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This content explains the basics and applications in electrochemistry.

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Part 3: Positive Feedback

Laboratory Of Research & Development, BAS Inc.
Professor Noriyuki Watanabe

In the previous two editions, the potentiostat was talked and the continuation will be presented. The positive feedback is a topic here. This is an optional function which is not always necessary, but you may select to use it, if you need.

The purpose of using positive feedback is to alleviate the unfavorable effect caused by the high electric resistance of the electrolyte solution.

When the current flows through the electrolysis cell, a voltage drop proportional to the product of solution resistance and the current value occurs. The potentiostat can not recognize a potential drop proportional to the solution resistance between the working electrode and the reference electrode, while controls the potential of the working electrode.

Since the set up potential is just applied between the working electrode and the reference electrode including this partial of the potential drop, the actual potential applied to working electrode is insufficient by the amount of potential drop.
Fig. 3.1 Insufficient applied potential due to uncompensated solution resistance (Ru), W, C, R are the working electrode, the counter electrode and the reference electrode.
Fig. 3.1 Insufficient applied potential due to uncompensated solution resistance (Ru), W, C, R are the working electrode, the counter electrode and the reference electrode.

This is commonly called uncompensated solution resistance and is marked as Ru. It can be schematically depicted as shown in Fig. 3.1. It shows the state when the current i flows a cell. Subscript u comes from the meaning of either “unkown” or “uncontrollable”.

Even with a potentiostat, such potential drop can not be controlled completely. The only alternative is to deal with incomplete methods with positive feedback as described below.

The unfavorable effect caused by this uncompensated solution resistance is, for example, a peak potential shift and an extra spread of the peak potential width in a CV (Cyclic Voltammetry) measurement.

The deviation of the peak potential causes a difference between the absolute values of the oxidation peak current and the reduction peak current, and the redox potential estimated from the average of the two peak potentials will slightly deviate. Besides, the electron transfer speed calculated from the peak potential width will be slower. For this reason, it is necessary to reduce the effect of uncompensated solution resistance. The method is positive feedback.

The positive feedback circuit can be configured with three op amps as shown in Fig. 3.2. An additive circuit (op 1) is added to the prototype basic circuit (configured by two op amp) prototype which was mentioned before.

Currently, the commercially available instrument equips a Ru measuring function, and if the user selects the positive feedback function, it will measure the Ru automatically. In addition, if the feedback ratio f is inputted, the potentiostat will do the Ru compensation. The noticed point is how to determine the value of f. If it is too large, the potentiostat oscillation is possibly occurred when the positive feedback is on. It is prudent to set it to about 80%.

This makes a considerable improvement for the electrochemical measurement. Please do not forget that positive feedback function is not perfect.
        Fig. 3.2 The positive feedback circuit for iR compensation.
Fig. 3.2 The positive feedback circuit for iR compensation.