Large-Amplitude Controlled-Current Techniques

William R. Heineman University of Cincinnati, Cincinnati, Ohio 

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

Controlled-current techniques in stationary solution saw extensive development and application in the 1960s. However, they were largely supplanted by con-trolled-potential techniques, especially cyclic voltammetry, in the 1970s. Today, controlled-current techniques in stationary solutions are used occasionally. 

The concentration profile of product R is determined by the flux of R away from the electrode surface. The flux of R is controlled by the flux of O to the electrode, so that 

The shape of the potential-time response is determined by the concentration changes of O and R at the electrode surface during electrolysis. The potential is related to Cq/Cr via the Nernst equation for a reversible system. The initial potential before current application is simply the rest potential or open-circuit potential (E ) of the solution, which reflects the initial Cq/C in solution. At the instant of current application, this ratio becomes finite and the potential changes to a value consistent with the Nernst equation. Early in the 

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Heineman, W. P. Kissinger, P. T., Large-Amplitude Controlled Current Techniques, in Laboratory Techniques in Electroanalytical Chemistry, Kissinger, P. T. Heineman, W. R., eds., Marcel Dekker, New York, 1984, pp. 129-142. 

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. 

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