Theory of Square-wave Voltammetry of two Electrode Reactions Coupled by Reversible Chemical Reaction

An ECE mechanism in which chemical reaction is permanently in equilibrium is investigated theoretically. The calculations were performed by the method of numerical integration for fast and reversible electrode reactions as well as for the reactions that are controlled by the electron transfer kinetics. In square-wave voltammetry the response depends on the difference in standard potentials of individual electron transfers and on the dimensionless equilibrium constant of the chemical reaction. Depending on the concentration of compound X-, which is one of reactants in chemical reaction, either a single peak or the split response may appear. In the first case the peak potential depends linearly on the logarithm of X- concentration, with the slope -2.3RT/2F. The potential of the first peak of the split response is also linear function of, but the slope is -2.3RT/F. The relationship is determined between standard potentials and the critical value of the equilibrium constant above which the response splits in two peaks. Under the influence of electrode kinetics, the responses change with square-wave frequency. The first electron transfer is slow and the second one is fast if two peaks appear at the lowest frequency and merge into a single peak at the highest frequency. If the second charge transfer is slower that the first one, the difference in peak potentials increases with the increasing frequency.