The kinetics of the oxidation of some inorganic species by peroxodisulphate

Abstract

The reactions of poroxodisulphate with manganese(II), nitrite and chromiurn(III) have been studied in the region of 60°C.

In the main introduction a survey is made of published work on the reaction of peroxodisulphate with mainly inorganic substrates.

The oxidation of manganese(II) to manganese(IV) was carried out in sulphate media for most of the kinetic investigation. The reaction was found to be first order with respect to peroxodisulphate and zero order with respect to manganese(II). Despite this, however, the observed first order rate constant is greater than that of the thermal decomposition of peroxodisulphate in the absence of manganese(II). The effect of added salts on the rate of the reaction is negligible though the reaction is sensitive to some impurities, added surface and free radical scavengers which all retard it. The activation energy which is 31.5 kcal/mole is only 2 kcal/mole lower than that for the thermal decomposition of peroxodisulphate alone. A chain mechanism consistent with the observed kinetics is proposed.

The oxidation of nitrite was carried out in phosphate media. The redox reaction is of the second order, being first order with respect to each reactant. Cations catalyze the reaction and the extent of catalysis depends on the nature and concentration of cation. The activation energy for the redox reaction is 14 kcal/mole. E.s.r. spectroscopy did not reveal any stable or detectable free radicals. Reaction mechanisms are proposed.

The peroxodisulphate-chromium(III) reaction was carried out in acid-perchlorate media and acid-sulphate media. The oxidation of chromium(III) to chromium(VI) is faster in the second medium than in the first.