Electrochemical oxidation of sulfite and sulfur dioxide at a renewable graphite electrode

Electrochemical oxidation of S(IV) species which are present in the aqueous solutions of sodium sulfite or are formed during the dissolution of gaseous sulfur dioxide was studied on a renewable graphite electrode as the dependence on the concentration of these particles, electrode potential and pH of solution. The voltammograms in acid and alkaline sulfite solutions have different shapes. In acid solutions, this is a voltammograms with a sole maximum of current near 1.0 V. In alkaline solutions, this is a curve with two maxima; the first of them appears around 0.6 V, while the second one is near 1.0 V, similarly to the case of acid solutions. Analysis of the behavior of j-E curves and calculations of the equilibrium composition of the solution indicate that the maximum of current near 0.6 V is connected with the oxidation of SO32- ions. The maximum of current near 1.0 V is mainly due to the oxidation of HSO3- ions which are present in acid solutions in bulk, while in alkaline solutions they might be formed in the near-electrode layer of solution as a product of the chemical reaction that follows the reaction of oxidation of sulfite ions. The results give use reason to assume that acid or basic surface oxides that are formed on graphite surface participate not only in the processes of oxygen evolution on this electrode but also in the processes of oxidation of S(IV) species. Their participation causes substantial difference in the potentials of oxidation of the indicated particles in alkaline and acid solutions. (C) 2017 Elsevier Ltd. All rights reserved.