Aqueous solubilities of praseodymium,europium, and lutetium sulfates

The aqueous solubilities of finely divided Pr₂(SO₄)₃·8H₂O(cr), Eu₂(SO₄)₃·8H₂O(cr), and Lu₂(SO₄)₃·8H₂O(cr) have been measured as a function of time at 25°C using isothermal saturation. Solubilities of the latter two salts showed a steady decrease with time, whereas Pr₂(SO₄)₃·8H₂O(cr) showed no such variation within the accuracy of the determinations. The turbidities of these filtered saturated solutions also decreased with time, and indicate that some colloidal rare earth sulfates were present. These colloidal particles (<0.2 m) have a large surface area, which contributes to the Gibbs energy of the solid phase, thus giving rise to enhanced solubilities. The micro-particles also grow with time, thereby reducing the surface area contribution to the Gibbs energy and also leaving fewer particles to pass through the filters. Extrapolation of solubilities to infinite time gives the solubilities of macrocurstalline Eu₂(SO₄)₃·8H₂O and Lu₂(SO₄)₃·8H₂O. Previous solubility data for Lu₂(SO₄)₃, at 20 and 40°C, yield an interpolated value at 25°C that is about 30% low. Densities were also measured at several concentrations of each salt.