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

Abstract

Oxidation of Ni²⁺,_aq, by S₂O₈^2- to nickel(IV) in the presence of molybdate ion, as in the analogous manganese system, involves the formation of the soluble heteropolymolybdate anion [MM_ogO₃₂]^2- (M = Ni, Mn ). The nickel(IV) product crystallized as (NH₄)₆ [NiM_ogO₃₂].6H₂O from the reaction mixture in the rhombohedra1 space group R3, a 15.922(1), c 12.406(1) Å ; 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 C over the pH range 3.0-5.2; a linear dependence on [S₂O₈^2-] and a non-linear dependence on l/[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 dequately to the expression k_obs = k_H [H⁺]/(K_a+[H⁺]) where k_H = 0.0013 dm³ mol^-1 s^-1 and K_a = 4-0x10^-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.