Voltammetric speciation studies of systems where the species diffusivities differ significantly

Abstract

A rigorous and simple analytical solution is reported for the square scheme in single pulse techniques at (hemi)spherical electrodes when the electron transfers are reversible and the coupled chemical reactions are at equilibrium. The solution presented imposes no restriction to the values of the diffusion coefficients of the different species, and then it fully describes the cases where the coupled chemical processes involve significant changes of diffusivity. Simple criteria are discussed to understand and predict the effects of electrode radius, experiment time-scale and chemical thermodynamics on the results obtained in normal pulse voltammetry and derivative voltammetry. Unlike at macroelectrodes and ultramicroelectrodes, the position of the voltammograms (in these and other voltammetric techniques) is time dependent at spherical electrodes of intermediate size when the diffusion coefficients of the species are unequal. An analytical expression for the half-wave potential is given to describe this behaviour and also to assist the quantitative determination of the diffusion coefficients, formal potentials and equilibrium constants with the electrochemical techniques abovementioned.