Journal of Atmospheric Chemistry 47: 287–303, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 287 Aqueous Oxidation of Sulfur(IV) Catalyzed by Manganese(II): A Generalized Simple Kinetic Model BOŠTJAN PODKRAJŠEK, GORAZD BER ˇ CI ˇ C, JANJA TURŠI ˇ C and IRENA GRGI ´ C ⋆ National Institute of Chemistry, Hajdrihova 19, PO Box 660, SI-1001 Ljubljana, Slovenia, e-mail: irena.grgic@ki.si (Received: 5 August 2003; accepted: 3 February 2004) Abstract. The reaction kinetics of S(IV) autoxidation catalyzed by Mn(II) in the pH range 3–5 typi- cal 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 S(IV) = k ·  MnHSO (a−1)+ 3  2 · 1  H +  0.5 , the concentration of complex is calculated from the stability constant K and concentrations of reac- tants 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. Key words: catalysis, HSO − 3 /SO 2− 3 oxidation, influence of pH, kinetic model, manganese. 1. Introduction Recent laboratory studies of chemical reactions involving S(IV) species in aqueous solution have greatly improved our understanding of the processes responsible for the oxidation of SO 2 in atmospheric liquid water (Brandt and van Eldik, 1995; War- neck et al., 1996). However, most laboratory experiments were carried out under ⋆ Corresponding author.