Kinetics and mechanism of oxidations by peroxodiphosphate ions. Part I. Oxidation of arsenic(III) in aqueous perchloric acid

Abstract

Peroxodiphosphate, [P₂O₈]^4–, oxidises arsenious acid in perchloric acid medium according to equation (i). H₄P₂O₈+ H₃AsO₃+ H₂O → 2H₃PO₄+ H₃AsO₄(i) The mechanism essentially involves hydrolysis of the species [H₂P₂O₈]^2–, [H₃P₂O₈]^–, and H₄P₂O₈, and the rate of the redox reaction is independent of the arsenic (III) concentration. The rate law when 2.5 > [HClO₄] > 0.1 mol dm^–3 is as in (ii), and the values of k₂ and k₃ are (2.7 ± 0.2)× 10^–4 and (8.65 ± 0.3)× 10^–3 s^–1 respectively at 45 °C. –d[P₂O₈]/dt=k₂[H₃P₂O₈^–]+k₃[H₄P₂O₈](ii) The rate of reaction is independent of the ionic strength. Acrylamide, F^–, Cu²⁺, Mn²⁺, Fe²⁺, and phosphate have no effect on the reaction, but Ag⁺ catalyzes the reaction.