Journal of Chemical Thermodynamics, Vol.35, No.11, 1775-1792, 2003
Thermodynamics of formation of double salts M2SO4 center dot M' SO4 center dot 6H(2)O and M2SeO4 center dot M' SeO4 center dot 6H(2)O where M denotes Rb, or Cs and M' denote Co, Ni, or Zn
This paper describes a chemical model that calculates (solid + liquid) equilibria in the (m(1) Rb2SO4 + M2CoSO4)(aq), (m(1)Rb(2)SeO(4) + m(2)CoSeO(4))(aq), (m(1)Rb(2)SO(4) + M2NiSO4)(aq), (m(1)Rb(2)SO(4) +m(2)ZnSO(4))(aq), (m(1)Rb(2)SeO(4) + m(2)ZnSeO(4))(aq), (m(1)Cs(2)SO(4) +m(2)CoSO(4))(aq), (m(1)Cs(2)SeO(4) + m(2)CoSeO(4))(aq), (m(1)Cs(2)SO(4)+m(2)NiSO(4))(aq), (m(1)Cs(2)SeO(4)+M2NiSeO4)(aq), (m(1)Cs(2)SO(4)+m(2)ZnSO(4))(aq), and (m(1)Cs(2)SeO(4)+m(2)ZnSeO(4))(aq) systems, where m denotes molality at the temperature T = 298.15 K. The Pitzer ion-interaction model has been used for thermodynamic analysis of the experimental osmotic and solubility data presented in the literature. The thermodynamic functions needed (binary and ternary parameters of ionic interaction, thermodynamic solubility products) have been calculated and the theoretical solubility isotherm has been plotted. The mixing parameters {theta(MN) and psi(MNX)} have been chosen on the basis of the compositions of saturated ternary solutions and data on the binary solubility of the sulfate M2SO4.M'SO4.6H(2)O double salts in water. To validate the mixing solutions model two different approaches have been used in evaluation of the ternary parameters: (1) preserving the same value of the binary mixing theta(MN) for the corresponding chloride, bromide, sulfate, and selenate systems with the same cations, and (11) with constant theta(MN) value (set equal to -0.05) for the all 11 sulfate and selenate systems. Very good agreement between experimentally determined and model predicted solubilities has been found. Important thermodynamic characteristics (thermodynamic solubility products, standard molar Gibbs free energy of formation) of the solid phases (simple salts, six sulfate -M2SO4 . M'SO4.6H(2)O, and five selenate M2SeO4 . M'SeO4.6H(2)O - double salts) crystallizing in the systems under consideration are determined. (C) 2003 Elsevier Ltd. All rights reserved.
Keywords:pitzer model;solubility diagram;rubidium;and cesium sulfate and selenate;cobalt;nickel;and zinc sulfate and selenate;M2SO4 center dot M ' SO4 center dot 6H(2)O- and M2SeO4 center dot M ' SeO4 center dot nH(2)O-double salts;Gibbs free energy