OXIDATION OF MANGANESE(II) BY PEROXOMONOSULFURIC ACID IN AQUEOUS-SOLUTION IN THE PRESENCE OF MOLYBDATE - CRYSTAL-STRUCTURE OF THE K6[MNMO9O32].6H2O PRODUCT

Oxidation of Mn2+ aq by HSO5- to manganese(IV) in the presence of molybdate ion in acetate buffer involves the formation of the soluble heteropolymolybdate anion [MnMo9O32]6-. This was crystallized as K6 [MnMo9O32].6H2O from the reaction mixture in the rhombohedral space group R32, a 15.569(1), c 12.432(1) angstrom, and the structure was determined by X-ray diffraction methods, refined to a residual of 0.028 for 1032 independent 'observed' reflections. Unlike the oxidation reaction in the absence of molybdate, which displays classical autocatalytic kinetics and generates manganese dioxide, no MnO2 is formed with Mn/Mo less-than-or-equal-to 1:12, the red heteropolymolybdate being the only isolated product. The kinetics of the oxidation were examined at 40-degrees-C over the pH range 4.0-5.3, and non-linear dependences on [HSO5-] and 1/[H+] observed. The influence of variation of the manganese-to-molybdenum ratio between 1:12 and 1:50 on the observed rate constant was very small at pH 4.54, a result supporting the view that the manganese exists initially as the known [MnMo6O24H6]4- or a close analogue in solution. A rate expression of the form -d[Mn(II)]/dt = k0[HSO5-]+k1[HSO5-]2 where k0 = 0.021 dm3 mol-1 s-1 and k1 = 0.041 dm6 mol-2 s-1 was observed at pH 4.54. Cyclic voltammetry identified the oxidation of the manganese(II) cluster as an irreversible process which occurs at +1.035 V (upsilon. Ag/AgCl) at pH 3.95, becoming progressively more negative with increasing pH (+0.84 V at pH 5.3), the variation presumably related to variation in protonation of the cluster, which also governs the non-linear dependence of the chemical oxidation on [H+].