The electrochemical reaction mechanism of arsenic on gold analyzed by anodic stripping Square-wave voltammetry

Abstract

The reaction mechanism of the oxidative stripping of As(0) accumulated on a polycrystalline gold electrode is studied theoretical and experimentally by Square-wave voltammetry (SWV). A mathematical model was developed specifically to describe this reaction mechanism. A reaction scheme E(ad)C is considered in this model, where As(0) corresponds to the adsorbed reduced species. The electrochemical oxidation of As(0) produces a soluble species that undergoes a chemical reaction with pseudo-first order kinetics. Although the reduction of hydrogen at the electrode surface can contribute to the deposition of As(0), significant evolution of H2 would compromise this process. From the fits of experimental SW voltammograms, it is possible to estimate the value of the standard charge-transfer rate constant ks = (4 ± 2) s−1, α = (0.20 ± 0.01), the chemical reaction equilibrium constant K = 3 × 10−4, the adsorption constant Kad = 1 × 10−2, the formal potential of the redox step E°’ = (0.055 ± 0.002) V, and the surface concentration of accumulated As(0), Γr * = 3.3 × 10−11 mol cm−2. The value of K corresponds to the ratio between the concentrations of the oxidized species and the product of the coupled chemical reaction Y. It was also found that the formation of Y involves a fast forward chemical reaction, since the kinetic constants of the chemical reaction can be estimated as k1 = 1 × 105 s−1 and k−1 = 30 s−1. Finally, it was found that the electrochemical reaction involves 3 electrons and that the lost of the first electron of As(0) is the rate-determining step of this multiple-step multiple-electron electrochemical reaction.