The hexacyanomanganate(IV)–hydrogen peroxide reaction. Kinetic determination of vanadium

Abstract

The reaction between hexacyanomanganate(IV) and hydrogen peroxide in acidic medium is strongly catalysed by vanadium. The stoichiometry has been proved to be Δ[Mn(IV)]/Δ[H 2O 2]=1. S-shaped absorbance–time curves are obtained for a wide range of conditions, which seems to be due to the transient formation of hydroperoxyl radicals, HO 2·; therefore, the reaction is autoinduced. The reaction kinetics is first order on the hexacyanomanganate(IV) concentration. Complex dependence of the initial rate on the H 2O 2 as well as H + concentrations are observed both for the uncatalysed and for the catalysed reactions, but the rate laws are given and reaction mechanisms are proposed. The catalytic effect of vanadium has been used to determine traces of vanadium (the (IV) and (V) oxidation states are determined together). By applying the initial rate method a detection limit of 0.9 ng ml −1 (18 nM), and a linear range up to 50 ng ml −1 were found. The relative standard deviations for the 4 and 25.5 ng ml −1 levels were 5.2 and 2.5% respectively. Positive kinetic interferences from Cr(VI), Hg(I) and Ag(I) have been observed; other interferences are less severe and Cr(IIl), Fe(II) and Fe(III), among many others, do not interfere. The Mn(IV) solution must be prepared daily and its conservation needs some care (0°C) but the proposed method has been applied to the determination of vanadium in a phosphate rock (reference material, BCR No. 32) without previous separations. Recovery experiments have also been performed; excellent results were obtained in both cases.