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 degreesC. The proposed mechanism includes a rapid reaction (k(1) = 3.0 x 10(7) 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 SO4. 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-, where k(5)(SO2/ClO2-) = 6.26 x 10(6) M(-)1 s(-1) and k(6)(SO3H-/ClO2-) = 5.5 M-1 s(-1). Direct reaction between ClO2- and SO32- is not observed. The presence or absence of general-acid catalysis leads to the proposal of different connectivities for the initial reactive intermediates, where a Br-S bond forms with BrO2- and SO32-, while an O-S bond forms with ClO2- and SO3H-.