OXIDATION OF NICKEL(II) AND MANGANESE(II) BY PEROXODISULFATE IN AQUEOUS-SOLUTION IN THE PRESENCE OF MOLYBDATE - CRYSTAL-STRUCTURE OF THE (NH4)6[NIMO9O32].6H2O PRODUCT

Oxidation of Ni2+aq by S2O82- to nickel(IV) in the presence of molybdate ion, as in the analogous manganese system, involves the formation of the soluble heteropolymolybdate anion [MMo9O32]6- (M = Ni, Mn). The nickel(IV) product crystallized as (NH4)6 [NiMo9O32].6H2O from the reaction mixture in the rhombohedral space group R3, a 15.922(1), c 12.406(1) angstrom; the structure was determined by X-ray diffraction methods, and refined to a residual of 0.025 for 1741 independent 'observed' reflections. The kinetics of the oxidation were examined at 80-degrees-C over the pH range 3.0-5.2; a linear dependence on [S2O82-] and a non-linear dependence on 1/[H+] were observed. The influence of variation of the Ni/Mo ratio between 1:10 and 1:25 on the observed rate constant was very small at pH 4.5, a result supporting the view that the precursor exists as the known [NiMo6O24H6]4- or a close analogue in solution. The pH dependence of the observed rate constant at a fixed oxidant concentration (0.025 mol dm-3) fits adequately to the expression k(obs) = k(H)[H+]/(K(a)+[H+]) where k(H) = 0.0013 dm3 mol-1 s-1 and K(a) = 4.0 x 10(-5). The first-order dependence on peroxodisulfate subsequently yields a second-order rate constant of 0.042 dm3 mol-1 s-1. Under analogous conditions, oxidation of manganese(II) occurs eightfold more slowly than oxidation of nickel(II), whereas oxidation of manganese(II) by peroxomonosulfuric acid is 16-fold faster than oxidation by peroxodisulfate under similar conditions.