Influence of conditions on the potential of the couple of nonoxygenated U⁵⁺/U⁴⁺ ions bound in complexes with the P₂W₁₇O ¹⁰⁻₆₁ and SiW₁₁O ⁸⁻₃₉ anions

Abstract

The formal potential of the couple of nonoxygenated U⁵⁺/U⁴⁺ ions bound in complexes with unsaturated heteropoly anions (HPAs) P₂W₁₇O ¹⁰⁻₆₁ (I) and SiW₁₁ ⁸⁻_O39 (II) in 0–1 M NaNO₃ and 1 M (NaCl + HCl) in the range of pH 0.7–4.7 was measured. In 1 M NaNO₃ solutions at pH 4.7–3.0 for I and 4.3–3.9 for II, the formal potentials are constant: 0.820 and 0.730 V, respectively. They preserve approximately the same value with a decrease in pH to 0.7 in 1 M (NaCl + HCl). The potential noticeably decreases with a decrease in the NaNO₃ or NaCl concentration from 1 M to 0 (pH 4.1–4.7): to 0.09 and 0.05–0.06 V for I and II, respectively. Approximate constancy of the potential of the U⁵⁺/U⁴⁺ couple with a decrease in pH to 1 and lower distinguishes this couple from the M⁴⁺/M³⁺ couples (M = Ce, Am, Bk) whose potential appreciably grows with increasing acidity. This is due to the fact that the U⁵⁺ and M⁴⁺ ions in acid solutions remain in the form of complexes with the ratio M: HPA = 1: 2, whereas the M³⁺ ions pass into the form of 1: 1 complexes. Thus, variation of the formal potentials of all the M^{n + 1}/Mⁿ⁺ couples in the presence of H⁺ and Na⁺ ions is associated with variation of the stability constants of the complexes M(HPA)₂, which, in turn, is caused by interaction of single-charged ions with HPA. However, the H⁺ and Na⁺ ions interact with HPA by different mechanisms and therefore affect the potential of the U⁵⁺/U⁴⁺ couple differently.