ISOPIESTIC DETERMINATION OF THE OSMOTIC AND ACTIVITY-COEFFICIENTS OF AQUEOUS LU2(SO4)3 AT 25-DEGREES-C

Osmotic coefficients have been measured for aqueous Lu2(SO4)3 solutions from 0.12402 to 0.89631 mol-kg-1 at 25°C by use of the isopiestic method; these measurements extend into the supersaturated molality region. Since there was a lack of activity data for Lu2(SO4)3 solutions at lower molalities, they were approximated by equating them to results for La2(SO4)3 from freezing temperature depression measurements. The combined osmotic coefficients were then used to derive mean molal activity coefficients for Lu2(SO4)3 solutions. The osmotic coefficients decrease to 0.307 as their minimum value and the mean molal activity coefficients decrease to 0.0069. When these activities were combined with our previously reported solubility of 0.6260±0.0017 mol-kg-1 for Lu2(SO4)3·8H2O, a thermodynamic solubility product of 2.3×10-10 was obtained. This value yields the Gibbs energy of formation ΔGf° (Lu2(SO4)3·8H2O, cr)=-5518.9±16.4 kJ-mol-1. © 1990 Plenum Publishing Corporation.