Kinetics and mechanism of the hexacyanomanganate(IV)–arsenic(III) redox reaction in acidic medium

The reaction between hexacyanomanganate(IV) and arsenic(III), in acidic media, proceeds with a stoichiometry Δ[Mn(IV)]/Δ[As(III)] = 1. The reaction in sulfuric acid media has been followed spectrophotometrically at 387 nm, where the Mn(IV) cyanide complex has an absorption maximum, and shows a first-order dependence on the hexacyanomanganate(IV) concentration, viz., −d[Mn(IV)]/dt = k[Mn(IV)], the value of k being dependent on the acidity and the arsenic(III) concentration. At As(III), 0.1 M; H + , 0.16 M; ionic strength, 2.0; and T, 30 °C, the value of the rate constant was found to be k = (1.04 ± 0.06) × 10 −5 s −1 . The proposed mechanism, through three parallel pathways, explains satisfactorily the experimental results observed.