ISOPIESTIC DETERMINATION OF THE OSMOTIC COEFFICIENTS OF LU-2(SO4)(3)(AQ) AND H2SO4(AQ) AT THE TEMPERATURE T=298.15 K, AND REVIEW AND REVISION OF THE THERMODYNAMIC PROPERTIES OF LU-2(SO4)(3)(AQ) AND LU-2(SO4)(3)CENTER-DOT-8H(2)O(CR)

Isopiestic vapor-pressure measurements were made for Lu-2(SO4)(3)(aq) at molalities from (0.35379 to 0.66478) mol . kg-(1) and for H2SO4(aq) at molalities from (0.27470 to 0.70512) mol . kg(-1) at the temperatu re T = 298.15 K using KCl(aq) as the isopiestic reference standard; re sults for Lu-2(SO4)(3)(aq) extend to slightly above saturation. These measurements encompass the water activity ranges a(w) of 0.9902 less t han or equal to a(w) less than or equal to 0.9783 and 0.9902 less than or equal to a(w) less than or equal to 0.9741, respectively. Molality analyses used for previous solubility, density (J. Solution Chem. 198 8, 17, 499-517), and isopiestic measurements (J. Solution Chem. 1990, 19, 525-541) for Lu-2(SO4)(3)(aq) from this laboratory were found to b e slightly high due to absorption of water vapor during the analyses, as was that of the NaCl(aq) isopiestic reference standard from incompl ete drying during the dehydration analysis. Those results were correct ed and combined with the present isopiestic measurements to yield reco mmended values of osmotic coefficients, water activities, and mean act ivity coefficients of Lu-2(SO4)(3)(aq), and the solubility and thermod ynamic solubility product of Lu-2(SO4)(3) . 8H(2)O(cr), all at the tem perature T = 298.15 K. A detailed study was made of the extent of the retention of water by NaCl(s) during drying and of absorption of water vapor by Lu-2(SO4)(3)(s) and other anhydrous rare earth sulfates as t hey are weighed in air. Methods were developed for accurately correcti ng each total mass of rare earth sulfate for absorbed water. In additi on, isopiestic measurements were performed for H2SO4(aq) in the molali ty region where previous isopiestic results were found to be somewhat discordant in the critical review by Clegg et al. (J. Chem. Sec. Farad ay Trans. 1994, 90, 1875-1894). (C) 1996 Academic Press Limited