General theory for migrational voltammetry. Strong influence of diversity in redox species diffusivities on charge reversal electrode processes.

High sensitivity of the microelectrode response to the difference between the substrate and the product diffusion coefficients is predicted for the charge reversal processes. This effect is anticipated from the general theoretical model developed for the diffusional-migrational transport to microelectrodes. The model predicts the voltammetric wave heights for any type of electrode processes carried out in the presence of any number and concentration of nonelectroactive ions. It involves changes in diffusion coefficients of the redox species and assumes no homogeneous complications. Handy, analytical expressions for the limiting current and limiting potential can be derived for a system of a univalent product and univalent ions of supporting electrolyte. This case covers charge reversal processes of the following type: S(z) --> P(+/-) + ne (n + z = sgn(n), absolute value(n) > or = 2). It has been shown that under migrational conditions the change in the ratio of the product and the substrate diffusivities (D(P)/D(S)) by as little as 10% results in significant changes in the voltammetric wave height. For 2-e charge reversal processes, a 10% increase in D(P) versus D(S) leads to a drop in the voltammetric wave height of 18.3% compared to that calculated for equal diffusion coefficients. The reversed change, i.e., the 10% decrease of the D(P) value with respect to D(S), increases the voltammetric wave height by 30.5% compared to that obtained for equal diffusivities. The theoretical predictions were confronted with our recent experimental results obtained for the 2-e oxidation of sodium (6,8-diferrocenylmethylthio)octanoate, which process can be classified as the charge reversal reaction. The best fit was obtained for D(P)/D(S) equal to 0.71.