Theory of square-wave voltammetry for the analysis of an EC reaction mechanism complicated by the adsorption of the reagent

Abstract

A theoretical model about the voltammetric response of electrode processes complicated by adsorption of reactant and a following coupled chemical reaction of soluble species (E(ad)C(sol) mechanism) is presented. The results are focused on reactions studied by square-wave voltammetry (SWV) where the electrochemical and the chemical kinetics affect the voltammetric response. The quasi-reversible maximum is a well-known characteristic of systems that involve adsorbed species and that are assessed by SWV. This characteristic usually helps researchers to estimate the value of ks. In this manuscript some warnings about the overuse of the quasi-reversible maximum are presented. Special attention should be paid for those systems where the quasi-reversible maximum does not have the shape of theoretical curves or where the frequency of the maximum shifts when the pH or another experimental variable is modified. All these outcomes are pointing out the presence of a reaction mechanism more complicated than a simple reaction scheme of an adsorbed reagent that releases a soluble product. This type of E(ad)C(sol) reaction mechanism would explain several aspects associated with the reduction of metal cations complexed with organic ligands. To the best of our knowledge, systems of this complexity have been not modeled so far. In this regard, this manuscript presents one step forward for the deconvolution of these not that simple reactions.