A reinvestigation of hexachloroiridate (IV) oxidation of arsenous acid in acid medium: Intra-contradictions in the earlier study

Abstract

In the oxidation of arsenious acid, As(III), by [IrCI6] - ion in perchloric acid (µ = 1 mol dm ^{- 3},NaClO₄), Δ[IrCl]/ Δ[As(OH)3]= 1.99±0.07. The reaction is first order both with respect to [As(llI)] and [Ir(IV)] ions. The k_ObS> the pseudo-first order rate constant ([As(lII)]. » [Ir(IV)]), is independent of ionic strength (µ), [H⁺], [Ir(III)] and [As(V)]. A rapid scan spectra of the reaction mixture indicates the formation of a weak intermediate which is probably a chlorine-bridged complex between [Ir(IV) and As(III). The kinetic viability of such an intermediate is also indicated by the change in the absorbance of the reaction mixture compared to that of Ir(IV) solution of the same concentration as noted from stopped-flow measurements. The rate determining step is the disproportionation of the weak complex which is followed by the rapid oxidation of H₂AsO₃, radical intermediate by a second Ir(IV) ion. The hydrolysis of [IrCl6]^3- - over the period of time (the reaction mixture was left overnight for completion) resulted in the formation of [IrCls(OH2)]^2- which was spectrophotometric ally detected in quantitative yield. The contradictory results of an earlier study are discussed and rationalised by suggesting an alternate mechanism in which [IrCls(OH)]^2- and As(OH)₃ are the reactive Ir(IV) and As(lII) species respectively.