SOLID-PHASE PRECIPITATES AND ANIONIC AQUEOUS THORIUM FLUORIDE COMPLEXES IN THE NA-NH4-TH-F-H2O SYSTEM TO HIGH-CONCENTRATION

The solubility of thorium(IV) fluoride compounds in aqueous solutions of NaF and NH4F was studied over a wide range of aqueous fluoride concentrations, from 0.001 M to 1.0 M, and over equilibration periods extending to almost three months. These solubility data show that at aqueous fluoride concentrations of less-than-or-equal-to 0.01 M the equilibrium solid phase is a hydrated ThF4 . nH2O(c) Compound. At higher dissolved fluoride concentrations a series of ThF4 salts form, including NaF . ThF4 . H2O(c) and possibly 3NaF . ThF4(S) in NaF solutions, and NH4F . ThF4(s) in aqueous NH4F. The solubility data indicate that, as expected from previous studies, ThF3+ and ThF4(aq) are the dominant aqueous Th(IV) species, at least from 0.0001 M to 0.003 M dissolved fluoride concentration. However, at higher dissolved fluoride concentrations the solubility data support the existence of anionic Th - F complex species including ThF5- and ThF62-, for which there are no available thermodynamic data. Values are proposed for the standard chemical potentials of these anionic Th - F complexes and the equilibrium solid phases. The Pitzer ion-interaction parameters necessary to accurately describe these solubility data are also included.