Thermodynamic study of the co-crystallization of ammonium, sodium and potassium alums and chromium alums

The Pitzer binary and mixing ion-interaction parameters determined in previous studies of the author were used for determining the thermodynamic characteristics of the formation process of mixed crystals of the ammonium and potassium alums, ammonium and potassium chromium alums, and of the ammonium alum and ammonium chromium alum. The ternary K2SO4–Cr2(SO4)3–H2O system has been re-parameterized using a different experimental value of the pure water solubility of the potassium chromium alum. The component activities of the saturated ternary (NH4)2SO4Al2(SO4)3–K2SO4Al2(SO4)3–H2O, (NH4)2SO4Cr2(SO4)3–K2SO4Cr2(SO4)3–H2O and (NH4)2SO4Al2(SO4)3–(NH4)2SO4Cr2(SO4)3–H2O solutions and in the mixed crystalline phase were determined and the change of the molar Gibbs energy of mixing ΔmixGmo(s) of crystals was determined as a function of the solid phase composition. It was established that at T=298.15 K, the mixed (K,NH4)2SO4·Al2(SO4)3·24H2O, (K,NH4)2SO4·Cr2(SO4)3·24H2O, and (NH4)2SO4·(Al,Cr)2(SO4)3·24H2O crystals show negative deviations from ideal mixed crystals. The basic Pitzer ion-interaction model was used for solubility calculations in ternary (NH4)2SO4Al2(SO4)3–Na2SO4Al2(SO4)3–H2O, and in quaternary (NH4)2SO4–Na2SO4–Al2(SO4)3–H2O systems, from which phases with constant stoichiometric composition (simple and double salts) crystallize. Good agreement between model calculations and experiment was found. The model predictions presented in this paper provide another step for the validation of the binary- and ternary-mixing solution parameters of our aluminum- and chromium-sulfate model.