Small-Amplitude Controlled-Potential Techniques

Peter T. Kissinger Purdue University and Bioanalytical Systems, Inc, West Lafayette, Indiana 

Thomas H. Ridgway University of Cincinnati, Cincinnati, Ohio 

The net effect of the presence of the solution resistance on potential excitation methods is that the potential seen by the electrode solution interface is different from the potential applied by the potentiostat. This difference is current-dependent and the current is itself potential-dependent. The resistance also makes it more difficult to separate current components arising from the double-layer capacitance from the faradaic process. Similar complications arise for current excitations. 

The objective of most electrochemical experiments is to allow the experimenter to investigate one or more of three types of parameters (1) the concentration and identity of one or more solution components, (2) the kinetics of chemical, charge transfer, or adsorption processes, and (3) the nature of the double-layer capacitance associated with the electrode-solution interface. Historically, most small-amplitude techniques have been developed in an attempt to allow an easier separation of the contributions of these basic parameters. 

In order for a method to be useful for these purposes, it is at least necessary to be able to predict what the current-time-voltage-concentration relationship should be like for a given set of conditions. Only then can one evaluate experimental results and determine concentration or kinetic contributions. 

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Refs. [i] Kissinger PT, Ridgway TH (1996) Small-amplitude controlled-potential techniques. In Kissinger PT, Heineman WR (eds) Laboratory techniques in electroanalytical chemistry, 2nd edn., Marcel Dekker, New York, pp 141 [ii] Smith DE (1966) AC polarography and related techniques theory and practice. In Bard AJ (ed) Electroanalytical chemistry, vol. 1. Chap. 1 Marcel Dekker, New York... 

Kissinger PT, Ridgway TH (1996) Small-amplitude controlled-potential techniques. In Kissinger PT, Heineman WR (eds) Laboratory techniques in elec-troanalytical chemistry, 2nd edn. Dekker, New York... 

Kissinger, P. T., Small-Amplitude and Related Controlled-Potential Techniques, in Laboratory Techniques in Electroanalytical Chemistry, Kissinger, P. T. Heineman, W. R., eds., Marcel Dekker, New York, 1984, pp. 143-161. 

The classification of methods for studying electrode kinetics is based on the criterion of whether the electrical potential or the current density is controlled. The other variable, which is then a function of time, is determined by the electrode process. Obviously, for a steady-state process, these two quantities are interdependent and further classification is unnecessary. Techniques employing a small periodic perturbation of the system by current or potential oscillations with a small amplitude will be classified separately. 

In early 1983, Bioanalytical Systems introduced a new class of integrated processor-driven instrumentation based on a concept first developed by Faulkner and his co-workers [1] at the University of Illinois. This unit (Figs. 6.22 and 6.23) has evolved over the years and now includes a repertoire of some 35 electrochemical techniques, including the most popular large-amplitude (Chap. 3) and small-amplitude (Chap. 5) controlled-potential methods. The unit also is capable of determining electrocapillary curves and can automatically measure and compensate for solution resistance (R in Fig. 6.5). Thus in a single instrument it is possible to utilize virtually all of the diagnostic criteria introduced in Chapters 3 and 5 and also to explore quickly which technique is optimum for... 

Kinetic studies of ECE processes (sometimes called a DISP mechanism when the second electron transfer occurs in bulk solution) [3] are often best performed using a constant-potential technique such as chronoamperometry. The advantages of this method include (1) relative freedom from double-layer and uncompensated iR effects, and (2) a new value of the rate constant each time the current is sampled. However, unlike certain large-amplitude relaxation techniques, an accurately known, diffusion-controlled value of it1/2/CA is required of each solution before a determination of the rate constant can be made. In the present case, diffusion-controlled values of it1/2/CA corresponding to n = 2 and n = 4 are obtained in strongly acidic media (i.e., when kt can be made small) and in solutions of intermediate pH (i.e., when kt can be made large), respectively. The experimental rate constant is then determined from a dimensionless working curve for the proposed reaction scheme in which the apparent value of n (napp) is plotted as a function of kt. 

This term is used to cover a range of techniques in which the mean potential is controlled potentiostatically and swept over a range while a small amplitude, relatively high frequency alternating potential superimposed on the slowly-varying sweep is used to excite a sinusoidal response in the current, which is... 

Modem potentiostats are able to apply various potential programs to a working electrode. In EIS measurements, a small sinusoidal perturbation of a controlled amplitude and frequency must be applied together with the dc electrical program. The system impedance may be measured using various techniques ... 

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