A kinetic and mechanistic study of the chromium (VI) reduction by hydrogen peroxide in acidic aqueous solutions

Abstract

The reduction of hexavalent chromium, Cr(VI), by hydrogen peroxide in both buffered and non-buffered aqueous solutions was investigated as a function of concentration, pH, ionic strength, effect of radical scavengers, temperature and pressure. The rate of the reaction between Cr(VI) and hydrogen peroxide exhibited a strong dependence on the pH of the reaction mixture, viz. a decrease in reaction rate with increasing pH from 1.0 to 7.0. For a 2.5 x 10^-4 mol·dm^-3 Cr(VI) solution, a H₂O₂ concentration of at least 10 times the initial Cr(VI) concentration was required for complete reduction in this pH range. Neither the ionic strength of the reaction mixture, nor the presence of a radical scavenger had an effect on the rate of the ratedetermining step. From the temperature dependence of the reaction the activation enthalpy (ΔH^≠) was calculated to be 10.4 ± 0.5 kJ·mol^-1 and the activation entropy (ΔS^≠) to be -186 ± 3 J K^-1·mol^-1 for the rate-determining step. The volume of activation (ΔV^≠) was found to be -6.1 ± 0.5 cm³ ·mol^-1 from the pressure dependence of the reaction rate. The empirical data could be fitted to: k_obs = k[H⁺][H₂O₂]/(K_a + [H⁺]), with k = (48.4 ± 1.4) x 10³ dm³·mol^-1·s^-1 and K_a the acid dissociation constant of H₂CrO₄. A reaction mechanism in which a Cr(VI)/H₂O₂ adduct is formed in the rate determining step, is proposed. The theoretical rate law that can be derived from this mechanism is in agreement with the empirical rate law.

Keywords: kinetics, mechanism, chromium (VI), reduction, hydrogen peroxide, activation parameters