Revision of the Osmotic Coefficients, Water Activities and Mean Activity Coefficients of the Aqueous Trivalent Rare Earth Chlorides at T = 298.15 K

The analysis by Wang et al. (J. Solution Chem. 35:1137–1156, 2006 ) of the osmotic and activity coefficients of fourteen of the aqueous rare earth trichlorides at the temperature 298.15 K is based mainly on the isopiestic results reported by Spedding et al. (J. Chem. Eng. Data 21:341–360, 1976 ) and Rard and Spedding (J. Chem. Eng. Data 27:454–461, 1982 ), but corrected for more recent critical reviews of the osmotic coefficients of the KCl(aq) and CaCl 2 (aq) reference solutions used in these measurements. Also included were the potential difference (emf) measurements by Malatesta et al. (Phys. Chem. Chem. Phys. 4:121–126, 2002 ) for LaCl 3 (aq) using cells with a liquid ion-exchange membrane electrode and earlier results for concentration cells with transport for the rare earth trichlorides. Our reexamination of osmotic coefficients calculated from these potential differences of cells with a liquid ion-exchange electrode indicates their values are skewed relative to osmotic coefficients obtained more directly from the isopiestic measurements, being significant higher for molalities between (0.025 and 0.3) mol·kg −1 and significantly lower at higher molalities. As a consequence, the combined results yielded mean, molality-based, activity coefficients that are systematically too large for the light and intermediate rare earths. Although restricted to a narrower molality region of about (0.02–0.04) mol·kg −1 , potential difference measurements for aqueous rare earth trichlorides from concentration cells with AgCl(s) electrodes appear to yield more reliable results. The extended ion-interaction model parameters of Wang et al. have been reevaluated using the cited isopiestic results together with osmotic coefficients derived from the concentration cells with transport as reported by Spedding et al. (J. Chem. Eng. Data 21:341–360, 1976 ). Mean activity coefficients calculated using these new model parameters should be more accurate than those previously reported.