KINETICS AND MECHANISM OF THE OXIDATION OF DODECATUNGSTO-COBALTATE(II) BY PEROXODISULFATE AND PERIODATE IN AQUEOUS ACIDIC SOLUTION

The kinetics of oxidation of [CoW12O40]6- by peroxodisulphate and periodate have been studied spectrophotometrically in aqueous acidic media. In the case of peroxodisulphate the reaction is zero order in [complex] and follows a rate law -d[CoW12O40(6-)]/dt = 2k(s)[S2O8(2-)]. However, with periodate the reaction is first order in [complex] and a limiting dependence on both [oxidant] and [H+] is obtained. An interesting stoichiometric result was also obtained: at high [complex]:[periodate] ratio (> 6:1) the stoichiometry is 2, but under pseudo-first-order conditions with excess of periodate the stoichiometry is less than 1. An inhibition of the reaction by alkali-metal salts added to adjust the ionic strength has also been observed. All these results have been explained successfully by considering a free-radical mechanism where the free radical produced in the rate-determining step either may decompose thermally or react with a second complex molecule. An inverse dependence of the rate on the concentration of alkali-metal cation and a high value of the association constant obtained from periodate variation suggest an inner-sphere pathway for this reaction.