Objective: Oxidation of manganese(II) substituted lacunary phophotungstate and
phosphomolybdate by bromate was investigated in aqueous sulphuric acid.
Method: The reaction was found to involve an induction period due to initiation through the
oxidation of traces of bromide by bromate. Under the reaction conditions the reductants exit
as their unprotonated anions [PMnW11O39]5- and [PMnMo11O39]5- respectively. The orders in
both the bromate and hydrogen ion were found to be two each. Both the anions were oxidized
by the HBrO2 free radical generated during the induction period and thereafter the reaction
follows the pseudo-first-order nature. The higher order with respect to bromate and hydrogen
ion is due to the formation and disproportionation of the HBrO2 free radical.
Results: The values of the activation parameters determined indicate an outer sphere nature
of the reaction between the anions and the free radical. The effect of ionic strength and
change in the solvent polarity did not affect the rate of the reaction.
Conclusion: The higher stability of the anions and their products results into a simple rate law
even though the redox potential of the anions was sufficient enough to get oxidized by bromate
directly. A probable mechanism was proposed justifying the kinetic results.