Kinetics and Mechanism of the Oxidation of HSO₃⁻ by O₂. 2. The Manganese(II)-Catalyzed Reaction

The Mn 2+ -catalyzed oxidation of HSO 3 - by O 2 has been studied in the pH region 4.5 and at bisulfite ion concentrations from 1.5 × 10 - 3 to 1.2 × 10 - 2 M. The reaction was found to obey a three-term rate law: −d[O 2 ]/d t = k α [HSO 3 - ] 2 + k β [HSO 3 - ][Mn 2+ ] + k γ [Mn 2+ ] 2 with k α = 3.6 × 10 - 3 M - 1 s - 1, k β = 1.23 M - 1 s - 1, and k γ = 98.6 M - 1 s - 1 at pH 4.50, 25 °C, and ionic strength 0.050 M. The kinetic behavior of the reaction resembles markedly that of the uncatalyzed reaction. The rate of the reaction is independent of oxygen concentration, S 2 O 7 2 - and HSO 5 - are intermediates in the reaction, and the reaction is catalyzed by S 2 O 8 2 - and strongly inhibited by methanol. The experimental results can be quantitatively explained by the addition to the uncatalyzed reaction mechanism of a chain propagation reaction involving Mn 2+ and SO 5 • - . Among several alternatives, the manganese propagation may be represented as follows: (I) Mn 2+ + SO 5 • - → Mn(III) + HSO 5 -; (II) Mn(III) + HSO 3 - → Mn 2+ + SO 3 • - . The resulting mechanism leads to the three-term rate law where the first term is the uncatalyzed rate and the second term can be predicted quantitatively from the uncatalyzed rate and the last term. It was inferred from the yield of S 2 O 7 2 - that, unlike the reaction between SO 5 • - and HSO 3 -, there is little or no branching of the first reaction to form the SO 4 • - radical. The ratio of the rate constant of (I) to that of the reaction of SO 5 • - with HSO 3 - in the uncatalyzed reaction mechanism has been determined to be 124. The quantitative agreement between the experimental data and predictions for the effect of S 2 O 8 2 - has substantiated the validity of the proposed mechanism. The pH dependence of the reaction rate can be almost entirely accounted for by the pH dependence of the reaction between HSO 5 - and HSO 3 -, indicating that reaction I is independent of the hydrogen ion concentration in the pH region studied. While methanol inhibits the reaction, the quantitative discrepancy between predictions and experiments suggests that the reactions with alcohols are more complex than previously thought. Further work in this area is needed to understand fully the reaction mechanism when alcohol is present.