Kinetics and mechanism of the oxidation of dodecatungsto-cobaltate(II) by peroxodisulphate 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[CoW12O406–]/dt = 2ks[S2O82–]. 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.