MODELING OF THERMODYNAMIC PROPERTIES AND PHASE EQUILIBRIUM IN SYSTEM OF LiCl - H2O WITHIN FRAME OF PITZER MODEL

Lithium chloride is used as a solid electrolyte in chemical sources of current, organic synthesis, pyrotechnics and as a desiccant. To describe the thermodynamic properties of aqueous electrolyte solutions, the Pitzer's model is most widely used. Model equations are simple and contain a small number of variable parameters that have a certain physical meaning. They are linear combinations of virial coefficients in the expansion of the excess free Gibbs energy of the solution of the number of moles of ions. The results of thermodynamic modeling of the LiCl - H2O system within the frame of this model are presented in the article. Equations for calculating the average ionic coefficient of activity for the component of the LiCl system and the osmotic coefficient of water are derived. The energy parameters of the model are calculated taking into account the experimental data on the osmotic coefficient of water. on the composition of the solution at 25 degrees C and on the temperature for the LiCl - H2O system. To verify the adequacy of the Pitzer's model, the inverse problem was solved, i.e. the values of the osmotic coefficient of water and the average ionic coefficient of the solution composition were calculated. The calculated values are in good agreement with the experimental data. The values of the standard Gibbs energies for the formation of crystalline hydrates LiC center dot H2O, LiCl center dot 2H(2)O are determined. A double phase diagram of LiCl - H2O is constructed. On the calculated diagram, there is a eutectic point with coordinates of 25.3 wt.% LiCl, t = -80 degrees C and two peritectics of 48.2 wt. % LiCl, t = 50 degrees C; 56.5 wt. % LiCl, t = 102 degrees C. The excess partial molar enthalpies and entropies of the solution components were calculated. As follows from the data obtained, aqueous solutions of lithium chloride are formed with heat absorption, the system is stabilized by increasing entropy. The solution experiences positive deviations from ideality.