Isopiestic determination of the osmotic and activity coefficients of aqueous Lu2(SO4)3 at 25°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 G_f^° (Lu₂(SO₄)₃·8H₂O, cr)=–5518.9±16.4 kJ-mol^–1.