Interaction of U(VI), Np(VI), and Pu(VI) ions with unsaturated heteropolytungstates of the 17th and 11th series in aqueous solutions

The stability constants of the complexes of U(VI), Np(VI), and Pu(VI) with the heteropolyanions (HPAs) P₂W₁₇O ₆₁ ^10? , SiW₁₁O ₃₉ ^8? , and PW₁₁O ₃₉ ^7? in solutions with pH from ?0.3 (2 M H⁺) to 5–5.5 in the presence of Na or K salts (up to 2 M) and without them were measured. All the complexes have exclusively the 1: 1 composition; their stability constants β^M(VI) in neutral solutions at a low ionic strength are close to 10⁸ 1 mol^?1. In 0.1–2.0 M acid solutions, log β^M(VI) for the complexes with P₂W₁₇O ₆₁ ^10? is within 1.4–3.9. The slope of the pH dependence of log β^M(VI) does not exceed 1.75; this fact suggests that no more than two protons are displaced from HPA upon complexation in acid solutions. In the presence of 1–2 M sodium salts, the β^M(VI) values reach a maximum at pH ～3 and drastically decrease with a further increase in pH. Actinides(VI) interact with HPAs appreciably more weakly than do actinides(III), which is apparently due to the fact that the denticity of HPAs in the complexes with An(VI), apparently, does not exceed 2.