Solubility products and other parameters for the samarium and gadolinium fluoride-water systems by radiometric and other methods

The solubilities, solubility products and thermodynamic functions for the dissolution of SmF30.5H2O(s) and GdF30.5H2O(s) in water at 250C have been measured using radiometric, conductometric and potentiometric methods for comparison with literature values that differ among themselves by a factor of 103. The radiometric value for KSO is the lowest in both cases. However, all of the pKSO values which were computed using free ion concentrations are in general agreement with each other. The effect of the aging of the precipitate in the mother liquor and of the nature of the precipitant on solubility may explain the huge difference in solubility products reported in the literature. The standard enthalpy change and the free energy change for the disssolution of both fluorides are positive as is the case for other lanthanon fluorides. Standard entropy changes computed from the standard enthalpy and standard free energy changes at 250C, on the other hand, were negative in both cases. The stability constants for the mono- and difluoride complexes of samarium and gadolinium have also been determined using a simplified potentiometric method. (author)