STANDARD THERMODYNAMIC PROPERTIES AND HEAT CAPACITY EQUATIONS OF RARE EARTH HYDROXIDES: II. CE(III)-, PR-, SM-, EU(III)-, GD-, TB-, DY-, HO-, ER-, TM-, YB-, AND Y-HYDROXIDES. COMPARISON OF THERMOCHEMICAL AND SOLUBILITY DATA

Abstract

Standard thermodynamic properties at 298.15 K, 1 bar for the solid hydroxides of Y, Ce(III), Pr, Sm, Eu(III), Gd, Tb, Dy, Ho, Er, Tm and Yb were calculated from thermochemical data available in the literature. `3Rn' heat capacity equations [Fei, Y., Saxena, S.K., 1987. An equation for the heat capacity of solids. Geochim. Cosmochim. Acta 51, 251–254.] were generated for Y-, Pr-, Eu(III)-, Gd-, Tb-, and Ho-hydroxides from low-temperature heat capacity measurements. Standard entropy and enthalpy of formation were calculated using literature data and correlations developed by Bratsch and Lagowski [Bratsch, S.G., Lagowski, J.J., 1985. Lanthanide thermodynamic predictions: 6. Thermodynamics of gas-phase ions and revised enthalpy for solids at 298.15 K. J. Phys. Chem. 89, 3310–3316]. Critically assessed literature data on solubility products for freshly precipitated and slightly aged, and aged-precipitated and well-crystalline Ln-hydroxides (data for the La- and Nd-hydroxides from Diakonov et al. [Diakonov, I.I., Tagirov, B., Ragnarsdottir, K.V., 1997. Standard thermodynamic properties and heat capacity equations of rare earth hydroxides: I. La(OH)3(s) and Nd(OH)3(s). Comparison of thermochemical and solubility data. Radiochim. Acta, in press.]) showed that (a) the former are approximately two orders of magnitude more soluble, and (b) the solubility products (Ks10b) for aged-precipitated and well-crystalline Ln-hydroxides decrease sharply from 10−22.29 for La-hydroxide to 10−25.98 for Nd-hydroxide and then changes gradually to 10−26.99 for Lu-hydroxide. Experimental solubility products for aged (well-crystalline) Pr-, Nd-, Sm-, Eu-hydroxides at 298.15 K, 1 bar were found to be up to three orders of magnitude less compared to those calculated from thermodynamic data for hydroxides from this study and for aqueous lanthanide ions accepted in the literature [Morss, L.R., 1994. Comparative thermochemical and oxidation–reduction properties of lanthanides and actinides. In: Gschneider, K.A., Jr., Eyring, L., Choppin, G.R., Lander, G.H. (Eds.), Handbook of Physics and Chemistry of Rare Earths. Elsevier, 18, 239–291.]. Consequently, these REE are less mobile in natural and technological aqueous fluids than previously predicted.