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

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 H-2 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 k(s) = (4 +/- 2) s(-1), alpha = (0.20 +/- 0.01), the chemical reaction equilibrium constant K = 3 x 10(-4), the adsOrption constant K-ad = 1 x 10(-2), the formal potential of the redox step E degrees' = (0.055 +/- 0.002)V, and the surface concentration of accumulated As(0), Gamma(Gamma)* = 3.3 x 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 k(1) = 1 x 10(5) 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. (C) 2016 Elsevier Ltd. All rights reserved.