398-401The 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 pH 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