Computer Coupling of Phase Diagrams and Thermochemistry 27 (2003) 213–219 C A L P H A D 1 9 7 3 www.elsevier.com/locate/calphad The thermodynamic study of the system LiCl–KCl–H 2 O at the temperature 298.15 K Mohamed El Guendouzi ∗ , Asmaa Benbiyi, Abderrahim Dinane, Rachid Azougen Laboratoire de Chimie Physique, D´ epartement de Chimie, Facult´ e des Sciences, Ben M’sik, Universit´ e Hassan II Mohammedia, B.P 7955, Casablanca, Morocco Received 14 July 2003 Abstract The mixture electrolyte {yLiCl + (1 - y)KCl}(aq) has been studied by the hygrometric method at 298.15 K. The water activities are measured at total molalities from 0.3 mol kg -1 to about saturation for different ionic-strength fractions y of LiCl with y = 0.33, 0.50, and 0.67. The results allow the deduction of osmotic coefficients. The experimental results are compared with the predictions of the Zdanovskii–Stokes–Robinson, Kusik and Meissner, Robinson and Stokes, Lietzke and Stoughton, Reilly–Wood and Robinson, and Pitzer models. From these measurements, the new Pitzer mixing ionic parameters are determined and used to predict the solute activity coefficients in the mixture. The excess Gibbs energy is also determined. © 2003 Elsevier Ltd. All rights reserved. 1. Introduction The behaviour of aqueous solutions of the electrolyte is interesting because of their importance in chemical [1, 2], process engineering [3–5], biological [6], and process atmospheric [7–9] areas. Free energy values, species distribution, water activity, activity and osmotic coefficient, are of fundamental importance to the understanding and the prediction of behaviour in aqueous multicomponent systems. In this work, the thermodynamic properties for the mixed solutions of lithium and potassium chlorides have been investigated at the temperature 298.15 K, by the hygrometric method. This method has been used for determining thermodynamic properties of binary aqueous chlorides [10], nitrates [11], and ternary mixed electrolytes [12, 13]. The main objectives are to determine thermodynamic properties for the ternary solutions of {yLiCl + (1 - y)KCl}(aq) by this method. The measurements of water activities for total molality range of 0.3 mol kg -1 until the saturation of one of the solutes was performed for the ternary system, of different ionic-strength fraction y of LiCl ranging from 0.33, 0.50, and 0.67 at a temperature of 298.15 K. The osmotic ∗ Corresponding author. E-mail address: elguendouzi@yahoo.fr (M.El Guendouzi). coefficients are also evaluated for these solutions from the water activities. The obtained results are compared with data from other commonly used thermodynamic models Zdanovskii– Stokes–Robinson (ZSR), Kusik and Meissner (KM), Robin- son and Stokes (RS), Lietzke and Stoughton (LSII), Reilly– Wood and Robinson (RWR), and Pitzer. The experimental data are used to determine the solute activity coefficients using the Pitzer model with our newly obtained ionic mixing parameters. The results are used to calculate the excess Gibbs energy. 2. Experimental The water activity was determined by a hygrometric method previously described in our earlier work [14]. The apparatus used in this study of mixed electrolyte solutions is essentially the same as that used for the investigation of aqueous single electrolyte solutions. It is based on the measurement of the relative humidity over aqueous solution containing non-volatile electrolytes. The apparatus used is a hygrometer in which a droplet of salt solution is maintained on a thin thread (Fig. 1). The diameter measurement of the previously calibrated droplet permits therefore the knowledge of the relative humidity of aqueous solutions. 0364-5916/$ - see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.calphad.2003.08.001