Operational amplifier potentiostat

E. R. Brown, H. L. Hung, T.G. McCord, D. E. Smith, and G. L. Booman, A Study of Operational Amplifier Potentiostats Employing Positive Feedback for iR Compensation II. Application to AC Polarography, Anal. Chem. 40 1411 (1968). 

Figure 25-2 is a schematic showing the components of a modern operational amplifier potentiostat (sec. Section 24C-1) for carrying out linear-scan voliammeiric measurements. The cell is made up of three electrodes immersed in a solution containing the analyte and also an excess of a nonreactive electrolyte called asupporC electrolyte. One of the three eiecirodes is the work-... 

Brown ER, Smith DE, Booman GL (1968) Operational amplifier potentiostats employing positive feedback for IR compensation I Theoretical analysis of stability and bandpass characteristics. Anal Chem 40 1411-1423. doi 10.1021/ac60266a024... 

FIGURE 25-2 An operational amplifier potentiostat. The three-electrode cell has a working electrode (WE), reference electrode (RE), and a counter electrode (CE). 

Reference electrode (RE) and potentiostatic setpoint are fed to the inverting and noninverting input of an operational amplifier. The counter-electrode (CE) is connected to the output of the operational amplifier. I (EC) electrochemical current. 

Fig. 18b.2. (a) A simple three-electrode system with a variable voltage source, (b) A potentiostat made of three operational amplifiers. 

It is usual in electrochemical measurements to control the potential of the working (or indicator) electrode or the electrolytic current that flows through the cell. A potentiostat is used to control electrode potential and a galvanostat is used to control electrolytic current. Operational amplifiers play important roles in both of these. 

Fig. 5.43 Voltage control circuit with the aid of an operational amplifier (a) and a circuit of potentiostat (b). VF voltage follower CF current follower.

In a potentiostatic arrangement, a three-electrode cell is always used connected to a potentiostat, as shown schematically in Fig. 9. The idea is that the potential difference between the indicator electrode and the reference electrode is controlled to be a chosen function of time, for example the step function to be treated in this section. The first potentiostat was devised as early as 1942 by Hickling [40]. In modern designs, use is made of solid-state operational amplifiers for both the part that controls the potential and the part that measures the resulting current. Technical details and further references can be found in recent handbooks, e.g. refs. 21 and 22. 

If one of the main points of this chapter is to be put in the form of a maxim, it would be An operational amplifier is not a potentiostat a potentiostat is a system comprising operational amplifiers and a cell. 

G. L. Booman and W. B. Holbrook, Optimum Stabilization Networks for Potentiostats with Application to a Polarograph Using Transistor Operational Amplifiers, Anal. Chem. 37 795 (1965). 

Several types of reference electrodes are convenient for use in analytical electrochemistry. The use of high-input-impedance operational amplifiers in the reference electrode inputs of potentiostats ensures that very low levels of current are drawn from the reference electrode (see Chap. 6). This permits the use of reference electrodes that do not have to contain a large number of redox equivalents in order to ensure a constant reference potential and are therefore very small. Three reference-electrode designs that are convenient for use in analytical electrochemistry are shown in Figure 9.4. Saturated calomel and silver-silver chloride (of various concentrations of chloride) are among the most common commercially available or conveniently fabricated reference electrodes. 

The potentiostatic control, aimed at compensating a major fraction of the cell resistance, is accomplished with a three-electrode system and a combination of operational amplifiers and feedback loops (Fig. 4.3). Here, the reference electrode is placed as close as possible to the working electrode and... 

Using modem electronics and/or well-designed potentiostats, both controlled-potential coulometry and controlled-current coulometry can be performed, as can gravimetric determinations of analytes. Figure 11.68 shows a potentiostat that uses three operational amplifiers. 

Potentiostat with three operational amplifiers A (adder control), B (voltage follower), and C (current follower). Adapted from Skoog et al. [56],... 

In most electrochemical measurements of corrosion kinetics a potentiostat is used. This description will cover the rudimentary operation of a potentiostat using the concept of an ideal operational amplifier (op amp) as a basis. An op amp is a three-terminal device as shown in Fig. 16 with two input terminals and one output terminal. A perfect op amp follows five basic rules (19) ... 

Polarographic measurements were made with either a three-electrode potentiostat, based on the use of solid state operational amplifiers (55), or a Sargent model XV polarograph. Controlled potential electrolyses were accomplished by use of a Wenking model 61-RH potentiostat. 

Potentiostat — A potentiostat is an electronic amplifier which controls the potential drop between an electrode (the -> working electrode, (WE)) and the - electrolyte. The WE is normally connected to ground potential the potential of the electrolyte is measured by a special probe, the -> reference electrode (RE). Effects of the -> counter electrode (CE), (e.g., potential drop at the CE electrolyte interface) and the electrolyte (esp. the solution resistance) can be suppressed by this technique. Potentiostats are based on -> operational amplifiers (OPA) the simplest circuit is given in Fig (a). The difference between the desired potential Ureference electrode potential Ure is amplified, resulting in currents via counter and working electrode until this difference becomes (almost) zero. 

Operational amplifiers provide the fotmdation for electrochemical instrumentation. The aim of this chapter is to describe the main properties of an operational amplifier so as to imderstand the principles of potentiostats and galvanostats and to imderstand how they can be used for impedance measurements. 

Electrochemical interfaces consist of potentiostats and galvanostats. These devices can be described in terms of combinations of operational amplifiers and resistors. 

Remember 6.2 A basic potentiostat can consist of two operational amplifiers one to control potential and one to follow current. 

For most potentiostats, the assumption Av >> 1 becomes invalid at frequencies above 1 to 10 kHz. In this case, the noise terms are still additive, but the interaction between the gain of the operational amplifier jmd the cell impedance results in additional correlation between the input and output chaimels. 

To completely eliminate changes in reference electrode half-cell potentials, a three electrode potentiostat is often employed. In simple terms, the potentiostat appHes a voltage to the working electrode, which is measured versus a reference electrode via a zero current potentiometric type measurement, but the current flow is between the working electrode and a third electrode, called the counter electrode. Thus if reduction takes place at the working electrode, oxidation would occur at the counter electrode but no net reaction would take place at the surface of the reference electrode, since no current flows through this electrode. A potentiostat circuit is relatively simple to construct using modern operational amplifiers. 

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