Thermodynamic Model for the Solubility of NdF3(cr) in the Na+–NH4+–Nd3+–F−–H2O System at 25 °C

The major objective of this study, based on critical review and experimental studies, was to develop a reliable thermodynamic model for the Nd–F system at 25 °C. The SIT model was used to convert concentration constants reported in the literature to constants at zero ionic strengths for cross comparison and selection of reliable values. The critically evaluated thermodynamic constants for the formation of NdF2+ and NdF 2 + were then used to interpret the extensive NdF3(cr) solubility data in NaF and NH4F solutions, ranging in concentrations from extremely low values to as high as 1.0 mol·kg−1, equilibrated for different periods ranging up to as long as 72 days. These efforts have resulted in \( \log_{10} \beta_{n}^{0} \) for the reaction [Nd3+ + nF− ⇌ NdF n 3−n ] of (3.81 ± 0.10), (5.89 ± 0.77), and <12.48 for n values of 1–3, respectively. The \( \log_{10} K_{\text{sp}}^{0} \) for the solubility of NdF3(cr) (NdF3(cr) ⇌ Nd3+ + 3F−) was determined to be (−20.49 ± 0.37). Because (1) Nd is an excellent analog for trivalent actinides—An(III) (i.e., Pu(III), Am(III), and Cm(III)), and (2) the available data for the An(III)–F system, especially the solubility products of AnF3(cr), are of extremely poor quality, the critical literature review in combination with the experimental Nd–F system data have been used to assign thermodynamic constants for the An(III)–F reactions until good quality specific data for them becomes available.