TECHNICAL NOTE 200
Getting To Know Your Potentiostat (Part I)
Potentiostat Stability Considerations Or Should Your Potentiostat be on Thorizine
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 273, 273A, 263 and the Versastat.
In the old days, the 173 and similar instnunents used a single-ended electrometer design to control the potential of the electrode. 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.
In moving to a design of the current follower used in more recent vintages of the potentiostats. like the
273A , the I/E converter is such that the working 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 electromettr design is that during periods of overload of the current, the potential
control of the cell is maintained.
The Versastat and the Model 263 Potentiostat also have differential electromettrs despite the fact that
they have virtual ground I/E converters. The electrometer in the 263, however. can also operate as
single-ended. avoiding the problem discussed below.
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 ohms or more. Some of the situations where this can result are when the 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 than the 50,000 ohms 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. This will incorporate a 1 MOhm resistor into the working input of the electrometer (See Figure 4) 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.
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. This will short the high reference resistance. This will also attenuate the mains interference that may also 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 uFarad 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. If the experiment that needs to be run is a corrosion type experiment, then placing a 0.1 - 1 uFarad capacitor between the working and the ground leads will allow
the high frequency ac signal causing the problem to by-pass 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 and measuring small currents, this solution will have the additional benefit of removing unwanted ac current noise. However, this solution is no good for impedance experiments.
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 bridge 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 uFarad capacitor in series with this wire will allow the high frequency component of the signal to bypass the reference electrode (if a 1 uFarad capacitor is used, anything over 2 KHz will be shunted) while the dc component will be passed through the reference electrode. This will work for impedance experiments.
These are a few of the suggestions that have been offered to improve the response of a differential
electrometer in a cell environment which 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 the actual electrode potential is
then not known.
References: 1. Stephen Fletcher and Michael Home, J.Electroanal. Chem., 297,(1991), 297-299. Short Communication
Operated by: vinces@ozemail.com.au