Transient techniques

The classical electrochemical methods are based on the simultaneous measurement of current and electrode potential. In simple cases the measured current is proportional to the rate of an electrochemical reaction. However, generally the concentrations of the reacting species at the interface are different from those in the bulk, since they are depleted or accumulated during the course of the reaction. So one must determine the interfacial concentrations. There are two principal ways of doing this. In the first class of methods one of the two variables, either the potential or the current, is kept constant or varied in a simple manner, the other variable is measured, and the surface concentrations are calculated by solving the transport equations under the conditions applied. In the simplest variant the overpotential or the current is stepped from zero to a constant value; the transient of the other variable is recorded and extrapolated back to the time at which the step was applied, when the interfacial concentrations were not yet depleted. In the other class of method the transport of the reacting species is enhanced by convection. If the geometry of the system is sufficiently simple, the mass transport equations can be solved, and the surface concentrations calculated. The interpretation becomes complicated if several reactions take place simultaneously. Since the measured current gives only the sum of the rate of all charge-transfer reactions, the elucidation of the reaction mechanism and the measurement of several rate constants becomes an art. A number of tricks can be used, such as complicated potential or current programs, auxiliary electrodes, etc., which work for special cases. There are several good books on the classical electrochemical techniques. Here we give a brief outline of the most important methods. We mostly restrict ourselves to the study of simple reactions, but will consider one example in which the charge-transfer reaction is preceded by a chemical reaction. The measurement of current and potential provides no direct information about the microscopic structure of the interface, though a clever experimentalist may make some inferences.