Kinetics and mechanism of the oxidation of aquated sulfur dioxide by hydrogen peroxide at low pH

A stopped-flow kinetic study of the oxidation of sulfur dioxide by hydrogen peroxide was performed over the pH range 0.0-4.5. A rate expression of the following form was verified experimentally: d[S(VI)]/dt = _lK_(a1)[H_2O_2][S(IV)](k_2[H^+] + /((k-l+ k2[H+]+ k_3[HA])/{k_(-1) + k_2[H^+]+ k_3[HA])(k_(al) + [H^+]) The following kinetic parameters at 15 ºC were determined: k_l = (2.6 ± 0.5) x 10^6 M^(-1) s^(-1), k_2/k_(-l) = 16 ± 4 M^(-1), k_2/k_3 = (5 ± 1) x 10^2 (HA = acetic acid), ΔH^*_1 = 37 ± 2 kJ mol^(-1), and ΔS^*_1 = 4 ± 4 J K^(-1) mol^(-1). The reaction proceeds via a nucleophilic displacement of HSO_3^- by H_2O_2 to form a peroxymonosulfurous acid intermediate which undergoes acid-catalyzed rearrangement to form product: SO_2.H_2O ⇌ HSO_3^- + H^+ (k_(al)), H_2O_2 HSO_3^- ⇌ HOOSO_2^- (k_1,k_(-1) HOOSO_2^- + HA → HA + HSO_4^- k_3. Application of the above rate expression to occurring in hydrometeors is discussed.