Please use this identifier to cite or link to this item: http://hdl.handle.net/1959.13/25258
- Reaction kinetics and mechanism of formation of MnW6O24 (8-) by hypochlorous acid oxidation of Mn(II)(aq) in the presence of tungstate
Nolan, Annette L.;
Irwin, Jennifer A.;
Burns, Robert Colin;
Lawrance, Geoffrey A.
- Oxidation of Mn(II) by HOCl in weakly acidic solution, in the presence of tungstate, results in the formation of the soluble heteropolyoxotungstate [MnW6O24](8-). The kinetics of formation of this species was studied from 15-35degreesC over the pH range 4.9-5.4 and found to exhibit solution autocatalytic behaviour. The oxidation kinetics was found to follow an expanded rate expression consisting of two terms: +d[MnW6O248-]/dt = k(AC(1))[Mn2+][MnW6O248-][HWO4-](2)[OCl-] + k(AC(2))[Mn2+][MnW6O248-][HWO4-](2)[HOCl] where k(AC(1)) and k(AC(2)) are 1.7(1) x 10(15) and 6.9(4) x 10(12) dm(12) mol(-4) s(-1) at 25.0degreesC and are the autocatalytic rate constants for the OCl- and HOCl oxidation pathways, respectively. A mechanism involving the formation of a transition state based on either a tungsten-based equivalent of the [(MnMo9O32)-Mo-IV](6-) structure or a partially-built Mn(IV)-centred polyoxotungstate Keggin structure is proposed, which must contain one Mn(II) and eight W(VI) addenda atoms in addition to the centrally located Mn(IV). The OCl- (or HOCl) is coordinated to the peripherally-positioned Mn(II) addendum atom. Following oxidation of the Mn(II) to Mn(IV), rapid separation of the two Mn(IV) centres must occur, which is then followed by fast assembly of a polyoxotungstate framework around each Mn(IV) to give the observed product. The speciation of Mn(II) in solution prior to oxidation was investigated by ESR spectroscopy and it was shown that weak interaction of Mn(II) with tungstate-containing species occurs, which is consistent with the formation of a Mn(II)-[HWO4](-) complex prior to oxidation as indicated by the rate law and derived mechanism.
- Inorganic Reaction Mechanisms Vol. 5, no. 3, p. 181-195
- City Publishing
electron spin relaxation;
- Resource Type
- journal article