Solubility products of the trivalent rare-earth phosphates

This paper reports on the solubility and solubility products of 14 rare-earth phosphates in aqueous solution that have been determined at 25 {plus minus} 1{degree} C. The most soluble compound is yttrium phosphate (pK = 24.76). The least soluble ones are lanthanum phosphate (pK = 26.15) and ytterbium phosphate (pK = 26.17). The rate-earth phosphates were prepared by precipitation from homogeneous solution. Saturated solutions were analyzed for pH, the concentrations of some rare-earth cations, and the total concentration of phosphate. Activity products were calculated from the experimental data. The solubilities of the rare-earth phosphates are so low that the solubility products and the activity products are essentially the same value. The rare-earth phosphates are more insoluble at higher temperatures. Lanthanum phosphate has a pK of 26.49 at 70{degrees} C. Twelve values for the activity product of rare-earth phosphates at elevated temperatures were determined.