Linearization of the current/voltage characteristic of an electrode process. application to relaxation methods

Abstract

Generally, an electrode process is assumed to behave as a linear element at potential departures from equilibrium not exceeding 5 mV. In this paper a rigorous examination is presented of the maximum overpotential at which linearization of the i/E relationship for charge-transfer and mass-transfer controlled electrode processes is allowed. It appears that the 5-mV limit is an over-estimate. Mostly the excursion from equilibrium potential should not exceed 0.5 mV.

Study of kinetics of electrode reactions with pulse methods is not feasible at such low potential levels. A considerable improvement of these methods can be obtained if a two-indicator electrode cell is used, one electrode functioning as the cathode and the other as the anode. Then, first-order deviations from linearity of each electrode cancel. The maximum value of the applied overpotential may be extended by one order of magnitude. In the faradaic impedance methods first-order deviations, giving rise to second harmonics, are not important provided higher harmonics than the fundamental are rejected by a tuned amplifier. In this case a 5-mV excursion from the equilibrium potential is allowed.