Electrochemical detection of As³⁺ and As⁵⁺ by anodic stripping voltammetry at a gold electrode

In this article, electrochemical detection of arsenite (As³⁺) and arsenate (As⁵⁺) by stripping voltammetry at a gold electrode has been investigated. As³⁺ and As⁵• are reduced to arsine (AsH₃) in 0.1 M sodium borohydride before deposition on the electrode surface. 0.1 M HCl was used as the supporting electrolyte, where Cl^- ions act as an ionic bridge, facilitating reduction at the working electrode. As³⁺ and As⁵⁺ can be deposited as arsine by simple electrochemical reduction at -0.5 V (versus Ag/AgCl). Anodic stripping voltammograms of arsine at a gold electrode showed similar peak potentials for As° oxidation (≈0.31 V versus Ag/AgCl) under the optimum conditions (deposition potential, deposition time, and scan rate of -0.5 V, 60 s, and 200 mV/s, respectively). The stripping voltammograms for As³⁺ and As⁵⁺ presented similar peak shapes and current intensities. A reaction mechanism involving reduction to As⁰ at a mild potential in a strong acid, followed by oxidation of As° to As³⁺ at the gold electrode, was proposed. The method developed offers a wide linear concentration range for As³⁺ (0-5 ppm) and As⁵⁺ (0-20 ppm) and very low limits of detection for As³⁺ (0.85 ppm) and As⁵⁺ (6.07 ppm).