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 conc entrations, from 0.001 M to 1.0 M, and over equilibration periods exte nding to almost three months. These solubility data show that at aqueo us fluoride concentrations of less-than-or-equal-to 0.01 M the equilib rium solid phase is a hydrated ThF4 . nH2O(c) Compound. At higher diss olved fluoride concentrations a series of ThF4 salts form, including N aF . ThF4 . H2O(c) and possibly 3NaF . ThF4(S) in NaF solutions, and N H4F . ThF4(s) in aqueous NH4F. The solubility data indicate that, as e xpected from previous studies, ThF3+ and ThF4(aq) are the dominant aqu eous Th(IV) species, at least from 0.0001 M to 0.003 M dissolved fluor ide concentration. However, at higher dissolved fluoride concentration s the solubility data support the existence of anionic Th - F complex species including ThF5- and ThF62-, for which there are no available t hermodynamic data. Values are proposed for the standard chemical poten tials of these anionic Th - F complexes and the equilibrium solid phas es. The Pitzer ion-interaction parameters necessary to accurately desc ribe these solubility data are also included.