Abstract: | A method of finding the activity coefficients of salts, anhydrous or hydrated, in binary solid solutions, described in an earlier paper as it applies to continuous series, has been applied to discontinuous series. The salts must differ with respect to only one ion. The method requires isothermal distribution data for equilibria between liquid (aqueous) and solid solutions in the ternary system consisting of the two salts and water. The following salt pairs were used for illustration: K(I/Br) at 0, 15, 25, 35, and 50°C., (NH4/K)SCN at 0, 30, 60, and 90°C., (K/Tl)C103 at 10°C., and (NH4/K)SO3NH2, (NH4/K)Br, (Mg/Co)SO4-7H2O, and (Mn/Cu) SO4.n H2O-all at 25°C. Two kinds of behavior were noted and treated differently: systems in which the two series have the same, and those in which they have different crystal lattices. For two salts, A and B, which have the same lattice, and whose rational activity coefficients, fA and fB, can be described by 2-suffix Margules equations (regular solutions), lnfA=BsxB2 and lnfB=BsxA2 to be partially miscible, Bs>2, but this requirement does not apply if the lattices are different. In each series, distribution constants for the equilibria were also determined. Where possible, the calculated activities of the salts or the Gibbs excess energies of the solid solutions were compared with values reported by others who determined them by other methods. All the salt pairs studied show slight or strong positive deviations from ideality. |