Rare Earth and Yttrium Phosphate Solubilities in Aqueous Solution

Rare earth and yttrium phosphate solubility products range over more than 1 order of magnitude. Minimum solubilities are observed for light rare earths between Ce and Sm. For the elements Ce, Pr, Nd, and Sm solubility products (log K _sp ⁰ ( M ) = log ([ M _i ³⁺ ] [PO ₄ ³⁻ ])) at zero ionic strength and 25°C can be approximated as log K _sp ⁰ ,( M ) = −26.3 ± 0.2. Rare earth phosphate solubility products for well-aged, coarse precipitates increase substantially between Sm and Lu, with log K _sp ⁰ (Lu) estimated as −24.7. The solubility product of Y is similar to that of Ho (log K _sp ⁰ (Y) = −25.0) and is much higher than those of all light rare earths. The solubility product of La is substantially larger than that of Cc (log K _{sp ⁰} (La) − log K _sp ⁰ (Ce) ≈ 0.5). Solubility products are strongly dependent on the conditions of solid phase formation. Fresh precipitates are much more soluble than slowly formed, well-aged, coarse precipitates. The pattern of rare earth and yttrium phosphate solubility products is generally similar to the fractionation patterns which are developed during phosphate coprecipitation.