Oxidation of Arsenous Acid by 12-Tungstocobaltate(III) Catalysed by Ruthenium(III) in Aqueous Acid Medium

Ruthenium(III)-catalysed arsenic(III) oxidation by 12-tungstocobaltate(III) was found to proceed with transfer of an electron from catalyst to oxidant generating an intermediate ruthenium(IV). In the next step ruthenium(IV) abstract an electron from arsenic(III) leading to form arsenic(IV), which further reacts with oxidant to give arsenic(V). The active catalyst species was RuCl5(H2O) and due to formation of the inactive higher chloro-complex, RuCl6, inhibition of the reaction occurs as chloride ion concentration increases. There was no effect of hydrogen ion concentration on the reaction due to absence of any protonation equilibria under experimental conditions. The reaction was also catalysed by alkali metal cations due to ion pair formation between 12-tungstocobaltate(III) and alkali metal cations.