Differential enthalpies of reduction, -β r H 4j , have been found
f(m j ) depends primarily on the pH of the salt solution, rather than on the nature of the cation. Reduction and oxidation at 343 K for 0.2 m aqueous heteropolyacids H 3+x PV x Mo 12-x -O 40 (HPA-x, where 1 Υ
x Υ
6) and their salts by hydrazine of HPA-x and their salts are reversible reactions. In HPA-x solutions, VO 2 + in the outer sphere is reduced first, and V V hydrate (HH). The -β r H 4j values depend on x and m, where
in the inner coordination sphere of the heteropoly anion is reduced next. Differential enthalpies for the oxidation of the 14 kJ mol -1 HH for all HPA-x. For HPA-x with 3 Υ
x Υ
6, the curves -β r H 4j = f(m j ) have maxima associated with formation reduced HPA-x by dioxygen (-β r H 2j ) have been calculated. The sequence of the oxidation of different V IV forms in the and reduction of the outer-sphere cation V V V IV O 3 3+ . A similar maximum is observed on reduction of 0.1 M VO 2 ClO 4 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. in 4 m HClO 4 . No maxima are observed for the reduction of HPA-1, HPA-2, and salts of HPA-3. For HPA-3 salts, -β r H 4j = It is very important to exclude any direct contact between sia