CRITICAL-EVALUATION OF THE THERMODYNAMIC PROPERTIES OF AQUEOUS CALCIUM-CHLORIDE .1. OSMOTIC AND ACTIVITY-COEFFICIENTS OF 0-10.77 MOL-CENTER-DOT-KG(-1) AQUEOUS CALCIUM-CHLORIDE SOLUTIONS AT 298.15-K AND CORRELATION WITH EXTENDED PITZER ION-INTERACTION MODELS

Isopiestic vapor-pressure measurements were performed at 298.15 K for CaCl2(aq) solutions at 66 molalities from (4.3235 to 10.253) mol.kg(-1 ), using H2SO4(aq) as the reference standard, with emphasis given to t he accurate characterization of the osmotic coefficients near and abov e saturation. Published isopiestic molalities, direct vapor pressures, and emf results for CaCl2(aq) have been critically reevaluated and re calculated in an internally consistent manner. This critically-assesse d database was used to evaluate the parameters of Fitter's equations a nd various extended forms at 298.15 K and 0.1 MPa. Neither the standar d equations nor empirically extended versions were able to represent t he thermodynamic results over more than part of the molality range wit hout large cyclic systematic deviations. It was possible, however, to obtain essentially quantitative agreement between experiment and model over the full molality range if (1) the presence of CaCl+(aq) ion pai rs was included explicitly and (2) higher order virial terms were incl uded. One such quantitative model is presented here in detail. Without these higher-order virial terms the same model is able to represent t he phi(CaCl2) fairly well only to about 8 mol.kg(-1). The osmotic coef ficient of CaCl2(aq) goes through a minimum around 0.11 mol.kg(-1), wh ich is followed by a regular increase with molality to a broad maximum in the supersaturated molality region where phi(CaCl2) is nearly cons tant at 3.169-3.173 from about (8.5 to 9.5) mol.kg(-1). It then decrea ses slightly by about 0.3% at higher molalities.