Thermochemical study of reduction and oxidation reactions of molybdovanadophosphoric heteropolyacids in aqueous solutions

Differential enthalpies of reduction, -Delta(r)H(4jr) have been found at 343 K for 0.2 m aqueous heteropolyacids H3+xPVxMO12-xO40 (HPA-x, where 1 less than or equal to x less than or equal to 6) and their salts by hydrazine hydrate (HH). The -Delta(r)H(4j) values depend on x and m, where m = [V-IV]/[HPA-x]. At m approximate to 0, the initial -Delta(r)H(4j) value is 436 +/- 14 kJ mol(-1) HH for all HPA-x. For HPA-x with 3 less than or equal to x less than or equal to 6, the curves -Delta(r)H(4j) = f(m(j)) have maxima associated with formation and reduction of the outer-sphere cation (VVO33+)-V-V-O-IV. A similar maximum is observed on reduction of 0.1 M VO2ClO4 in 4 m HClO4. No maxims are observed for the reduction of HPA-1, HPA-2, and salts of HPA-3. For HPA-3 salts, -Delta(r)H(4j) = f(m(j)) depends primarily on the pH of the salt solution, rather than on the nature of the cation. Reduction and oxidation of HPA-x and their salts are reversible reactions. In HPA-x solutions, VO2+ in the outer sphere is reduced first, and V-V in the inner coordination sphere of the heteropoly anion is reduced next. Differential enthalpies for the oxidation of the reduced HPA-x by dioxygen (-Delta(r)H(2j)) have been calculated. The sequence of the oxidation of different V-IV forms in the reduced H(m)HPA-x solutions is the reverse of that for the reduction of the V-V forms in the initial HPA-x solutions.