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+)][PO43-])) at zero ionic strength and 25 degrees 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 Ce (log K-sp(0)(La) - log K-sp(0)(Ce) approximate to 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. Copyright (C) 1997 Elsevier Science Ltd.