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 ionic solid solutions: III. An addendum

In two earlier papers (C.A. 117:259320(1;121:19411y) the activity coefficients of the salts in binary solid solutions at 25‡C for 38 salt pairs, in which the members of each pair differ with respect to only one kind of ion, were determined. While the activity data are correct, the conclusions regarding deviations from ideality for eight of these pairs, namely those in which there are two moles of replaceable ion per mole of salt, require modification in order to be consistent with ideal entropies of mixing. By changing the formulation of the component salts to one-half of what is usual, the inconsistencies disappear. This half-mole approach, applied to the salt pairs CU_1/2(NH₄/K)SO₄-3H₂O, Mg_1/2NH₄(SO₄/ CrO₄)-3H₂O, Mg_1/2NH₄(SeO₄/SO₄)-3H₄O, Mg_1/2NH₄(SeO₄/CrO₄)-3H₄O, Mg_1/2 (K/NH₄)SeO₄-3H₂O, (NH₄/K)(SO₄)_1/2, and Ba_1/2(ClO₃/BrO₃)-1/2 H₂O shows that these solid solutions exhibit positive, not negative, deviations from ideality at 25‡C. Only the system Pb_1/2(C1/Br) still deviates negatively.