Thermodynamics of the LiCl + H2O System

Literature data for the solubility of lithium chloride salts (anhydrous LiCl, LiCl·H2O, LiCl·2H2O, LiCl·3H2O, and LiCl·5H2O) in pure water have been compiled and critically evaluated. These data have been represented by empirical temperature−molality expressions from which the coordinates of the eutectics and of the peritectics have been calculated. The thermodynamic properties of the LiCl salts have been calculated from their solubilities in pure water using two different models of aqueous LiCl solutions (Pitzer's ion interaction model and the mean spherical approximation model) which allow the calculation of the activity of water and of the LiCl(aq) activity coefficient to very low temperatures (199 K) and/or very high concentrations (up to 30 M), characteristic of the LiCl + H2O system. The water−ice equilibrium constant has been determined to 199 K. Results of the Pitzer−Holmes−Mesmer ion interaction model are reliable only for LiCl molalities below 11 M. At higher molalities (corresponding to the solubilities of LiCl·2H2O(s), of LiCl·H2O(s), and of anhydrous LiCl for temperatures between 273 and 433 K), the mean spherical approximation model has been used. Entropies and standard enthalpies of formation of the various solids have been retrieved from fits of their solubility products with respect to temperature. Our values are in good agreement with the NBS values. There is a linear correlation between the entropies and standard enthalpies of formation and the number of water molecules in the LiCl hydrates, as already reported for MgCl2·nH2O, MgSO4·nH2O, and Na2CO3·nH2O.