REACTION-KINETICS AND MECHANISM OF FORMATION OF [MNMO9O32]6- BY HYPOCHLOROUS ACID OXIDATION OF MN2+ (AQ) IN THE PRESENCE OF MOLYBDATE

Oxidation of Mn2+ (aq) by HOCl in weakly acidic solution, in the prese nce of molybdate, results in the formation of the soluble heteropolymo lybdate [MnMo9O32]6-. The speciation of Mn2+ in solution prior to oxid ation has been examined using ESR spectroscopy and it has been establi shed that the principal species is [Mn(OH2)6]2+, there being evidence for weak interactions only with molybdate and isopolymolybdate anions. In keeping with this conclusion, the secondary manganese(II) polymoly bdate species Na4[MnMo8O27].2OH2O Was crystallized from an acidic (pH 4.5) solution containing Mn2+ (aq) and isopolymolybdate ions by slow v apour diffusion of ethanol into the aqueous phase. The compound crysta llizes in the triclinic space group P1BAR, a = 9.568(2), b = 9.868(2), c = 11.703(3) angstrom, alpha = 103.60(1), beta = 100.86(1) and gamma = 96.05(1)-degrees, and the structure was determined by X-ray diffrac tion methods to an R of 0.026 (R' = 0.040) for 3182 independent observ ed reflections. It consists of polymerized octamolybdate units linked through the sharing of common oxygen atoms and hydrated Mn2+ groups re sulting in a sheet structure, with hydrated Na+ ions also attached to the octamolybdate units. Uncomplexed water is also present. The kineti cs of oxidation of Mn(II) by HOCl was studied at 5-20-degrees-C over t he range pH 4.0-5.4 and found to exhibit solution autocatalytic behavi our. The oxidation kinetics followed the expanded rate expression +d[M nMo9O326-]/dt = k(AC)[Mn2+][MnMo9O326-][HOCL]-[MoO42-][HMoO4-] based o n an examination of [HOCl], pH and total [MoO42-] dependences, and arg uments based on molybdate speciation under the reaction conditions. An approximate value of k(AC) at 25-degrees-C, 1.36(10) x 10(8) dm12 mol -4 s-1, was estimated from an extrapolated value of the second-order a utocatalytic rate constant using an Arrhenius plot and the reported fo rmation constants describing the speciation of molybdate, isopolymolyb date and their protonated analogues in solution. A mechanism involving sequential one-electron oxidation steps of Mn2+ through to Mn4+ via M n3+, together with the equilibrium Mn2+ + Mn4+ half arrow right over h alf arrow left 2Mn3+, which favours Mn2+ and Mn4+, is able to account for the observed autocatalysis.