PASCO Specialty & Mfg. CI-6727 User's Manual

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PASCO Specialty & Mfg. CI-6727 User's Manual | Manualzz
Instruction Manual and
Experiment Guide for the
PASCO scientific
Model CI-6727
012-06601A
9/97
CALCIUM SELECTIVE ELECTRODE
© 1997 PASCO scientific
$7.50
®
better
10101 Foothills Blvd. • P.O. Box 619011 • Roseville, CA 95678-9011 USA
Phone (916) 786-3800 • FAX (916) 786-8905 • web: www.pasco.com
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teach science
Calcium Ion Selective Electrode
012–06601A
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012–06601A
Calcium Ion Selective Electrode
Table of Contents
Introduction......................................................................................................................................... 1
Theory .................................................................................................................................................. 1
Equipment ........................................................................................................................................... 2
Included ........................................................................................................................................ 2
Additional Required ...................................................................................................................... 2
Required Solutions .............................................................................................................................. 2
General Preparation ........................................................................................................................... 3
Electrode Preparation.................................................................................................................... 3
Electrode Slope Check Using Science Workshop .......................................................................... 3
Measurement ....................................................................................................................................... 4
Measuring Hints ............................................................................................................................ 4
Sample Requirements ................................................................................................................... 4
Units of Measurement ................................................................................................................ 4-5
Measurement Procedure .................................................................................................................... 5
Direct Measurement ...................................................................................................................... 5
Low Level Calcium Determination .............................................................................................. 6
Titration ............................................................................................................................................... 7
Titration of Calcium ................................................................................................................... 7-8
Electrode Characteristics ................................................................................................................... 9
Reproducibility ............................................................................................................................. 9
Interferences.................................................................................................................................. 9
Complexation .............................................................................................................................. 10
Temperature Influences............................................................................................................... 10
Electrode Response ..................................................................................................................... 11
Limits of Detection ..................................................................................................................... 11
pH Effects ................................................................................................................................... 11
Electrode Life ............................................................................................................................. 11
Electrode Storage ........................................................................................................................ 11
Troubleshooting Guide ..................................................................................................................... 12
Glassware/Plastic-ware ............................................................................................................... 12
Electrode ..................................................................................................................................... 12
Standards & Reagents ................................................................................................................. 12
Sample ........................................................................................................................................ 12
Technique .................................................................................................................................... 13
Troubleshooting Hints ...................................................................................................................... 13
Technical Support ............................................................................................................................. 15
i
Copyright, Warranty, and Equipment Return
Please—Feel free to duplicate this manual
subject to the copyright restrictions below.
Copyright Notice
The PASCO scientific 012-06601A manual is
copyrighted and all rights reserved. However,
permission is granted to non-profit educational
institutions for reproduction of any part of the
Calcium Ion Selective Electrode manual providing
the reproductions are used only for their laboratories
and are not sold for profit. Reproduction under any
other circumstances, without the written consent of
PASCO scientific, is prohibited.
Limited Warranty
PASCO scientific warrants the product to be free from
defects in materials and workmanship for a period of
one year from the date of shipment to the customer.
PASCO will repair or replace at its option any part of
the product which is deemed to be defective in
material or workmanship. The warranty does not
cover damage to the product caused by abuse or
improper use. Determination of whether a product
failure is the result of a manufacturing defect or
improper use by the customer shall be made solely by
PASCO scientific. Responsibility for the return of
equipment for warranty repair belongs to the
customer. Equipment must be properly packed to
prevent damage and shipped postage or freight
prepaid. (Damage caused by improper packing of the
equipment for return shipment will not be covered by
the warranty.) Shipping costs for returning the
equipment after repair will be paid by PASCO
scientific.
Credits
Author: Peter Boyle
Editor: Steve Miller
ii
Equipment Return
Should the product have to be returned to PASCO
scientific for any reason, notify PASCO scientific by
letter, phone, or fax BEFORE returning the product.
Upon notification, the return authorization and
shipping instructions will be promptly issued.
NOTE: NO EQUIPMENT WILL BE
ACCEPTED FOR RETURN WITHOUT AN
AUTHORIZATION FROM PASCO.
ä
When returning equipment for repair, the units must
be packed properly. Carriers will not accept
responsibility for damage caused by improper
packing. To be certain the unit will not be damaged in
shipment, observe the following rules:
➀ The packing carton must be strong enough for the
item shipped.
➁ Make certain there are at least two inches of packing
material between any point on the apparatus and the
inside walls of the carton.
➂ Make certain that the packing material cannot shift in
the box or become compressed, allowing the
instrument come in contact with the packing carton.
Address:
Phone:
FAX:
email:
web:
PASCO scientific
10101 Foothills Blvd.
P.O. Box 619011
Roseville, CA 95678-9011
(916) 786-3800
(916) 786-3292
[email protected]
www.pasco.com
012–06601A
Calcium Ion Selective Electrode
Introduction
The PASCO Calcium Ion Selective Electrode is used to quickly, simply, accurately, and economically measure
calcium concentration in aqueous solutions.
Theory
The Calcium Ion Selective Electrode consists of an electrode body containing an ion exchanger in a sensing
module. This sensing module contains a liquid internal filling solution in contact with a gelled organophilic
membrane containing a calcium ion selective exchanger .
An electrode potential develops across the membrane. When the membrane is in contact with a calcium
solution. Measurement of this potential against a constant reference potential, using an ISE Amplifier and
Science Workshop interface, depends on the level of free calcium ion in solution. The level of calcium ions,
corresponding to the measured potential, is described by the Nernst equation:
E = E 0 + S log X
where:
E = measured electrode potential
E0 = reference potential (a constant)
S = electrode slope (~26 mv/decade)
X = level of calcium ions in solution
The activity, X, represents the effective concentration of the calcium ions in the solution. Total calcium
concentration, Ct includes free calcium ions, Cf plus bound or complexed calcium ions, Cb, since the calcium
electrodes only respond to free ion, the free ion concentration is:
C f = Ct – Cb
The activity is related to the free ion concentration, Cf, by the activity coefficient, γ, by:
X = γ Cf
Activity coefficients vary, depending on total ionic strength, I, defined as:
I = 1 2 Σ C xZ x 2
where:
C x = concentration of ion X
Z x = charge on ion X
Σ = sum of all of the types of ions in the solution
In the case of high and constant ionic strength, relative to the sensed ion concentration, the activity coefficient, γ,
is constant and the activity, X, is directly proportional to the concentration.
To adjust the background ionic strength to a high and constant value, ionic strength adjuster (ISA) is added to
samples and standards. The recommended ISA solution for calcium is potassium chloride, KCl. Solutions other
than this may be used as ionic strength adjusters as long as ions that they contain do not interfere with the electrode’s
response to calcium ions.
1
Calcium Ion Selective Electrode
012–06601A
The reference electrode must also be considered. When two solutions of different composition are brought into contact
with one another, liquid junction potentials arise. Millivolt potentials occur from the inter-diffusion of ions into the
two solutions. Electrode charge will be carried unequally across the solution boundary resulting in a potential
difference between the two solutions, since ions diffuse at different rates. When making measurements, it is important
to remember that this potential be the same when the reference is in the standardizing solution as well as in the sample
solution or the change in liquid junction potential will appear as an error in the measured electrode potential.
The composition of the liquid junction, filling solution in the reference electrode is most important. The speed with
which the positive and negative ions in the filling solution diffuse into the sample should be equiltransferent. No
junction potential can result if the rate at which positive and negative charge carried into the sample is equal.
Equipment
• PASCO Science Workshop Computer Interface
Included:
• Calcium Ion Selective
Electrode
• Calcium Ion Selective
Electrode fill solution
• pipette for fill solution
Additional Required:
Required Equipment
• PASCO CI-6738 ISE (Ion
SelectiveElectrode)Amplifier
• Science Workshop 2.25 or
higher
to ISE
Amplifier
Calcium Ion Selective Electrode
filling
solution
RO
E
O10
13 SAMPL
Re
feren
ce Fill Solutio
4MKCI
filling pipette
Figure 1
Included Equipment
2
n
• Semi-logarithmic 4-cycle graph paper for preparing calibration
curves (Linear graph paper is recommended for low level
measurements)
• magnetic stir plate
• Lab-ware made of plastic, not glass, for all low level
measurements.
Required Solutions
The stock solutions listed in this section may be created as described
in the text or ordered directly form PASCO. The solutions available
for order and their respective prices are listed on the ‘ISE Working
Solution Price List’.
• Deionized or distilled water for solution and standard
preparation.
• Calcium Chloride solution 0.1M CaCl2
To prepare this solution, half fill a one liter volumetric flask
with distilled water and add 14.7 grams of reagent-grade
calcium chloride (CaCl2.2H20). Swirl the flask gently to
dissolve the solid. Fill to the mark with distilled water, cap,
and upend several times to mix the solution.
• Calcium Chloride solution 1000 ppm Ca+2
To prepare this solution, half fill a one liter volumetric flask
with distilled water and add 3.67 grams of reagent-grade
calcium chloride (CaCl2.2H20). Swirl the flask gently to
dissolve the solid. Fill to the mark with distilled water, cap,
and upend several times to mix the solution.
• Calcium Chloride solution 100 ppm Ca+2 as CaCO3
To prepare this solution, half fill a one liter volumetric flask
with distilled water and add 0.15 grams of reagent-grade
calcium chloride (CaCl2.2H20). Swirl the flask gently to
dissolve the solid. Fill to the mark with distilled water, cap,
and upend several times to mix the solution.
012–06601A
Calcium Ion Selective Electrode
• Ionic Strength Adjuster (ISE), 4 M KCL
To prepare this solution, half fill a 1000 ml volumetric flask with distilled water and add 298 grams of
reagent-grade potassium chloride (KCl). Swirl the flask gently to dissolve the solid. Fill to the mark with
distilled water, cap, and upend several times to mix the solution.
• EDTA titrant, 1 M stock solution, for the titration of calcium
To prepare this titrant, add 37.2 grams of reagent grade Na2EDTA.2H20, ethylenediaminetetraacetic acid
dihydrate, disodium salt, to a 100 ml volumetric flask, add about 75 ml of distilled water, and swirl the flask
gently to dissolve the solid. Fill to the mark with distilled water, cap, and upend several times to mix the
solution.
General Preparation
Electrode Preparation
1. Remove the rubber cap covering the electrode tip. Slide
the rubber sleeve down away from the filling hole of the
Calcium Selective Ion Electrode. Fill the electrode with
the included filling solution to a level just below the fill
hole. Slide the rubber sleeve back over the filling hole
(Figure 2a).
2. Gently shake the electrode downward in the same manner
as a clinical thermometer to remove any air bubbles
which might be trapped behind the calcium membrane.
Prior to first usage, or after lon-term storage, immerse the
calcium membrane in calcium standard for thirty minutes.
fill hole
a
rubber
sleeve
rubber cap
b
Insert DIN
connector into
analog channel
A or B.
3. Connect the Calcium Selective Ion Electrode to the ISE
Amplifier and insert the DIN connector of the ISE
Amplifier into analog channel A or B on a PASCO
Computer Interface (Figures 2b and 2c).
Electrode Slope Check Using ScienceWorkshop (check
electrodes each day)
1. To a 150 ml beaker, add 100 ml of distilled water. Place
the beaker on a magnetic stirrer and begin stirring at a
constant rate. Start Science Workshop, select the Ion
Selective Electrode sensor, open a Digital display, and
begin monitoring data. Lower the electrode tip into the
solution.
2. Using a pipette, add 1 ml of 0.1 M, 1000 ppm, or 100 ppm
(as calcium carbonate) standard and 2 ml of ISA to the
beaker. When the reading has stabilized, record the
voltage reading indicated in the Digits display.
c
Rotate ring
one-quarter
turn to secure
ISE Amplifier
electrode
connector
Figure 2
Equipment Setup. a: filling the electrode
with filling solution; b & c: connecting the
electrode to the ISE Amplifier and to the
computer interface
3
Calcium Ion Selective Electrode
012–06601A
3. Using a pipette, add 10 ml of the same calcium standard used above to the beaker. When the reading has
stabilized, record the voltage reading indicated in the Digits display.
4. Determine the difference between the two readings. The electrode is operating correctly if the potential
has changed by 26 ± 2 mV, assuming the temperature is between 20°C and 25°C. See the Troubleshooting
sections if the potential change is not within this range.
➤ Note: Slope is defined as the change in potential observed when the concentration
changes by a factor of 10.
Measurement
Measuring Hints
• All samples and standards should be at the same temperature for precise measurement. A difference of
l °C in temperature will result in a 4% measurement error.
• The sensing membrane is normally subject to water uptake and might appear milky. This has no effect on
performance.
• Constant, but not violent stirring is necessary for accurate measurement. Magnetic stirrers can generate
sufficient heat to change the solution temperature. To counteract this effect, place a piece of insulation
material, such as a styrofoam sheet, between the stirrer and beaker.
• Always rinse the electrodes with distilled water and blot dry between measurements. Use a clean, dry tissue
to prevent cross-contamination.
• For samples with high ionic strength, prepare standards whose composition is similar to the sample.
• Always check to see that the membrane is free from air bubbles after immersion into the standard or sample.
• A slow responding electrode may be caused by interferences to the electrode. To restore proper
performance, soak the electrode in distilled water for about 5 minutes to clean the membrane, rinse, and soak
in diluted standard solution for abut 5 minutes.
• Dilute concentrated samples (over 0.1M) before measurement.
• Recalibrate every few hours for routine measurement.
Sample Requirements
• All samples must be aqueous and not contain organics which can dissolve the membrane or extract out the
liquid ion exchanger.
• The temperature of the standard solutions and of the sample solutions should be the same and below 40°C.
About a 2% error in the slope will occur for each l°C difference in temperature.
• The pH range for the calcium ion electrode is 3-10. Neutralize samples outside this range with NaOH or
HCl to bring them in range.
• Interferences should be absent. If they are present, use the procedure found in the Interferences and
Electrode Response sections to remove them.
Units of Measurement
Calcium concentrations are measured in units of ppm as calcium, ppm as CaCO3, moles per liter, or any other
convenient concentration unit. Table 1 indicates some concentration units and conversion factors.
4
012–06601A
Calcium Ion Selective Electrode
TABLE 1
Concentration Unit Conversion Factors
ppm Ca+2
ppm CaCO3
4.01
10.0
1.0 X 10-4
10.00
24.9
2.5 X 10-4
40.10
100.1
1.0X10-3
400.80
1000.9
1.0 X 10-2
moles/liter
Measurement Procedure
Direct Measurement
Direct measurement is a simple procedure for measuring a large number of samples. A single meter reading is all
that is required for each sample. The ionic strength of samples and standards should be made approximately the
same by adjustment with ISA. The temperature of both sample solution and standard solution should be the same.
Direct Measurement of Calcium
ä Note: A calibration curve is constructed on semi-logarithmic paper. The measured
electrode potential (linear axis) is plotted against the standard concentration (log axis). In
the linear region of the curve, only two standards are necessary to determine a calibration
curve. Calibration standards close to the anticipated value of the “unknown” should be
chosen. In the non-linear region, additional points must be measured. The direct
measurement procedures given are for the linear portion of the curve. The non-linear
portion of the curve requires the use of low level procedures.
1. By serial dilution, prepare three standard solutions from the 0.1M, 100 ppm, or 100 ppm standard. The
resultant concentrations should be 10-2 M, 10-3 M, and l0-4 M or 100, 10 and 1 ppm standards. Add 2 ml
of ISA to each 100 ml of standard.
2. Place the most dilute solution (1.0 X 10-4M or 1 ppm) in a 150 ml beaker on the magnetic stirrer and begin
stirring at a constant rate. After assuring that Science Workshop is operating, lower the electrode tip into
the solution. When the reading has stabilized, record the voltage reading indicated in the Digits display.
3. Place the mid-range solution (1.0 X 10-3M or 10 ppm) in a 150 ml beaker on the magnetic stirrer and begin
stirring. Add 2 ml of ISA. After rinsing the electrodes with distilled water, blot dry, and immerse the
electrode tips in the solution. When the reading has stabilized, record the voltage reading indicated in the
Digits display.
4. Place the most concentrated solution (1.0 X 10-2 M or 100 ppm) in a 150 ml beaker on the magnetic stirrer
and begin stirring. After rinsing the electrodes with distilled water, blot dry, and immerse the electrode
tip in the solution. When the reading has stabilized, record the voltage reading indicated in the Digits
display.
5. Using the semi-logarithmic graph paper, plot the voltage reading (linear axis) against the concentration
(log axis). Extrapolate the calibration curve down to about 2.0 X 10 -6 M. A typical calibration curve can
be found in Figure 3.
5
Calcium Ion Selective Electrode
012–06601A
+100
10-fold change
+60
~26 mV
electrode +20
potential
(mV)
-20
-60
10
1
.1
10
1
10 -5
10 - 4
1000
100
1000
100
10 -3
10 -2
10 -1
Ca+2 concentration (M)
Figure 3
Typical calcium electrode calibration curve
6. To a clean, dry, 150 ml beaker, add 100 ml of the sample, 2 ml of ISA, and 2 ml of ISA. Place the beaker
on the magnetic stirrer and begin stirring at a constant rate. Rinse the electrode with distilled water, blot
dry, and lower the electrode tip into the solution. When the reading has stabilized, record the voltage
reading indicated in the Digits display. Using the calibration curve, determine the sample concentration.
7. The calibration should be checked every two hours. Assuming no change in ambient temperature,
immerse the electrode tip in the mid-range standard. After the reading has stabilized, compare it to the
original reading recorded in step 3 above. A reading differing by more than 0.5 mV or a change in the
ambient temperature will necessitate the repetition of steps 2–5 above. A new calibration curve should
be prepared daily.
Low Level Calcium Determination
This procedure is recommended for solutions with ionic strengths less than 1.0 X 10-2 M. If the solution is high in
ionic strength, but low in calcium, use the same procedure, but prepare a calibration solution with a composition
similar to the sample.
1. Dilute 10 ml of the 0.l M standard to 1000 ml to prepare a 1.0 X 10-3 M standard solution for measurements
in moles per liter. Dilute 10 ml of the 1000 ppm or 1 ml of the 100 ppm standard to 1000 ml to prepare
a 10 ppm standard solution for measurements in ppm.
2. Soak the calcium electrode for at least 1 hour in 1.0 X 10-3 or 100 ppm calcium standard solution.
3. To a 150 ml beaker, add 100 ml of distilled water. Place the beaker on the magnetic stirrer and begin
stirring at a constant rate.
4. Place the electrode tips in the solution. Assure that the meter is in the mV mode.
6
012–06601A
Calcium Ion Selective Electrode
5. Add increments of the l.0 X 10-3 M or 10 ppm standard as given in Table 2 below.
6. After the reading has stabilized, record the voltage reading indicated in the Digits display after each addition.
TABLE 2: Step-wise Calibration For Low Level Calcium Measurements
Step Pipette
1
2
3
4
5
6
7
A
A
A
A
A
B
B
Added
Volume (ml)
0.1
0.1
0.2
0.2
0.4
2.0
2.0
Concentration
M
ppm
1.0 X 10-6
2.0 X 10-6
4.0 X 10-6
6.0 X 10-6
9.9 X 10-6
2.9 X 10-5
4.8 X 10-5
1.0 X 10-2
2.0 X 10-2
4.0 X 10-2
6.0 X 10-2
1.0 X 10-1
2.9 X 10-1
4.8 X 10-1
Pipet A = 1 ml graduated pipette
Pipet B = 2 ml pipette
Solutions: additions of 10 ppm or 1.0 X l0-3 M standard to 100 ml of distilled water.
7. On semi-logarithmic graph paper, plot the mV reading (linear axis) against the concentration (log axis)
as in Figure 1.
8. Rinse the electrodes in distilled water and blot dry.
9. Measure out 100 ml of the sample into a 150 ml beaker and place the beaker on the magnetic stirrer and
begin stirring. Lower the electrode tips into the solution. After the reading has stabilized, record the mV
reading and determine the concentration from the low level calibration curve.
10.Prepare a new low level calibration curve daily. Check the calibration curve every 1-2 hours by repeating
Steps 2-7 above.
Titration
The progressive and quantitative addition of a reagent to a measured sample until neither active species (reagent
or sample) is in excess. Ion selective electrodes are excellent endpoint detectors since they are not influenced by
solution color or turbidity. Though titration is more time consuming than direct measurement, it is about 10 times
more accurate.
Titration of Calcium
The method outlined in this section makes use of the calcium ion electrode as a highly sensitive endpoint detector
for calcium-containing samples. The titrant used is EDTA.
EDTA complexes calcium as well as other cations. The sample pH can be adjusted to pH 10 by adding ammonia
to eliminate unwanted ion complexes. Masking agents can be added in some cases.
1. Soak the calcium ion electrode tip in l0-3M or 100 ppm calcium standard solution for a minimum of one
hour prior to use.
7
Calcium Ion Selective Electrode
012–06601A
2. Prepare the stock EDTA titrant as given in the section Required Solution. Dilute the EDTA to 10 to 20
times as concentrated as the suspected sample concentration. The sample should contain at least
l.0Xl0-3M calcium for a good detection of the endpoint.
3. Fill a 50 ml buret with the EDTA solution. Pipette 100 ml of the sample into a 150 ml beaker, place the
beaker on the magnetic stirrer and begin stirring at a constant rate. Adjust the sample to pH 10 by adding
ammonia.
4. Position the buret tip in the beaker, slightly above the liquid level in the beaker and slightly off center.
Position the electrode tips in the solution about half way between the center of the beaker and the beaker
wall.
5. Begin adding the EDTA in 0.5 ml to 1.0 ml increments and about 0.1 ml to 0.2 ml increments as the
potential begins to change more rapidly. Record the mV potential after each addition. Continue the
additions several milliliters past the endpoint.
6. Plot the milliliters of EDTA added against the mV potential on standard coordinate graph paper (See
Figure 4). The point of greatest potential change is the endpoint .
Figure 4
Typical titration of 100 ml of 5 X 10-3M CaCl, (ph adjusted to ~10 with
ammonia with 0.1 M Na, EDTA
+40
+30
+20
electrode
potential
(mV)
endpoint
+10
0
-10
-20
2
4
6
8
10
ml of 0.1 M Na2 EDTA added
7. The calcium ion concentration from the unknown is calculated as follows:
M Ca +2 =
where:
M Ca +2 =
Vt =
Mt =
VCa +2 =
8
VtM t
VCa +2
concentration of calcium ion in the unknown (moles/liter)
volume of EDTA added at endpoint
EDTA concentration (moles/liter)
volume of unknown sample
012–06601A
Calcium Ion Selective Electrode
Electrode Characteristics
Reproducibility
Electrode measurements reproducible to ±4 % can be obtained if the electrode is calibrated every hour. Factors such
as temperature fluctuations, drift, and noise limit reproducibility. Reproducibility is independent of concentration
within the electrode’s operating range.
Interferences
Table 3 lists some common cautions that, if present in high enough levels, will cause electrode interferences and
measurement errors or electrode drift when using the calcium electrodes.
Electrode drift and slow response could indicate the presence of high interferences from the ions listed. Soak the
electrode in distilled water for five minutes, then for five minutes in calcium standard solution to restore proper
response.
Table 3
Concentration of Possible Interferences causing a 10% Error at Various Levels of Calcium
Interferences
(moles/liter)
10-2M Ca+2
10-3M Ca+2
10-4M Ca+2
Mg+2
1.0X10+1
1.0X100
1.0X10-1
Zn+2
1.0X10+1
1.0X100
1.0X10-1
Ba+2
7.0X100
7.0XlO-1
7.0X10-2
k+1
4.0X100
4.0XlO-1
4.0X10-2
Na+1
2.0x100
2.0XlO-1
2.0X10-2
Ni+2
5.0X10-1
5.0Xl0-2
5.0X10-3
Cu+2
4.0x10-1
4.0X10-2
4.0X10-3
Fe+2
2.0X10-2
2.0X10-3
2.0X10-4
Sr+2
6.0X10-2
6.0Xl0-3
6.0X10-4
H+1
4.0X10-2
4.0X10-3
4.0X10-4
Hg+2
4.0X10-2
4.0X10-3
4.0X10-4
Pb+2
1.0X10-4
1.0X10-5
1.0X10-6
9
Calcium Ion Selective Electrode
012–06601A
Interferences
(ppm)
1000 ppm CaCO3
100 ppm CaCO3
10 ppm CaCO3
Mg+2
2.43X105
2.43X104
2.43X103
Zn+2
6.53X105
6.53X104
6.53X103
Ba+2
9.60X105
9.60X104
9.60X103
K+1
l.56X105
l.56X104
l.56X103
Na+1
4.60X104
4.60X103
4.60X102
Ni+2
2.94X104
2.94X103
2.94x102
Cu+2
2.54X104
2.54X103
2.54X102
Fe+2
l.l1XlO4
l.11X103
l.11X102
Sr+2
5.20X103
5.20X102
5.20X101
H+1
1.4 pH
2.4 pH
3.4 pH
Hg+2
8.0X102
8.0X102
8.0X101
Pb+2
2.0X101
2.0
2.0Xl0-1
Complexation
Sulfate, bicarbonate, and carbonate are the most common species that complex calcium ions. The level of calcium
ions, the level of the complexing ion, the pH of the solution, and the total ionic strength of the solution determine
the extent of the complexation. Complexation reduces the free calcium ion concentration and, since the electrode
responds only to free calcium ions, a false reading results .
To avoid formation of CaSO4, the sulfate concentrations must be less than 5X10-4M (50 ppm) . To avoid formation
of CaCO3 or formation of the CaHCO3 complex, the pH of the solution should be less than 7, and the total carbonate/
bicarbonate concentration should be less than 3X10-3M (280 ppm carbonate) .
Temperature Influences
Samples and standards should be at the same temperature, since
electrode potentials are influenced by changes in temperature.
A l°C difference in temperature results in a 4% error at the
1.0 X 10-3 M level .
Provided that temperature equilibria has occurred, the calcium
electrodes can be used at temperatures from 0°-40°C. Room
temperature measurements are recommended, since
measurements at temperatures quite different from room
temperature may require equilibrium times up to one hour.
Table 4 indicates the variation of the theoretical slope with
temperature.
10
TABLE 4: Temperature vs.
Theoretical Values for the
Electrode Slope
Temp ( °C)
S
0
10
20
25
30
40
27.10
28.09
29.08
29.58
30.07
31.07
012–06601A
Calcium Ion Selective Electrode
Electrode Response
Plotting the electrode potential against the calcium concentration on semi-logarithmic paper results in a straight line
with a slope of about 26 mV per decade. (Refer to Figure 3)
The time needed to reach 99% of the stable electrode potential reading, the electrode response time, varies from one minute
or less for calcium concentration above 1.0 X 10-4M to several minutes near the detection limit. (Refer to Figure 5)
+50
10-3 M to 10-2 M CaCl2
+40
+30
electrode
potential +20
(mV)
+10
10-3 M to 10-4 M CaCl2
0
-10
10-3 M to 10-6 M CaCl2
-20
1
2
time (minutes)
3
4
Figure 5
Typical electrode time response to step changes in CaCl2
Limits of Detection
The upper limit of detection in pure calcium chloride solutions is 1M. In the presence of other ions, the upper limit
of detection-is above 1.0 X 10-1M, but the possibility of a liquid junction potential developing at the reference
electrode and the “salt extraction effect” are two limiting factors. Some salts may infuse into the electrode membrane
at high salt concentrations causing deviation from theoretical response. Calibrate the electrode at four or five
intermediate points, or dilute the sample, to measure samples between 1.0 X 10-1M and 1M.
The lower limit of detection is influenced by the slight water solubility of the ion exchanger used in the sensing
portion of the electrode. Refer to Figure 1 for a comparison of the theoretical response to actual response at low levels
of calcium chloride.
pH Effects
The operating range of the calcium electrode is from pH 3 to pH 10. Use at other pH values can adversely affect
the membrane. Hydrogen ion interferes with measurements of very low levels of calcium. Hydroxide ion will
complex calcium ions.
Electrode Life
The calcium electrode will last six months in normal laboratory use. On-line measurement might shorten operational
lifetime to several months. In time, the response time will increase and the calibration slope will decrease to the point
calibration is difficult and electrode replacement is required..
Electrode Storage
The calcium electrodes may be stored for short periods of time in 1.0 X 10-2M calcium standard. For longer storage
(longer than two weeks), rinse and dry the calcium membrane and cover the tip with any protective cap shipped with
the electrodes. The reference portion of the combination electrode (or the outer chamber of the reference electrode)
should be drained of filling solution, if refillable, and the rubber insert placed over the filling hole.
11
Calcium Ion Selective Electrode
012–06601A
Troubleshooting Guide
The goal of troubleshooting is the isolation of a problem through checking each of the system components in turn:
the glassware, the electrodes, the standards and reagents, the sample, and the technique.
Glassware/Plastic-ware
Clean glassware is essential for good measurement. Be sure to wash the glassware/plasticware well with a mild
detergent and rinse very well with distilled or deionized water. Clean glassware will drain without leaving water
droplets behind.
Electrode
The electrodes may be checked by using the procedure found in the sections entitled Electrode Slope Check.
1. Be sure to use distilled or deionized water when following the procedures given in Electrode Slope
Check.
2. If the electrode fails to respond as expected, see the sections Measuring Hints and Electrode Response.
Repeat the slope check.
3. If the electrode still fails to respond as expected, substitute another calcium electrode that is known to
be in good working order for the questionable electrode.
4. If the problem persists, the reagent may be of poor quality, interferences in the sample may be present
or the technique may be faulty. (See Standards & Reagents, Sample, and Technique sections below.)
5. If another electrode is not available for test purposes, or if the electrode in use is suspect, review the
instruction manual and be sure to:
-
Clean and rinse the electrodes thoroughly.
Prepare the electrodes properly.
Use the proper filling solution.
Adjust the pH and the ionic strength of the solution by the use of the proper ISA.
Measure correctly and accurately.
Review Troubleshooting Hints.
Standards & Reagents
Whenever problems arise with the measuring procedure that has been used successfully in the past, be sure to check
the standard and reagent solutions. If in doubt about the credibility of any of the reagents, prepare them again. Errors
may result from contamination of the ISA, incorrect dilution of the standards, poor quality distilled/deionized water,
or a simple mathematical miscalculation.
Sample
Look for possible interferences, complexing agents, or substances which could affect the response or physically
damage the sensing electrode if the electrodes work perfectly in the standard, but not in the sample.
Try to determine the composition of the samples prior to testing to eliminate a problem before it starts. (See
Measuring Hints, Sample Requirements, and Interferences.)
12
012–06601A
Calcium Ion Selective Electrode
Technique
Be sure that the electrode’s limit of detection has not been exceeded. Be sure that the analysis method is clearly
understood and is compatible with the sample.
Refer to the instruction manual again, particularly the General Preparation and Electrode Characteristics sections.
If trouble still persists, call PASCO Technical Support.
Troubleshooting Hints
Symptom
Possible Causes
Next Step
Out of Range Reading
defective electrode
check electrode operation
electrodes not plugged in
properly
unplug electrodes and reseat electrodes
reference electrode not filled
be sure reference electrode is filled
13
Calcium Ion Selective Electrode
14
012–06601A
Technical Support
Feedback
Contacting Technical Support
If you have any comments about the product or
manual, please let us know. If you have any suggestions
on alternate experiments or find a problem in the
manual, please tell us. PASCO appreciates any
customer feedback. Your input helps us evaluate and
improve our product.
To Reach PASCO
For technical support, call us at 1-800-772-8700
(toll-free within the U.S.) or (916) 786-3800.
fax:
(916) 786-3292
e-mail:
[email protected]
web:
www.pasco.com
Before you call the PASCO Technical Support staff,
it would be helpful to prepare the following
information:
➤ If your problem is with the PASCO apparatus,
note:
­ Title and model number (usually listed on the
label);
­ Approximate age of apparatus;
­ A detailed description of the problem/
sequence of events (in case you can’t call
PASCO right away, you won’t lose valuable
data);
­ If possible, have the apparatus within reach
when calling to facilitate description of
individual parts.
➤ If your problem relates to the instruction manual,
note:
­ Part number and revision (listed by month and
year on the front cover);
­ Have the manual at hand to discuss your
questions.

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