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

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(log₁₀ K_s0 (298 K), 0.95 level of confidence) were evaluated to log₁₀ K_s0 (TPHPH, 298 K) = (71.2 ±1.3), while the Δ_f⁰G (TPHPH, 298 K) value reached (5200 ± 23) kJmol^-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: Δ_fH⁰ (TPHPH) =-(5640.5 ± 16.0) kJ mol^-1 and Sm = (298 ± 68) J K^-1mol^-1, respectively. (authors)