**1.2.3 Solubility prediction for the phase equilibria of salt-water systems**

Pitzer and co-workers have developed an ion interaction model and published a series of papers (Pitzer, 1973a-b, 1974a-b, 1975, 1977, 1995, 2000; Pabalan & Pitzer, 1987) which gave a set of expressions for osmotic coefficients of the solution and mean activity coefficient of electrolytes in the solution. Expressions of the chemical equilibrium model for conventional single ion activity coefficients derived are more convenient to use in solubility calculations (Harvie & Weare, 1980; Harvie et al.1984; Felmy & Weare, 1986; Donad & Kean, 1985).

In this chapter, as an example, the stable and metastable phase equilibria in the salt-water system (NaCl - KCl – Na2B4O7 - K2B4O7 - H2O), which is of great importance to describe the metastable behavior in order to separate and purify the mixture salts of borax and halosylvite were introduced in detail. The stable phase diagrams of the sub-ternary systems (NaCl - Na2B4O7 - H2O), (KCl –K2B4O7 - H2O), (Na2B4O7 - K2B4O7 - H2O) at 298.15 K and the metastable phase diagrams of the sub-ternary systems (NaCl - Na2B4O7 - H2O) at 308.15 K for the mentioned reciprocal quaternary system were systematically studied on our previous researches under several scientific funding supports. The theoretical prediction for the stable solubility of this reciprocal quaternary system was also briefly introduced based on the ion-interaction model.
