Oxidation of Manganese(II) by Peroxomonosulfuric Acid in Aqueous Solution in the Presence of Molybdate. Crystal-Structure of the K₆[MnMo₉O₃₂].6H₂O Product

Abstract

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 rhombohedra1 space group R32, a 15.569(1), c 12.432(1) Å , 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 ≤ 1:12, the red heteropolymolybdate being the only isolated product. The kinetics of the oxidation were examined at 40ºC over the pH range 4.0-5.3, and non-linear dependences on [HSO₅-] and l/[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[Mn¹¹dt = k_o [HSO₅^-]+ k₁ [HSO₅^-]² where k₀ = 0.021 dm³ mol^-1 s^-1 and k₁ = 0.041 dm⁶ mol^-1s^-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 (v. 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⁺].