Aqueous Oxidation of Sulfur(IV) Catalyzed by Manganese(II): A Generalized Simple Kinetic Model

Abstract

The reaction kinetics of S(IV) autoxidation catalyzed by Mn(II) in the pH range 3–5 typical for atmospheric liquid water, was investigated. For reactions with pH maintained constant during the reaction course, the predictions obtained by a simple integral approach cover kinetic results only for concentrations of HSO 3 − up to 0.2 mM at pH 4.5. Thus, a generalized simple kinetic model, which can be used for predicting the reaction kinetics in wider concentration, pH and temperature ranges, was derived. This model is based on the assumption that the reaction rate is proportional to the concentration of a transient manganese-sulfito complex formed in the initial step of a radical chain mechanism. In the proposed power law rate equation $${}^{ - r}{\text{S}}({\text{IV}}) = k \cdot [{\text{MnHSO}}_{\text{3}}^{{\text{(}}a - 1{\text{) + }}} ]^2 \cdot \frac{1} {{[{\text{H}}^ + ]^{0.5} }},$$ the concentration of complex is calculated from the stability constant K and concentrations of reactants at a specific reaction time. This rate equation adequately predicts the reaction kinetics in the pH range 3–5, in the concentration ranges 0.1 ≤ [HSO 3 − ] ≤ 0.4 mM and 2 ≤ [Mn(II)] ≤ 14.6 μM. For the temperature range 15–35 °C, the estimated value for activation energy is 92.0 ± 0.1 kJ mol−1 and the Gibbs free energy of formation of the manganese-sulfito complex is −20.4 ± 0.3 kJ mol−1. Furthermore, the kinetics for catalytic reactions with pH maintained constant during the reaction course as well as with initial pH adjusted only at the start of the reaction, is described satisfactorily by the present model.