Redox potentials of molybdovanadophosphoric heteropoly acids in aqueous solutions

The redox potentials (E) and the pH values for 0.01-0.2 M solutions of moly belovanadophosphoric heteropoly acids (H3+/-nPVmMo12-nO40)-Mo-V (HPA-n, 1 l ess than or equal to n less than or equal to 4) and of their reduced forms (H3+m+nPVmVn-mMo12-nO40)-V-IV-Mo-V (H(m)HPA-n, 0 less than or equal to m le ss than or equal to n) were measured. The E values are determined by the re dox potential of the VO2+/VO2+ pair, which exists in an equilibrium with th e heteropoly anions. The equilibrium fraction of the VO2+ cation (beta(IV) = [VO2+](eq)/[V-Sigma(IV)] detached from the heteropoly anion H(m)HPA-n is greater than the fraction of the detached VO2+ cation (beta(V) = [VO2+](eq) /[V-Sigma(V)]). The beta(IV)/beta(V) ratio decreases as it increases and pa sses through a maximum with an increase in m. The resulting dependences wer e interpreted in terms of the concept that the HPA-n and H(m)HPA-n anions p articipate in one-electron transfer.