The influence of isosaccharinic acid on the solubility of Np(IV) hydrous oxide

Several sets of solubility experiments were conducted to determine the infl uence of isosaccharinic acid (ISA) on the solubility of NpO2(am). The solub ility of NpO2(am) was measured either as a function of pH (ranging from 2 t o 8) at fixed ISA concentrations (0.008 M and 0.0016 M) or as a function of the concentration of ISA (ranging from 0.00008 M to 0.94 M) at a fixed pH (about 5). We observed that the presence of ISA significantly increased the solubility of NpO2(am) in the pH region under study. The increase in the s olubility of NpO2(am) was interpreted in terms of the formation of Np(IV) h ydroxy isosaccharinate complexes Np(OH)(3)ISA(aq) and Np(OH)(2)(ISA)(2)(aq) . Using Pitzer's ion-interaction approach, the logarithm of the formation c onstants were calculated to be 43.2 for Np4+ + 3OH(-) + ISA(-) reversible a rrow Np(OH)(3)ISA(aq) and 36.9 for Np4+ + 2OH(-) + 2ISA(-) reversible arrow Np(OH)(2)(ISA)(2)(aq).