Critical Evaluation of the Thermodynamic Properties of Aqueous Calcium Chloride. 1. Osmotic and Activity Coefficients of 0−10.77 mol·kg-1 Aqueous Calcium Chloride Solutions at 298.15 K and Correlation with Extended Pitzer Ion-Interaction Models

Abstract

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 the accurate characterization of the osmotic coefficients near and above saturation. Published isopiestic molalities, direct vapor pressures, and emf results for CaCl2(aq) have been critically reevaluated and recalculated in an internally consistent manner. This critically-assessed database was used to evaluate the parameters of Pitzer's equations and various extended forms at 298.15 K and 0.1 MPa. Neither the standard equations nor empirically extended versions were able to represent the thermodynamic results over more than part of the molality range without 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 pairs was included explicitly and (2) higher-order virial terms were included. One such quantitative model is presented here in detail. Without these higher-order virial terms the same model is able to represent the φ(CaCl2) fairly well only to about 8 mol·kg-1. The osmotic coefficient of CaCl2(aq) goes through a minimum around 0.11 mol·kg-1, which is followed by a regular increase with molality to a broad maximum in the supersaturated molality region where φ(CaCl2) is nearly constant at 3.169−3.173 from about (8.5 to 9.5) mol·kg-1. It then decreases slightly by about 0.3% at higher molalities.