Kinetics and mechanism of oxidation of arsenous acid by Waugh-type enneamolybdomanganate(IV)

Abstract

The reaction between As(III) and Waugh-type enneamolybdomanganate(IV) has been studied in perchloric acid medium. The reaction follows a second order kinetics with an order of unity each on both the reactants and proceeds with a complimentary two-electron outer-sphere path. The reaction is accelerated by [H⁺] ions due to protonation equilibria of the oxidant. The first two protonation constants of the enneamolybdomanganate(IV) anion have been determined by <i>p</i>H metric titration and found to be 2.0 × 10³ and 316 dm3 Mol^-1. The product of reaction, enneamolybdomanganous(II) has been synthesized to study the effect of the added product. Both ammonium salts of enneamolybdomanganate(IV) and enneamolybdomanganous(II) have been characterized by FTIR and AAS. The reaction is

unaffected by change in ionic strength and the free radical test is negative which suggests direct two-electron transfer from As(III) to oxidant. Di-protonated form of enneamolybdomanganate(IV) has been found to be active oxidant species. The solvent polarity and the thermodynamic parameters suggest the outer sphere mechanism.