OXIDATION OF SULFITE AND THIOSULFATE BY THE COBALT(III) COMPLEX OF DIPICOLINATE IN AQUEOUS-MEDIUM

The oxidation of sulfite and thiosulfate by Co(dipic)2- (where H2dipic = 2,6-pyridinedicarboxylic acid) was studied in aqueous medium. The reactions were carried out by varying reagent concentrations, pH, and ionic strength. For thiosulfate oxidation, the rate is zero order in complex and second order in thiosulfate. The reaction rates were found to increase with the acidity and the general rate law is expressed as k(obs) = {k0' + k1'[H+]}[S2O3(2-)]2 where k0' and k1' denote the acid independent and dependent paths respectively. The values of rate parameters are k'0 = (2.87 +/- 0.20) x 10(-8) mol dm-3 s-1 and k1' = (0.57 +/- 0.02) mol-2 dm6 s-1 at 30-degrees-C. A plausible mechanism is suggested by considering the formation of an intermediate active species in the rate-determining step. The sulfite oxidation was, however, first order both in complex and reagent. An inverse dependence on [H+] was obtained for this reaction indicating a higher reactivity of the anionic species in solution. The general rate law is given by k(obs) = 2(k1[SO2.H2O] + k2[HSO3-] + k3[SO3(2-)]) and the evaluated second-order rate constants are: k1 = (5.60 +/- 0.04) x 10(-5) mol-1 dm3 s-1, k2 = (3.02 +/- 1.21) x 10(-3) mol-1 dm3 s-1 and k3 = (4.91 +/- 0.05) x 10(-2) mol-1 dm3 s-1 at 60-degrees-C. The reactivity order SO3(2-) > HSO3- > SO2.H2O has been explained successfully by considering the two isomeric structures for these species in solution. An overall rate law has been derived to interpret these results over the whole range of pH. The theoretical pH rate profile calculated from the different rate parameters shows an excellent agreement with the experimental one suggesting the validity of the proposed mechanism.