Kinetic studies of the oxidation reaction of arsenic(III) to arsenic(V) by peroxodisulphate ion in aqueous alkaline media

Abstract

The arsenic(III) ion was oxidized to arsenic(V) by the peroxodisulphate ion in aqueous alkaline solutions of various pH (>8.6) at 20–40 °C. The only reactive species at pH 8.6–11.6 was the deprotonated species H₂AsO₃^–; the mechanisms (i) and (ii) and the rate law (iii) are proposed to, H₃AsO₃[graphic omitted]H₂AsO₃^–+ H⁺(i), H₂AsO₃^–+ S₂O₈^2– [graphic omitted]HAsO₄^2–+ 2SO₄^2–+ 3H⁺(ii), –d[S₂O₈^2–]//_dt=kK_a[OH^–][S₂O₈^2–][AS^III]//_{Ka[OH^–]+Kw}(iii) account for the results obtained. The first acid dissociation constant K_a of H₃AsO₃ and the rate constant k for the reaction between the S₂O₈^2– and H₂AsO₃^– ions were determined under various conditions. The activation enthalpy and entropy, ΔH^‡ and ΔS^‡, for the reaction of H₂AsO₃^– with S₂O₈^2– were 47.6 kJ mol^–l and –130.4 J K^–1 mol^–1, respectively, whereas ΔH_a and ΔS_a, for the acid dissociation of H₃AsO₃ were 2.8 kJ mol^–1 and –171.4 J K^–1 mol^–1, respectively. The addition of acrylamide and acrylonitrile as radical scavengers produced no polymer and did not affect the reaction rate. The radicals SO₄˙^– and As^IV, the expected reaction intermediates, could not be detected by the addition of acrylamide and acrylonitrile monomers, and thus, the reaction between S₂O₈^2– and As^III is considered to be a two-electron transfer reaction. In strong basic medium (pH 12.0), the HasO₃^2– ion as well as H₂AsO₃^– reacted with the peroxodisulphate ion with the rate constant (1.6 ± 0.3)× 10^–2 dm³ mol^–1 s^–1 at an ionic strength of 0.1 mol dm^–3, at 30 °C.