Solubility product of the thorium phosphate hydrogen-phosphate hydrate (Th2(PO4)2(HPO4)·H2O, TPHPH)

- Pure thorium phosphate hydrogen-phosphate hydrate (TPHPH) was synthesized.
- Solubility experiments were performed under various conditions.
- The standard solubility constant was determined at several temperatures.
- The thermodynamic data of dissolution and formation of TPHPH were determined.
- The thermodynamic properties of thorium phosphate compounds need to be re-evaluated.

The standard solubility constant of TPHPH, Th2(PO4)2(HPO4)·H2O was evaluated at 298 K by performing dissolution experiments of a synthetic sample in different nitric acid solutions at a constant ionic strength of 1 mol L−1. The average value of the solubility constant and the associated expanded uncertainty U(log10Ks,0∘ (298 K), 0.95 level of confidence) were evaluated to log10Ks,0∘ (TPHPH, 298 K) = −(71.2 ± 1.3), while the ΔfG∘ (TPHPH, 298 K) value reached −(5200 ± 23) kJ·mol−1. From the experiments performed at different temperatures, the standard molar enthalpy as well as the standard molar entropy of the dissolution reaction was determined. The solubility of TPHPH was found to be retrograde over the investigated range of temperatures (298-363) K. The values of the standard enthalpy of formation and the standard molar entropy of TPHPH are equal to: ΔfH∘(TPHPH)=-(5640.5±16.0)kJ·mol-1 and Sm = (298 ± 68) J K−1 mol−1, respectively.