Quantitative characterization of desorptive stripping voltammograms complicated by surface dimerization reactions.: Application to the reductive desorption of thiols from mercury

The influence of analyte adsorption and of a follow-up surface dimerization reaction on desorptive stripping voltammograms has been investigated. A modified version of the spline orthogonal collocation technique has been developed to compute the voltammetric response under Langmuirian reversible conditions. Simple analytical expressions derived for surface-confined redox processes are shown to reproduce the stripping voltammograms only in the presence of significant (0(R)(*) > 0.01) analyte adsorption. In the absence of analyte adsorption, asymmetrical waves are obtained as an intrinsic characteristic of the stripping protocol. Empirical equations relating the stripping peak potential and the voltammetric half-height width with scan rate and adsorbed product coverage are presented. Voltammetric scan rate dependence is shown to provide a quantitative estimate of the analyte adsorption extent, whereas its dependence on adsorbed product coverage allows one to identify the presence of surface dimerization processes. Maximum surface excesses and dimerization equilibrium constants can readily be obtained by plotting the reciprocal half-height width versus the surface concentration of stripped species. In the absence of analyte adsorption, a complementary convolutive analysis is also proposed. Application is made to the cathodic stripping of three mercaptocarboxylic acids differing in their hydrocarbon chain-length. The extent of oxidized product dimerization is shown to display a strong dependence on the solution pH, so that higher peak current sensitivities are found in acidic solutions, where oxidized products are present as dimers. (C) 2000 Elsevier Science S.A. All rights reserved.