Thermodynamics of ionic solid solutions: II. Discontinuous series: A new treatment of existing distribution data

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, f _A and f _B , can be described by 2-suffix Margules equations (regular solutions), lnf _A =B_sx _B ² and lnf _B =B_sx _A ² to be partially miscible, B_s>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.     

Thermodynamics of ionic solid solutions. A new treatment of existing distribution data

Excess amounts of two isomorphous salts, which differ with respect to only one ion (the exchanging ions), added to water and equilibrated produce an aqueous solution of the two salts and a solid solution of one salt in the other. The ratio of the two salts in the liquid phase is, in general, different from that in the coexisting solid. Data for twenty-nine such systems, including pairs of double salts (alums and picromerites), and pairs of simple salts, at or near 25°C, have been reviewed. By making the plausible assumption that the activity coefficients of the exchanging ions in the liquid phase are equal, it has been possible to derive the activity coefficients of the salts in the solid phase in addition to the thermodynamic equilibrium constants for the distributions. The interpretation of the data is compared with that of an earlier paper which drew different conclusions and was based on what is shown to be an erroneous premise. Solid solutions of alums are found, for the most part, to be ideal or nearly so. Of nine picromerite solid solutions containing sulfate as the only anion, seven pairs deviate positively from ideality, one deviates negatively, and one shows both positive and negative deviations. Three other picromerite pairs involving exchange of sulfate, selenate, and chromate ions, show only negative deviations. For the pairs [(NH₄)₂,K₂]SO₄, (NH₄,K)Cl, Ag(Cl,Br), K(Br,Cl), Rb(Br,Cl), (Rb,K)Br, and (Rb,K)Cl the deviations are positive; those for Pb(Cl,Br)₂ are negative, and those for Ba(ClO₃)₂·H₂O are both positive and negative. Independent supporting evidence for some of the conclusions is presented.     

Thermodynamics of mixed electrolyte solutions. VI. An isopiestic study of a pseudo-ternary system: NaCl−KCl−MgCl2−H2O at 25°C

Activity coefficients for magnesium chloride in the aqueous pseudo-ternary system sodium chloride-potassium chloride-magnesium chloride were derived from isopiestic measurements at 25°C. The isopiestic data were treated by both McKay-Perring and Scatchard methods, and results obtained agree fairly well over the ionic strength range of 1–6. At constant ionic strength, the activity coefficient of MgCl₂ increased with addition of other salts. Interaction coefficients were obtained from Scatchard's and Friedman's formalisms. The excess free energy of mixing was calculated and compared with similar systems.     

Thermodynamics of mixed electrolyte solutions. VIII. An isopiestic study of the ternary system KCl−MgCl2−H2O at 25°C

The osmotic coefficients of aqueous solutions of mixtures of potassium and magnesium chlorides were derived from isopiestic measurements at 25°C. The isopiestic data were treated by the methods of both Scatchard and Friedman, and the results obtained agree very well over the ionic strength range of 1–5. Interaction coefficients were obtained from both formalisms. Excess free energies of mixing were calculated and compared with those of similar systems.