Reduction of vanadium(v) by oxalic acid in aqueous acid solutions

The reduction of vanadium(V) to vanadium(IV) by oxalate in acidic media proceeds at 50 °C via two parallel pathways that involve activated states of compositions {VO2+; 2H+; 2C2O42−}≠ and {VO2+; 2H+; 3C2O42−}≠, resulting in an apparent change in the partial order on oxalate as its concentration increases and a maximum in the rate/pH profiles. The best explanation for these results assumes outer sphere electron transfer as the rate determining step in both pathways, without formation of vanadium(III). The implications for the bioinorganic chemistry of vanadium are discussed.