Technical Note 200 - Princeton Applied Research
Technical Note 200
Subject: Getting to Know Your Potentiostat: Potentiostat
Stability Considerations
INTRODUCTION
When a company sells corrosion and impedance
measurement instrumentation to customers that use the
equipment to explore a wide variety of test cells and
environments, there is always a chance that the equipment
will be used in situations that will compromise the
performance. One such situation has been observed more
and more. This problem is a result of using a differential
electrometer in the 263A, 273A and PARSTATs.
In moving to the design of the current follower, used in
more recent vintages of the potentiostats, the I/E
converter is such that the ground electrode is no longer
held at virtual ground. This requires that a differential
electrometer be used to control the potential. Figure 2
shows the design employed in the 273A. The differential
electrometer requires potential input not only from the
reference electrode, but also from the working electrode.
One benefit from this particular electrometer design is
that during periods of overload of the current, potential
control of the cell is maintained.
In the old days, the 173 and similar instruments used a
single-ended electrometer design to control the potential
of the electrode (see Figure 1). This design was possible
due to the fact that the I/E converter was such that the
working electrode was held at virtual ground. This style
electrometer slowed down the response of the potentiostat
and masked many of the problems encountered when
using real cells.
-
POWER
AMPLIFIER
+
COUNTER
ELECTRODE
SINGLE-ENDED
ELECTROMETER
Rref
+
-
WHITE PIN JACK
REFERENCE
ELECTRODE
WORKING
ELECTRODE
+
VIRTUAL GROUND
I/E CONVERTER
-
FIGURE 2: Simplified schematic of the 273A potentiostat.
The PARSTATs and Model 263A potentiostats also have
differential electrometers despite the fact that they have
virtual ground I/E converters.
FIGURE 1: Simplified schematic of a 173 potentiostat.
Princeton Applied Research, 801 S. Illinois Avenue, Oak Ridge, TN 37830.
Tel: (865) 425-1289 Fax: (865) 481-2410 Web: www.princetonappliedresearch.com
PROBLEM
One condition that could cause severe problems for a
differential electrometer is when the reference impedance
(Rref) becomes very large, on the order of 50,000 Ω or
more. Some of the situations where this can result are
when Vycor frits used on the reference electrode and the
bridge tube are dried out and reused, when the solution
resistance is extremely high, when the bridge tube filling
solution is highly resistive or when a luggin capillary
arrangement for the reference is employed. In these
instances, the reference circuit impedance could be
increased to a value greater that the 50,000 Ω believed to
be required for this problem to occur. This increased
impedance, coupled with the stray capacitance of the
reference circuit, slows down the negative feedback
(stability generating) side of the electrometer operational
amplifier. The positive feedback (stability destroying)
side of this electrometer is not similarly slowed. When
this occurs, oscillation of the potentiostat is likely to
result.
Most of the time, this problem can be addressed by
examining your reference electrode. Making sure that the
frits are new, using a bridge tube filling solution with
higher conductivity and making sure there are no air
bubbles in the reference electrode or the bridge tube will
possibly eliminate the problem. However, sometimes
these remedial actions are not sufficient to remove the
problem.
How does one overcome this potentiostat or experimental
design limitation? One way to gain some insight into the
problem is to remove the black shorting plug that shorts
the working and sense leads together on the front panel of
the electrometer (Model 273A). This will incorporate a 1
MΩ resistor into the working input of the electrometer
(see Figure 3) and may increase the time constant of the
working input to match that of the reference. This should
reduce or possibly eliminate the oscillations. However,
by removing the shorting plug from the electrometer, you
have compromised the capability of the electrometer and
may produce errors in measurement. Thus, this is not a
solution to the problem but should indicate the source of
the problem. There are, however, a number of solutions
to this problem.
+
+
-
GREY PIN
JACK
JUMPER
GREEN
PIN JACK
+
-
FIGURE 3: Effect of removing the shorting plug.
SOLUTIONS
If the problem occurs because of the restricted volume
(poor conductance) of the luggin capillary, the solution
may be as simple as placing a platinum wire inside of the
capillary, beginning at the liquid junction of the reference
electrode and terminating as close as possible to the
luggin tip (see Figure 4).1 This will short the high
reference resistance. This will also attenuate the mains
interference that may be present and reduce the need for
Faraday shields. This method will also work when the
wire is placed inside of the bridge tube that is sealed with
a Vycor tip.
Perhaps one of the easiest ways of addressing this
problem is to place a 0.1 µF capacitor between the
counter electrode and the reference electrode (see Figure
5). This has the effect of slowing down the potential
controlling op amp (summing amplifier) and does not
allow the potential control to outrace the feedback
response.
Princeton Applied Research, 801 S. Illinois Avenue, Oak Ridge, TN 37830.
Tel: (865) 425-1289 Fax: (865) 481-2410 Web: www.princetonappliedresearch.com
and measuring small currents, this solution will have the
additional benefit of removing unwanted ac current noise.
However, this solution will not work for impedance
experiments.
poor-conductivity
solution
good-conductivity
solution
high porosity sinter Pt wire shunt
or ether low
resistance liquid
junction
calomel
mercury
Pt “horseshoe” contact
For impedance experiments, use a platinum wire in
parallel with the reference electrode that is normally used.
One end of the platinum wire should terminate close to
the tip of the reference electrode or bride tube, whichever
is closer to the specimen. The other end of the wire is
connected to the input jack of the reference lead before it
goes into the electrometer. The placement of a 0.1 -1 µF
capacitor in series with this wire will allow the high
frequency component of the signal to bypass the
reference electrode (if a 1 µF capacitor is used, anything
over 2 kHz will be shunted) while the dc component will
be passed through the reference electrode.
FIGURE 4: Placing Pt wire inside the reference bridge
tube.
+
+
+
WORKING
ELECTRODE
0.1 µF
0.1 µF
+
+
-
WHITE PIN JACK
-
BLACK LEAD
+
-
FIGURE 6: Shorting of the working and ground leads.
FIGURE 5: Shorting the reference and counter electrodes.
If the experiment that needs to be run is a corrosion type
experiment, then placing a 0.1 -1 µF capacitor between
the working and the ground leads will allow the high
frequency ac signal causing the problem to bypass the
current measuring resistors. This will tend to slow down
the potentiostat’s current measurement and would
eliminate the oscillations. When using a large electrode
Princeton Applied Research, 801 S. Illinois Avenue, Oak Ridge, TN 37830.
Tel: (865) 425-1289 Fax: (865) 481-2410 Web: www.princetonappliedresearch.com
+
0.1 µF
+
WHITE PIN JACK
-
Pt
WIRE
+
-
FIGURE 7: High frequency bypass of the reference.
These are a few of the suggestions that have been offered
to improve the response of a differential electrometer in a
cell environment that produces a high reference
impedance. We are always open for new, improved ways
of eliminating this problem. Of course, when all else
fails, the use of a “pseudo-reference” electrode, like a
platinum wire alone, will work but then the actual
electrode potential is not known.
REFERENCES
1. Fletcher, S.; Horne, M. J. Electroanal. Chem., 297
(1991), 297-299, Short Communication.
Princeton Applied Research, 801 S. Illinois Avenue, Oak Ridge, TN 37830.
Tel: (865) 425-1289 Fax: (865) 481-2410 Web: www.princetonappliedresearch.com
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