Kinetics and mechanism of oxidation of arsenous acid by 12-tungstocobaltate(III) ion
Abstract
The oxidation of arsenous acid by 12-tungstocobaltate(III) ion, [CoIIIW]5–, studied in perchloric acid solution at constant ionic strength (1.0 mol dm–3), can be expressed by the rate equation (i) where k and k1kobs= 2(k+k1Ka[H+]–1)[H3AsO3](i) are the rate constants for reaction paths involving As(OH)3 and [AsO(OH)2]– and Ka is the ionisation constant of arsenous acid. The activation parameters for the two paths are evaluated. Based on a rapid-scan spectrum of the reaction mixture, the formation of an intermediate between [MCoIIIW]4– and [AsO(OH)2]– in equilibrium prior to its rate-limiting decomposition is suggested where [MCoIIIW]4– is an ion pair between [CoIIIW]5– and alkali-metal ion M+(Li+, Na+ or K+). The rate is accelerated by alkali-metal ions in the order K+ > Na+ > Li+. The equilibrium constant for the ion-pair formation between M+ and [CoIIIW]5– is also evaluated.