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This content explains the basics and applications in electrochemistry.

The topics are listed below:

Part 1: Potentiostat circuit configuration and its features

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

It is well know that a potentiostat is essential in electrochemical measurements.

Comprehending or uncomprehending the basic constitution principle of ordinary potentiostat will be quite differ in electrochemical applications. In previous technical note "Counter Electrode", it was mentioned that the comprehending of potentiostat was useful to distinguish the difference between counter electrode and working electrode. This time only the operation principle and the role of the potentiostat were introduced.
The Op amps (operational amplifiers) are used for the potentiostat. An overview of Op amp is introduced here, instead of the detailed commentary. The symbol of Op amp is a triangle mark with + and - two input terminals and one output terminal.
The basic circuit diagram of potentiostat.

               Fig. 1 The basic circuit diagram of potentiostat.
The features of Op amp are the large DC current gain, the large input impedance, the small output impedance, and the large amplification degree amplifier. Usually it is possible to perform various calculations through feedback from the output side to the input side (such as addition, subtraction, differentiation, integration, and voltage follower etc. impedance conversions.).
As a result, there are no currents flowing into and out of the two input terminals (because the input impedance is very large) and the two input terminals have the same voltage (the potential difference between the two input terminals is zero). If the both points were memorized, the circuit configurations would be just understood.
Due to these features of the Op amp, the potentiostat can be consisted by the combination of at least two operational amplifiers (In Figure 1. dashed circle is the mark for the cell, while W, C, Ref are represented for working electrode, counter electrode and reference electrode respectively).
The fundamental functions of the potentiostat are summarized into ① the working electrode potential regulation versus the reference electrode, ② measuring the current flowing through the working electrode, ③ no current flowing to the reference electrode, these three points.
First of all the voltage applied from the outside (setting potential, applied potential) ei is same to the voltage applied to the reference electrode (because the voltages at both input terminals of Op-1 is equal). On the other hand, the voltage of the working electrode is the ground voltage (+ input terminal is grounded, - input terminal is floating, whereas the two input terminals are the same potential, the potential is equivalent to ground. This is called a virtual ground.). That is, the working electrode is at the potential of -ei respecting to the reference electrode. Through this configuration, i) the working electrode potential regulation versus the reference electrode is established.
The function is a circuit that outputs a voltage proportional to the current i flowing through the working electrode in Op-2, ② measuring the current flowing through the working electrode is realized (function 2).
The reference electrode is connected independently to the minus input end of Op-1. Because the impedance at the input end is extremely large, no current flow. That is, ③ no current flowing to the reference electrode is realized.
Above are the three fundamental functions of the potentiostat.

From what you see, the reason why the counter electrode and the working electrode must be connected to completely different points in terms of circuit inside the potentiostat can be understood.
As mentioned in previous "Counter Electrode" technical note, in the case of two-electrodes system, it is difficult to differentiate between the counter electrode and the working electrode, whereas in three-electrodes system using potentiostat, they are clearly distinguished.