Comments on the shape of voltammetric plots of reversible stoichiometric reactions for linear potential scan

Theoretical studies of the shape of the linear potential sweep voltammogram for a reversible electron-transfer reaction have been overviewed. Two different expressions for the current–potential dependences have been proposed therein. Our analysis has shown that this divergency originates from the implicit difference of the assumptions on the pretreatment procedure before the beginning of the potential sweep. Two principal variants of such pretreatment have been considered in detail: (1) potentiostatic exposure to the potential equal to the starting one for the subsequent sweep, E = Ei; (2) OCP (zero current) regime. In the former variant, the potentiostatic regime during the pretreatment period shifts the surface concentrations from their bulk values to the steady-state ones (dependent on Ei). It induces current passage of a diminishing amplitude due to the diffusion-layer expansion. If the pretreatment period is sufficiently long (well over 10 s in water) a time-independent distribution of the reactant and product concentrations is established across the steady-state diffusion layer accompanied by the current stabilization, i ≅ iss. At the starting moment of the potential sweep, t = ti, both the potential and the surface concentrations are continuous as functions of time so that the amperometric response for t > ti increases in time from its steady-state value, i ≅ iss, due to the potential sweep, with its gradual approach to a universal dependence which corresponds to the response of the system if the potential sweep starts very far from the half-wave potential where the equilibrium concentration of the reaction product is extremely low. In the latter variant of pretreatment there is a stepwise change of the potential and of the surface concentrations at t = ti; it leads to a Cottrell-type contribution to voltammetric current which in the course of the potential sweep is supplemented by another contribution corresponding to the above universal response where the sweep starts very far from the half-wave potential. As a result, after a relatively short relaxation period the current responses in both variants approach its universal behavior. Method of a direct determination of the number of transferred electrons per consumed reactant species has been proposed. Approximate analytical expression for the dependence of current on time after passage of the current's peak has been derived. Predicted features have been verified with the use of a well-known reversible reaction: ferrocene oxidation in acetonitrile.