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 mo dified version of the spline orthogonal collocation technique has been deve loped to compute the voltammetric response under Langmuirian reversible con ditions. Simple analytical expressions derived for surface-confined redox p rocesses are shown to reproduce the stripping voltammograms only in the pre sence of significant (0(R)(*) > 0.01) analyte adsorption. In the absence of analyte adsorption, asymmetrical waves are obtained as an intrinsic charac teristic of the stripping protocol. Empirical equations relating the stripp ing peak potential and the voltammetric half-height width with scan rate an d adsorbed product coverage are presented. Voltammetric scan rate dependenc e is shown to provide a quantitative estimate of the analyte adsorption ext ent, whereas its dependence on adsorbed product coverage allows one to iden tify the presence of surface dimerization processes. Maximum surface excess es and dimerization equilibrium constants can readily be obtained by plotti ng the reciprocal half-height width versus the surface concentration of str ipped species. In the absence of analyte adsorption, a complementary convol utive analysis is also proposed. Application is made to the cathodic stripp ing of three mercaptocarboxylic acids differing in their hydrocarbon chain- length. The extent of oxidized product dimerization is shown to display a s trong dependence on the solution pH, so that higher peak current sensitivit ies are found in acidic solutions, where oxidized products are present as d imers. (C) 2000 Elsevier Science S.A. All rights reserved.