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

Standard thermodynamic properties at 298.15 K, 1 bar for the solid hyd roxides 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 equatio n for the heat capacity of solids. Geochim. Cosmochim. Acta 51, 251-25 4.] 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 correl ations developed by Bratsch and Lagowski [Bratsch, S.G., Lagowski, J.J ., 1985. Lanthanide thermodynamic predictions: 6. Thermodynamics of ga s-phase ions and revised enthalpy for solids at 298.15 K. J. Phys. Che m. 89, 3310-3316]. Critically assessed literature data on solubility p roducts for freshly precipitated and slightly aged, and aged-precipita ted and well-crystalline Ln-hydroxides (data for the La- and Nd-hydrox ides from Diakonov et al. [Diakonov, I.I., Tagirov, B., Ragnarsdottir, K.V., 1997. Standard thermodynamic properties and heat capacity equat ions of rare earth hydroxides: I. La(OH)(3)(s) and Nd(OH)(3)(s). Compa rison of thermochemical and solubility data. Radiochim. Acta, in press .]) showed that (a) the former are approximately two orders of magnitu de more soluble, and (b) the solubility products (K-s10(b)) for aged-p recipitated and well-crystalline Ln-hydroxides decrease sharply from 1 0(-22.29) for La-hydroxide to 10(-25.98) for Nd-hydroxide and then cha nges 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 fr om this study and for aqueous lanthanide ions accepted in the literatu re [Morss, L.R., 1994. Comparative thermochemical and oxidation-reduct ion 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, t hese REE are less mobile in natural and technological aqueous fluids t han previously predicted. (C) 1998 Elsevier Science B.V. All rights re served.