Solubilities, solubility products and solution chemistry of lanthanon trifluoride–water systems

Abstract

Solubilities and solubility products of 14 lanthanon trifluorides (LnF₃· 0.5H₂O) have been determined using conductometric, potentiometric and radiometric techniques. The solubilities and the averages of selected pK_sp values from literature and from our own work, that do not differ by more than one unit, show three maxima when plotted against the atomic number of lanthanons. Standard enthalpy change measured from the temperature-dependence plots of K_sp and the free-energy change calculated from K_sp of the trifluorides fall within the range 30–60 kJ mol^–1 and 78–103 kJ mol^–1, respectively, in most cases. Standard entropy change calculated from the enthalpy and free energy changes ranges from –47 to –294 J K^–1 mol^–1. The stability constants determined for the monofluoride complexes fall within the range of reported values, but those for the difluoride complexes are much higher than the fewer values that are found in literature. Lanthanon trifluorides were found to be least soluble in water at pH 3–4.