriptions of the models.................................................................................................................. 85
4.14 CURRENT DENSITY .......................................................................................................................... 95
4.15 WE2 VERSUS WE PLOT ................................................................................................................... 95
4.16 ENDPOINT COULOMETRIC TITRATION .............................................................................................. 95
5. EDITING OF MEASURED DATA ................................................................................................ 97
5.1 SMOOTH ............................................................................................................................................ 97
5.2 CHANGE ALL POINTS ......................................................................................................................... 98
5.3 DELETE POINTS .................................................................................................................................. 98
5.4 BASELINE CORRECTION ..................................................................................................................... 98
5.5 SUBTRACT DISK FILE ......................................................................................................................... 99
5.6 SUBTRACTION OF SECOND SIGNAL FROM FIRST SIGNAL. .................................................................... 99
5.7 DERIVATIVE ...................................................................................................................................... 99
5.8 INTEGRATE ........................................................................................................................................ 99
5.9 FOURIER TRANSFORM ...................................................................................................................... 100
5.10 CONVOLUTION TECHNIQUES .......................................................................................................... 100
Detection of overlapping peaks ........................................................................................................ 102
Determination of formal potential and the number of electrons involved ........................................ 104
Irreversible homogeneous reaction consuming the product of the electrode process ...................... 105
Investigations of factors controlling the transport to the electrode.................................................. 106
Algorithms for convolution ............................................................................................................... 108
5.11 CONVOLUTION IN PRACTICE .......................................................................................................... 109
5.12 IR DROP CORRECTION .................................................................................................................... 110
APPENDIX I GPES DATA FILES................................................................................................... 111
APPENDIX II DEFINITION OF PROCEDURE PARAMETERS............................................... 113
APPENDIX III COMBINATION OF GPES AND FRA ................................................................ 127
APPENDIX IV MULTICHANNEL CONTROL............................................................................. 129
Installation and test .......................................................................................................................... 129
Program operation ........................................................................................................................... 130
APPENDIX V TECHNICAL SPECIFICATIONS.......................................................................... 133
Interface for mercury electrodes (IME 303 and IME663) ................................................................ 134
Burettes............................................................................................................................................. 134
Hardware specifications of optional modules .................................................................................. 134
SCAN-GEN, SCAN250: analog scan generator module .................................................................. 134
ADC750: dual channel fast ADC module......................................................................................... 135
ECD: low current amplifier module ................................................................................................. 135
BIPOT, ARRAY and BA: (bipotentiostat) module............................................................................. 135
FI20: filter and integrator module.................................................................................................... 135
BSTR10A or Booster20A: current booster ....................................................................................... 135
Table of contents
3
INDEX ................................................................................................................................................. 137
Chapter 1
Principles of operation
5
1. Principles of operation
1.1 Preface
Autolab and the General Purpose Electrochemical System software (GPES) provide a
fully computer controlled electrochemical measurement system.
It can be used for different purposes, i.e.:
•
general electrochemical research
•
polarographic analysis in conjunction with a dropping or static mercury drop
electrode
•
voltammetric analysis with solid electrodes, such as glassy carbon or rotating disk
electrodes
•
research of electrochemical processes like plating, deposition and etching
•
electrochemical corrosion measurements
•
electrochemical detection in Flow Injection Analysis (FIA) and High Performance
Liquid Chromatography (HPLC).
The instrument is controlled by a personal computer equipped with an IBM/PC or AT
I/O expansion bus. All the Autolab configurations are supported by GPES:
•
µAutolab or µAutolab Type II, the compact version of a standard Autolab with
potentiostat
•
Autolab with potentiostat/galvanostat PGSTAT10/12/20/30/100 and other,
optional, modules.
The GPES combines the measurement of data and its subsequent analysis. GPES runs
under MS-Windows 95, 98 and NT. Its installation is described in the "Installation
and Diagnostics" guide.
The user should be familiar with MS-Windows.
The GPES program consists of two distinct parts i.e.:
•
The user-interface, graphics and data-analysis software.
•
The routines which perform all the communication with the Autolab instrument.
Both parts communicate via shared memory. The measurement tasks run with the
highest priority. All the spare time is left for MS-Windows applications.
Familiarisation with GPES is best obtained by experimenting. Most of the required
help which might be necessary to perform the measurements and the data analysis is
provided for by the on-line help within the program.
This manual concentrates more on explaining the general concepts and backgrounds
than on guiding the user through the program. Moreover, this manual tries to explain
the possibilities of GPES. The "Installation and Diagnostics" guide explains the
hardware aspects, the computer requirements and the installation.
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1.2 The concept
The design of GPES Windows has been based on the following ideas:
•
GPES should incorporate the facilities electrochemists need.
•
The user should have full and easy control over the Autolab instrument via the
computer.
•
'How to perform experiments' should be easy and clear.
•
Actions should require only a few clicks.
•
The introduction learning period should be short.
•
All important electrochemical techniques should be available.
•
GPES should be a full and standard Windows application.
•
Series of unattended experiments, using different techniques and/or procedures,
should be possible.
The GPES screen consists of several windows: one for manual control over the
potentiostat/galvanostat, one for data presentation and manipulation, one for entering
the experiment parameters and one for collecting results of data analysis. Surrounding
windows, menu options and tool bars give extra facilities like cell-diagnosis,
accessory control, Autolab configuration, access and data transfer to programs like
Excel and MS-Word.
The MS-Windows related terminology used in this manual is in agreement with the
standard as described in the book "The GUI Guide - international terminology for
Windows Interface" (Microsoft Press, Washington ISBN 1-55615-538-7). It is a good
book to become acquainted with the Windows vocabulary.
The following mouse conventions are used:
•
Quickly pressing and releasing the mouse button is called "clicking". A click of
the left mouse button on a menu option, a button, an input item on the screen,
etceteras will result in an action.
•
Clicking and holding down the left mouse button is called "dragging" and is used
for several purposes. You can focus on an item on the screen without an action,
you can drag a window when the mouse pointer is in its title bar. It can be used to
shrink or to enlarge a window when the mouse pointer is on the border of a
window. Finally, you can drag a scroll bar, a slider or a zoom-panel.
•
A double-click of the left mouse button is used to perform particular actions.
Except for the standard uses in window actions, it is used to edit the graph in the
Data presentation window.
•
A click of the right mouse button is used to open a zoom panel in the Data
presentation window or to shrink or enlarge the Graphics panel in the Setup
Template option in the Print menu window, which appears after selecting Print
from the File option in the GPES Manager window.
Chapter 1
Principles of operation
The following keyboard functions are supported:
•
RETURN/ENTER key:
jump to next data input field;
select menu option; or
click button with focus.
•
left and right arrow key:
move cursor in data input field.
•
up and down arrow:
move up and down in potential/current level input in chronomethods; or
move up and down in a menu.
•
ALT:
puts focus on the menu bar of the window with the focus;
typing a subsequent underlined character will move the cursor to the
corresponding menu item, a RETURN/ENTER will select the menu item.
•
ESC: aborts the execution of the measurement procedure.
•
F1: access Help.
•
F4: plot rescale.
•
F5: starts the execution of the measurement procedure.
•
F6 and shift F6: change focus to the next window.
This manual does not describe the background of the electrochemical methods. We
would like to refer to the ‘Electrochemical methods’-manual and some excellent
textbooks:
•
C.M.A. Brett and A.M.O. Oliveira Brett,
Electrochemistry
Oxford science publications ISBN 0-19-855388-9
•
Allen J. Bard and Larry R. Faulkner,
Electrochemical Methods: Fundamentals and Applications
J. Wiley & Sons ISBN 0-471-05542-5
•
R. Greef, R. Peat, L.M. Peter, D. Pletcher and J. Robinson,
Instrumental Methods in Electrochemistry
Ellis Horwood Limited ISBN 0-13-472093-8.
7
Chapter 2
Getting started with GPES
9
2. Getting started with GPES
In this chapter some basic examples are given to become familiar with GPES. The
possibilities and the options of the software are described. Some of the examples
contain a measurement with the Autolab dummy cell, so before you start with the first
example, please connect the dummy cell box to the cell cable by putting the banana
plug connector into the matching colour connector on the dummy cell. The red banana
plug should be connected to WE(a).
As soon as you start GPES, by clicking the icon in the Autolab application window,
you will see the standard layout of the GPES software, which consists of three
windows and two bars.
Fig. 1 Default layout of GPES windows
The two bars are:
•
The GPES manager bar, with the menus and the tool buttons.
•
The Status bar at the bottom of the screen, which contains the start and stop
buttons for measurement and displays the system messages.
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The three windows are:
•
The Edit procedure window, which specifies all the experimental parameters.
Changed parameters are automatically saved when leaving the software and will
appear as default parameters on the next occasion. For most of the techniques, this
window will consist of two pages.
•
The Manual control window, which manually controls the settings of the
potentiostat/galvanostat.
•
The Data presentation window, which gives a graphical display of the measured
data, and allows you to do data analysis and/or modification.
The options and possibilities of these three windows will be explained in detail in the
next chapter.
2.1 Recording a cyclic voltammogram with the dummy cell
1. Before starting with this (and with the other examples) please check the hardware
configuration of your system. You can do this by executing the hardware
configuration program. In the Autolab hardware configuration window, you can check
if all the modules in your instrument are also selected in the software, if so, you can
close this window by clicking ‘OK’. See also the Installation and diagnostics manual.
2. From the Method menu of the GPES manager bar, please select ‘Cyclic
voltammetry (staircase) Normal’.
3. Select ‘Open procedure’ from the File menu, and open the ‘testcv.icw’ from the
\autolab\testdata directory.
Fig. 2 The Open procedure window
Chapter 2
Getting started with GPES
11
4. In the Edit procedure window, you will now find all the measurement parameters.
By clicking ‘Start’ the program will start the dummy cell measurement. During the
measurement you can automatically rescale the curve in the Data presentation window
by typing F4 on your keyboard.
Fig. 3 Results of procedure TESTCV
5. After the measurement is done, the curve should look like the curve in figure
‘Results of procedure TESTCV’, i.e. a straight line, if not, please consult the
“Installation and Diagnostics” guide in this manual.
6. In the Edit procedure window, please select ‘Number of scans’ and change the
value from 1 to 100. If you now press start again, the program will start to do 100
scans. You can always stop the measurement by using ‘Esc’ on your keyboard, or by
clicking the Abort button. Please do so after a few scans. After stopping, the Data
presentation window will show you the last scan. You can also select one of the
previous scans by using the Work scan option.
7. You are able to change some measurement parameters during the measurement by
using the Send option in the Edit procedure window. Please (re)start a measurement
with 100 scans and during the measurement change the ‘Scan rate’ from 0.1 V/s to 1
V/s. After clicking ‘Send’ the speed of the measurement will increase. You can again
stop the measurement by using the ‘Esc’ button.
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Fig. 4 Edit procedure window
Version 4.9
Chapter 2
Getting started with GPES
13
8. After finishing a measurement, the program allows you to save more than one scan
by selecting ‘Save scan’ and then selecting a scan that you would like to save. For the
following scans you want to save, please select ‘Save scan as’. The ‘Save data buffer’
option saves all the scans that the PC has in its memory.
2.2 The use of the Manual control window
Fig. 5 Manual control window: the appearance depends on the Autolab configuration
The Manual control window allows you to control the potentiostat manually, instead
of via a measurement procedure. With the dummy cell still connected to the cell
cable, and the procedure “testcv” loaded, you can try the following:
1. Clicking the highest current range (10 mA for a PGSTAT10/µAutolab, 100 mA for
a PGSTAT12/100 and 1 A for a PGSTAT20/30) results in the selection of all current
ranges except the 100nA range. This allows automatic current ranging using all the
‘checked’ ranges, during the execution of a measurement procedure. The green circle
indicates which current range is active. Check this by clicking on one of the circles
and see what happens on the front panel of the potentiostat.
2. The cell can be switched on and off manually, please click the cell on/off button,
and check the result on the potentiostat.
3. By using the slider below ‘Potential’ it is possible to set a potential value. Make
sure that the cell is ‘on’ and use the slider to set a potential of 1 V. In the Manual
control window the current and the potential are given. By clicking the Clock on/off
button the program will start making a graph of the current versus time. Switch the
clock off again, and use the window below the slider to set a potential of 0 V. Switch
the cell off again.
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2.3 Data manipulation of a cyclic voltammogram.
1. Choose ‘File’ and ‘Load scan’ and load the datafile ‘democv01’ in the
\autolab\testdata directory. Enlarge the graph by clicking the maximise button on the
Data presentation window.
2. Double click the curve in the Data presentation window. A plot parameter window
appears, in which you can change the colour of the curve, or change from a ‘line’
display to a ‘scattered’ display.
Please try this by changing the colour and the style of the line. Select the settings that
you feel are the most suitable for this curve.
3. With the peak search option, the software allows you to determine all peak
parameters of the CV. Choose ‘Analysis’ and ‘Peak search’. When the Peak search
window appears, click the Options>> button, the program now allows you to set a
number of options. Start by selecting ‘Curve cursor’ and ‘Lin. front baseline’, click
close and press the search button in the Peak search window. You are now asked to
set two markers for a baseline in front of the peak, please do so and press OK. The
program shows the result of the peak search in the ‘Peak search results window’ and
shows the peak in the curve.
4. Please repeat the above after selecting ‘Automatic’ and ‘Linear baseline’ in the
Options>> window. Click search, and have a look at the results.
5. Close the Peak search results window and choose ‘Window’ and ‘Analysis results’
from the GPES manager bar. In this window all the data analysis results are kept as
long as you do not exit the GPES software. Please check under ‘File’ that you are able
to ‘Save’, ‘Print’ or ‘Clear’ the results. Close the Analysis results window.
6. Under ‘Edit data’ choose the option ‘Subtract disk file’. Select the same file that
has been loaded: ‘democv01’. Note that, as might be expected, the result is a
horizontal line at I=0. By selecting ‘Plot’ and ‘Resume’ the original curve is retrieved.
This option always allows you to get back to the original curve after editing or
analysing the data.
7. Under ‘Edit data’ choose the option ‘Baseline correction’. Select ‘Linear baseline’
at the settings, and press ‘Set markers’. You are now asked to set two markers for the
baseline you want to correct. Set the markers on the horizontal part of the forward
curve before the peak and press ‘OK’. Note that the Data presentation window now
shows you both the original and the corrected curve (in black). By clicking OK you
accept the corrected curve and the original is removed from the window. Using the
‘Resume’ option, however, gives you back the original.
8. The option ‘Wave Log analysis’ under ‘Analysis’ allows you to determine the halfwave potential and the number of electrons for S-shaped voltammogram, for example
a Normal pulse voltammogram. The file ‘democv01’ may be transferred to an Sshaped curve by choosing ‘Edit data’, ‘Convolution’ and ‘Time semi-integral’. For
more details on the convolution techniques, please read the relevant chapter in this
manual.
Chapter 2
Getting started with GPES
15
Fig. 6 Results of time semi-integral convolution
After selecting ‘Wave log analysis’, you are asked to set markers for the baseline and
the limiting current line for the forward (or black) curve. Please set two markers for
the baseline and press OK and do the same for the limiting line. Now the Wave log
analysis window gives you a value for the half-wave potential and the height of the
curve. By clicking ‘Continue’ the curve transforms and you are again asked to set two
markers. After doing this and clicking OK, you get the results for the analysis in the
Wave log analysis window.
Close this window and choose ‘Plot’ and ‘Resume’.
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Fig. 7 Wave log analysis option
9. From the GPES manager bar, choose ‘File’ and ‘Print’, the Print menu allows you
to print the measured data, one or more of the windows, and a Template. Select
‘Template’ and ‘Set-up template’. With this option you can print a measured curve
together with the most important measurement parameters on one sheet. With ‘Insert’
and ‘Field’ you can add or remove fields with parameters from the template. You can
also drag fields around to put them on another place on the sheet. By clicking ‘File’
and ‘Print preview’ the values for the parameters and the curve are shown. The size of
the curve may be changed by clicking it with the right mouse button and then moving
your mouse (without pressing a button!). The way in which the parameters are
displayed may be changed by double clicking a field and choosing for example
‘scientific’ and then setting a precision. (Precision -1, means that the value is printed
with the format used in the Edit procedure window). Close the Template window.
Chapter 2
Getting started with GPES
Fig. 8 Results of printing of the template
2.4 Calculation of a corrosion rate.
Before you start, make sure that the method is still Cyclic Voltammetry, Normal.
1. Load the datafile ‘democv02’ from the \autolab\testdata directory.
2. In the Data presentation window, double click the vertical axis. The vertical axis
window appears.
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Fig. 8a Vertical axis window
3. Change the scale from the axis from linear to Lg, i.e. 10log. Please note that you
are also able to change the range of the axis and the position of the intercept of the
axis in this window. Close the window, and note the change of the curve.
4. From the Analysis menu, choose ‘Corrosion rate’. The Corrosion rate window
appears.
Chapter 2
Getting started with GPES
19
Fig. 9 Corrosion rate analysis
In this window the program shows a first value for the corrosion potential, as well as
for the polarisation resistance. Furthermore you can specify values for the surface
area, the equivalent weight and the density of the material you are using. For this
example you can set these values to 1. Click the Tafel slopes button. You are asked to
set markers on the anodic branch and on the cathodic branch. After you have done so,
the Corrosion rate window appears, with a list of parameters, among which the
corrosion rate in mm/year. By clicking the Start fit button, the software will adjust the
parameters until a best fit of the original curve is found. This fitted curve is shown in
black, and the final values for the parameters are given.
Click close and transform the vertical axis of the curve back to linear.
2.5 Noise reduction
Make sure that the method is Cyclic voltammetry (staircase), normal.
1. Load the datafile ‘democv04’ from the \autolab\testdata directory.
2. From the Edit data menu, choose the Smooth option. The Smooth window appears,
and gives you the possibility to choose from different smoothing methods. Choose
FFT (Fast Fourier Transform) with linear graph. Click the Smooth button, the curve is
now transformed to the frequency spectrum and a marker window appears. You are
asked to set one marker for the cut-off frequency: set this marker at ca 7 Hz and press
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OK. All frequencies above 7 Hz will now be filtered out. Please note that the 7 Hz is
not the frequency of the potential or current noise. The scaling is arbitrary. Note the
smoothed curve in black, with OK you accept the curve, and the noisy original will
disappear.
Fig. 10 Data smoothing using FFT
2.6 Data analysis with Chrono-amperometry.
From the Method menu on the GPES manager bar, choose Chrono-amperometry
(interval time < 0.1 s).
1. Load the datafile ‘democx01’ from the \autolab\testdata directory.
2. The curve shown is the result of a double potential step experiment. To do data
analysis it is more convenient to choose just one of both potential levels at a time. To
do so, from the Plot menu in the Data presentation window, choose select potential,
and select the -0.7 V level. The data shown now are the result of the selected potential
step.
3. In Chrono-amperometry the current is proportional to 1/Square root of time. In
order to visualise this, please double click the horizontal axis and change the scale to
1/square root. The curve is now linear. From the Analysis menu, choose ‘Linear
regression’ and set two markers for the beginning and the end of the linear regression.
The Linear regression window now gives you the results. From the slope of this line,
it is possible to calculate for example the diffusion coefficient.
Chapter 2
Getting started with GPES
21
Fig. 11 Transition time analysis
4. From the Plot menu choose ‘Resume’ and the original curve will reappear.
2.7 Data analysis with differential pulse voltammetry.
From the Method menu, choose voltammetric analysis and then Differential Pulse.
1. Load the datafile ‘demoea01’ from the \autolab\testdata directory.
2. From the Analysis menu choose Peak Search. Under the options, choose the
automatic search, close the Options window and press the search button. The Peak
search results window will now show parameters for four peaks. For the first three the
results are reasonable, but for the peak at the highest potential the linear baseline is
not the best option.
3. With the Set window option under the Plot menu, you are able to extract the last
peak from the curve. Set the markers so that only the last peak is visible. Now there
are two options (please start each option after setting the window around the last
peak):
a. From the Edit data menu choose ‘baseline correction’ and select the polynomial
basecurve. Click the set markers button and set the markers for the baseline (one on
each side of the peak). After ‘OK’, the corrected (black) curve is shown with a more
horizontal baseline. You can now use the automatic peak search option to find the
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peak parameters. Please do so and check the difference with the automatic search on
the non-corrected curve by opening the Analysis results window.
b. From the Analysis menu, choose the Peak search option. Under options, choose
‘curve cursor’ with a polynomial baseline. After pressing the Search button you are
asked to set two markers for the baseline, please do so. Now the Peak search results
window will give you the parameters of this search. Please open the Analysis results
window to compare this option with the result of option a. As should be expected, the
results of these two options are very similar. Close the Analysis results window.
Fig. 12 Example of polynomial baseline correction
2.8 Analysis of Electro Chemical Noise
From the Method menu, choose Electrochemical noise and then Transient.
1. Load the datafile ‘demoecn1’ from the \autolab\testdata folder.
2. From the Analysis menu choose Spectral noise analysis.
3. Choose a Hanning Window, check subtract offset, and the Result type: E(f) and
I(f).
4. By clicking OK, a spectral analysis is performed on the Potential and the Current
components of the noise signal.
Chapter 2
Getting started with GPES
23
Fig.13 Example Electrochemical Noise analysis
2.9 iR-compensation
When your Autolab is equipped with a PGSTAT12/20/30/100, you are able to
measure the uncompensated resistance in your electrochemical cell and to compensate
for this resistance. The GPES software provides two methods to do this: iR Interrupt
and Positive feedback (see Chapter 3 for more details). This example is meant to
make you more familiar with both options. Before starting, please connect the WE
connector of the cell cable to WE(c) on the dummy cell.
I-interrupt
a. Under the Utilities option in the GPES window, please choose I-interrupt, the iRcompensation window will appear. In the manual control window please type 1.0 V in
the potential panel, and check the 1 mA Current range checkbox. You can now switch
the cell on, by clicking the button in manual control.
b. In the iR-compensation window, please give the following values: Range =10V;
Duration of interrupt =0.001 s; First marker =15; Second marker =35. The current will
now be interrupted for 0.001 s, and the decay of the potential in time is measured.
Please click the Measure button. After a short time, in the result panel a value of about
100 should appear. When you want to compensate for 90% of this value (never use
100% of the measured value!), you can do this by typing the value in the iRcompensation panel in the Manual Control window. After clicking the iRcompensation button the program will automatically compensate the measurements
for the value given.
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Positive feedback
With positive feedback you can give in values for the resistance yourself, and you can
see when the current starts to oscillate, i.e. when you have overcompensated the
resistance.
a. Please choose the Positive feedback option in the Utilities menu, an iRcompensation will appear in which you can type the following values:
Potential pulse = 0.1 V; Duration = 0.01 s. Connect dummy cell (C) and put the
current range to 1 mA. After pressing ‘start’ the program will start applying potential
pulses. By giving different values in the iR-compensation panel and watching the
change in the i-t curve you can check how high the uncompensated resistance is.
Please check the change after typing a value of “95”. Now do the same after typing
“130”, you will see oscillations appearing, you have now done overcompensation.
Please note that if you reach a value where the current starts to oscillate, you should
use 90% of this value during your measurements.
Fig. 14 Effect of overcompensation of the iR-drop
2.10 Detection of noise problems
In the GPES software an option is available to detect noise problems. Since noise is
encountered frequently in electrochemical research, it is useful to become familiar
with the detection of problems caused by noise. In GPES, the Check Cell option under
the Utilities menu, provides the option to detect noise.
Chapter 2
Getting started with GPES
25
Please connect the red banana plug to the WE(c) connection on the dummy cell. In
order to generate some noise, please connect an unshielded cable between the blue
banana plug and the RE connector on the dummy cell, and place part of this
unshielded cable over the monitor of your computer. In manual control please check
the 100 nA Current range checkbox.
After selecting the Check Cell option, the Check Cell window appears. After pressing
the measure button, the program will start checking the electrode connections, and
will then measure the noise level. With the unshielded cable over the monitor, you
will see the current levels in red and the software will give you a warning that the
noise level is too high. Please redo the measurement without the unshielded cable.
You will now see the current levels in black indicating that the noise level is
acceptable.
Please use this option if you have doubts about the noise level in your system.
Chapter 3
The GPES windows
27
3. The GPES windows
3.1 GPES Manager window
The title bar of the GPES Manager window contains several options i.e. File, Method,
Utilities, Project, Window, Help.
File menu
This menu contains options which are usually present in Windows programs.
Fig. 15 The File menu
Open procedure
A procedure is a file containing all the experimental parameters. It contains
measurement parameters, potentiostat/galvanostat settings, and graphics display
values. The extension of the file which is mentioned in the "File name" field should
not be changed.
The directory in which the procedure file is stored is called the procedure directory.
When the directory in the Open procedure window is changed and a procedure file is
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successfully loaded from this new directory, this new directory becomes the new
default procedure directory.
Normally only files with the current method are shown in the load window. By
selecting Show all GPES files in File dialog box (Utility menu), all procedures are
displayed and can be selected.
It is also possible to load procedure files from the DOS version GPES 3. If this is
required, click the "List Files of Type" drop down button and select the proper option.
It is also possible to load a procedure from other methods/techniques than the current
one. The program will change automatically to the method described in the selected
procedure. See the Dropdown menu called "List Files of Type".
Save procedure
This option will save a procedure under its current name in the procedure directory.
Save procedure as
Allows to store a procedure on disk in the procedure directory with a different name
as the current one. Please use the default file extension as mentioned in "File name"
field or omit the extension. In the latter case the correct extension will be added.
Print
The Print menu window appears after selecting this option. The Print select panel
allows to choose between the print-out of the measured data, a dump of a window,
and the print of a template consisting of a user-defined set of measurement parameters
and a copy that can be scaled of the current graph.
The Setup template option allows to edit the template. The parameters on the template
can be selected using the Insert menu option.
The parameter position can be dragged over the screen with the mouse. The comment
text as well as the attribute of the item with focus can be edited by double clicking on
it. If you have changed the template according to your requirements, please do not
forget to save it (see corresponding File option).
The rectangle in the template is the Graph frame. The focus is on the frame after a
click in the frame. If the left hand mouse button is pressed down within this frame it
can be dragged.
If the right hand button is clicked, the lower right corner of the frame jumps to the
mouse cursor and is subsequently attached to the mouse cursor. This allows you to
adjust the frame size. After a click with the left hand mouse button the attachment is
broken. The appearance of the graph in the print out of the template depends on the
actual size and shape of the Data presentation window.
The print of the template will cover half a page if printed in ‘portrait’ and a full page
if printed in ‘landscape’. See print setup. On the print-out the parameter values also
appear. The print-out can also be previewed (see corresponding File option).
Load data (sometimes called Load scan)
Allows to load previously measured data from disk.
It is also possible to load data files from the DOS version GPES3 or data files of other
methods or techniques than the current one. If this is required, click the "List Files of
Type" drop down button and select the proper option. You can select multiple files at
a time by using <Shift> or <Control>, combined with the mouse action. This allows
Chapter 3
The GPES windows
29
you to load the work data as well as 10 overlay files. After this action it is possible to
exchange the work data by clicking ‘Work scan’ on the Data presentation window.
Save data
Store the most recent measured data under the current procedure name on disk. In
case of cyclic and linear sweep voltammetry the user is asked to select a scan number
first. The data are stored in the so-called data directory, together with the
corresponding procedure parameters.
When more than one scan is recorded in Cyclic voltammetry, it is possible to save the
previously measured scan while the measurement is going on. This option is available
at the File menu on the Data presentation window. The option is called ‘Quick save of
previous scan’. This option can also be activated by typing ‘SAVE’ on the keyboard.
The path and the name of the file can be specified on page two of the Edit procedure
window (‘Direct output filename’). The last five characters of the file name will be
used as the scan number.
Please note: These files can be overwritten during another measurement session with
the same procedure.
Save scan as
This options allows to save a scan in Cyclic and Linear sweep voltammetry.
First, if more than one scan is recorded, a menu is shown from which the user can
select the number of the scan to be saved.
Save data as
Similar as the previous option but the name of the file name containing the data can
be specified. For cyclic and linear sweep voltammetry a submenu is presented from
which the required data format can be selected.
Save data buffer as
This option can only be selected for cyclic and linear sweep voltammetry. The whole
data memory is dumped on disk in binary format in the data directory under a user
specified procedure name, together with the corresponding procedure parameters.
This is the only save option which stores all relevant data. If a buffer is reloaded, all
data treatment and save option are open for use.
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Export to scanno. vs Q+,Q- file
When a cyclic voltammogram is observed with more than one scan it is possible to
save the observed cathodic and anodic charge data against the scan number. The file,
with the extension .Q&Q, has the following layout:
ScanNr
1
2
3
4
5
6
7
8
9
10
.
.
Q+ (C)
1.697E-07
1.670E-07
1.671E-07
1.672E-07
1.674E-07
1.675E-07
1.669E-07
1.667E-07
1.674E-07
1.675E-07
Q-(C)
-2.351E-07
-2.365E-07
-2.363E-07
-2.357E-07
-2.356E-07
-2.361E-07
-2.356E-07
-2.358E-07
-2.364E-07
-2.355E-07
Export Chrono data
This option is only available for cyclic voltammetry. It allows to store the chronoamperometric data which are recorded at the vertex potential. See the input
parameters under the heading "Chrono-amperometry" in the Edit procedure window
Export to BAS-DigiSim data
This option is only available for cyclic voltammetry. Save the current active in such a
format that it can be read by the program DigiSim. This is an ASCII-file with the
default extension .TXT.
Export data buffer to text file
This option can only be selected for cyclic and linear sweep voltammetry. As in the
previous option the whole data memory is stored on disk, but in this case in a readable
ASCII-format. The file consists of several columns. The first column is the potential
(or current in galvanostat mode) and the other columns are the measured currents for
subsequent scans (potentials in galvanostat mode). The first row indicates the scan
number. The separator c.q. delimiter between two columns is a TAB character. The
default extension of the file is .TXT. They are meant to be read by MS-Excel, they
cannot be read by GPES. In order to create a nice and properly columned file, each
scan should have the same number of points. This means that the Reverse button
should not have been clicked. Also an interruption of a scan by pressing ESC should
be avoided.
Chapter 3
The GPES windows
31
The layout of the text file saved with the ‘Export data buffer to text file’ also contains
some important procedure parameters:
Date: 26-May-97
Time: 14:12:17
Exp. Conditions:
Linear sweep voltammetry
Begin potential (V) = .0000
End potential (V) = 1.0000
Step potential (V) = .02441
Scan rate (V/s) = .99992
Equilibration time (s) = 10
Number of Data Points = 42
Potential
0.024414
.
.
Scan 1
-1.191406E-07
Scan 2
-1.232910E-07
Load data buffer
This option can only be selected for data from cyclic and linear sweep voltammetry.
Allows to load the complete set of scans (see Save data buffer). This is only possible
if in the start-up menu "Autolab applications" has been chosen.
Delete files
This option allows to delete procedures and measured data files. The File window
shows only the procedure files. A selected procedure will be deleted from disk
together with corresponding data files. A delete action can not be undone.
Exit
The GPES window will be closed and the program is exited. The program settings are
stored on disk.
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Method
The required electrochemical technique can be selected with the Method menu The
type of experiment parameters in the Edit procedure window will change depending
on the selected technique.
For more information see the manual section on “Electrochemical methods”.
Fig. 16 The Method menu
The settings in the File menu and the data analysis also depend on the type of
technique. More details about the available methods can be found in a separate
chapter.
Utilities
The Utilities window allows to select electrode control, burette control, I-interrupt,
positive feedback, hardware, check cell, plate, sleep mode, set colour defaults, and
options.
Chapter 3
The GPES windows
33
Fig. 17 The Utilities menu
Electrode control
The Electrode control option allows to operate a static mercury drop electrode which
is connected via an IME-interface to the Autolab. The stirrer can be switched on and
off, the purge valve can be opened and closed, and a mercury drop can be created.
The Reset button will reset the digital I/O port of the Autolab instrument. The Purge
and Stirrer will be switched off. This option is not accessible when no static mercury
drop electrode is connected to the Autolab.
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Fig. 18 The Electrode control
window
Burette control
The burette control option allows to control motorburettes connected to Autolab via
the DIO48 module. Consult the "Installation and Diagnostics" guide about the type of
burettes that can be connected. First click the Setup button. Then select the burette.
Fig. 19 The Burette control window
The displayed Burette setup window allows to define the connected burette. Please
consult the manual of your burette for the parameters.
Chapter 3
The GPES windows
35
The ‘Maximum time to check for Ready’ is the maximum wait time for the software
to receive a "ready" signal from the burette.
The DIO port used is shown on your Autolab front.
The Dose button will dose the amount specified above. The dosed volume is
displayed.
The Dose on button will dose with the speed displayed above.
The Reset button will give a ‘reset’-command to the burette and sets the dosed
volume to zero.
RDE-control
In order to control an external Rotating Disk Electrode (RDE), an option is available
in the Utilities menu of the GPES manager. In Hardware configuration, an external
RDE should be specified. After selecting the RDE control item the following window
appears:
Fig. 20 The RDE control window
With the scroll bar it is possible to control the rotation speed of the RDE. You can
also enter the number of rotations per minute by changing the r.p.m. edit field or enter
the rotation speed in radials per second in the rad/s edit field.
After pressing the Setup button the RDE setup window appears:
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Fig. 21 The RDE setup window
In this screen you can configure the RDE.
MUX control
The channel number of the SCNR16A, SCNR8A or MULTI4 module can be selected
manually by the operator before starting the measurement procedure:
1. Open the MUX control dialog by selecting MUX control from the Utility menu.
The dialog screen shown in the figure below will pop up.
2. Enable the checkbox “Use Multiplexer Module”.
3. Choose the desired channel.
4. Pressing <Apply> or closing the dialog screen will set the selected channel.
5. The active channel number will be indicated in the Manual control window.
Fig. 21a The MUX control window
Chapter 3
The GPES windows
37
If you want to return to direct connections, you can disable the “Use Multiplexer
Module” checkbox.
I-interrupt
The I-interrupt option provides a method to determine the Ohmic resistance of the
cell. This option is only available when the Autolab is equipped with a
PGSTAT12/20/30/100 potentiostat/galvanostat. The technique involves switching off
the current and measuring the potential-time curve. As soon as the current is switched
off, the potential difference across the Ohmic resistance is zero and the charged
double layer is discharged. By extrapolating the curve following a straight line to the
beginning (time is zero), the iR-drop is calculated. Since the current is measured just
before switching off the cell, the uncompensated resistance is calculated.
It will be clear that for a proper calculation of the uncompensated Ohmic resistance
Ru, the current must be known very precisely. Proper measurement must be done at a
potential where the current is high enough to be measured and the applied current
range must be adequate to measure the current. For proper measurements, the current
must be at least in the order of 1 mA.
Also make sure that the current at the applied potential, before the current
interruption, can be measured sufficiently accurate. Therefore select a proper current
range, which means that the current should be in the order of the selected current
range.
It is recommended to switch off the iR-compensation, see Manual control window.
In order to get an accurate value for the uncompensated Ohmic-drop, the I-interrupt
measurements should be done at the highest possible speed. If an ADC750 module is
present in the Autolab system it is possible to use this module, in order to speed up the
measurements to 750 kHz.
Before measuring you need to specify the potential range at which the I-interrupt is
performed. If the potential is within the limit of -1V to 1V, specify 1 V range. If the
potential is outside this range, specify 10 V range.
On the iR-compensation window that appears, several parameters need to be
specified.
Duration of the interrupt: The interruption period; a reasonable value is .001 to .01.
The shorter the better.
First/second marker: The data point numbers between which a straight line is fitted.
In total 20 points are measured.
After the parameters have been specified, the Measure button can be clicked. The
measured data are subsequently plotted and the straight line is drawn. The calculated
uncompensated resistance is printed in the Result panel.
Now the Set marker button is available. When clicked you can change the First and
Second marker value by clicking two data points on the measured curve.
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Fig. 22 Example of the results of a current-interrupt measurement
The calculated uncompensated resistance can be used as an estimated start value to be
used in the Positive feedback option. See next section.
WARNING: Too high Ohmic drop compensation can cause oscillation of the
potentiostat, which may cause damage to the working electrode.
An example of the use of I-interrupt is given in the chapter 'Getting started with
GPES'.
Positive feedback
The Positive feedback option provides an interactive method for determination and
compensation of the Ohmic resistance of the cell. This option is available only when
the Autolab is equipped with a PGSTAT12/20/30 or 100 potentiostat/galvanostat. The
technique of measurements is based on measuring the current response after applying
a potential pulse. The current response is displayed on the screen. The current
response depends on the actual values of the Ohmic resistance and the doublelayer
capacitance. Compensation of the Ohmic resistance results in a faster decaying of the
charging current. When the compensation is near 100%, the measured current
response will show damped oscillation.
Three parameters need to be specified.
Potential range: Can be either 10 Volt or 1 Volt. If the expected measured potential is
< 1 Volt or > -1 V, select the 1 Volt range. Otherwise select 10 V range.
Potential pulse: The height of the applied potential pulse. A reasonable value is 0.1V.
Duration: The period during which the current versus time data are measured. This is
twice the duration of the pulse. A reasonable value is 0.01 s.
Chapter 3
The GPES windows
39
When the Start button is clicked, current versus time measurements are done
repeatedly and the iR-compensation of the potentiostat is switched on. Now the
compensated resistance can be varied with the iR-compensation slider on the Manual
control window.
When "Switch iR-compensation off at current overload" is checked, the cell will be
switched off when the current exceeds about 8 times the current range value. This
normally occurs when the potentiostat oscillates because the compensated resistance
is too high.
WARNING: Too high Ohmic drop compensation can cause oscillation of the
potentiostat, which may cause damage of the working electrode.
fig 22a. Effect of overcompensation of the iR-drop
Calibrate pH-Electrode
This window allows to calibrate pH electrodes. It is possible to specify two buffer
solutions and the calibration temperature. Measuring the pH as a 2nd Signal gives the
possibility to specify a measurement temperature. The pH is corrected for
temperature.
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Fig. 22b The Calibrate pH-Electrode window
When the value for a pH buffer is Ok press the Accept button. The OK button will
actuate the calibration for the measurement.
Check cell
The Check cell option allows to check the electrode connections and the noise level.
When selecting this option an empty window appears with a Cancel and a Measure
button. First apply a proper electrode potential and current range on the Manual
control menu. Subsequently click the Measure button. The window will subsequently
give information about the Electrode connections by comparing the applied and
measured electrode potential.
Also during 0.100 second the current is sampled at the highest rate possible.
Fig. 23 The Check cell window
Chapter 3
The GPES windows
The next figure shows a noisy signal displayed after pressing Measure. The plot
clearly shows periodic noise with a frequency of 50 Hz. After optimising the cell
simply by removing the unshielded cable of the reference electrode, the same
measurement shows a better signal-to-noise ratio.
Fig. 24a A noisy signal
Fig 24b A better signal-to-noise ratio
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The measured current and the five average values over one power cycle, normally
0.020 second, are plotted in the Measure window. The obtained five average values
and their standard deviations are given in the Check cell window. A judgement about
the noise level and the selected current range are given. See also Chapter 16 of the
"Installation and Diagnostics" guide. It is possible, after an improvement of e.g. the
cell configuration, to re-Measure. By pressing Cancel the Check cell window
disappears.
Plate
The Plate option will display a window in which three plating potentials, a 'cell off'
wait time and a final potential can be specified.
Fig. 25 The Plate window
The three plating potentials are alternated with the 'cell off' time. Subsequently the
'final' potential is applied.
Sleep mode
When the Sleep mode option is clicked, newly measured data will no longer be
displayed in the Manual control window and the Data presentation window. This
option is useful when during slow but time consuming measurements a spreadsheet
program or word processor is activated. The Sleep mode will minimise the time
required by GPES. During the sleep mode, the measuring part of GPES will stay
active. In this way data are measured but not displayed.
Chapter 3
The GPES windows
43
Project
The Project option allows to execute a large number of electrochemical experiments
unattended. A project encompasses a number of tasks which have to be executed
sequentially. Sometimes this is called batch mode processing. A measurement
procedure is normally activated by clicking the Start button in the lower left corner. It
is also possible to start a procedure by creating and subsequently executing a project.
A project can be created by selecting the Project edit option. First you have to indicate
whether a new project should be made (New option) or an existing project file should
be opened (Open option). An example of a project is delivered with the GPES4
program in the testdata directory.
After editing a Project it can be stored on disk under its current name (Save option) or
under a new name (Save as option).
When Edit is selected the Edit project window appears with two options on the main
menu bar. The Check option checks whether there are syntax errors in the project
commands. The Edit option provides the standard Cut, Copy and Paste option.
Below you will find the Project script language definitions and rules.
Project command rules
•
•
•
•
•
•
Both upper and lower case characters can be used in command lines.
Space characters are ignored.
If during the execution an error occurs the project continues with the next line.
An error message will be printed in the Results window.
One line per command.
The following commands are allowed:
; <string>
rem <string>
Procedure!Method = <method id>
Procedure!Open("<filename>")
Procedure!SaveAs("<filename>")
Procedure!Start
Procedure!AddToStandby(<real>)
Procedure!AddToPotlevel(<real>)
Procedure!AddToPotlevelEx
(<n>,<real>)
:
:
:
:
:
:
comment
define the electrochemical method
open a procedure file
save the procedure file
start the execution of the procedure
Add a value to the standby potential.
Only available for Chrono-Amperoand Chrono-Coulometry experiments.
: Add a value to all specified potential
levels. Only available for ChronoAmpero- and Chrono-Coulometry
experiments.
: Add a value to a specific potential
level (n). Only available for ChronoAmpero- and Chrono-Coulometry
experiments.
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Procedure!AddToCurlevelEx
(<n>,<real>)
Dataset!Open("<filename>")
Dataset!SaveAs("<filename>")
Dataset!AutoNum = <n>
Dataset!AutoReplace ("<string>")
: Add a value to a specific current
level (n). Only available for ChronoPotentiometry experiments.
: open a previously measured data file
: save the measured data
: enable auto-numbered files names,
starting with number <n>
: specify the string which should be
replaced by a number in the
<filename> for auto-numbered files.
Example (see below)
;Start file numbering with number 5
DataSet!AutoNum = 5
;Replace string xxx
DataSet!AutoReplace("xxx")
;Save Automatic number file
DataSet!SaveAs("c:\autolab\data\filexxx")
;The first file is now saved as
c:\autolab\data\file005.ocw
Please note:
When a FRA-project is started from GPES and the FRA-project and both projects
contain the command 'DataSet!AutoNum = <n>', then the number of the FRA-project
is overruled by the number in the GPES-project.
Dataset!PeakSearch
Dataset!Selectscan = <scanno>
Dataset!MinMax
Dataset!Smooth = <smooth level>
Dataset!SaveQ&Q(“<filename>“)
: perform automatic peaksearch with
baseline correction
: select a recorded scan number in
cyclic or linear sweep voltammetry
for further processing
: find the minimum and maximum
value in a dataset
: smooth the data using the Savitsky
and Golay algorithm. The smooth
level can be an integer number
between 0 and 4. Note that after the
execution of the project the smoothed
data are replaced by the originally
measure data
: store the anodic and cathodic charge
versus scan number (Q&Q files) for
cyclic voltammetry data. The filename
must be specified without an
extension.
Chapter 3
Dataset!SaveImpedance
(“<filename>“)
Dataset!Subtract("<filename1>",
<filename2>","<filename3>")
The GPES windows
45
: project command to store impedance
data measured for AC-voltammetry.
The extension is .IMP. The filename
must be specified without an
extension.
Utility!SetRDERPM = <rpm>
: subtract files and save the result in
another file. <filename3> =
<filename1> - <filename2>
: set the Rotating Disk Electrode to a
specific rotation speed. The set-up of
the RDE is made in the RDE control
option under Utilities.
Results!Clear
Results!SaveAs
: clear the Results window
: save the Results window
System!Run("<filename>")
: execute another program and wait
until it terminates.
The System!Run command search for
the program file with the next
sequence:
.PIF
.EXE
.COM
.BAT
System!Beep
Print!Template
: give a beep
: print a hardcopy according to the
template
: print a hardcopy of the Plot window
: print a hardcopy of the measurement
procedure
: print a hardcopy of the Results
window
: start a FRA project file from GPES
"<filename>" should be a FRA
project file
Print!Plot
Print!Procedure
Print!Results
FRA!Start("<filename>")
Utility!Channel = <n>
Utility!NextChannel
: sets the active channel to <n>. The
MUX will be automatically enabled
when necessary.
: increase the active channel number
with one. If the channel is not
available, the active channel number
is set to 1.
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Please note:
The last 2 commands are available in the GPES and FRA programs. However, for
FRA projects that are called from within GPES projects, all channel switching
commands in the FRA project scripts are ignored. In such cases, the GPES project
will have exclusive control over the channel selection.
Utility!Delay = <n>
Repeat(<n>)
EndRepeat
ForAllChannels("<filename>")
: hold the project for <n> seconds.
: with the Repeat and EndRepeat
commands it is possible to repeat a
part of the project <n> times. You can
nest these commands maximal 5
times.
: executes the active measurement
procedure for all available MUXchannels and store the results in the
<filename> adding 3 characters to the
filename as channel number counter,
for example: fname001, fname002,
etc. .
DIO!SetMode("<Connector>",
"<Port>","<Mode>") : set the mode of a port of the DIO.
DIO!SetBit("<Connector>","<Port>",
"<n>","<Bit>")
: set a single pin of the DIO on or off.
DIO!SetByte("<Connector>","<Port>"
,"<n>")
: set a port of the DIO to the specified
value.
DIO!WaitBit("<Connector>","<Port>",
"<n>","<Bit>")
: wait until a single pin of the DIO is
set on or off.
DIO!WaitByte("<Connector>","<Port>",
"<n>")
: wait until a port of the DIO is set to
the specified value.
Burette!DoseVolume (<Burette number>
,<Dose volume>) : dose a specified volume to the
specified burette.
Burette!Fill (<Burette number>)
: Fill the burette.
Burette!Flush (<Burette number>
,<Number of flushes>) : flush the burette.
Burette!Reset (<Burette number>)
: Will give a 'reset'-command to the
burette.
Chapter 3
•
The GPES windows
47
The <method id> can be:
VA
: voltammetric analysis
CV
: cyclic or linear sweep voltammetry
CM
: one of the chronomethods
ECD
: multi mode electrochemical detection
ECN
: electrochemical noise
SAS
: steps and sweeps
PSA
: potentiometric stripping analysis
<string>
: line of text
<filename> : a filename without extension, but including a directory name
<scanno>
: the number of a recorded scan
<rpm>
: rotations per minute
The FRA project file can only be executed if the FRA-program is already running.
For more information about the combination of GPES and FRA, see Appendix III in
the GPES manual.
A special case occurs when the measurement should start at the open circuit potential.
Normally the user is asked to click the Continue button, but in automatic mode the
program continues by itself after 1 second.
When no scan number is selected in cyclic or linear sweep voltammetry, the program
uses the last recorded scan as default.
Examples of projects can be found in the files ‘Demo01.mac’ and ‘Demo02.mac’
present in the \AUTOLAB\TESTDATA folder.
Project wizard
The Project wizard provides an easy way of editing and/or defining a project. This
option allows the user to pick project command lines from a list of all commands,
insert them in a project and define the parameters. The window below gives a project
Wizard overview.
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Version 4.9
Fig 26. An example of a project inside the Project wizard
Every project command can be inserted in the project, deleted or moved to another
place. A short description of the command is given in the information and syntax box.
Using the parameter button one can define the parameters that belong to that specific
command.
Project examples
Example 1: Cyclic voltammetry on MUX channel 2 and 4
The following example of a GPES project will perform the "c:\autolab\testdata\testcv"
procedure on channels 2 and 4, and stores the results in “c:\autolab\data\test channel
1” and in ”c:\autolab\data\test channel 4” :
Procedure!Method = CV
Procedure!Open("c:\autolab\testdata\testcv")
Utility!Channel = 2
Procedure!Start
Dataset!SaveAs("c:\autolab\data\test channel 1")
Utility!Channel = 4
Procedure!Start
Dataset!SaveAs("c:\autolab\data\test channel 4")
Example 2: Chrono amperometry on consecutive MUX channels
Chapter 3
The GPES windows
49
The following example will perform the "c:\autolab\testdata\testca" procedure on
channels 1 to 4, and stores their results using automatic filename numbering. The
result will be stored as “test scanner with cm 001”, “test scanner with cm 002”, “test
scanner with cm 003” and “test scanner with cm 004” with the number corresponding
to the channel:
Procedure!Method = CM
Procedure!Open("c:\autolab\testdata\testca")
Dataset!Autonum = 1
Dataset!Autoreplace("xxx")
Utility!Channel = 1
Repeat(4)
Procedure!Start
Dataset!SaveAs("c:\autolab\data\ test scanner with cm xxx")
Utility!NextChannel
Endrepeat
Example 3: Provide or receive trigger signals to or from DIO ports
These are the commands to set any of the pins on a DIO port of the Autolab
instrument. They can for example be used to control a Metrohm 730 Sample Changer.
Any of the pins can be set from low to high or the other way around, and can also be
used to receive an input trigger. With the SetMode command, one specifies whether
one wants to send or receive a trigger. SetBit allows to set one of the pins to on or off.
SetByte can be used to set multiple pins to on or off.
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DIO!SetMode("P1", "A", "OUT")
;On connector P1, port A, the mode is set to OUT, so ready to
send a trigger.
DIO!SetBit("P1", "A", "4", "ON")
;On connector P1, port A, Pin number 5 (pin number 1 has value 0)
is set ON (i.e. sends a trigger).
DIO!SetBit("P1", "A", "4", "OFF")
;On connector P1, port A, Pin number 5 is set OFF again.
DIO!SetMode("P1", "A", "IN")
;On connector P1, port A, the mode is set to IN, so ready to
receive a trigger.
DIO!WaitBit("P1", "A", "2", "ON")
;The project will wait for an input trigger on P1, port A, Pin
number 3.
DIO!SetMode("P1", "A", "OUT")
;On connector P1, port A, the mode is set to OUT, so ready to
send a trigger.
DIO!SetByte("P1", "A", "3")
;On P1, port A, both pin 1 (2^0) and 2 (2^1) are set ON. In case
one wants to set Pin 3 and 5, one needs to set the value 40
(=2^3+2^5) instead of 3.
DIO!SetMode("P1", "A", "IN")
;On connector P1, port A, the mode is set to IN, so ready to
receive a trigger.
DIO!WaitByte("P1", "A", "3")
;On P1, port A, the project is waiting for an input trigger on
both pin 1 and 2 .
;In case one wants to receive the trigger on Pin 4 and 6, one
needs to set the value 80 (=2^4+2^6) instead of 3.
Fig. 26a Schematic overview of both DIO ports with PIN numbering for the different
sections. Pin 25 is always the digital ground.
Port1
1
2
3
4
Section A
5
6
7
8
1
2
Section C UPPER
3
4
1
Section C LOWER
Port 2
2 Section C LOWER
3
4
1
2
3
4
Section B
5
6
7
8
25 dgnd
Chapter 3
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51
Options
The Options menu encompasses the following items.
Trigger
Under this item the option Trigger is present. After selecting this option the following
window appears. In this window the trigger pulse can be configured.
Fig. 27 The Trigger option window
After enabling the trigger pulse option, the 'Start' button has to be clicked. The
program will go through pretreatment and equilibration and will then wait for the
trigger-signal.
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pretreatment…. equilibration
Version 4.9
measurement
end of measurement
high
low
Rescale after measurement.
Enable or disable automatic rescaling directly after the measurement of a
voltammogram.
Rescale during measurement
When this option is activated, the graph in the Data presentation window is rescaled
when a measured data point is outside the boundaries of the plot.
Procedure name in Data presentation Window
With this option it is possible to print the procedure file name on the Data presentation
window. This is useful to identify the graph when it is dumped on a printer.
Show all GPES files in File dialog box
If this option is activated, all files of all techniques are shown in the File dialog boxes
of the File menu. The program will switch automatically to the appropriate method.
Window
The Window option allows the selection of windows which should be shown on the
screen. The Tile option gives the default partitioning of the screen.
The Close all option will delete all the GPES windows except for the status bar and
the GPES Manager window.
Help
The Help option is the top entry point in the help structure. For most topics on the
screen Help is available. By pressing F1 the specific information about the part of the
screen on which has been focused is given.
Tool bar
The tool bar contains a list of buttons, the current electrochemical method, and the
name of the current measurement procedure.
The buttons give short cuts to various menu options which are frequently used. Place
the mouse pointer on top of a button. Its meaning will appear in yellow, if pressing the
button is allowed.
Chapter 3
The GPES windows
53
3.2 Status bar
The lowest part of the screen is reserved for the status bar. The Start button starts the
execution of a measurement procedure. After clicking this button, other buttons
appear which allow to advance to a next stage or to abort a measurement procedure.
The Status and Message panel give important control information.
After the Start button is clicked, the cell is switched on and the measurement starts
with a pre-treatment.
If an automatic mercury drop electrode is connected to Autolab, the following control
sequence is executed: the solution is purged, if the purge time exceeds 0 s.
Subsequently a new drop will be created. Then the cell will be switched on and the
pre-treatment potentials are applied when their duration is not zero. During these
periods, the stirrer will be on. Before the measurement starts, the stirrer is switched
off and the initial or standby potential is applied and the equilibration period starts in
order to stop convection of the solution.
During the pre-treatment period, the measured dc-current is printed in the Manual
control window.
3.3 Manual control window
The Manual control window gives full control over the potentiostat/galvanostat of the
Autolab instrument, including the optional modules:
•
the low current module ECD
•
the bipotentiostat module BIPOT
•
the 10 A current booster BSTR10A
•
the integrator/filter module FI20 for the PGSTAT12/20/30/100.
Fig. 28 Manual control window
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It is also possible to perform potential/current/charge measurements as a function of
time. Note that some of the Autolab settings are part of the measurement procedure.
The Manual control window consists of several panels.
Current range
In the Current range panel the green 'LED' indicates the actual current range. A mark
in the neighbouring check box indicates whether the current range can be selected.
Only a joined column of selectable current ranges is allowed. The software always
checks whether the row is closed. If a range separated from another range is checked,
the intermediate ranges are checked automatically. When a check box is clicked
again, the check disappears. The allowed current ranges are stored on disk as part of
the procedure. Sometimes a current range annotation is coloured red. This means that
it is advised not to select this current range during execution of the current procedure.
The response times of this current range is too high for the specified measurement
procedure.
Settings
In the Settings panel, the mode of operation can be defined.The text on the button
represents the current situation. The following buttons might appear (depending on
the type of potentiostat/galvanostat):
Cell on/off: allows to switch cell on or off. In the ‘off’ position the connection of the
potentiostat with the potentiostat/galvanostat is broken, so no current can flow
between the counter and working electrode.
High Sens off/on: it indicates whether the gain 100 of the amplifier of the ADconverter is used. If "off" only gain 1 and 10 are used.
The current resolution is improved by a factor of 10. The disadvantage of the high
sensitivity "on" is that the measurements are somewhat slower and that they are more
susceptible to overloads. It is recommended to switch the high sensitivity "on" only
when the limits of the digital resolution show. For example, at the current range
100nA the current resolution is improved from 3pA to 0.3 pA, when high sensitivity
is switched on. See also the chapter about the digital base of Autolab in the
"Installation and Diagnostics" guide.
High Stability/High Speed: The potentiostats/galvanostats can be used in either the
high speed, with high bandwidth, mode or the high stability, with low bandwidth,
mode. The bandwidth at high stability is about 10 kHz. Some electrochemical cells
may cause stability problems with the instrument in high speed mode. Especially cells
with high capacity and low resistance may cause oscillations. Using the instrument in
high stability mode may prevent oscillations. It is advised to use high stability in all
experiments, except when high bandwidth is needed. High speed or high bandwidth is
needed when frequencies higher than 10 kHz or sampling or interval times below 100
µs are used.
Potentiostat/Galvanostat: allows switching from potentiostatic to galvanostatic mode.
It is highly recommended to switch the cell off before the mode change. In case of
potentiostatic control, the output of the DAC module corresponds to an applied
potential level. In case of galvanostatic control, the output of the DAC module
corresponds to an applied current.
Chapter 3
The GPES windows
55
iR-comp. on/off: switches iR-compensation’ on’ or ’off’. A more elaborate description
is found in the dedicated paragraph in this chapter (only available with a
PGSTAT12/20/30/100).
Potential
The Potential panel contains a slider and a text box. With these tools the applied
potential can be specified. The slider box can be dragged to change the value. A click
on the arrows and slider bar changes the value by a distinct increment. The increment
is different for the arrows and for the bar.
In the two panels below the measured current, potential and/or time can be displayed,
depending on the option button selected.
The option button "2nd Sig." appears, in case a chronomethod or the method cyclic or
linear sweep voltammetry is selected and the Record second signal option is checked
on page 2 of the procedure parameter list.
The Clock off/on button in the lowest of the two panels starts the timer, and displays
the measured data from the panel above in the Data presentation window. This makes
it possible to display graphically what is going on. These measured data can be
replotted, printed, and the graph can be stored. However, the data can neither be
saved, analysed, nor edited.
Noise meters
The noise levels for current and potential signals are visualised by 2 noise meters at
the signal panels. When these VU-meters are active, the first green LED or a grey
background is shown.
The VU-meter for the current signal is only active when the cell is switched on. The
VU-meter for the potential signal is also active when the cell is switched off i.e. no
current can flow. During the execution of the procedure (except for pre-treatment
stage) the VU-meters are inactive.
In case more than 4 LED's of the VU-meter are on, it is advised to take precautions.
You can select a higher current range or minimise the noise of your electrochemical
cell. High potential noise levels are often caused by the reference electrode.
iR-compensation
The iR-compensation panel appears only when the Autolab is equipped with a
PGSTAT12/20/30/100 potentiostat/galvanostat. In order to perform iR-compensation
the iR-compensation button on the Settings panel should be switched to "iR-comp.
on". Subsequently the Ohmic resistance can be specified using the slider or by typing
in the textbox. Note that when the iR-compensation is switched on, automatic current
ranging is no longer possible. The checked current range box becomes the actual
current range.
In case of a manual change of the current range, the compensated resistance will be
kept as constant as possible. Because internally the compensation is proportional to
the current range, the value for the compensation needs to be recalculated when the
current range is changed. With a change of current range the resolution of the
compensation will alter. The maximum compensation of a current range equals two
times the measurement resistance. This means that for the 1 A range the maximum
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compensated resistance is 2 Ohm and for the 1 µA range it is 2 MOhm. The resolution
is always 0.05% of the measurement resistance.
Integrator
The Integrator panel appears only when the Autolab is equipped with an analog
integrator. This is not the case with a PGSTAT12/20/30/100 potentiostat/galvanostat
unless the Autolab is equipped with a FI20-module. For further use, see the section
about chronocoulometry.
Filter panel
The Filter panel appears when the Autolab is equipped with an ECD or an FI20
module. The following option buttons can be clicked "Off", "5s", "1s", ".1s". The
effect of a filter constant of e.g. 5s is that 5 seconds after a potential perturbation has
been applied, the current response can be measured correctly.
The “Remote led” indicates when it is possible to edit the parameters in Manual
Control. In cases where it is red, it is not possible to edit the parameters in this
window.
3.4 Data presentation window
The Data presentation window serves several functions:
•
display of data
•
data analysis
•
data manipulation
•
communication with other programs like Paintbrush, Excel or MS-Word.
The window consists of a menu bar, a graphical display, and a message line.
As mentioned earlier the measured data are kept in a shared data memory block with
the data acquisition software. During the measurement the measured data points are
also copied to the memory block of the Data presentation window. After the
measurements the data in this memory block can be modified by options in the Data
presentation window. However, it is always possible to resume the measured data.
Note that the save options of the File menu of the GPES Manager window always
save the measured data. Also note that for cyclic and linear sweep voltammetry,
during the execution of a procedure, the measured data are plotted, but after the
measurements only the last measured scan is visible. The data, which can be modified
in the Data presentation window, are called work data and can be stored from the File
menu of the Data presentation window in a work data file. This file cannot be
distinguished from the files with measured data. Both types of files have the same
format and layout. Some care should be taken with saving the work data. For
example, as is described further on, the display of the current values can be changed
to a square root of the current. If the work data are then saved, not the current values
but the square root of the current is saved.
Chapter 3
The GPES windows
57
On the message line at the bottom of the graphical display important text about the
required user actions during analysis of editing data appears. If no message is
displayed, the currently measured potential and current are displayed.
File
The file option allows to create data files based op on data presented in the Data
presentation window. It allows to save the so-called work data file as discussed
above. The following options are available.
Save work file
This option was discussed above.
Save I(forward) and Save I(backward)
The Electrochemical techniques Square wave, Differential pulse and Differential
Normal pulse provides the forward and backward current data. These curves can be
plotted but not analysed. The separate curves can be stored, and retrieved with ‘File’,
‘Load data’ from the Gpes manager window to do analysis.
Save impedance data
When an AC voltammetry measurement is done, the impedance data for each data
point can be saved to disk. This option is present in the File menu of the Data
presentation window. The file, with the extension .IMP, has the following layout:
Date: 02-May-97
Time: 13:24:39
Frequency (Hz) : 252.020
Amplitude (Vrms): .001
Phase sensitive : Yes
Phase (degree) : 20.000
E/V
i(ac)/A
.
.
.
Z/ohm
phi/deg
Rs/ohm
Cs/F
NOTE: These data are additional data and can only be obtained after the
measurement, NOT after loading a data file.
Quick save of previous scan
When a Cyclic or Linear sweep voltammetry measurement with more than one scan is
going on it is possible to save the previous measured scan. This option can also be
activated by typing 'SAVE' on the keyboard. This option work similar to the
procedure parameter ‘Save every nth cycle’.
Save as Chrono data
Save a Step segment of Steps and Sweeps as Chrono methods data. This data file can
be read in Chrono methods in order to perform data analysis.
Save as Linear sweep data
Save a Sweep segment of Steps and Sweeps as Linear sweep data. This data file can
be read in the Linear sweep method in order to perform data analysis.
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Copy
The Copy option allows to copy the graph to clipboard or to dump the graph in a
bitmap file (.BMP). These files can be read by programs like Paintbrush, Excel or
MS-Word. These programs allow editing of the graphs. The best result is obtained by
doing this from a maximised Data presentation window. By default GPES only draws
dots. It is sometimes better to draw lines. This can be achieved by double-clicking the
data points in the graph. For further information, see the paragraph on Editing
graphical items.
Plot
The Plot option contains all kind of possibilities to manipulate the graph like plot
refresh, automatic scaling, zooming, setting a Data window, display of a previously
measured signal. Sometimes, not all options are selectable because they are not
applicable or intervene with current active data analysis options. Also when the
execution of a procedure is going on, not all options are selectable.
New plot
The New plot option removes objects, such as lines and markers, from the screen.
During measuring a cv, this option removes the previous measured data points from
the screen.
Fig. 29 The Plot menu
Chapter 3
The GPES windows
59
Some sub-options require explanation.
Automatic
This option automatically rescales the data during a measurement.
Resume
The Resume option makes a fresh copy of the measured data into the Data
presentation window. This allows you to get back to the original data after doing data
analysis.
Zoom
Clicking the Zoom option has the same effect as pressing the right mouse button.
When this option is activated a magnifying glass appears. When subsequently the left
mouse button is clicked and held down a Zoom window can be created.
Set window
The Set window option allows to define a part of the data set. Any further data
manipulation and display will be applied to this part of the data. With Chrono
methods the x-values will be normalised to 0, so the x-axis always starts at t=0.
First- and Second signal
The First, and Second signal options are selectable, when, next to the current or
potential signal, a second signal is recorded. A marker in front of these options means
that they are displayed. When both are displayed no further data analysis is possible.
E vs t plot and dt/dE vs E plot
These options appear only when the method is Potentiometric stripping analysis. It
allows switching between these types of plots. The E vs t plot can be used to do
Transition time analysis (see the chapter on the analysis of measured data) in order to
analyse the kinetics and reversibility of the electrode process. The dt/dE vs E plot can
be used for electroanalytical purposes.
Show I (forward)
This option shows for the techniques square wave and differential pulse voltammetry
only. With square wave voltammetry it allows to toggle the display of the current
measured in the pulse which is applied in the scan direction. With differential pulse
voltammetry it allows to toggle the display of the current before applying the pulse.
Show I (backward)
This option shows for the techniques square wave and differential pulse voltammetry
only. With square wave voltammetry it allows to toggle the display of the current
measured in the pulse which is applied in the opposite of the scan direction. With
differential pulse voltammetry it allows to toggle the display of the current measured
in the pulse.
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Load overlay file
This option allows the making of a graphical overlay of one or more data sets. A
maximum of 10 overlays can be made. You can select multiple files at a time by using
<Shift> or <Control>, combined with the mouse action.
The overlay legends shown in the Data presentation window, will also be visible in a
print-out. After clicking the New plot option or the Resume option, the overlays will
disappear. It is possible to load overlays together with the main data (see Load
data/scan option of the File menu)
Reverse axes
This option will reverse the direction of the horizontal as well as the vertical axis. It
allows peaks always to be displayed upwards.
Enter text
When this option is clicked the text "Text field" appears in the left top corner of the
graph. This text can be dragged over the graph. After double-clicking the text field,
the text itself, and the format can be modified. The first text line of the Paste buffer
can be inserted on the text field as well. Thus a line of text from the Analysis results
window can be copied to the Paste buffer and subsequently inserted there. Please note,
that the text cannot be stored.
Analysis
The Analysis option contains an elaborate set of methods to extract essential
parameters from the measured data. The background is described in the special
chapter about this subject. The available data analysis options depend on the selected
electrochemical method.
Edit data
The Edit data option gives the opportunity to modify the presented data. The
backgrounds are explained in the special chapter about this subject.
Work scan
The Work scan option, which is only present for cyclic and linear sweep voltammetry,
allows the selection of a scan for further data analysis.
Work potential
The Work potential option, which is only present for multi mode electrochemical
detection, allows the selection of a potential for further data analysis and peak search.
Editing graphical items and viewing data
Except for the available options, items of the graph can be edited by double-clicking
them. The following items can be double-clicked:
•
the axis annotation
•
the axis itself
Chapter 3
The GPES windows
Fig. 30 Horizontal axis window
the axis description
•
the plot title and subtitle
•
the data.
Colours, sizes, marker types, text, formats, axis position: all these things can be
changed. Please take some care with changes in Colours. E.g. do not make the data
colour the same as its background colour.
•
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Fig. 31 Plot parameter window
By double-clicking the data points a window appears, which among the standard
graphical operations also gives the possibility to view the data values itself and to edit
them. Moreover the data can be copied to clipboard and subsequently be entered into
e.g. a spreadsheet program.
Fig. 32 Graph parameter window
Double-clicking the axis itself allows scaling and positioning of the axis and selection
of the axis function. Data can be displayed, among others, as linear inverse, 10log,
natural log, square root, inverse square root. Except for the linear and 10log, the value
Chapter 3
The GPES windows
63
of the presented data is modified in real. So all subsequent operations are really
performed on e.g. the square root of the data.
In case of the 10log, not the values but the axis is changed to a logarithmic axis.
When the button "| 1 |" in the upper right corner is clicked, the Graph parameter
window appears. This window allows modification of the relative scale parameters of
the so called graph and plotting area, and their background colours.
All the changes made to colour and sizes are stored in the default graphics display
file. The other changes are kept in the procedure file.
3.5 Edit procedure window
The Edit procedure window consists of 2 pages. In Page 1 the most common
parameters can be specified. Page 2 contains the other parameters. The meaning of
each parameter is clarified by the Help program.
A list of parameter descriptions is given in the appendix about this subject.
In the option bar, the Send option is displayed. This option can only be clicked during
the execution of a measurement procedure. It is relevant when a parameter is changed.
The Send option activates the modified value. The modified value is accepted when a
beep sounds.
The layout and setup of the Edit procedure window is more or less the same for all
electrochemical methods.
For most methods on Page 1, the following type of parameters can be specified:
•
the first pre-treatment and subsequent equilibration
•
the definition of the type of measurement
•
the potential or current level parameters
•
the title and subtitle.
The items on page 2 depend on the method.
For voltammetric analysis the following extra items are available:
•
number of voltammetric scans which will be recorded subsequently and averaged
•
a number of comment lines.
For cyclic and linear sweep voltammetry the following items are extra available:
•
extra pre-treatment stages
•
special measurement conditions
•
special display option to swap x- and y-axis (Tafel plot)
•
a number of comment lines
•
current boundaries for reversing the scan direction
•
optional chronoamperometric measurement conditions at the vertices
•
optional recording of an external signal.
For the chronomethods the following extra items are available:
•
extra pre-treatment stages
•
special measurement conditions
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optional recording of an extra signal
a number of comment lines.
For electrochemical detection the following extra items are available:
•
extra pre-treatment stages
•
a number of comment lines
•
differential pulse condition (if applicable).
For potentiometric stripping analysis the following extra items are available:
•
smooth level during differentiation of potential versus time data
•
a number of comment lines.
A full list of input parameters is given in appendix II.
3.6 Analysis results window
The Analysis option of the Data presentation window allows the making of an
analysis of the data. In some cases the results are displayed in a special window which
differs per analysis technique. In all cases a report of analysis is printed in the
Analysis results window.
The Analysis results window contains all the results of the analysis of the data. Only
when the GPES Manager window is closed, the Analysis results window is cleared.
The File option of this window allows the user to clear, save or print the content of the
window. The Edit option allows the user to remove (Cut) the selected part of the text.
Text can be selected by keeping the left mouse button pressed and moving it over the
window. The Copy option copies the content of the window to the paste buffer
including a so called DDE link. For example, MS-Word can, via Paste special option
makes a Paste Link. This means that any change to the content of the window will
automatically be copied to the MS-Word document until the link is broken via the
Links... option of MS-Word.
The Paste option will include text from the paste buffer.
It is possible to copy results of Analysis into the graph of the Data presentation
window. This is useful if you want to have a complete printout of the graph. You can
include the Analysis text as follows:
•
You can select (a part of) the text,
•
Under the option Edit click the option Copy (or Cut). Now the text is present in
the paste buffer,
•
Now select the Copy menu on the Data presentation window,
•
Select Paste text.
The text will be included into the graph.
Please note:
In the Data presentation, you can drag the text to the position you want. The text in
Data presentation can not be changed, but can be cleared by double clicking and
typing in a space in the Text field. You can also change the text style in the window.
For a proper outlined text a change to the font ‘Courier’ is sometimes required.
Chapter 4
Analysis of measured data
65
4. Analysis of measured data
Under the Analysis option on the Data presentation menu there are a number of
facilities to analyse measured data. Some analysis techniques are specific for an
electrochemical method.
The results of the analysis are sometimes displayed in a specific window. In all cases
the results are printed in the Analysis results window. This window can be made
visible from the Window option on GPES Manager menu bar.
In most cases not all options are selectable. This may have the following causes:
•
The option is not relevant for the electrochemical method.
•
The execution of a measurement procedure is in progress.
•
Another conflicting option is already selected.
•
Just after the recording of more than one cyclic or linear sweep voltammogram, a
selection of the scan to be analysed can be made. See Work scan option on the
Data presentation window. By default the last measured full scan is displayed.
•
When a second signal has been recorded concurrent, first a selection has to be
made which signal should be analysed. See Plot menu on the Data presentation
window.
•
The option is not relevant or cannot activated because an ambiguous situation is
present, i.e. it is not clear on what data the analysis should be done.
4.1 Peak search
When the Peak search option is selected, the Peak search window appears. If the
curve shows distinct peaks, you can often simply click the Search button in the right
panel and the results are shown in an appearing Results panel. The number of digits is
the same as the precision of the axis.
The Clear button will erase the Results window and will refresh the plot. The Close
button closes the Peak search window. The Show results button will show the Results
window.
The Peak search options window gives several options to search for peaks in a
voltammogram. It opens when the Option button is clicked. If Automatic option
button is 'checked', the peak search algorithm needs two input parameters to be
specified: the minimum peak height and the peak width. Otherwise only a value for
the minimum peak height is required. These parameters have to be specified in the
Minimum panel. Peaks with a height below the minimum peak height will be omitted.
The peak width parameter determines the width of the window which moves through
the data set. Peaks with a width (at half peak height) which are smaller than the
specified peak width might not be found.
It is possible to find peaks that are present as shoulders on a steep base curve.
For this purpose the Peak search option contains an Include shoulders option. The
option is found at the Peak search options window and can be selected only in
automatic search mode.
When this option is clicked on, the Peak search is performed after a basecurve
correction on the background, according to the moving average baseline correction
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method. This method can also be performed separately. See Baseline correction
option in the Edit data menu.
Fig. 42 Peak search window
Fig. 43 Peak search options window
Fig. 44 Set results format window
Chapter 4
Analysis of measured data
67
Fig. 45 Peak search results window
Peaks are normally searched in the scan direction. If this is not required, the Reverse
option in the upper right corner of the window should be checked.
In the Baseline panel it can be 'checked' whether or not the peaks found will be
corrected for the baseline. This baseline is determined by means of the tangent fit
method.
The tangent is drawn from the left side to the right side of a peak if ‘linear baseline is
checked’. In cyclic and linear sweeps it may be better to use only the front of the peak
to construct the tangent. This is done when ‘Lin. front baseline’ is checked.
If the automatic peak search method fails, the Automatic search should be switched
off. Two alternatives are present: curve cursor and free cursor. In both modes two
points need to be marked; the beginning and the end point of the peak. In curve cursor
mode the markers are automatically put on the voltammogram. In free cursor mode
the marks can be put everywhere on the graph. In curve cursor mode the two markers
several types of basecurves can be drawn: linear, 3rd order polynomial, or
exponential. Moreover, in case of double peaks, only peakheight of the front peak, the
rear peak, or highest in the whole selected peak area can be determined.
Fig. 46 Example of polynomial basecurve in peak
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Furthermore it is also possible to draw a linear baseline in case only the front part or
only the rear part of the baseline is known. The options Lin. front baseline or Lin. rear
baseline on the Baseline panel should then be ‘checked’. Now two points on the front
part respectively rear part of the peak should be marked. In free cursor mode only a
linear baseline can be drawn.
When the options 'Curve cursor, linear baseline' and 'Free cursor' are selected,
baselines can be constructed manually. After the first point of the baseline has been
marked, a line connected to this point will be dragged until a second marker is
clicked.
The Set format button can be clicked to adjust the formats in the Set results format
window. The defaults in this window are the formats of the axis-labels.
The results of the peak search are:
Position
Height
: potential at which the current with respect to the baseline has a
maximum.
: maximum current with respect to baseline.
Fig. 47 Example of a linear front baseline
Peak area
Derivative
Ep - Ep/2
: area of the peak corrected for the baseline. For data from
cyclic linear sweep voltammetry the area is expressed in
Coulombs. This means that the area is divided by the scanrate.
: the sum of the absolute values of the maximum and the
minimum in the derivative of peak.
: the difference between peak potential and the potential at half
height. This value is only printed for data from cyclic and
linear sweep voltammetry. It is useful to derive kinetic
parameters. See the book of C.M.A. Brett and A.M.O.
Oliveira Brett, Electrochemistry Oxford science publications
ISBN 0-19-855388-9.
Chapter 4
Analysis of measured data
69
In quantitative voltammetric practice the peak height is the most widely used
parameter to determine concentrations. The peak area is less sensitive to noise, but if
the peak is not completely isolated from another peak, the error in the peak area will
be high. The sum of the derivatives of the peak is less sensitive to peak overlap.
However, derivation of an experimental curve will increase noise. For more details
see Ref.: J. Tacussel, P. Lectere and J.J. Fombon, J. Electroanal. Chem.,214 (1986)
79-94.
4.2 Chronoamperometric plot
The Chronoamperometric plot option produces a special plot for two sequential
potential steps in a chronoamperometric experiment. The first step should be the
forward potential step and the second the reverse potential step in a (quasi-)reversible
electrode reaction. The book of A.J. Bard and L.J. Faulkner, Electrochemical
Methods, Fundamentals and Applications, Chapter 5 (ISBN 0-471-05542-5) gives
more details.
If data for more than two potential steps are present, a selection of the first (forward)
potential step can be made. This option is only active if it is applicable.
4.3 Chronocoulometric plot
The Chronocoulometric plot option produces a special plot for two sequential
potential steps in a chronocoulometric experiment. The first step should be the
forward potential step and the second the reverse potential step in a (quasi-) reversible
electrode reaction. The book of A.J. Bard and L.J. Faulkner, Electrochemical
Methods, Fundamentals and Applications, Chapter 5 (ISBN 0-471-05542-5) gives
more details. If data for more than two potential steps are present, a selection of the
first (forward) potential step can be made. This option is only active if it is applicable.
The charge flown since the beginning of the potential step is plotted versus
the square root of the time since the start of the step.
The kinetic parameters can be obtained by selecting the Linear regression option (see
below). A line should be fitted for the forward as well as for the reverse plot. The fit
parameters appear in the Analysis results window as well as in the results panel.
4.4 Linear regression
The Linear regression option allows to fit a straight line through a part of the
measured curve.
When the option has been selected, two windows appear. One is the Linear regression
window and the other is the Markers window. When the user has marked the begin
and end point of the line on the measured curve and has clicked the OK button, a line
is drawn so that the sum of squares of the differences between measured and
calculated value is minimum.
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Fig. 48 Linear regression window
The slope of the line (dY/dX), its inverse, the intercept (the value at X = 0), the
number of points between begin and end point, the standard deviations and the
correlation coefficient are displayed in the Linear regression window. More lines can
be fitted when the Set line button is clicked.
4.5 Integrate between markers
This operation will determine the area under the curve between two selected points.
With cyclic voltammetry, the area is expressed as charge (C). Thus the calculated area
is divided by scanrate.
4.6 Wave log analysis
The Wave log analysis option is active for voltammetric analysis and cyclic and linear
sweep voltammetry.
The half wave potential E½ can be determined and Tafel slope analysis can be done
for a S-shaped cyclic voltammograms or convoluted voltammograms (see
Convolution option). After having selected this option, the user is asked to define a
base line and a limiting line. Subsequently E½ is calculated from the crossing between
the average line of the base line and the limiting line with the measured curve. The
limiting current at E½ is calculated as two times the current at E½ with respect to the
base current.
After pressing the Continue button on the Wave log analysis window, a Tafel line
analysis can be done from the plot of ln {(id - i )/i} versus the potential, where "i"
stands for current, "id" for limiting current and "ln" for natural logarithm. If a cyclic
voltammogram has been deconvoluted the "i" is replaced by "m". The markers for the
line should be selected not too far from zero on the Y-axis.
The intercept is at E = 0. The parameter Alpha * n (αn) has been calculated from the
slope:
Chapter 4
Analysis of measured data
71
slope = αnF/RT
F = Faraday constant
= 96484.6 C/mol
R = Gas constant
= 8.31441 J/mol/K
T = temperature
= 298.15 at 25°C
n = no. of transferred electrons
α = 1 for reversible reactions
α = transfer coefficient for irreversible reactions.
An example of wave log analysis is described in Chapter ‘Getting started with GPES’.
4.7 Tafel slope analysis
This option is only available for Cyclic and Linear sweep voltammetry. When the
option is selected, the current is plotted on a logarithmic scale. The user is asked to set
markers for the anodic and cathodic branch on the curve. In principle this option
works similar to Corrosion rate analysis without the fitting part.
4.8 Corrosion rate
This option allows the determination of the corrosion rate and the polarisation
resistance.
If the current versus the potential curve passes the zero current line more than once,
the user is asked to define a window of interest around the point where the anodic
current balances the cathodic current. Before doing this, it might be useful to draw the
horizontal axis through the origin of the vertical axis. This can be done by doubleclicking the horizontal axis and subsequently selecting the "Origin" in the Intercept
position panel.
If the curve passes the zero current line only once, the whole curve is used for the
analysis.
Subsequently the graph is transformed in a logarithmic scaled current versus potential
plot and the Corrosion rate window appears. This window shows the corrosion
potential and the polarisation resistance at the corrosion potential.
In this window the surface area (SA), equivalent weight (EW), and the density (D) of
the electrode material can be specified. These data are used to calculate the corrosion
rate in terms of current density (Icorrosion) and millimetres/year (CR):
Icorrosion = icorrosion /SA A/cm2
and
CR = 3272*icorr*EW/(SA * D)
The polarisation resistance Rp is determined by taking the reciprocal value of the
derivative di/dE. The derivative is obtained from a 2nd order polynomial fit through
the corrosion potential and its neighbours. From this Rp value the corrosion rate can
be obtained:
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icorrosion = B/Rp where B is normally an empirical constant.
B can also be obtained from the Tafel slopes (M. Stern and A.L. Geary, J.
electrochem. Soc. 1957, 104, 56).
After clicking the Tafel slope button, the Marker window appears. Now the Tafel line
for cathodic branch has to be defined by marking two points. After the OK-button on
Marker window has been clicked, the same has to be done for the anodic branch.
After the Tafel lines have been set, a second Corrosion window appears with the
results:
1. The corrosion current, corrosion current density and the corrosion rate.
2. The Tafel slopes ba and bc.
3. The corrosion potential at zero current and the corrosion potential as calculated
from cross-point of the two Tafel lines.
4. The Polarisation resistance Rp obtained from the equation:
Rp = B/icorrosion where B =
1

S
Fig. 49 Corrosion rate analysis
and S = 2.303 *
 1
1 
  +  
 ba
bc 
Chapter 4
Analysis of measured data
73
The corrosion rate is determined on the basis of the equation:
i = icorrosion
where





 exp s1(E - Ecorr)  - exp -s2(E - Ecorr) 





s1
s2
Eeq
icorrosion



= slope of the anodic branch = 2.303/ba
= slope of the cathodic branch = 2.303/bc
= the equivalence or corrosion potential
= the corrosion rate or exchange current in Ampere
When the Start fit button is clicked, a fit is performed on the slopes and the corrosion
current. The observed corrosion potential, i.e. the potential where i = 0, is taken as the
corrosion potential Ecorr.
The fitting is performed according to the non-linear least square fit method of
Levenberg/Marquardt. The values obtained from the Tafel lines are used as start
parameters. After some iterations the fit results are presented on the screen. Also the
number of iterations and the goodness of fit parameter chi-square are given. Chisquare is the sum of the squares of the differences between measured and calculated
data. The fitted slopes are s1 and s2. The fitting procedure can be interrupted by
clicking the Stop button.
The comparison between the observed and the calculated curve is shown.
The Tafel slope parameter α can be obtained from the slopes:
b = 2.303 RT/3αnF
F
R
T
n
2.303
= Faraday constant = 96484.6 C/mol
= Gas constant
= 8.31441 J/mol/K
= temperature
= 298.15 at 25°C
= no. of transferred electrons
= ln (10)
It is sometimes possible to improve the fit by clicking the Restart button.
4.9 Spectral noise analysis
After recording current- and potential- noise transients, it is commonly desired to
perform a statistical or frequency analysis on the results. The GPES software enables
the calculation of a frequency spectrum for current and potential or impedance. It can
be activated in the Analysis menu of the Data presentation window.
The frequency spectrum is calculated by means of a FFT algorithm. Since it requires
the dataset to be a power of 2, the number of datapoints is automatically extended
when necessary. In case of an interrupted measurement (<Abort> was pressed), the
dataset is padded with zeros.
In principle, the Fourier method should only be used on datasets that are periodical
and “fit” exactly in the time duration of the recorded scan. For noise signals, this is of
course not true. Therefore for some cases, it would be advisable to apply a so-called
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“Window function”. These operations will counteract the effects that were mentioned
above. A total of 5 functions are available:
none/Bartlett/Hanning/Hamming/Blackman.
A range of literature is available on the theoretical background of signal processing.
See:
“Numerical Recipes”, W.H.Press et al., Cambridge University Press,
Cambridge 1997
4.10 Find minimum and maximum
The Find minimum and maximum option shows the minimum and maximum Y-value
with their corresponding X-values.
4.11 Interpolate
The Interpolate option allows the user to calculate one or more X-values or Y-values
which corresponds to a given value on the other axis. A linear interpolation is used to
calculate intermediate values.
4.12 Transition time analysis
The Transition time analysis option only appears for chronopotentiometric data or
data from Potentiometric stripping analysis. The background of this analysis is fully
described in the book of C.M.A. Brett and A.M.O. Oliveira Brett, Electrochemistry
Oxford science publications ISBN 0-19-855388-9.
After selecting this option, a graph of the time versus potential is presented and the
user is asked to specify two marker points for subsequently the baseline, the transition
line, and the limiting line.
The time difference between the crosspoints of the transition line and the base line
respectively the limiting line is defined as the quantity 'tau'. The crosspoint of the
transition line with the baseline is called t-base of 'tau'. Finally also the quantity E3/4 E1/4 is given. This is the difference in potential at three quarters of 'tau' and at one
quarter of 'tau'.
Subsequently the plot can be transformed dependent on whether the measured system
is thought to be reversible or irreversible. Finally a linear regression can be done to
extract the kinetic parameters from fitting a straight line. An example of transition
time analysis is given in the chapter ‘Getting started with GPES’.
4.13 Fit and simulation
The Fit and simulation option is located in the Analysis menu of the Data Presentation
window. It provides the method to determine parameters of electrochemical
processes, like formal redox potential, heterogeneous rate constant, transfer
coefficient α etc., as well as to simulate theoretical current-potential curves. The table
Chapter 4
Analysis of measured data
75
below summarises the models (i.e. combinations of experimental techniques and
reaction mechanisms) for which fitting and simulation are currently available.
The models available for fitting and simulation are extended with the mechanisms in
which two parallel reactions are involved. The models are called 'two-component'
models and are available for the cyclic voltammetry technique.
Table 2 Models available for fitting and simulation.
staircase cyclic
voltammetry
reversible
quasi-reversible
differential pulse
voltammetry
reversible
quasi-reversible
normal pulse
voltammetry
reversible
quasi-reversible
irreversible
ErCi (irreversible
chemical reaction)
irreversible
ErCi (irreversible
chemical reaction)
irreversible
ErCi (irreversible
chemical reaction)
square wave
voltammetry
reversible
quasireversible
irreversible
ErCi
(irreversible
chemical
reaction)
two-component
In all the above models it has been assumed that the experiment is carried out on a
stationary electrode in an unstirred solution. The fit and simulation works only with
staircase voltammograms but not with linear scans. Moreover, in cyclic voltammetry,
the current measured during a potential step depends on the “history” of the scan, i.e.,
on the number, height and duration of all preceding steps. This means that for the sake
of speed, the number of potential steps should in the scan should be kept small.
The simulation method
Digital simulation of current-potential curves is based on finite difference method.
Equations of the transport of electroactive substances to the electrode surface are
solved by Crank-Nicolson technique, a method widely used by electrochemists and
renowned for its accuracy and stability. The time-dependent concentration profiles
obtained from these equations are used for the calculation of the current.
The advantage of digital simulation is its versatility. This feature is easily visible
when the simulation method is compared with analytical equations describing currentpotential curves, which have a number of restrictions on their validity.
The fitting method
Fitting is carried out using Marquardt nonlinear least-squares method. The model
functions are either calculated from analytical equations (wherever possible) or
obtained by digital simulation of the electrode processes. The convergence criteria are
based on the value of χ2 , Σ(Y-Yfit)2, and its change during the last iteration, as well as
on the requested precision of the fitted parameters.
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Elements of the Fit and Simulation Window
Fig. 50 Fit and simulation window
In the upper part of the Fit and simulation window there is the drop-down menu with
a list of available models, five action buttons, and two option buttons to switch
between the fit and the simulation mode of the program. The list of models that can be
used depends on the experimental technique chosen.
The middle part of the window contains three display fields. 'Init. Guess' button starts
the calculation of the initial guesses of fitable parameters. Fast Fit performs a fit on a
reduced data set. See the chapter ‘Fitting in more detail’ about the data reduction and
other fast fit parameters. The Full Fit button starts a fit of the model on all data points.
This button is replaced by the Simulate button in case the simulation option is chosen.
The Stop Fit button will interrupt a running fit procedure. Fields show the status of
calculations, the value of χ2 and the elapsed time.
The lower part of the fit and simulation window displays the parameters. Normally,
only fitable parameters are shown (it is possible to display all parameters by choosing
the Extended setup option from the Option menu). Each line contains the name of the
parameter, its value and a checkbox to indicate whether the parameter should be fitted
or not. In extended setup, in each line there are additional fields for the value and the
type (absolute, relative or disabled) of the convergence criterion.
Fitting and simulation step by step
Fitting
1
Load a cyclic voltammogram file,
C:\AUTOLAB\TESTDATA\DEMOCVO3.0CW.
2
From Analysis menu (Data presentation window) select Fit and Simulation.
3
Select the model 'reversible' from the drop-down menu in the top part of the
appearing window.
Chapter 4
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Analysis of measured data
77
Make sure the switch right to the model name is set to Fit.
Get the initial guesses for the parameters either by clicking Init. guess button,
pressing Alt-G or by choosing Initial guess from the Option menu.
Check whether the number of exchanged electrons is correct.
Select parameters to fit by checking the boxes next to parameter names. If
necessary, adjust the starting value of parameters by clicking it with the mouse
pointer and entering the new value (make sure that the number of electrons is set
to 2).
If necessary, switch to extended setup (Option menu, or Ctrl-E) to edit other
parameters or to change parameter’s convergence criteria. Parameters that are
visible in the extended setup but invisible in the standard setup are not fitable, i.e.,
their values are not changed during the fit process (see "Fitting: choosing
parameters to fit" and "Fitting: convergence criteria" for more detailed
information).
Select Full Fit control parameters from the Option menu and adjust their values
(see "Fitting: advanced options" for details).
Click Fast Fit button on a reduced number of data points.
The fitting proceeds until the convergence criteria are satisfied or the maximal
number of iterations is reached, whatever comes first. It is possible to stop fitting
at any moment by clicking Stop fit button. It is possible, that the program will
need a few seconds to complete the iteration before stopping.
During the fit, the field Status shows the number of the iteration, field Chi-square
shows the χ2 value and field Elapsed time shows time elapsed from the start of the
fit.
If convergence is reached, the Status field contains information "ready" and the
number of iterations.
If the maximal number of iterations has been carried out without reaching the
demanded convergence criteria values, the status field displays information
"stopped" and the number of iterations.
If fit has been stopped by Stop fit button, the status field displays information
"interrupted".
Pressing Full Fit starts the new cycle of fitting, taking all data into account, with
as start values the values visible on the screen. The number of iterations and the
elapsed time counter are reset.
Fit parameters can be saved using option Save fit parameters and reloaded using
the option Load fit parameters (both options from File menu).
To replace the work data by the fitted curve use Make work data option from the
File menu.
To quit the fit window click the Close button, press Alt-C or select Close from the
File menu.
During fitting the results of each iteration are shown in the Data Presentation window
together with the original data set. It is possible, that some iterations will go in the
wrong direction delivering worse approximations than the previous one. In this case
the fitting procedure steps back and tries to obtain another, better approximation. The
process of stepping back can take few iterations, during which curves observed on the
screen may differ severely with the analysed data. However, this is not a reason to
worry, as the fitting procedure finally comes with a better approximation.
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Simulation
1
Load the differential pulse voltammogram C:\autolab\testdata\demoea01.
2
Set a window (see Plot menu on Data presentation window) between -1.150 and 0.850.
3
From Analysis menu (Data presentation window) select Fit and Simulation.
4
Select the model reversible from the drop-down menu in the top part of the
appearing window and set the simulation mode by activating radio-button
Simulation.
5
Switch on extended setup (option Extended setup in the Option menu or Ctrl-E).
6
Set values of parameters.
7
Check whether the type of the process (oxidation/reduction) is set correctly.
8
Each parameter has its allowed range. If a value is entered that exceeds this range,
it is automatically adjusted to fit within it.
9
If necessary, adjust simulation options (Fit control parameters in the Options
menu). For details on these options see "Simulation: advanced options".
10
The Init. guess button can be used all the time to obtain estimates of parameters
for the work data.
11
Press Simulation button or Alt-S to start calculations.
12
When simulation is completed, the status field shows message "simulation ready".
The χ2 field displays the sum of squares of datapoints values, ∑ yi2 .
i
13
The comparison can be improved by switching to ‘Fit’ and selecting ‘Full Fit’.
To make simulation results permanent, select the Make work data option from the File
menu. Close the fit and simulation window (Close button or Alt-C or Close option
from the File menu), or switch to fitting (by pressing Fit button close to the model
name), or select another model. In the last case a new list of parameters appears with
their default values.
Chapter 4
Analysis of measured data
79
Fitting: advanced options
Fig. 51 The Fit control parameters
It is possible to fine-tune the fitting process. Parameters influencing the fit can be set
by choosing Fit control parameters (Option menu) or by pressing Ctrl-P. The
following parameters appear on the screen:
Fit control parameters
Maximum change of chi-square (scaled):
The convergence is reached when the last change in χ2 is not larger than the given
fraction of χ2 value.
Maximum fitting time:
The time limit of fit procedure. If it is set to 0 (default) no checking of the time is
done. Change default value of this parameter only if there is a clear reason to do so.
Maximum number of iterations:
The limit for the number of iteration (fitting stops earlier if all convergence criteria
are satisfied).
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Number of iterations per fitting step:
Indicates how many iterations are carried out before a signal to break from the user
can be processed and the data on the screen is updated.
Extended parameter setup allows to change default settings of individual convergence
criteria for fitted parameters. A convergence criterion is satisfied when the change in
the parameter value does not exceed the predefined level. Convergence criteria may
be absolute or relative (type of the criterion is indicated by the buttons visible in the
extended setup). Selecting No button for any parameter disables its convergence
criterion (numerical value of previous setting remains visible).
Simulation control parameters
Most of the simulation options are set automatically by the program. In the choice of
their values the program assumes that the parameters of the electrode process have
moderate values and tries to find a reasonable compromise between results precision
and the calculation time. However, when extreme values of parameters (or extreme
combinations of values) are set, or when very high accuracy is desired, automatic
settings may be insufficient.
The simulation options are set by choosing Fit control parameters from the Option
menu. The middle part of the appearing window contains parameters that control the
way the calculations are carried out. These parameters are presented below:
Minimal number of simulation steps per potential value:
The minimal number of simulations cycles carried out for each value of the potential.
If this value is set to N, then each potential step time is divided into N subintervals and
the simulation is carried out for each of these subintervals with time step equal to 1/N
fraction of the step time. The default value is 4.
Maximal number of simulation steps per potential value:
The upper limit for the previous parameter.
Number of points in concentration gradient calculation:
The value of the current is calculated from the derivative of the reactant concentration
at the electrode surface. In digital simulation, the values of concentrations are discrete
and defined only in grid points, and in calculations of concentration gradient the
specified number of points is used. Use of many points usually gives a better precision
of the calculated current. On the other hand, the increase of number of the datapoints
significantly increases the execution time. The default value for this parameter is 2.
Parameter A in space transformation y=ln(1+Ax):
To speed up the calculations, the space grid exponentially expands with the distance
from the electrode surface. This is obtained by transforming the space in such a way,
that the increasing distances in the real space correspond to equally spaced distances
in the transformed space. The program uses Feldberg’s function y=ln(1+Ax) for space
transformation and the rate of expansion, thus also the number of points in
concentration profile, is controlled by parameter A. This parameter should be in the
range 2...3, and its default setting is 2.
Chapter 4
Analysis of measured data
81
Use LU decomposition for boundary condition:
This is the option to provide an alternative way to solve the matrix representing
boundary condition at the electrode surface. The matrix is highly sparse and it is
usually solved by direct method. However, it can happen that the LU (=Lower/Upper)
decomposition method gives better results in some cases, but it is significantly slower
when the number of points used to calculate concentration gradient increases. Default
setting is not to use LU method.
Fast Fit parameters
See "Fitting in more detail, Full and Fast fit".
Data reduction factor:
Allows to carry out fast fit with a data set reduced by this factor (in the reduced data
set points are evenly spaced). The actual value of this factor depends also on the
minimal number of points for fast fit (For more information consult "Fitting: Fast and
Full fit").
Minimum number of points for Fast Fit:
This is the limit for the data reduction factor. The data set used for fast fitting cannot
have fewer points than set by this parameter.
Maximum number of iterations for Fast Fit:
The limit for the iteration number. After reaching it, the fitting procedure switches to
regular fit with all data points.
Fitting in more detail
Fast and Full fit
When working with larger data sets (over 200 points), often obtained in cyclic
voltammetry, it may be attractive to speed up fitting by getting raw values of
parameters with a reduced data set and then to refine them using full set. For this
purpose one can use the reduced data set fitting feature of the fit and simulation
program.
In the reduced data set only every N-th point is used in the fitting. The number N is
called Data reduction factor. Certainly, this factor cannot be very high because fitting
with very few points is likely to deliver parameter values that are not much better than
the initial guesses the program can make. Therefore the user can define the minimal
number of points that the reduced data set must contain. The actual reduction factor is
the smaller number from the data reduction value set and the (total
datapoints)/(minimal datapoints) ratio. Data reduction factor equal to 1 means that no
Fast Fit is performed.
Parameters to control Fast Fit can be set in a window activated by option “Fit control
parameters” from Option menu. In the lower part of this window the data reduction
factor, minimal number of points in the reduced data set and the maximal number of
Fast Fit iterations can be set. Using Fast Fit can be particularly useful when initial
guesses do not deliver good estimates of the parameters, or when fitting process tends
to oscillate.
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Choosing parameters to fit
Each model is defined by a number of parameters. These parameters are measurement
parameters (e.g., start potential, scan rate), general parameters (e.g., temperature) and
specific parameters (e.g., diffusion coefficient, standard redox potential etc.).
From these parameters only some can be fitted and they are called fitable parameters.
The general rule is that only specific parameters can be fitted. From them, parameters
that can acquire only certain values (for example: the number of electrons involved in
the process) are not fitable.
It is only possible to fit successfully parameters that are independent on each other,
i.e., change in the model due to variation of a certain parameter cannot be obtained by
combination of variations of other parameters. Although models are built in such a
way that interdependent parameters are avoided, it can happen that the particular data
set renders two parameters dependent or partially dependent. An example of such
situation is a data set obtained in cyclic voltammetry with a quasi-reversible system: if
scan potentials are chosen so that only one peak (cathodic or anodic) is visible, it is
impossible to determine both the heterogeneous reaction rate ks and the formal
potential Eo of the redox couple.
How to detect dependence of parameters is explained in "Finding interdependence of
fitted parameters".
Initial guesses
The program provides initial guesses for most of the fitable parameters. The exact list
of parameters for which initial guesses are calculated is available in the description of
the models.
To calculate initial guesses of the parameters, click the button Init. guess, select
option “Initial guesses” from the Option menu or press Alt-I. Initial guesses will
appear on the screen. Also the type of the process (reduction or oxidation) is set to the
default value: reduction for scans going toward negative potentials, and oxidation for
scans going toward positive potentials. The type of the process is an extended setup
parameter, that can be inspected and modified when extended parameter mode is on
(Ctrl-E or the Extended setup option from the Options menu).
To check the correctness of initial guesses before starting the actual fit, switch the
mode to simulation (option button near the model name) and then click the Simulate
button. A curve will be simulated with the current parameter settings, and displayed in
Data Presentation window. If initial guesses are satisfactory, switch back to fit mode
and proceed with fitting. Special care in checking is required for the number of
Exchanged electrons and the Dimensionless electrode radius.
Adjust step size
The fit and simulation option for CV automatically searches for the potential step in
the CV-data. However the data can sometimes have an non-equidistant step in the
potential data (i.e. if measured with ADC750). The fit and simulation software
assumes an equidistant step in the potential. Therefor the fit can give error messages.
In order to get rid of these error messages, an option to adjust the step to the step
potential from the procedure is available. The button is visible on the Fit and
simulation window when the ‘Extended setup’ is activated. Please note that the kinetic
parameters, as a result of this fit, are not reliable anymore.
Chapter 4
Analysis of measured data
83
Convergence criteria
Fig. 52 Convergence criteria
Fitting process is carried out until convergence criteria are satisfied or either the
iteration limit or time limit is reached. There are two types of convergence criteria:
based on χ2 and related to the parameter value change.
Criteria based on χ2 demand that:
1. change of χ2 in the last iteration step must be negative (χ2 decreases)
2. value of χ2 (weighted with σ0) should be less than 1 (or less than ∑ yi2 for
i
unweighted data)
3. the last change in χ2 is so small, that it can be neglected. The value that can be
neglected is defined by the user as the maximal relative in χ2 (Option menu, “Fit
control parameters”). The default maximal change is 0.01 (=1%) of χ2 value.
Criteria related to fitted parameters value require, that the change to the parameter
value in the last iteration should not exceed a certain value. This value can be defined
as an absolute, or as a relative one (a fraction of the parameter’s value). In the
extended setup (Option menu) it is possible to define the value and the type
(absolute/relative) of the convergence criterion for each fitable parameters. The
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Convergence field contains the criterial value, and the buttons right to the value field
can be used to select the absolute type of the criterion, the relative one or to disable
the criterion for this parameter. The fitting process finishes when all the convergence
criteria are satisfied. If the convergence is not reached in spite of many interactions
done, this can be due to the following reasons:
1. Demanded precision of the parameters is unrealistic. The level of noise in the data,
or precision of the measured variable (usually current) may keep the variation of
the parameter of interest above the demanded level.
2. The type of the criterion is not appropriate. For example, in datasets where the
background is very small it is better to use the absolute criterion than the relative
one for background value, and relate it to the measured peak or wave current.
The model is not applicable. In this situation the value of χ2 is usually larger than 1
(or larger than ∑ yi2 for unweighted data) while all other criteria can be met. It is then
i
advisable to either change the model, of re-examine settings for non-fitable
parameters, like the number of electrons involved in the electrode reaction.
Finding interdependence of fitted parameters
Dependence of parameters can be detected by inspection of the covariance matrix
(Covariance matrix in the Option menu or Ctrl-M). The diagonal terms are unity. If a
term corresponding to a pair of parameters is significant, there is a serious chance that
these two parameters are interdependent.
Problems can arise if two or more parameters are interdependent. The matrices used
during fit can become singular and an error occurs. Also, the fitted values of
interdependent parameters are meaningless, or the program oscillates and it cannot
deliver the final values. Finally, the computational time unnecessary increases,
because fitting of each parameter requires the calculation of the derivative of the fitted
curve with respect to this parameter. Some derivatives can be calculated analytically,
but some not, and in the latter case the derivative must be obtained numerically. This
requires an additional simulation per iteration step.
Fit and simulation error messages
error -1: “xxxxxxx”:
Internal error of the fit and simulation program. String “xxxxxxx” contains specific
information about the problem. Please report circumstances under which this error
appeared.
error 1: Not enough memory:
There is not enough memory available to carry out the simulation. Try to free some
memory by closing other applications.
error 10: Technique not supported:
An operation has been requested that is not supported for the currently selected
technique. Please report circumstances under which this error appeared.
error 11: Mechanism not supported:
An operation has been requested that is not supported for this electrode reaction
mechanism. Please report circumstances under which this error appeared.
Chapter 4
Analysis of measured data
85
error 12: Model not supported:
An operation has been requested that is not supported for the model chosen. Please
report circumstances under which this error appeared.
error 20: Negative reactant concentration obtained:
Negative concentration of the reactant at the electrode surface has been obtained
during the simulation of concentration profiles. This is usually due to the extreme
value of the potential, at which the ratio of reactant to product concentration is either
very small or very large. The general remedy is to shorten the potential range used. If
it is not possible, try to use another settings for advanced simulation parameters. If
this doesn’t help, please report the problem.
error 21: Negative product concentration obtained:
Negative concentration of the product at the electrode surface has been obtained
during the simulation of concentration profiles. This is usually due to the extreme
value of the potential, at which the ratio of reactant to product concentration is either
very small or very large. The general remedy is to shorten the potential range used. If
it is not possible, try to use another settings for advanced simulation parameters. If
this doesn’t help, please report the problem.
error 51: GAUSSJ: Singular Matrix-1:
Two or more parameters in the model are dependent or nearly dependent.
error 52: GAUSSJ: Singular Matrix-2:
Two or more parameters in the model are dependent or nearly dependent.
Descriptions of the models
General remarks
All models of electrode reactions assume diffusion to an electrode with finite
dimensions. The size of the electrode is characterised by the dimensionless electrode
radius
rd = re / DR τ
where re is the radius in meters, DR is the diffusion coefficient of the reactant in m2/s
and τ is the characteristic time parameter of the technique. The time parameter τ is
equal to RT/nFV (in staircase and cyclic voltammetry, V=scan rate), to pulse time (in
normal pulse voltammetry and chrono techniques), to modulation time (in differential
pulse techniques) and to inverse of frequency (in square wave voltammetry).
If the electrode is large or the τ parameter is small (fast experiments), i.e., only linear
diffusion takes place, the dimensionless electrode radius should be set to zero. This
value indicates that the radius of the electrode is irrelevant. All electron transfer rates
are normalised according to equation
k = k τ DR
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and all homogeneous (chemical) reaction rates are normalised by multiplication by τ
k chem = k chem τ
Use of normalised constants allows carrying out fitting without the knowledge of
diffusion coefficients.
In all equations in the description of the models the log() function refers to 10-based
logarithm, and ln() function to natural (e-based) logarithm.
Cyclic voltammetry: reversible electrode process
ne−
Reaction equation: R ←
→ P
Fitable parameters:
redox potential E0 (V)
normalised current Inorm (I = Inorm*χ(at)) (A)
constant background current (A)
Initial guesses available for: number of exchanged electrons, E0, Inorm, background
current
Comments: According to the theory, the peak current is equal to
nFAcbulk(πaD)1/2*χ(at), where the first term is equal to the fitable parameter Inorm and
the function χ(at), a=nFV/RT, V being scan rate, represents the shape of the
voltammetric peak. The peak value of this function is 0.4463 for linear sweep
voltammetry, while for voltammetry utilising the staircase voltage ramp the exact
value depends on the step height, step time and the current sampling parameter α.
Details regarding the function χ(at) can be found in literature1.
Cyclic voltammetry: quasi-reversible electrode process
ne− ,k
Reaction equation: R ←s → P
Fitable parameters:
redox potential E0 (V)
Log (normalised electron transfer rate), log( k s )
transfer coefficient α
normalised current Inorm (I = Inorm*χ(bt)) (A)
constant background current (A)
Initial guesses available for: number of exchanged electrons, log(ks), E0, Inorm,
background current
Comments: The parameter log( k s ) is a 10-base logarithm of the electron transfer rate,
normalised with respect to the time scale of the experiment k s = k s RT nFVD ,
where V is the scan rate and ks is the electron transfer rate used in Butler-Volmer
equation
1
R. S. Nicholson and I. Shain, Anal. Chem., vol. 36, 1964, page 706.
Chapter 4
Analysis of measured data
(
I = k s c R exp( −αnF RT ( E − E 0 ) ) + c P exp( ( 1 − αnF ) RT ( E − E 0 ) )
87
)
According to the theory, the peak current is equal to nFAcbulk(πbD)1/2*χ(bt), where the
first term is equal to the fitable parameter Inorm and the function χ(bt), b=αnFV/RT,
represents the shape of the voltammetric peak. The peak value of this function
depends on the electron transfer rate ks, the transfer coefficient α, the step height, step
time and the current sampling parameter α.
Cyclic voltammetry: irreversible electrode process
ne− ,k
Reaction equation: R s → P
Fitable parameters:
( )
Log(Normalised electron transfer rate at E0=0), log k fh0
(exchanged electrons)*(transfer coefficient) αn
normalised current Inorm (I = Inorm*χ(bt)) (A)
constant background current (A)
Initial guesses available for: E0, αn, Inorm, background current
( )
Comments: The parameter log k fh0 is a 10-base logarithm of the electron transfer rate,
normalised with respect to the time scale of the experiment k fh0 = k 0fh RT nFVD ,
where V is the scan rate and k0fh is the electron transfer rate at E=0, used in reduced
Butler-Volmer equation
(
)
(
I = k 0fh c R exp ( −αnF RT ) E = k s c R exp ( −αnF RT )( E − E 0 )
)
According to the theory, the peak current is equal to nFAcbulk(πbD)1/2*χ(bt), where the
first term is equal to the fitable parameter Inorm and the function χ(bt), b=αnFV/RT,
represents the shape of the voltammetric peak. The peak value of this function is equal
to 0.4958 for linear sweep voltammetry, and for staircase voltammetry it depends on
the electron transfer rate ks, the transfer coefficient α, the step height, step time and
the current sampling parameter α. Details regarding the function χ(bt) can be found in
literature.
There is no need to set the number of exchanged electrons, because the term αn is
fitted as a whole.
Cyclic voltammetry: reversible electrode process followed by irreversible
chemical reaction (ECi)
ne−
kcf
→ P → B
Reaction equation: R ←
Fitable parameters:
redox potential E0 (V)
forward chemical reaction rate kc(foll)-> (norm.) = kcf
normalised current Inorm (I = Inorm*χ(bt)) (A)
constant background current (A)
Initial guesses available for: number of exchanged electrons, E0, Inorm, background
current
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Comments: The forward chemical reaction rate is normalised with respect to the time
scale of the voltammetric experiment, k cf = k cf ( RT nFV ) . Similarly to previous cases,
the voltammetric peak current is expressed as a product of the peak shape function
χ(bt) and the term including the electrode area, concentration, diffusion coefficient
and the number of exchanged electrons. Details regarding this mechanism can be
found in literature.
Cyclic voltammetry: two-component models
Two-component models represent the situation when two electroactive species are
reduced or oxidised at the electrode independently from each other. In such situations
their peaks can overlap, what hinders the extraction of relevant parameters for
separate reactions.
There is a number of two-component models available, differing in the degree of
reversibility of each electron transfer (reversible, quasi-reversible or irreversible
mechanism). The parameters are denoted with numbers 1 and 2 to indicate to which
component they correspond.
Reaction equations for single component:
ne −
Ri ←→ Pi
(reversible process)
ne − , k
Ri ←s → Pi
(quasi-reversible process)
ne − , k s
Ri → Pi
(irreversible process)
Fitable parameters
•
reversible case:
redox potential E0 (V)
normalised current Inorm (I = Inorm*χ(at)) (A)
•
quasi-reversible case:
redox potential E0 (V)
Log(normalised electron transfer rate), log( k s ) )
transfer coefficient α
normalised current Inorm (I = Inorm*χ(bt)) (A)
•
irreversible case:
( )
Log(Normalised electron transfer rate at E0=0), log k fh0
(exchanged electrons)*(transfer coefficient) αn
normalised current Inorm (I = Inorm*χ(bt)) (A)
•
common parameter:
constant background current (A)
Initial guesses available for:
Comments: According to the theory, the peak current is equal to
nFAcbulk(πaD)1/2*χ(at) (reversible processes) or nFAcbulk(πbD)1/2*χ(bt) (irreversible
processes), where the first term is equal to the fitable parameter Inorm and the functions
χ(at) and χ(bt) represent the shape of the voltammetric peak. The parameters a and b
are respectively a=nFV/RT, b=αnFV/RT, V being the scan rate. The peak values of
these function are 0.4463 (reversible) and 0.4958 (irreversible) for linear sweep
voltammetry. If voltammetry with staircase voltage ramp is used, the exact value
Chapter 4
Analysis of measured data
89
depends on the step height, step time and the current sampling parameter α. Details
regarding the function χ(at) and χ(bt)can be found in literature.
The electron transfer rates k s and k fh0 are defined as follows:
k fh0 = k 0fh RT nFVD and k s = k s RT nFVD
while the Butler-Volmer equations for respectively quasi-reversible and irreversible
processes are
I = k s c R exp( −αnF RT ( E − E 0 ) ) + c P exp( ( 1 − αnF ) RT ( E − E 0 ) ) (quasireversible)
(
)
(
)
(
)
I = k 0fh c R exp ( −αnF RT ) E = k s c R exp ( −αnF RT )( E − E 0 )
(irreversible)
Due to the possible complication of two-component voltammograms, it can happen
that the initial guesses are inferior to those obtained in simpler models. However, it is
always possible to select datapoints (by setting the window in such a way that only
one peak is covered), do initial guesses for one-component model and transfer the
results to two-component model.
When electron transfer processes are different, for example Er+Ei, all parameters
marked with (1) refer to electron transfer Er, while all marked with (2) - to electron
transfer Ei. The initial guess procedure can have problems with appropriate
assignment of peaks to the components. This assignment can be changed by swapping
the values referring to the first component and to the second component. In case of
irreversible electron transfer it is necessary, however, to adjust the value of k fh0 using
the relationship ∆k 0fh = αnF∆E p RT log e .
Normal pulse voltammetry: reversible electrode process
ne−
→ P
Reaction equation: R ←
Fitable parameters:
halfwave potential E1/2 (V)
limiting current Ilim (A)
constant background current (A)
Initial guesses available for: number of exchanged electrons, E1/2, Ilim, background
current
Comments: The theoretical expression for the limiting current at a large flat electrode
is Ilim=nFAcbulk(DR/πtp)1/2, where tp is the pulse time. The halfwave potential should be
equal to polarographic halfwave potential.
Normal pulse voltammetry: quasi-reversible electrode process
ne− ,k
Reaction equation: R ←s → P
Fitable parameters:
redox potential E0 (V)
Log(normalised electron transfer rate), log( k s )
transfer coefficient α
limiting current Ilim (A)
constant background current (A)
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Initial guesses available for: number of exchanged electrons, E0, Inorm, background
current
Comments: The parameter log( k s ) is a 10-base logarithm of the electron transfer rate,
normalised with respect to the time scale of the experiment k s = k s t p D , where tp is
the pulse time and ks is the electron transfer rate used in Butler-Volmer equation
(
I = k s c R exp( −αnF RT ( E − E 0 ) ) + c P exp( ( 1 − αnF ) RT ( E − E 0 ) )
)
The theoretical expression for the limiting current at a large flat electrode is
Ilim=nFAcbulk(D/πtp)1/2.
Normal pulse voltammetry: irreversible electrode process
ne− ,k
Reaction equation: R s → P
Fitable parameters:
Characteristic potential E* (V)
( )
Log(Normalised electron transfer rate at E0=0), log k fh0
(exchanged electrons)*(transfer coefficient) αn
limiting current Ilim (A)
constant background current (A)
( )
Initial guesses available for: log k fh0 , αn, E*E, Inorm, background current
Comments: There is no need to set the number of exchanged electrons, because the
term αn is fitted as a whole. The theoretical expression for the limiting current at a
large flat electrode is Ilim=nFAcbulk(D/πtp)1/2, where tp is the pulse time. The
characteristic potential E* is defined as E * = ( RT αnF ) ln k fh0 = E 0 + ( RT αnF ) ln k s ,
where k fh0 = k 0fh t p D and k s = k s t p D . ks and k0fh are the electron transfer rates
used in Butler-Volmer equation
(
)
(
I = k 0fh c R exp ( −αnF RT ) E = k s c R exp ( −αnF RT )( E − E 0 )
)
Normal pulse voltammetry: reversible electrode process followed by
irreversible chemical reaction (ECi)
ne−
kcf
Reaction equation: R ←
→ P → B
Fitable parameters:
redox potential E0 (V)
forward chemical reaction rate kc(foll)-> (norm.) k cf
limiting current Ilim (A)
constant background current (A)
Initial guesses available for: number of exchanged electrons, E0, Inorm, background
current
Chapter 4
Analysis of measured data
91
Comments: The theoretical expression for the limiting current at a large flat electrode
is Ilim=nFAcbulk(D/πtp)1/2, where tp is the pulse time. The forward chemical reaction
rate is normalised with respect to the time scale of the experiment, k cf = k cf t P .
Normal pulse voltammetry: reversible electrode process (analytical), quasireversible electrode process (analytical) and irreversible electrode process
(analytical)
These models differ from the previous only by the fact, that each point of the normal
pulse voltammogram is calculated from analytical expressions for
chronoamperometry under the following assumptions:
•
the current is measured at the end of the potential pulse
•
before each potential pulse concentrations of the species at the electrode surface
are the same as in the bulk of the solution
The latter means that no significant electrode reaction occurs at the base potential, and
that the interval time is long enough to restore the initial concentrations (or the
dropping mercury electrode is used). In addition to this, it is assumed in the reversible
model that the product of the electrode reaction is initially absent in the solution.
Thanks to analytical expressions the speed of calculations is much higher than in the
regular model based on finite-difference simulation. Therefore, if the mentioned
assumptions are valid, this model should be preferred.
Differential pulse voltammetry: reversible electrode process
ne−
→ P
Reaction equation: R ←
Fitable parameters:
peak potential Ep (V)
peak current Ip (A)
constant background current (A)
Initial guesses available for: number of exchanged electrons, Ep, Ip, background
current
Comments: The approximate expression for the peak height, valid under linear
diffusion conditions, is
Ip = nFAcbulk(D/πtm)tanh(nF∆E/4RT)
where tm is the modulation time and ∆E is the modulation amplitude.
Differential pulse voltammetry: quasi-reversible electrode process
ne− ,k
Reaction equation: R ←s → P
Fitable parameters:
peak potential Ep (V)
Log(normalised electron transfer rate), log( k s )
transfer coefficient α
peak current Ip (A)
constant background current (A)
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Initial guesses available for: number of exchanged electrons, Ep, log( k s ) , α, Ip,
background current
Comments: The parameter log( k s ) is a 10-base logarithm of the electron transfer rate,
normalised with respect to the time scale of the experiment, k s = k s t m DR , where V
is the scan rate and ks is the electron transfer rate used in Butler-Volmer equation
(
(
(
))
(
(
I = k s cR exp − αnF RT E − E 0 + cP exp (1 - α )nF RT E − E 0
)))
The general expression for the peak height does not exist, expressions for particular
situations are complicated.
Differential pulse voltammetry: irreversible electrode process
ne− ,k
Reaction equation: R s → P
Fitable parameters:
Characteristic potential E* (V)
( )
Log(Normalised electron transfer rate at E0=0), log k fh0
(exchanged electrons)*(transfer coefficient) αn
peak current Ip (A)
constant background current (A)
( )
Initial guesses available for: number of exchanged electrons, Ep, log k fh0 , αn, Ip,
background current
Comments:
There is no need to set the number of exchanged electrons, because the term αn is
fitted as a whole.
The characteristic potential E* is defined as
E * = ( RT αnF ) ln k fh0 = E 0 + ( RT αnF ) ln k s , where k fh0 = k 0fh t m DR and
k s = k s t m DR . ks and k0fh are the electron transfer rates used in Butler-Volmer
equation
(
)
(
I = k 0fh c R exp ( −αnF RT ) E = k s c R exp ( −αnF RT )( E − E 0 )
)
Differential pulse voltammetry: reversible electrode process followed by
irreversible chemical reaction (ECi)
ne−
kcf
→ P → B
Reaction equation: R ←
Fitable parameters:
redox potential E0 (V)
normalised forward chemical reaction rate kc(foll)-> (norm.), k cf
peak current Ip (A)
constant background current (A)
Initial guesses available for: Ep, Ip, background current
Chapter 4
Analysis of measured data
93
Comments: The forward chemical reaction rate is normalised with respect to the time
scale of the voltammetric experiment, k cf = k cf t m . No simple expression is available
for the peak height.
Square wave voltammetry: reversible electrode process
ne−
→ P
Reaction equation: R ←
Fitable parameters:
formal potential E0 (V)
peak current Ip (A)
constant background current (A)
Initial guesses available for: Ep, Ip, background current (fitting on net current)
Epf, Ipf, background current (fitting on
forward/backward current)
Comments: There is no simple expression for the net peak current. The interpretation
of the value of Ip depends whether fitting takes place on the net current or on
forward/backward currents: in the first case Ip corresponds to the height of the net
current, in the second - to the height of the forward peak. Switching between net
current and forward/backward current will therefore result in the difference in peak
heights.
The potential of the peak is very close to the polarographic halfwave potential.
Square wave voltammetry: quasi-reversible electrode process
ne− ,k
Reaction equation: R ←s → P
Fitable parameters:
formal potential E0 (V)
Log( normalised electron transfer rate), log( k s )
transfer coefficient α
peak current Ip (A)
constant background current (A)
Initial guesses available for: Ep, Ip, background current
Comments: The parameter log( k s ) is a 10-base logarithm of the electron transfer rate,
normalised with respect to the time scale of the experiment k s = k s fDR , where f is
the frequency and ks is the electron transfer rate used in Butler-Volmer equation
I = k s (cR exp(− αnF RT (E − E 0 )) + cP exp((1 − α )nF RT (E − E 0 )))
There is no simple expression for the net peak current. The interpretation of the value
of Ip depends whether fitting takes place on the net current or on forward/backward
currents: in the first case Ip corresponds to the height of the net current, in the second
- to the height of the forward peak. Switching between net current and
forward/backward current will therefore result in the difference in peak heights.
Square wave voltammetry: irreversible electrode process
ne− ,k
Reaction equation: R s → P
Fitable parameters:
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Characteristic potential E* (V)
( )
Log( normalised electron transfer rate at E0=0), log k fh0
(exchanged electrons)*(transfer coefficient) αn
peak current Ip (A)
constant background current (A)
Initial guesses available for: E*,Ip, background current
Comments: There is no need to set the number of exchanged electrons, because the
term αn is fitted as a whole.
The characteristic potential E* is defined as
E * = ( RT αnF ) ln k fh0 = E 0 + ( RT αnF ) ln k s , where k fh0 = k 0fh ( fDR ) and
k = k s ( RT nFV ) D
. where f is the frequency and ks and k0fh are the
k s = k s ( fDR ) s
electron transfer rates used in simplified Butler-Volmer equation
(
)
(
)
I = k 0fh c R exp ( −αnF RT ) E = k s c R exp ( −αnF RT )( E − E 0 )
There is no simple expression for the net peak current. The interpretation of the value
of Ip depends whether fitting takes place on the net current or on forward/backward
currents: in the first case Ip corresponds to the height of the net current, in the second
- to the height of the forward peak. Switching between net current and
forward/backward current will therefore result in the difference in peak heights.
Square wave voltammetry: reversible electrode process followed by
irreversible chemical reaction (ECi)
ne−
kcf
→ P → B
Reaction equation: R ←
Fitable parameters:
redox potential E0 (V)
normalised forward chemical reaction rate kc(foll)-> (norm.) k cf
peak current Ip (A)
constant background current (A)
Initial guesses available for: Ep, Ip, background current
Comments: The forward chemical reaction rate is normalised with respect to the time
scale of the experiment, k cf = k cf f , where f is the frequency.
There is no simple expression for the net peak current. The interpretation of the value
of Ip depends whether fitting takes place on the net current or on forward/backward
currents: in the first case Ip corresponds to the height of the net current, in the second
- to the height of the forward peak. Switching between net current and
forward/backward current will therefore result in the difference in peak heights.
Chapter 4
Analysis of measured data
95
4.14 Current density
The current density is calculated with the surface area, using the surface area on page
2 of the procedure window.
4.15 WE2 versus WE plot
When a BIPOT is present, Iring versus Idisk plots can be constructed with this option.
4.16 Endpoint Coulometric titration
After performing a coulometric titration experiment (Chrono method (>0.1s)
potentiometry (galvanostatic) with pX/pH signal) this option can be chosen.
The experiment time is converted into charge (the current applied is given from the
procedure) and only the pX/pH signal is shown on the y-axes. Furthermore the
‘Endpoint Coulometric titration’ window is opened, to find the endpoint. If you
follow the instructions the turning point will be shown as endpoint.
•
•
•
•
•
The endpoint is obtained from the zero crossing(s) in the 2nd derivative. The 2nd
derivative and endpoint are calculated using the last applied level. If only one
level is defined all calculations are done on this particular level.
The 2nd derivative values are normalised so do not pay attention to the absolute
values. The 2nd derivative plot is used for indicative purposes only.
The show 2nd derivative button is active after the Find endpoint button has been
pressed.
The ‘Show last level’ button appears only if more then one level is measured.
If the amount of measured point is poor the indicated end point in the curve can be
slightly different from the actual zero crossing in the 2nd derivative. The pointer
can only be set on a real data-point and not on an interpolated point in between.
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Fig 52a. Coulometric titration plot
Parameters:
Filter for derivative:
Filter factor to reduce noise on the 2nd derivative.
1%: no filtering – 25%: heavy filtering.
Window for zero crossings: Defines when a zero crossing should be noticed as a real
zero crossing. This window defines the amount of points
with different sign before and after the zero crossing.
Due to Faraday’s law the equivalent of generated titrant is proportional to the charge
and the equivalent of the analyte can be calculated (see also Application note
“Coulometric titration”, and “Installation and Diagnostics Guide: pX-module).
Chapter 5
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97
5. Editing of measured data
5.1 Smooth
Every measurement is disturbed by noise. In many cases the noise level will be low,
but especially at low current levels the amount of noise can be severe. In order to
enhance the signal to noise ratio of experimental data sets, a Smooth option is
supplied. The data files can be smoothed using either the Savitsky-Golay algorithm or
a FFT-algorithm.
Smoothing can be performed on the whole curve or a part of it. When only a part of
the curve should be smoothed, click the Smooth window button and select a part of
the curve. The remainder will not be smoothed. This option is only available for
Savitzky and Golay smoothing.
The popular Savitzky and Golay method is described in Anal. Chem.,36,1627 (1964).
Their method presumes that a number of points can be fitted to a polynomial so that
the best curve will pass through the experimental points. This method is also called
weighted moving averaging. Before the smooth routine of Savitzky and Golay is
applied to the data set, spikes in the set of data are removed.
The Smooth option in all programs first asks which smooth level has to be applied.
Valid levels are 0 to 4.
These levels are :
0- spike rejection only
1- spike rejection and a 5-point weighed moving average
2- spike rejection and a 9-point weighed moving average
3- spike rejection and a 15-point weighed moving average
4- spike rejection and a 23-point weighed moving average
The applicable smooth level heavily depends on the number of points of the data set.
The more points within the curve, the higher the smooth level can be without
modifying the curve too much.
Having selected the FFT option, a logarithmic or linear frequency domain plot is
displayed. Now a cutoff frequency has to be supplied, which should be less than the
dominant noise frequency. The FFT-smoothing algorithm assumes that the signal is
composed of n/2 sine waves of different frequencies, where n is the number of
measured data points, filled up to a power of 2 (512, 1024, etc.). The added data
points get a value of zero. A cut-off frequency of for instance 20 means that the
amplitudes of the 20 sine waves with the lowest frequencies are kept, all other
amplitudes are set to zero. After a back-transformation, both the original curve and the
smoothed curve are displayed and the question is posed whether the original data
should be replaced. The FFT-algorithm is very effective in removing noise originating
from the power-source. The FFT-algorithm is explained in the book "Numerical
Recipes", W.H. Press et al., Cambridge University Press, ISBN 0 521 30811 9. FFTsmoothing should not be used in data files with spikes or discontinuities. It works best
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if the noise only consists of a periodic disturbance with a higher frequency than the
real signal. The real signal should not change much within one period of this periodic
disturbance.
Please note that the presented frequency in the frequency domain plot is not the
frequency of the noise in the current or potential signal. The frequency is presented in
an arbitrary unit.
5.2 Change all points
This option allows to add a constant value to all data points or to multiply all
datapoints with a constant value.
5.3 Delete points
It is possible to remove points from the plot. You can specify up to 20 points.
This option can be used to remove spikes from the measured data. With resume (Data
presentation, Plot) the original data set will be loaded again. The Save work data
(Data presentation, File) can be used to save the adjusted data-set.
5.4 Baseline correction
Four types of baselines can be specified.
The first is the linear baseline. Two markers on the measured curve can be specified,
which then define a line. After acceptation of the markers, the corrected curve is also
drawn. By clicking either the Cancel or the OK button on the Baseline correction
window, the correction can be either ignored or accepted.
It is also possible to subtract a polynomial baseline. After selecting this option the
user is asked to mark between two and five data points as contact points between
baseline and curve. After accepting the markers the program will calculate a 3rd order
polynomial through the markers.
The third type calculates a connecting exponential curve through the specified begin
and end point. The whole curve is subsequently corrected for this baseline.
Finally, there is the so-called ‘Moving average baseline’. This is an automatic
baseline correction. This method is very effective when peaks show as shoulders on
steep flanks. After a baseline correction real peaks will show. The number of data
points is reduced by calculating the average within a step window. The step window is
the minimum peak width which can be specified on the Baseline correction window.
The baseline is subsequently calculated by comparing each point with the mean value
of its two neighbours. If the absolute mean value is lower, it replaces the current
value. This operation is repeated again and again until no data point is replaced
anymore. If more than 1000 iterations are required, a message is given and the process
stops. As a default minimum peakwidth a value of 30 mV or less is advised in most
cases. The process can be hindered by anomalies in the voltammogram. Please note
that this technique cannot be applied for Cyclic and Linear sweep voltammograms.
Chapter 5
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99
Fig 52b: Baseline correction window
5.5 Subtract disk file
This option allows to subtract a previously measured data set from the current one. In
case the spacing between the two data sets is different a linear interpolation method is
used.
5.6 Subtraction of second signal from first signal.
This option allows to subtract a simultaneously measured second signal using either
of the free ADC-channels from the current or potential signal (first signal). In case the
spacing between the two data sets is different a linear interpolation method is used.
The option is only enabled when a second signal is really available.
5.7 Derivative
First the data points are smoothed according to the Savitsky-Golay algorithm (see
above) using smooth factor 2. Subsequently the derivative is determined by the simple
algorithm:
 y(n) - y(n-1) 
dy/dx (n) = 0.5*    + 0.5*
 x(n) - x(n-1) 
 y(n+1) - y(n) 
 − 
 x(n+1) - x(n) 
For cyclic and linear voltammetry the time derivative is given, i.e. x is the time since
the start of the scan instead of the potential.
5.8 Integrate
The integral is determined using the trapezium rule, which assumes a straight line
between two data points.
For cyclic and linear voltammetry the time integral is given, i.e. x is the time since the
start of the scan instead of the potential.
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5.9 Fourier transform
The frequency spectrum is determined by means of a Fast Fourier method (FFT). The
real frequency is displayed on the x-axis.
5.10 Convolution techniques
Convolution voltammetry consists essentially of a voltammetric,
chronoamperometric, or chronocoulometric experiment followed by a mathematical
transformation - convolution. The technique delivers quantities directly related to the
concentration of electroactive species at the electrode surface (instead of the flux of a
compound, as in the case of the original techniques) and it is rather insensitive to iRdrop.
Fig. 53 Convolution menu
In a number of electroanalytical techniques, the current measured displays
proportionality to a t-½ function. The popularity of this type of dependence originates
from the solution of Fick's law in the case of semi-infinite linear diffusion, the most
common type of the transport of the reagent to the electrode. According to this
solution, the gradient of the concentration of a substance, consumed in the electrode
process, decreases with the square root of the electrolysis time and so does the current
which is proportional to this gradient. Such a dependence can be easily observed in
chronocoulometry, chronoamperometry, and in voltammetry (in this latter case in the
descending branch of the peak).
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Editing of measured data
101
Using a convolution method, the effect of the decrease of the concentration gradient
can be eliminated from the total response of the electrode. The surface concentration
of the product of an electrode reaction during the experiment can be obtained using
equation∗.
cs(t) = i(t)*g(t)/(nFAD½)
(eq. 1)
where i(t)*g(t) is a convolution operation defined as
f1(x)*f2(x) =
x
⌠
 f1(u)f2(x-u)du =
⌡
0
x
⌠
 f1(x-u)f2(u)du
⌡
0
(eq. 2)
The function g(t) depends on the transport conditions and the electrode geometry,
being in the simplest case (πt)½. The convolution of a voltammogram results in an Sshaped curve, where voltammetric peaks are replaced by waves, very similar to
polarographic ones. In the case of a fast and uncomplicated electron transfer, the wave
can be described using the equation
E = E½ + (RT/nF)ln[(md-m)/m]
(eq. 3)
where m denotes current convolution (for approximate description of kineticcontrolled processes the RT/nF value should be replaced by RT/αnF). The height of
the plateau is given by the formula
md = nFAD½C
(eq. 4)
It can be shown that such a result is independent of the scan rate used and that the
height of the wave is insensitive to iR-drop.
Convolution of voltammetric data with a t-½ function results in a curve equivalent to
the derivative of the previous one (up to a normalisation factor). Valuable features of
this new curve can be noted: symmetric, narrow peaks which are much better resolved
compared to asymmetric, "tailing" voltammetric ones. The obtained t-½ convolution
peak follows the function
∗
)
K.B. Oldham, Anal. Chem. 58 (1986) 2296, J.C. Myland, K.B. Oldham, C.G.
Zoski, J. Electroanal. Chem. 193 (1985) 3.
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e = nFAD½Ccosh-2 [nF/2RT(E-E°')]
Version 4.9
(eq. 5)
in case of a fast, reversible electron transfer.
It is also possible to use other convolution functions e.g. to separate the spherical
diffusion effect, the kinetic effect of the preceding homogeneous reaction etc∗ .
In chronoamperometry, convolution of current with t½ function results in a horizontal
line at the height equal to
e = nFAD½C
(eq. 6)
if the transport to the electrode follows semi-infinite linear diffusion. For
chronocoulometry, convolution with a t-½function leads exactly to the same result.
It is profitable to distinguish a class of convolutions with a g(t) function in the form
g(t)=t-u (u is a real number): such a convolution can be considered as a generalised
differentiation/integration (differintegration) operation with respect to the variable t.
In this approach, the value of the exponent denotes the order of integration (if
positive) or differentiation (if negative) and thanks to the convolution definition, the
value of u need not be integer. Differintegration is cumulative, i.e. d½/dt½ (d½i/dt½) =
di/dt or d-½/dt-½ (di/dt) = d½i/dt½. From the practical point of view, two forms of
convolution, with t½ and t-½, deserve special attention. They can be considered as,
respectively, semi-integration and semi-differentiation.
Another reason for mentioning differintegration is that there are special algorithms
allowing this operation to be performed rapidly. For more information please refer to
K.B. Oldham, J. Spanier, "The Fractional Calculus", Academic Press, N.Y., 1974.
As mentioned before, in case of semi-infinite linear diffusion the results of
convolution with the function t±½ (semi-integration and semi-differentiation) are welldefined and quite simple. This suggests that these methods can be used for the
investigation of variations of product concentration on the electrode surface as well as
detection and studies of phenomena, resulting in deviations from linear diffusion
transport. Other practical applications are the resolution of overlapping voltammetric
peaks, the determination of the formal potentials and numbers of electrons involved in
the reaction step, detection of the adsorption on the electrode as well as of the
irreversible homogeneous reaction consuming the product generated by the electron
transfer step.
Detection of overlapping peaks
The nature of the voltammetric peak causes overlap in case of complex
voltammograms. While the ascending branch of the peak rises rapidly and the
beginning of the rise can easily be found, the descending branch follows a t-½ function
and is characterised with a slow decrease. Even far away from the top of the peak, the
∗
)
F.E. Woodard, R.D. Goodin, P.J. Kinlen, Anal. Chem. 56 (1984) 1920, J.H.
Carney, Anal. Chem. 47 (1975) 2267.
Chapter 5
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103
value of the current differs significantly from zero. Due to this feature, all following
voltammetric peaks rise from the "tail" of the previous one.
If the separation of two voltammetric peaks is large enough, they can be detected
without any problems. The situation is difficult when the distance between peaks gets
smaller: below a certain distance, the first peak is reduced to a shoulder on the rising
part of the next peak. The extreme situation is shown in fig. 56 C, where the overlap is
very strong, so that only one peak can be observed and there is no indication for the
presence of more of them.
Fig. 54 Overlapping linear voltammetric peaks and their semi-derivatives
In most situations, except in those of extreme overlap, semi-derivative peaks are
clearly visible and their number can easily be found. There are, however, three
important limitations to this method. First, voltammograms that are to be semidifferentiated, should be background-corrected: semi-differentiation changes a
constant or a linear background into complicated forms in the semi-derivative domain.
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Secondly, semi-derivative peak artefacts have to be recognised: consider the case of
an uncomplicated slow electron transfer leading to the voltammogram presented in the
appropriate figure.
Fig. 55 Peak artefacts in semi-derivative voltammetry
The curve in the above-mentioned figure is the result of semi-differentiation, where
two peaks appear, one in the forward and one in the backward branch. The second
semi-derivative peak does not represent any hidden voltammetric peak, but is an
artefact resulting from semi-differentiation of a wave-like current decay. Such a peak
can only appear in the backward branch of semi-derivative voltammograms and has
characteristic features: in the potential range, where such a peak appears, (i) there is
no backward voltammetric peak, (ii) there is a forward peak and (iii) the sign of SCV
current values is the same in both branches. All peaks that satisfy these criteria, are
probably artefacts.
The third limitation of the method stems from the fact that results presented in the
above-mentioned figure concern an uncomplicated electron transfer under semiinfinite linear diffusion conditions. The form of the peak is different when the
mechanism of the reaction and the transport type change: usually peaks become less
symmetric and broader, resulting in a decrease in separation capability and, in certain
situations, leading to deformations of neighbouring semi-derivative peaks.
Determination of formal potential and the number of electrons involved
The equation 5 describes the form of a semi-derivative voltammetric peak in a case of
uncomplicated fast electron transfer under semi-infinite linear diffusion transport. It is
clear that the peak potential is equal to the formal potential of the reacting system and,
for cyclic voltammetry, that both anodic and cathodic peaks appear at the same
potential. This feature can be used as a simple and rapid test for reversibility of the
reaction. This test is superior to the well-known test based on the difference of
potentials of voltammetric peaks, as it does not require knowledge about the number
of electrons involved.
If the rate of electrode reaction is limited by the diffusion or by kinetics of the
electron transfer, the number of electrons involved can be determined from the halfwidth of the semi-derivative peak. This half-width should be
Chapter 5
w = 3.53RT/nF
Editing of measured data
105
(eq. 7)
for a diffusion-controlled process and
w = 2.94RT/αnF
(eq. 8)
for a rate-controlled process.
Irreversible homogeneous reaction consuming the product of the
electrode process
The criterion for the absence of an irreversible homogeneous reaction is restoration of
the initial state at the electrode surface after a cyclic change of electrode potential. If
such a reaction does not occur, the surface concentration of all species after the
experiment should be exactly the same as before.
Fig. 56 Semi-integration of voltammograms in case of the absence (A) and the
presence (B) of an irreversible homogenous reaction. Thin line - voltammogram,
thick line - semi-integral
As the convolution of the voltammetric current with a t½ function (semi-integration)
produces a value proportional to the surface concentration of the product of the
reaction, the convoluted value should return exactly to zero after completion of the
cycle, which means that the product of the reaction has been entirely converted back
to the substrate∗. If it does not return to zero, the consumption of the initially present
substance is suggested.
It should be stressed, however, that this method requires diffusion to be semi-infinite
and linear. In situations where this is not the case, corrections have to be made. Such a
correction is available (S.O. Engblom, K.B. Oldham, Anal. Chem. 62(1990)625) for
∗
)
F.E. Woodard, R.D. Goodin, P.J. Kinlen, Anal. Chem. 56 (1984) 1920, I.D.
Dobson, N. Taylor, L.R.H. Tipping in "Electrochemistry, Sensors and Analysis"
(M.R. Smyth, J.G. Vos, eds.), Elsevier, Amsterdam, 1986.
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spherical electrodes (mercury drops) under the name of spherical convolution it
requires the values of electrode radius and of diffusion coefficient, delivering the
concentration of the reaction product on the surface of a spherical electrode.
If the presence of an irreversible homogeneous reaction is detected, its rate can be
measured. For this purpose a so-called kinetic convolution can be used. In this
transformation the effect of consumption of the product by the reaction with the rate k
can be eliminated; by means of inserting different values of k one can obtain the result
in which a convoluted current at the end of the cycle approaches the same value as on
the beginning of the cycle.
Some problems can be expected if the substrate or the product of the reaction is
accumulated on (or in) the electrode by adsorption, deposition, or amalgamation. In
this case, the initial value of the surface concentration is not restored after the
completion of the voltammetric cycle and convoluted voltammograms will not deliver
proper results.
Investigations of factors controlling the transport to the electrode
Theoretically the most simple and quite commonly encountered transport type is
semi-infinite linear diffusion: the substance diffuses from the bulk of the solution,
where the concentration is constant, to the planar electrode, where it is consumed. The
flux of the substance depends on the gradient of the concentration at the electrode
surface; this gradient decreases with the rate proportional to the square root of the
electrolysis time.
Linear diffusion leads to the simplest description, but unfortunately its conditions are
rarely realised in the experimental setups used in electrochemistry. In case of mercury
electrodes, the surface of the electrode is not planar, and the diffusion can be
approximated using a linear model over a short period of time only. In case of solid
electrodes a so-called edge effect occurs: the contribution of spherical diffusion
appears. Apart from the geometry of electrodes, chemical processes taking place in
the solution can disturb the concentration profiles developed during the electrolysis,
for instance when electroactive species are produced by a homogeneous chemical
process; another example of deviations from the linear model may be caused by the
adsorption of the compound on the electrode surface.
It can be useful to consider different types of transport as deviations from the semiinfinite linear diffusion case. These deviations can then be classified into two groups:
deviations, causing an increase of the transport to the electrode and those causing a
decrease. In the first group, spherical diffusion and different kinetic effects are
included; the second group covers effects such as limited diffusion and reaction from
the adsorbed state.
Spherical diffusion enhances the transport because the spherical expansion of the
diffusion zone increases its volume faster than in the semi-infinite linear case. The
increased volume results in a larger amount of the substance that diffuses to the
electrode.
Kinetic effects occur when the electroactive compound is involved in a chemical
equilibrium. The local decrease of its concentration within the diffusion layer disturbs
the equilibrium and in consequence leads to the production of the compound in a
chemical process. This extra amount increases the flux of the substance to the
electrode surface. Such conditions can be called mixed linear diffusion - kinetic
effects. For a long electrolysis time, the kinetic increase of the flux can entirely
Chapter 5
Editing of measured data
107
compensate the decrease of the concentration gradient and may lead to steady-state
conditions provided that the amount of compound involved in reagent production is
present in large excess.
Another type of effect can be observed when the solution is present in the form of a
thin layer. Electrolysis under such circumstances first leads to the depletion of this
layer and then to the exhaustion of the entire solution volume. This effect can
relatively easily be observed in case of the dissolution of metals from a small
amalgam drop or from amalgam film electrodes: the process is initially controlled by
linear diffusion, but after some time the drop is depleted and the flux of the substance
through the electrode surface drops more rapidly than t-½. This is called limited
diffusion.
An extreme case of this situation is the reaction of a substance adsorbed on the
electrode or forming a monolayer on its surface. In such a case no transport is needed
and the whole amount of substance reacts within a very short time. The measured
current drops sharply to zero after exhaustion of the compound.
Cyclic chronoamperometry and chronocoulometry
If an electroactive compound reaches the electrode by means of semi-infinite linear
diffusion, and the potential of the electrode is such that the surface concentration of
the compound is kept zero, the current can be described using Cottrell's equation
i = nFAD½ Cπ-½ t-½
(eq. 9)
where n is the number of electrons involved, F - Faraday's constant, A - electrode
area, D and C the diffusion coefficient and the bulk concentration of the compound,
respectively, and t - time from the beginning of electrolysis. Integration of this
equation leads to the expression for the charge
i = nFAD½ Cπ-½ t½
(eq. 10)
Semi-integration of eq. 9 or semi-differentiation of eq. 10 leads to the formula
m = nFAD½C
(eq. 11)
In case of additional contributions enhancing the transport due to, for instance,
spherical diffusion or a kinetic effect, the semi-charge is greater than predicted from
the purely linear model and the line displays positive bias. The inverse effect appears
when the transport is slower than for limited diffusion or if the reagent is strongly
adsorbed on the electrode.
There are a number of experimental problems, that should be mentioned here. First,
the data used for studies of transport phenomena should be corrected for the
background otherwise deviations from linearity of the graphs can have other reasons.
In case of kinetic control of the process, the time scale of the experiment also
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determines the range of reaction rates and equilibrium constants that can be detected.
The quantitative discussion of influences of different experimental factors on the
convolution curve can be found in Goto M, Oldham KB, Anal. Chem. 46(1974)1522.
Linear and staircase voltammetry
The convolution techniques facilitate the interpretation of voltammograms,
particularly the information included in the peak shape. As already said, this shape
can be considered as produced by the convolution of two functions: function I,
describing the surface concentration of the reaction product, and function II,
representing the flux of the reaction substrate, depending on the geometry of the
measuring system.
In the simplest case of semi-infinite linear diffusion and fast electron transfer,
function I has the form of an S-shaped wave and function II is a t-½decay. The
character of function I is well-reflected in the ascending branch of the voltammetric
peak; function II is responsible for the t-½ - proportional decay of the descending
branch of the peak. Semi-integration (convolution of the SCV peak with a t½ function)
should therefore give a wave with a horizontal plateau.
When the transport to the electrode is enhanced or diminished compared to semiinfinite linear diffusion, the descending branch of the SCV peak can be approximated
using a tu function, where u>-0.5 for faster transport (slower decay) and u<-0.5 for
slower transport (faster decay). Changes in transport result in semi-integrated waves
with a biased plateau (negative for slower transport, positive for a faster one).
Algorithms for convolution
As mentioned before, there are special algorithms for differintegration as well as for
other convolution. Below, four algorithms used for differintegration and convolution
are described in short.
G0 algorithm (Grünwald-0)
This algorithm can be used to carry out differintegration to any order. The data must
be acquired in constant intervals. For the order = 1 the operation is equivalent to
differentiation, for -1 - to integration using rectangle method. For +½ the
G0 algorithm is the same as semi-differentiation. For -½ the G0 algorithm is the same
as semi-integration. Error in results increases with the length of the interval and
accumulates, i.e. error in latter points is larger than in earlier ones. Important
advantage is that this algorithm does not require the value of the function for t=0,
which makes it very well suited for transformation of chronoamperometric data
(where i(t=0)->∞. The disadvantage of the algorithm is that the total number of
operations is proportional to the square of the number of data points, so calculation
time grows fast with the length of the data set. The fundamentals of this algorithm are
described in Oldham KB, J. Electroanal. Chem. 121(1981) 431.
FRLT algorithm (Fast Riemann-Liouville Transform)
This is a fast, approximate algorithm based on a recursive digital filter. It is best suited
for differintegration in the range of 0.0...-0.5 (up to semi-integration). It is less precise
than G0 algorithm, but the number of operations is linearly related to the number of
Chapter 5
Editing of measured data
109
data points. For details refer to Pajkossy T, Nyikos L, J. Electroanal. Chem.
179(1984) 65.
Spherical convolution
The algorithm is used to carry out convolution of the data measured using a spherical
electrode and staircase potential waveform. Values of the diffusion coefficient, the
electrode radius as well as the delay between begin of the potential step and the
current sampling moment are necessary. The number of operations is proportional to
the square of data points. Details of the algorithm can be found in S.O. Engblom, K.B.
Oldham, Anal. Chem. 62(1990)625.
Kinetic convolution
This algorithm carries out kinetic convolution according to F.E. Woodard, R.D.
Goodin, P.J. Kinlen, Anal. Chem. 56 (1984) 1920. The number of operations is
approximately proportional to the square of the number of points. This convolution
requires the value of the rate constant of irreversible homogeneous follow-up reaction
(ECi mechanism).
5.11 Convolution in practice
The Convolution option can be selected for data measured with cyclic and linear
sweep voltammetry. This convolution menu offers a number of transformations of the
data set, like differentiation, integration, and convolutions.
There are three principal types of convolution available: differintegration (convolution
with t-u function, equivalent to fractional differentiation or integration, depending on
u), using G0 or FRLT algorithm, spherical convolution, and kinetic convolution. The
difference between G0 and FRLT algorithm is that G0 is more exact, while FRLT is
faster with large data sets. Two items: semi-integration and semi-differentiation
denote differintegrations using FRLT with the u value equal to -0.5 and 0.5,
respectively.
It is possible to carry out more transformations in succession. Because
differintegration is an operation that can be cumulated, double semi-differentiation is
equivalent to differentiation and the integration followed by semi-differentiation is
equal to semi-integration. Please note that some combinations, especially those
involving differentiation are not equivalent: differentiation+integration is not the same
as integration+differentiation.
In all convolutions the scale on the Y axis represents cs(t)nFAD½, where cs(t) is the
concentration of the product of the electron transfer step on the surface of the
electrode.
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Fig. 57 Example of a convoluted voltammogram
It is also possible to perform the convolution on part of the voltammogram (See Set
window option of the Plot menu).
It is recommended that the cyclic voltammogram starts at zero current. This can be
achieved by performing a baseline correction.
5.12 iR drop correction
iR drop correction allows for software correction of the potential data for the Ohmic
drop in the solution. This option can be used for data from cyclic and linear sweep
voltammetry.
After supplying a value for the solution resistance, the measured and corrected curve
are shown. The question appears whether the data are corrected.
Appendix I
GPES data files
111
Appendix I GPES data files
The following types of files are used by GPES
Graphical display settings for:
•
cyclic and linear sweep voltammetry
*.ici
•
chronomethods
*.ixi
•
voltammetric analysis
*.iei
•
multi mode electrochemical detection
*.idi
•
potentiometric stripping analysis
*.ipi
•
steps and sweeps
*.ifi
•
electrochemical noise
*.ini
These data files are in ASCII-format and are stored in the procedure directory and in
the data directory.
Experiment parameter settings for:
•
cyclic and linear sweep voltammetry
*.icw
•
chronomethods
*.ixw
•
voltammetric analysis
*.iew
•
multi mode electrochemical detection
*.idw
•
potentiometric stripping analysis
*.ipw
•
steps and sweeps
*.ifw
•
electrochemical noise
*.inw
These data files are in ASCII-format and are stored in the procedure directory and in
the data directory.
Measured data files for:
•
cyclic and linear sweep voltammetry
*.ocw
•
chronomethods
*.oxw
•
voltammetric analysis
*.oew
•
multi mode electrochemical detection
*.odw
•
potentiometric stripping analysis
*.opw
•
steps and sweeps
*.ofw
•
electrochemical noise
*.onw
These data files are in ASCII-format and are stored in the data directory.
Data memory buffer in binary format for:
•
cyclic and linear sweep voltammetry
*.bcw
*.cv1
•
*.cv2
•
*.cv3
These data files are in binary format and are stored in the data directory.
•
Data memory buffer in ASCII-format and BAS Digisim file for:
•
cyclic and linear sweep voltammetry
*.txt
These data files are in ASCII-format and are stored in the data directory.
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Project files containing command lines for automatic processing of measurement
procedures and data analysis:
*.mac
These data files are in ASCII format and are stored in the data directory.
Print template files:
*.def
These data files are in ASCII format and are stored in the Autolab directory.
File containing anodic and cathodic charges
in cyclic voltammetry:
*.q&q
These data files are in ASCII format and are stored in the data directory.
The GPES executable file:
The GPES binary help file:
gpes4.exe
gpes40.hlp
The system parameter file, ASCII-format:
Description file of sysdef40.inp, ASCII-format:
sysdef40.inp
sysdef40.txt
Fit & simulation parameter files:
*.efs
Appendix II
Definition of procedure parameters
113
Appendix II definition of procedure parameters
CM
CV
ECD
ECN
PSA
SAS
VA
: Chronomethods
: Cyclic and linear sweep voltammetry
: Electrochemical detection
: Electro Chemical Noise
: Potentiometric stripping analysis
: Steps and Sweeps
: Voltammetric analysis
ADC channel number: (Second signal, CV, CM)
The channel number which should be used for recording the output from an external
source.
Amplitude: (VA ac voltammetry)
The root-mean-square value of the applied potential sine wave perturbation.
Amplitude: (VA square wave)
Half of the peak to peak value in the squared wave perturbation.
Base potential: (VA normal pulse, differential normal pulse)
The base potential level. The pulse will be superimposed on this potential level.
Begin potential: (CV linear sweep)
The potential at which the ramp starts.
Cell off after measurement: (All)
If not 'checked' the cell switch will be left in the 'on' position after the measurement
procedure has been completed. The applied potential is the "stand-by potential".
Comment: (All)
A panel to type in several lines of text.
Conditioning potential: (VA)
This is the first potential applied after the start of the procedure. This potential is
normally applied to clean the electrode surface. This potential is not applied when its
duration is set to zero.
Correct iR-drop during dyn. iR: (CV, CM with dynamic iR compensation)
If ‘checked’ the potential will be corrected for the ohmic drop. If ‘not checked’ the
value of the ohmic drop is determined only.
Current range: (bipotentiostat, CV, CM)
The input parameter only appears when the Autolab is equipped with the
bipotentiostat module. The maximum range is 10 mA, the minimum range is 100 nA.
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Cutoff on charge: (CM interval time > .1 s)
If checked, in chrono-amperometry or -coulometry, the specified cutoff value is
charge, otherwise it is current. This feature allows to specify a cutoff value for the
charge in chrono-amperometry.
Cutoff value: (CM interval time > .1 s)
If the specified value is exceeded, the measurements will proceed with the next
potential level or, if it is the last potential level in the sequence, the measurements will
stop.
Cutoff value for 2nd signal >(V’): (CV and CM interval time > .1s)
This value is the upper limit for the 2nd signal, as soon as this limit is reached, the
experiment will stop.
Cutoff value for 2nd signal <(V’): (CV and CM interval time > .1s)
This value is the lower limit for the 2nd signal, as soon as the limit is reached, the
experiment will stop.
Cutoff value for time >: (CM interval time > .1s)
The measurement is aborted when the time exceeds the specified value. It is only
active when the option ‘Specify time limit’ is checked.
Define potential w.r.t. OCP: (CM)
If 'checked', the stand-by potential and the specified potential levels are
applied with respect to the open circuit potential (OCP). Before the equilibration
starts, the OCP is recorded. If sufficiently stationary, a button can be pressed to
continue.
Define start potential w.r.t. OCP: (CV)
If this item is checked the measurement starts with measuring the OCP. After
acceptance the Start potential will be corrected for the OCP (Start potential + OCP). If
you want to start at the OCP.
Define vertex potential w.r.t. OCP: (CV)
If ‘checked’, the vertex and start potentials are specified with respect to the open
circuit potential (OCP). Before the equilibration starts, the OCP is recorded. If
sufficiently stationary, a button can be pressed to continue.
Deposition potential: (VA)
This is the second potential applied after the start of the procedure. This potential is
normally applied to deposit the components to be analysed on the electrode. This
potential is not applied when its duration is set to zero.
Direct output filename: (CM interval time > .1 s)
If a file name (without extension) is specified, the measured data are directly written
to a data file with the extension ".oxw". This option may be useful for long duration
measurements. It prevents loss of data due to a failure in the power supply.
Appendix II
Definition of procedure parameters
115
If the number of measured data points exceeds the allowed maximum (default
10,000), the program will continue to store the data points on disk, although the data
points are no longer plotted on the screen and stored in the computer memory. Also
the data file becomes too long to be loaded by the GPES program.
Direct Output filename: (CV)
The path and the name of the file. The last five characters of the file name will be
used as the scan number. This filename will be used for the Save every nth Cycle
option.
Duration of Measurement: (ECN)
The total duration of the Measurement. It will be rounded to the next nearest power of
2 times the Interval time.
Dynamic iR amplitude: (CV, CM interval times > .1s)
The amplitude of the square wave in Dynamic iR compensation.
End potential∗ : (CV linear sweep, SAS)
The potential at which the ramp stops.
Equilibrate with potential pulses: (ECD)
If 'checked', the specified potential pulses are applied without recording data,
otherwise the stand-by potential is applied.
Equilibration time: (All)
The time to equilibrate the electrode at the start potential (CV, VA) or the stand-by
potential (CM, ECD, PSA).
Equilibration threshold level: (VA, CV, CM, ECD)
If enabled, the Equilibration stage will be aborted after reaching this specified current.
The measurements will start as soon as this threshold value is exceeded. This option is
not available for galvanostatic measurements.
Final rotation speed (rpm): (LSV staircase hydrodynamic)
The rotation speed applied during the last scan.
First conditioning potential∗ : (CV, CM, ECD)
The first potential which is applied after the Start button has been pressed. If the
corresponding "Duration" is zero, the potential is not applied.
First potential boundary: (CV)
Used, in combination with ‘Second potential boundary’, for automatic calculation of
the total positive and total negative charge. Only active when ‘Use boundaries for
Q+/Q- calc.’ is ‘checked’.
∗
)
In galvanostatic cyclic voltammetry or galvanostatic chronopotentiometry
'potential' should be read as 'current' and vice versa.
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First vertex potential∗ : (CV cyclic)
The potential goes from the start potential to the first vertex potential where it turns
around to go to the second vertex potential.
Frequency: (VA)
The number of times the square wave or sine wave perturbation is applied per second
in respectively square wave voltammetry and ac voltammetry.
Highest current range: (bipotentiostat, CV, CM)
The input parameter only appears when the Autolab is equipped with the bipotentiostat module. The maximum range is 10 mA, the minimum range is 100 nA.
The actual current range for the bi-potentiostat module will be automatically set
between the specified ‘highest’ and ‘lowest’ current range.
Initial rotation speed (rpm): (LSV staircase hydrodynamic)
The rotation speed applied during the first scan.
Interval time: (CM interval time > .1 s)
Normally the time between two recorded data points. If a maximum dE, di, or dQ
value is specified, the actual interval time can be less. For more information see the
chapter on the methods.
Interval time: (ECD)
The time between two current measurements in dc-amperometry.
Interval time: (ECN)
The time between two recorded current and potential samples. It should be >=
0.002 s.
Interval time: (VA)
Time between two measurements.
Linear(1) or square root(2) distr.: (LSV staircase hydrodynamic)
The rotation speed table is calculated with a Linear distribution (1) or with a square
root distribution (2). A linear distribution means that, when the initial speed is e.g.
100 and the final speed is e.g. 1000 with 10 scans, the subsequent rotation speeds will
be 100, 200, 300, ..... 1000.
Lowest current range: (bipotentiostat, CV, CM)
The input parameter only appears when the Autolab is equipped with the bipotentiostat module. The maximum range is 10 mA, the minimum range is 100 nA.
The actual current range for the bi-potentiostat module will be automatically set
between the specified ‘highest’ and ‘lowest’ current range.
Maximum dE, di, or dQ: (CM interval time > .1 s)
In case the box "Specify maximum dE, di, or dQ" is 'checked' and if the change in
current, charge, or potential exceeds the specified value, a data point will be recorded.
Appendix II
Definition of procedure parameters
117
Maximum time interval: (CV, stationary current)
After this period the current is supposed to be stationary.
Maximum time of measurements: (PSA)
The measurements will stop when duration of the measurement exceeds the specified
time.
Measurement temperature: (CV, CM with pH as second signal)
Temperature for pH correction with respect to the calibration temperature.
Minimum abs(di/i) per second: (CV, stationary current)
Every second the relative current change is determined. If during three seconds this
relative change is less than the specified value the current is supposed to be stationary.
The next potential is applied.
Minimum abs(di) per second: (CV, stationary current)
Every second the absolute current change is determined. If during three seconds this
relative change is less than the specified value the current is supposed to be stationary.
Minimum variation: (CM)
Value at which the experiment is stopped or the next step in the experiment will be
applied. This value is only active if the ‘Specify minimum variation’ is ‘checked’.
Modulation amplitude: (VA differential pulse)
The height of the potential pulse. The pulse direction is the same as the scan direction
when the specified amplitude is positive. If a negative amplitude is specified, the
pulse direction is reversed with respect to the scan direction.
Modulation amplitude: (VA differential normal pulse)
Potential superimposed on the sum of base potential and pulse amplitude.
Modulation time: (VA)
Time during which the modulation amplitude(differential pulse, differential normal
pulse) or the sine wave(ac voltammetry) is applied. A convenient value is 0.07 s for
differential pulse and differential normal pulse. For ac-voltammetry 0.5 s is
convenient.
Number of cycles: (CM interval times < .1 s)
The number of times the sequence of potential levels as specified in the potential level
table are applied. After the measurements only the last cycle is in the computer
memory. All the measured data of the previous cycles are lost. In most cases the
number of cycles will be one, but for e.g. pulse plating experiments a higher number
can be specified.
Number of cycles: (CM interval times > .1 s)
The number of cycles you want to measure. A cycle includes the pre-treatment. The
old data (previous scan) will be overwritten by the new one. The direct output file
(when specified) is appended with every scan. The time parameter also adds up.
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Between two scans the interval time is recorded by the computer clock and this time is
also added to the time parameter. After reaching the maximum number of points in
memory (=10000) the on-line plot option will stop. The actual data-points however,
will be in memory and are plotted after the ‘number of cycles’ has been reached or
after pressing ‘Abort’.
Number of equilibration scans: (CV Scan averaging)
Number of cycles of linear sweeps to reach an equilibrium. The averaging starts after
the specified number of scans. The equilibration scans are not kept in memory.
Number of potential steps: (CM)
During the measurements the potential steps from the stand-by potential to a number
of potential levels. These levels can be specified under header 'Potentials'
Number of pulses: (ECD)
The number potential levels which should be applied in multiple pulse or differential
pulse mode.
Number of scans: (CV)
The number of cycles or linear sweeps to be measured.
Number of scans: (LSV staircase hydrodynamic)
The number of scans with a different rotation speed.
Number of scans: (VA)
The number of times a voltammogram is recorded. The presented voltammogram is
the average of all recorded voltammograms.
Phase: (VA ac voltammetry)
If "Phase sensitive" field above is ‘checked’, the supplied value will be the phase shift
with respect to the applied ac potential at which the ac current is obtained.
Phase sensitive: (VA ac voltammetry)
If 'checked' a value for the phase should be supplied.
Potential: (bipotentiostat, CV, CM)
The input parameter only appears when the Autolab is equipped with the
bipotentiostat module. The constant potential which should be applied to the second
working electrode.
Potential limit: (PSA)
The measurements will stop when the potential passes the specified potential limit.
Potential shift: (CV)
The specified amount will be added to the recorded potentials of the voltammogram.
In this way it is possible to record potential versus present reference electrode, but
display them with respect to another.
Appendix II
Definition of procedure parameters
119
Potentials∗ : (CM)
A table of potential levels can be specified. The number of rows is equal to
the ‘number of potential levels’ specified on the field above. During a measurement
sequence, the potential steps from the stand-by potential to each of the specified
levels, The following columns can be specified in the potential table:
Potential: the required potential level
Duration: the time the potential level is applied
Sample time: the time between two current samples. This column is only present for
the chronomethod with interval times < .1s. See description of the methods.
Potentials table: (ECD)
In this table the potential levels to be applied, and their duration can be specified in
multiple pulse or differential pulse mode. The number of rows is equal to the
"Number of pulses". In multiple pulse mode, in a third column, it can be specified
whether the current should be recorded or not. In differential pulse mode the two
levels, specified on page 2, are recorded.
Pulse time: (VA Normal pulse)
Time during which the potential pulse is applied.
Purge time: (All)
The time the gas valve is positioned to flow the gas through the cell. This parameter
only appears when an automatic electrode is present (see Hardware configuration
program).
Quick save of previous scan: (CV)
When more then one scan is recorded in Cyclic voltammetry, it is possible to save the
previously measured scan. This option can also be activated by typing 'SAVE' on the
keyboard. The path and the name of the file can be specified as the ‘Direct output
filename’. The last five characters of the file name will be used as the scan number.
Record Bipotentiostat signal: (bipotentiostat, CV, CM)
The input parameter only appears when the Autolab is equipped with the
bipotentiostat module.
Record second signal: (CV, CM)
The primary signal, current or potential, is sampled via one of the channels of the
ADC164 or ADC124 analog to digital converter module.
If "Record second signal" is 'checked', the voltage level of an additional channel of the
ADC164 or ADC124 module is sampled as well. The channel number can be
specified.
The ADC164 or ADC124 module has 16 input channels which can be recorded. An
internal multiplexer allows switching from one channel to another. Four of them have
an external BNC-plug. Normally, input number three and four are free channels, i.e.
not used by GPES. They can be used to record the output of another instrument.
∗
)
In galvanostatic cyclic voltammetry or galvanostatic chronopotentiometry
'potential' should be read as 'current' and vice versa.
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The input parameters, related to the second signal, do not appear when the Autolab is
equipped with a bipotentiostat module.
Reverse scan for i> : (CV)
If "Specify current boundaries" is 'checked', the scan direction will be reversed if the
current exceeds the specified value.
Reverse scan for i< : (CV)
If "Specify current boundaries" is 'checked', the scan direction will be reversed if the
current becomes lower than the specified value.
Run time: (ECD)
The duration of the measurement.
Save every nth Cycle: (CV)
When more then one scan is to be recorded in Cyclic voltammetry, it is possible to
save scan at regular intervals during the measurements. If this parameter is zero, no
scans will be saved during the measurements, otherwise every nth scan will be stored
on disk. If, e.g. '5' is specified, scan 1, 5, 10, 15 are saved. The path and the name of
the file can be specified on page two of the Edit procedure window ('Direct output
filename'). The last five characters of the file name will be used as the scan number.
Please note: These files can be overwritten during another measurement session with
the same procedure.
Scan rate∗ : (CV Staircase)
The required speed of potential change. The lowest scan rate is 0.00001. The highest
acceptable depends on the speed of the AD-converter, the computer, and the step
potential. The essential number for the highest scan rate is the number of potential
steps per second i.e. (scan rate)/(step potential). The maximum value is 4,800 with
normal CV (2400 with Bipot or 2nd Signal). With Fast scan CV the maximum value is
45,000. The specified value is adjusted by the program, so that the number of steps
per second becomes equal to one of the discrete values of the Autolab hardware timer.
The maximum values might vary in combination with advanced options like “Specify
current boundaries”, “High sensitivity”, “alpha (different from 1)” and
“Chronoamperometry at vertexes”.
Scan rate: (CV Linear scan)
The required speed of potential change. The lowest scan rate is 0.001. This is a
hardware limitation of the SCAN-GEN module.
The highest scan rate is 10,000 V/s for the SCAN-GEN module. As stated above, no
more than about 4,800 samples per second can be taken in combination with the
ADC164 or ADC124 module. This limits the measurable scan rate to about 10 V/s.
Higher scan rates can be measured with ADC750 module, which allows to measure
750,000 samples per second.
Second conditioning potential∗: (CV, CM, ECD)
∗
)
In galvanostatic cyclic voltammetry or galvanostatic chronopotentiometry
'potential' should be read as 'current' and vice versa.
Appendix II
Definition of procedure parameters
121
The second potential which is applied after the Start button has been pressed. If the
corresponding "Duration" is zero, the potential is not applied.
Second potential boundary: (CV)
Used, in combination with ‘First potential boundary’, for automatic calculation of the
total positive and total negative charge. Only active when ‘Use boundaries for Q+/Qcalc.’ is ‘checked’.
Second vertex potential: (CV cyclic)
The potential goes from the start potential to the first vertex potential where it turns
around to go to the second vertex potential.
Show noise around zero Volt : (ECN, with ECN-module selected)
If ‘checked’ the potential noise will be plotted around zero volt in stead of around the
DC-potential.
Signal multiplier: (Second signal, CV, CM)
The recorded "second signal" is measured in Volts. It can be multiplied by a factor to
convert it into another unit.
Signal offset: (Second signal, CV, CM)
The recorded "second signal" is measured in Volts. An offset can be supplied to
convert it into another unit.
Smooth level: (PSA)
The potential - time data are recorded. Subsequently the data are smoothed using the
Savitsky-Golay algorithm and the derivative dt/dE is calculated.
See for further details the section on "smoothing".
Specify cutoff value for 2nd signal: (CV, CM interval times > .1s)
If ‘checked’, the 2nd signal will be checked on the ‘Cutoff value for 2nd signal >(V’)’
and ‘Cutoff value for 2nd signal <(V’)’. The measurement will stop (CV) or proceed
with the next potential level or stop if it is the last potential level in the sequence
(CM).
Specify current boundaries: (CV, normal, or stationary current mode)
If 'checked', the scan direction will be reversed, if the current exceeds one of the
values specified below. In case of linear sweep voltammetry, the recording of the scan
will be terminated.
Specify cutoff value: (CM interval time > .1s)
If 'checked' and the specified cutoff value is exceeded, the measurements will proceed
with the next potential level or, if it is the last potential level in the sequence, the
measurements will stop.
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Specify maximum dE, di, or dQ: (CM interval time > .1s)
If 'checked', a data point will not only be recorded after the specified interval time, but
also if the change in current, charge, or potential exceeds the specified value.
Specify minimum variation: (CM)
If ‘checked’, this parameter can be used to stop a chrono-amperometry or chronopotentiometry experiment as soon as the change in, respectively, current or potential
is less than the ‘Minimum variation’ value. In other words, as soon as the measured
signal has reached the ‘Minimum variation’, the experiment is stopped or the next
step in the experiment is applied.
Specify Time limit: (CM interval time > .1s)
If this option is checked, the measurement will stop when the time exceeds the value
specified for ‘Cutoff value for time >’. This option might be of use when a large
number of cycles is specified.
Stand-by potential∗ : (All)
The potential which is applied after the measurement in case of ‘cell on after
measurement’. Sometimes (CM, ECD) it is also the 'start' potential before a
measurement.
Start potential∗ : (CV, VA)
The potential at which the measurement, after the pre-treatment, begins.
Step potential∗ : (CV)
The potential increment between two successive current measurements. The specified
value is adjusted by the program, so that it becomes equal to the closest 16 bit
(DAC164) or 12 bit (DAC124) value and the number of steps per second becomes
equal to the closest value of the Autolab hardware timer. See also "Scan rate".
∗
)
In galvanostatic cyclic voltammetry or galvanostatic chronopotentiometry
'potential' should be read as 'current' and vice versa.
∗
)
In galvanostatic cyclic voltammetry or galvanostatic chronopotentiometry
'potential' should be read as 'current' and vice versa.
Appendix II
Definition of procedure parameters
123
Steps and sweeps table: (SAS)
A table in which up to 10 potential level or sweeps can be defined. The following
items can be specified.
Segment type:
Not used
This level will not be included in the measurement
Step
One potential can be applied during a given time. The current is
sampled with the specified interval time.
Staircase sweep
The potential sweeps from the previous applied potential to the
End potential with the specified scan rate. The current is
sampled at the end of every potential step. If the segment type
of the first level is specified as a sweep, the start potential
equals the Standby potential.
Linear sweep
Equal to Staircase sweep but performed with SCAN-GEN
module. If the SCANGEN module is present, it is only possible
to select Linear sweep segment (Staircase sweep is disabled).
Potential (V) (Step segment)
Sample time (s) (Step segment)
(the lowest possible value is 0.0002 s)
Total time(s) (Step segment)
End potential (V) (Sweep segment)
Scan rate (V/s) (Sweep segment)
(the highest possible value is 5000 times the
step potential value)
Step potential (V) (Sweep segment)
The parameters for the Step segments are similar to the chrono-methods with short
interval times (see items ‘Potentials’ in the appendix). The parameters for the Sweep
segment are similar to the parameters for Linear sweep voltammetry.
Stirrer off during conditioning: (VA)
Switch off the stirrer of the Automatic electrode during the conditioning stage.
Stirrer off during deposition: (VA)
Switch off the stirrer of the Automatic electrode during the deposition stage.
Stop Equilibrium at threshold: (VA, CV, CM, ECD)
Enable the option to abort the equilibration stage when the Equilibration threshold
level is reached.
Stop scan: (CV, linear sweep)
The recording of the sweep will be terminated if the current exceeds the upper or
lower specified limit.
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Surface area / cm^2: (CV & CM)
Surface area of the working electrode, with which the Current density can be
calculated in the Analysis menu of the Data presentation window.
Switch cell off when i=0 A:(CM interval time < .1 s)
If this option is checked the cell will be switched off during levels specified with a
current equals 0 A. This feature assures zero current. If this option is not checked, the
current is set to zero with the cell switch ‘on’. This means that a small offset current
of maximum 0.2% of the selected current range can flow.
Tafel plot: (CV)
If checked, the x-axis becomes the 10Log-axis of the measured current and the y-axis
becomes the potential axis. This option has been added to allow presentation of
traditional Tafel plots. It does not give any functional contribution. Unless required, it
is recommended to use this option. Normally recorded voltammograms can always be
converted to corrosion plots:
Load previously measured data; check corrosion plot; save the data again; reload the
data. Now the data should be presented as a corrosion plot.
Third conditioning potential∗ : (CV, CM, ECD)
The third potential which is applied after the Start button has been clicked. If the
corresponding "Duration" is zero, the potential is not applied.
Time to wait for OCP: (CV, CM)
The time you want to wait for acceptance of the Open Circuit Potential. If this time
has expired the program will continue using the OCP measured at that time. If this
parameter is 0(zero), the program will not continue unless the 'Accept' button is
pressed. If 0(zero) is specified in a procedure that is used in a project, the program
will wait for 2 seconds at the OCP and will use the OCP measured at that moment.
Title and subtitle: (All)
Text lines to describe the experiment.
These lines are the same as the ones displayed above the plot.
Type of signal: (CV, CM)
Aux signal
- Signal measured on selected ADC-channel
Charge
- Calculated charge
Potential
- Measured potential
Current
- Measured current
ESPR
- Measured response from ESPR device
pH
- Measured response of the pX module (converted to pH, see Utilities
menu, Calibrate pH-electrode)
pX
- Measured response of the pX module
∗
)
In galvanostatic cyclic voltammetry or galvanostatic chronopotentiometry
'potential' should be read as 'current' and vice versa.
Appendix II
Definition of procedure parameters
125
Use ADC750: (CV, CM)
When the ADC750 module for fast AD conversions is present in the Autolab
instrument, it can be used for fast CV or chrono measurements if an interval time is
smaller than 100 µs.
Use boundaries for Q+/Q- calc.: (CV)
If ‘checked’ this option enables the user to set potential boundaries for automatic
charge calculation. Between the first and second potential boundary the total positive
and total negative charge will be calculated automatically. Charge values are
displayed in the status bar, at the bottom of the screen. The option is mainly used for
Cyclic Voltammetric Stripping in electroplating research. If this option is not checked,
the ‘First and Second vertex potential’ are used as potential boundaries.
Use dynamic iR-compensation : (CV, CM interval times > .1s)
This options offers the possibility to measure and compensate for the Ohmic Drop
during the measurement. This is useful in systems where the Ohmic Drop changes
during the experiment. At every potential level, either a step in staircase cyclic
voltammetry or a step in chronoamperometry, a small amplitude (Dynamic iR
amplitude) high frequency square wave signal is added. By measuring the resulting
current responses, the Ohmic Drop is calculated.
Please keep in mind that the following limitations apply to this technique:
•
The sweep rate in cyclic voltammetry is limited.
•
The method cannot be used in combination with a Rotating Disk Electrode, an
ARRAY, ADC750, BIPOT, pX or ECD module or any other device (EQCM,
ESPR, etc.) that will result in an external signal.
•
Hardware adjustments are necessary for this option, so the option cannot be used
on an older instrument with new software only.
•
The method only works in High Speed mode.
Use ECN module: (ECN)
Use the ECN module to perform the noise measurements.
Use high ADC resolution:(CV staircase fast scan)
Fast scan measurements are done at a fixed gain. If this option is checked the
measurement are done at gain 10 of the ADC-module, otherwise gain 1 is used.
Use lowest possible interval time: (CM interval times < .1s)
If this option is checked, the sample time per level will disappear from the table at
page one. It is not possible to specify a sample time anymore. The sample time is
calculated just after the measurement and depends on the speed of the PC that is used
and on the type of AD converter included in the Autolab system. In general it is
possible to get an interval time of approximately 19 µs.
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User Manual
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Version 4.9
Value of alpha: (CV staircase)
Fraction of the time interval, between two potential steps, at which the current is
sampled. It should normally be 1. Only in cases where linear sweep voltammetry
should be compared with staircase voltammetry, this number should be different. The
number should be at least 0.25. Linear sweep voltammetry equals staircase
voltammetry for reversible systems when α = 0.25.
Ref.: M. Saralthan, R.A. Osteryoung, J. Electroanal. Chem. 222, 69 (1987).
Wait after first vertex: (CV, staircase cyclic voltammetry, normal or stationary current
mode)
If ‘checked’, scanning will stop at the first vertex potential and a chronoamperogram
will be recorded, with duration and sample time as specified. The chronoamperogram
is not displayed on the screen, but the data are stored in memory and can be saved on
disk using the File option on the Data presentation window.
Wait after second vertex: (CV, staircase cyclic voltammetry, normal or stationary
current mode)
If ‘checked’, scanning will stop at the second vertex potential and a
chronoamperogram will be recorded, with duration and sample time as specified. The
chronoamperogram is not displayed on the screen, but the data are stored in memory
and can be saved on disk using the File option on the Data presentation window.
Appendix III
Combination of GPES and FRA
127
Appendix III Combination of GPES and FRA
The FRA and GPES programs can be used at the same time. Moreover a FRA project
file can be executed from GPES. The command FRA!Start(<"filename">) is available
for this purpose.
However, in general it is important to note that both programs share the Autolab
instrument and the graphics part of the software. Moreover, both programs require a
considerable amount of the system resources. This means that when both programs
are active, hardly any system resources are left. The amount of free system resources
can be seen in option ‘About program manager’ in the Help menu of the Program
manager window.
Practical rules are:
•
The computer should be equipped with at least 32 MB RAM
•
It is not possible that both programs are measuring and controlling the Autolab
instrument
•
Before the FRA program starts measuring, the ‘sleep mode’ in GPES is
automatically switched on. This means that the GPES screen is no longer updated.
•
Do not use function keys when both programs are active, because they will cause
actions in both programs.
•
When a measurement procedure is being executed, user interaction with the
programs should be avoided.
•
Apart from GPES and FRA no other program/window should be active.
Appendix IV
Multichannel control
129
Appendix IV Multichannel control
It is possible to control the multichannel potentiostat with GPES. Activate the
multichannel option by starting GPES with the shortcut "Multichannel GPES". The
multichannel software is similar to the GPES software, however, some options are
different or not available. The differences are explained in this paragraph.
Installation and test
The Hardware Setup program contains a button for Multichannel setup. After pressing
this button the following screen appears:
Fig. 59 Multichannel configuration
The items above are factory settings and should normally not be changed. The test of
the software is similar to the test of GPES. The procedure TESTCV6 can be used for
more than one potentiostat.
If the multichannel system is equipped with ARRAY modules, one dummy cell is
available. After loading the procedure TESTCV6 from the \AUTOLAB\TESTDATAdirectory, the Multi channel control window shows that PGSTAT and ARRAY-2 are
“active”. Now connect the dummy cell. The lead from ARRAY-2 should be
connected to WE(b). After starting the execution of the procedure, the normal dummy
cell response should appear and the current response should be the same for both
channels. In a subsequent measurement ARRAY-3 can be connected to the dummy
cell in stead of ARRAY-2. After ARRAY-2 has been made inactive and ARRAY-3
active, the same current response is expected. In this way all channels can be checked.
In case of a multi-PGSTAT10 set-up two dummy cells should be available. The
PGSTAT10-2 should be connected to the second dummy cell. Then the same
procedure should be followed as for the multi-ARRAY set-up as described above.
The DIAGNOST test is available, but does not work properly. The “zero-test” of the
ADC does not work properly because the test-channel is used for one of the extra
channels.
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User Manual
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Version 4.9
Program operation
After starting the program an extra Window appears :
Fig. 60 Multi channel control window
When this window is behind other windows on the screen it can be shown with the
Window option from the GPES manager. The last item is Multichannel. After clicking
this item the multichannel window will be shown. Depending on the configuration,
this screen will be adjusted. The base potential for all arrays, including the PGSTAT,
can be set in the PGSTAT panel. If offset DAC’s are present, a specified offset
potential can be given.
With ‘Dependent current ranging’ the current range of all channels will be the same.
Changing the current range of ‘Array 4’ will lead to a current range change of all
channels. The Active-option button allows to select the channels that should be
measured. Note that the potential will always be applied to all channels.
The number of items on the Manual control window is considerably reduced
compared to normal GPES.
On the Data presentation window two additional items are present.
1. The Signal menu allows to select the signal from which channel is the active work
signal. All Edit and Analysis operation will be performed on this signal. The
currently selected signal is shown between brackets.
2. The Plot overlay signal option in the Plot menu allows to overlay several signals.
It operates similarly to the other overlay options.
The following methods are available in the multichannel mode:
Voltammetric analysis
•
Differential pulse
•
Square wave
•
Sampled DC
•
Normal pulse
Appendix IV
Multichannel control
131
Cyclic voltammetry
•
Normal
Linear sweep voltammetry
•
Normal
Chronomethods
•
Amperometry
•
Potentiometry
During the measurements, the sampling duration is the same as in the normal GPES.
This is described in the chapter about the Methods. However in the multichannel
mode all six possible channels sampled one after an other. So the number of samples
per channel from which the average registered current c.q. potential value is, one sixth
of the normal GPES. If a channel is specified as 'active', it only means that the
measured current c.q. potential is registered.
The minimum sampling time is equal to 6 ranged AD-conversion and this is 6 times as
long as in normal GPES. This has a consequence for the minimum interval time in
Chronomethods with short interval times. The minimum time is now about 800
microseconds (see also the information about the manual control window).
Specifications of the instrument are almost similar to those of the PGSTAT10
potentiostat.
Appendix V
Technical specifications
133
Appendix V Technical specifications
µAutolab
type III
Autolab with
PGSTAT12
Autolab with
PGSTAT302N
Autolab with
PGSTAT100
maximum output current
maximum output voltage
± 80 mA
± 12 V
± 250 mA
± 12 V
±2A
± 30 V
± 250 mA
± 100 V
potentiostat
galvanostat
yes
yes
yes
yes
yes
yes
yes
yes
potential range
applied potential accuracy
±5V
± 0.2% of setting
2 mV
150 µV
300 or 30 µV
± 10 V
± 0.2% of setting
2 mV
150 µV
300 or 30 µV
± 10 V
± 0.2% of setting
2 mV
150 µV
300 or 30 µV
± 10 V
± 0.2% of setting
2 mV
150 µV
300 or 30 µV
10 nA to 10 mA in
seven ranges
10 nA to 100 mA
in eight ranges
10 nA to 1 A
in nine ranges
10 nA to 100 mA
in eight ranges
± 0.2% of current
and ± 0.2% of
current range
0.015% of current
range
0.0003% of current
range
± 0.2% of current
and ± 0.2% of
current range
0.015% of current
range
0.0003% of current
range
± 0.2% of current
and ± 0.2% of
current range
0.015% of current
range
0.0003% of current
range
± 0.2% of current
and ± 0.2% of
current range
0.015% of current
range
0.0003% of current
range
30 fA
500 kHz
1 µs
30 fA
500 kHz
< 500 ns
30 fA
500 kHz
< 500 ns
high speed/
high stability
> 100 GΩ//< 8 pF
high speed/
high stability
> 100 GΩ//< 8 pF
30 fA
>1 MHz
< 250 ns
(with external source)
high speed/
high stability
> 1 TΩ//< 8 pF
< 1 pA
> 4 MHz
n.a.
< 1 pA
> 4 MHz
depending on selected
range: 0Ω-20Ω at 1 A
range to 0Ω -200 MΩ
at 10 nA range,
current interrupt and
positive feedback
available
0.025%
< 1 pA
> 4 MHz
depending on selected
range: 0Ω-200Ω at
100 mA range to 0Ω200 MΩ at 10 nA
range, current
interrupt and positive
feedback available
0.025%
applied potential resolution
measured potential resolution
current ranges
applied and measured
current accuracy
applied current resolution
measured current
resolution
- at current range
of 10 nA
potentiostat bandwidth (1)
- potentiostat risetime/falltime
(1 V step, 10-90%) (1)
potentiostat modes
input impedance of
electrometer
input bias current @25°C
bandwidth of electrometer
IR-compensation
high speed/
high stability
> 100 GΩ//< 8 pF
- resolution
n.a.
< 1 pA
> 4 MHz
depending on selected
range: 0Ω-200Ω at
100 mA range to 0Ω200 MΩ at 10 nA
range, current
interrupt and positive
feedback available
0.025%
four electrode control
front panel meter
no
no
yes
potential and current
yes
potential and current
yes
potential and current
Analog outputs (BNC
connector)
control voltage input
multichannel option
potential and
current
no
no
potential, current and
optionally charge
yes
multipleWE option
potential, current and
optionally charge
yes
multipleWE option
potential, current and
optionally charge
yes
no
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User Manual
GPES for Windows
Version 4.9
µAutolab
type III
Autolab with
PGSTAT12
Autolab with
PGSTAT302N
Autolab with
PGSTAT100
booster option
no
no
yes
analog integrator
- time constants
yes
10 and 100 ms,
1 and 10 s
optionally available
10 and 100 ms,
1 and 10 s
optionally available
10 and 100 ms,
1 and 10 s
on request BSTR10A
only
optionally available
10 and 100 ms,
1 and 10 s
interfacing
A/D converter
USB
16-bit with software
programmable gains
of 1, 10 and 100
USB
16-bit with software
programmable gains
of 1, 10 and 100
USB
16-bit with software
programmable gains
of 1, 10 and 100
USB
16-bit with software
programmable gains
of 1, 10 and 100
auxiliary input channels
D/A converter
1
16-bit
three channels
1
48
2
16-bit, four channels
(optionally eight)
1
48
2
16-bit, four channels
(optionally eight)
1
48
2
16-bit, four channels
(optionally eight)
1
48
26 x 26 x 10 cm³
3.6 kg (4.2kg / FRA2)
144 W
100-240 V, 50/60 Hz
52 x 42 x 17 cm³
18 kg
247 W
100-240 V, 50/60 Hz
51.5 x 41.6 x 16 cm³
18 kg
247 W
100-240 V, 50/60 Hz
52 x 42 x 17 cm³
21 kg
300 W
100-240 V, 50/60 Hz
auxiliary output channel
digital I/O lines
(W x D x H)
weight
power requirements
Notes: (1) Measured at 1 mA current range, 1 kOhm impedance, high speed mode when applicable. All specifications at 25°C.
Interface for mercury electrodes (IME 303 and IME663)
Supported electrodes
•
Metrohm VA Stand 663
•
EG&G PAR303(A)
•
dropping mercury electrodes with knock-off hammer
Control lines
•
new drop
•
purge on/off
•
stirrer on/off
Burettes
•
Metrohm Dosimat 665/765
•
Schott T90 and T100
Hardware specifications of optional modules
SCAN-GEN, SCAN250: analog scan generator module
scan range
vertex potentials
output offset
ranges of scan rates
scan rate
- resolution
- accuracy
- temperature dependence
- minimum
- maximum
hold mode
maximum number of scans
monitor output (BNC)
SCAN-GEN
SCAN250
± (0.2% full scale+500 µV/s)
<0.04%/K
100mV/s
10kV/s
available
32767
± (0.2% full scale+500 µV/s)
<0.04%/K
100mV/s
10kV/s
available
32767
scan signal
scan signal
± 5 V relative to initial
potential
2.5 mV resolution and 5 mV
accuracy
± 1 mV maximum
100 mV/s to 10 kV/s full
scale (6 ranges)
1 in 4096
± 5 V relative to initial
potential
2.5 mV resolution and 2 mV
accuracy
± 0.2 mV maximum
100 mV/s to 10kV/s full
scale (6 ranges)
1 in 4096
Appendix V
ADC750: dual channel fast ADC module
•
number of ADCs
•
maximum conversion rate
•
maximum integration time
•
basic resolution
•
resolution of measurements
•
memory
ECD: low current amplifier module
•
current ranges
•
•
•
•
•
current measurement
type of filter
filter time constants
compensation of current offset
monitor output (BNC)
Technical specifications
2, each with four input channels
750 kHz
5.5 ms (mean of 4096 AD conversions)
1 in 4096 (12 bit)
- potential
5 mV at range 10 V
2 mV at range 4 V
1 mV at range 2 V
- current
0.5%, 0.05% and 0.005% of full
scale
128000 samples per channel
(optionally 512000 samples)
100 pA to 100 µA full scale (seven ranges)
1 pA and 10 pA with selectable-gain amplifier
± 0.5% accuracy
third order Sallen-Key
RC-times 0 s, 10 ms, 100 ms and 500 ms
± 10 µA maximum
current
BIPOT, ARRAY and BA: (bipotentiostat) module
current ranges
current measurement
maximum current output
potential range
potential accuracy
monitor output (BNC)
FI20: filter and integrator module
•
filter section
- type of filter
- filter time constants
- output offset
- monitor output (BNC)
•
integrator section
- ranges
- charge measurement
- temperature dependence
- monitor output (BNC)
BIPOT/ARRAY
BA
100 nA to 10 mA full scale
(6 ranges)
± 0.2% of current ± 0.2%
of current range
± 35 mA
±5V
± (0.2% + 2 mV)
current
10 nA to 10 mA full scale
(7 ranges)
± 0.2% of current ± 0.2%
of current range
± 50 mA
± 10 V
± (0.2% + 2 mV)
current
third order Sallen-Key
RC-times 0 s, 10 ms, 100 ms and 500 ms
± 2 mV
filter output
10 ms, 100 ms, 1 s and 10 s
0.2% accuracy
< 0.04%/K
charge output
BSTR10A or Booster20A: current booster
BSTR10A
maximum output voltage
maximum output current
maximum output power
bandwidth
current measurement
dimensions (W x D x H)
weight
± 20 V
± 10 A
200 W
4 kHz full power
10 A full scale ± 0.5% accuracy
37 x 36 x 15.5 cm3
approx. 9 kg
Note: Specifications subject to change without notice. All specifications at 25°C.
Booster20A
± 20 V
± 20 A
400 W
20 kHz
.1% of full scale = 20mA
52 x 42 x 17 cm3
approx. 25 kg
135
Index
137
Index
A
ac voltammetry................................................................................113, 116, 117, 118
Analysis results ........................................................................................................64
Automatic ................................................................................................................59
axis annotation ...................................................................................................58, 60
B
BAS-DigiSim ...........................................................................................................30
basecurve .................................................................................................................67
baseline .....................................................................................44, 67, 68, 74, 98, 110
Batch mode ..............................................................................................................43
Burette control..........................................................................................................34
C
Calibrate pH-Electrode .............................................................................................39
Check cell ..........................................................................................................40, 42
Chronoamperometric plot.........................................................................................69
chronoamperometry................................................................................ 100, 102, 107
Chronocoulometric plot............................................................................................69
chronocoulometry............................................................................. 56, 100, 102, 107
chronomethods ............................................................................................... 7, 47, 63
colours ...............................................................................................................61, 63
Computer ........................................................................................5, 6, 115, 117, 120
configuration ....................................................................................................5, 6, 42
convolution ............................................................................................................108
Convolution .....................................................................................................70, 100
Copy ............................................................................................................ 43, 58, 64
corrosion .......................................................................................... 5, 71, 72, 73, 124
corrosion rate ......................................................................................... 19, 71, 72, 73
Coulometric titration ................................................................................................95
Crank-Nicolson ........................................................................................................75
Current density.........................................................................................................95
current range ........................................................................................ 37, 54, 55, 113
curve cursor..............................................................................................................67
cyclic voltammetry......................................................................................... 104, 126
D
Data buffer ...............................................................................................................31
Delete files ...............................................................................................................31
derivative ................................................................ 68, 69, 71, 99, 101, 103, 104, 121
differential pulse...............................................................................................64, 117
DIO ports .................................................................................................................49
dt/dE vs E plot..........................................................................................................59
E
E vs t plot .................................................................................................................59
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User Manual
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Version 4.9
Edit procedure ....................................................................................................32, 63
electrochemical detection ............................................................................... 5, 47, 60
Electrode control ......................................................................................................33
Enter text..................................................................................................................60
Exit ..........................................................................................................................31
Export Chrono data ..................................................................................................30
Export data buffer.....................................................................................................30
Export to BAS-DigiSim data ....................................................................................30
F
FFT ....................................................................................................................73, 97
Filter for derivative...................................................................................................96
First- and Second signal ...........................................................................................59
Fit ............................................................................................................................74
free cursor ..........................................................................................................67, 68
frequency spectrum ..................................................................................................73
G
galvanostat .......................................................................................................5, 6, 30
GPES 3 files.............................................................................................................28
GPES3 files..............................................................................................................28
graphics...................................................................................................... 5, 6, 27, 63
H
help .................................................................................................... 5, 7, 52, 63, 112
High sensitivity ........................................................................................................54
I
Idisk .........................................................................................................................95
I-interrupt .................................................................................................................37
integrate ...................................................................................................................99
integrator............................................................................................................53, 56
Interpolate ................................................................................................................74
iR drop ...................................................................................................................110
iR-compensation .......................................................................................... 37, 39, 55
Iring .........................................................................................................................95
L
linear regression ........................................................................................... 69, 70, 74
linear sweep voltammetry..................................................................... 29, 47, 63, 126
Load data ...........................................................................................................28, 31
Load scan .................................................................................................................28
M
manual control................................................................................................ 6, 37, 53
Marquardt ................................................................................................................75
Mercury drop electrode ............................................................................................53
Method menu ...........................................................................................................32
Metrohm 730 Sample Changer .................................................................................49
mouse...................................................................................................................6, 28
Index
139
MS-Windows .........................................................................................................5, 6
MS-Word ................................................................................................. 6, 56, 58, 64
MULTI4...................................................................................................................36
MUX control............................................................................................................36
N
New plot...................................................................................................................58
noise................................................................................................. 40, 42, 69, 97, 98
normal pulse................................................................................................... 113, 119
O
Open procedure ........................................................................................................27
overlay .....................................................................................................................60
P
peak search............................................................................................. 60, 65, 67, 68
pH buffer..................................................................................................................40
pH electrodes ...........................................................................................................39
Plate .........................................................................................................................42
plot title....................................................................................................................61
polarisation resistance ........................................................................................71, 72
Positive feedback .....................................................................................................38
potentiometric stripping analysis ........................................................................59, 64
potentiostat........................................................................................... 5, 6, 27, 38, 39
Print ............................................................................................................. 6, 28, 112
Procedure name in Data presentation Window..........................................................52
Project..............................................................................................................43, 112
Project command rules .............................................................................................43
Project examples ......................................................................................................48
Project wizard ..........................................................................................................47
pX/pH ......................................................................................................................95
Q
Quick save................................................................................................................57
R
RDE-control.............................................................................................................35
Rescale after measurement .......................................................................................52
Rescale during measurement ....................................................................................52
Resume ..............................................................................................................59, 60
Reverse axes.............................................................................................................60
S
Save data...................................................................................................... 29, 30, 57
Save procedure.........................................................................................................28
Save procedure as.....................................................................................................28
Save scan .................................................................................................................29
Scan averaging .......................................................................................................118
SCNR16A ................................................................................................................36
SCNR8A ..................................................................................................................36
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User Manual
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Version 4.9
second signal......................................................................... 55, 59, 65, 113, 119, 121
Set window ......................................................................................................59, 110
Show all GPES files in File dialog box .....................................................................52
Show I (backward) ...................................................................................................59
Show I (forward) ......................................................................................................59
simulation ................................................................................................................74
Sleep mode...............................................................................................................42
smooth ......................................................................................................... 44, 97, 99
Smooth...................................................................................................................121
Spectral noise analysis..............................................................................................73
square wave.................................................................................................... 113, 116
Start button...............................................................................................................53
stationary current.................................................................................... 117, 121, 126
Status bar .................................................................................................................53
surface area ..............................................................................................................95
T
Tafel slope.............................................................................................. 70, 71, 72, 73
tool bar.................................................................................................................6, 52
transition time ....................................................................................................59, 74
trigger ................................................................................................................49, 51
V
viewing data .............................................................................................................60
voltammetric analysis..................................................................................... 5, 63, 70
W
Wave log............................................................................................................16, 70
WE2 versus WE plot ................................................................................................95
Window for zero crossings .......................................................................................96
Window function......................................................................................................74
Work potential..........................................................................................................60
Work scan ..........................................................................................................60, 65
Z
zoom ....................................................................................................................6, 59
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