Kinetics and Mechanisms of S(IV) Reductions of Bromite and Chlorite Ions

The reaction of bromite with aqueous S(IV) is first order in both reactants and is general-acid catalyzed. The reaction half-lives vary from 5 ms (p[H+] 5.9) to 210 s (p[H+] 13.1) for 0.7 mM excess S(IV) at 25 °C. The proposed mechanism includes a rapid reaction (k1 = 3.0 × 107 M-1 s-1) between BrO2- and SO32- to form a steady-state intermediate, (O2BrSO3)3-. General acids assist the removal of an oxide ion from (O2BrSO3)3- to form OBrSO3-, which hydrolyzes rapidly to give OBr- and SO42-. Subsequent fast reactions between HOBr/OBr- and SO32- give Br- and SO42- as final products. In contrast, the chlorite reactions with S(IV) are 5−6 orders of magnitude slower. These reactions are specific-acid, not general-acid, catalyzed. In the proposed mechanism, ClO2- and SO3H-/SO2 react to form (OClOSO3H)2- and (OClOSO2)- intermediates which decompose to form OCl- and SO42-. Subsequent fast reactions between HOCl/OCl- and S(IV) give Cl- and SO42- as final products. SO2 is 6 orders of magnitude more reactive than SO3H...