Solubility and thermodynamic properties in mixed ternary aqueous solutions of hexafluoridosilicate salts (Na+, K+ or NH4+) at T= 353.15 K

Abstract

The thermodynamic properties of ternary aqueous solutions of hexafluoridosilicate salts {y M2SiF6 + (1-y) N2SiF6 + H2O}; with (M; N) = Na+, K+ and NH4+ were investigated at 353.15 K. The water activity measurements at various ionic strength fractions (y) with y = 0.20, 0.50, and 0.80 of each ternary system were carried out from dilute to saturation of the solution using the hygrometric method. From these measurements, the related thermodynamic properties were evaluated by development of an aqueous ion-interaction model. The mixing parameters θ and ψ of the mixed solutions were determined and used with the binary parameters to evaluate the osmotic and activity coefficients at the temperature T = 353.15 K.The experimental solubilities of saturated solutions {yK2SiF6 + (1-y)Na2SiF6 + H2O}; {yK2SiF6 + (1-y)(NH4)2SiF6 + H2O} and {yNa2SiF6 + (1-y)(NH4)2SiF6 + H2O} were also performed at different ionic strength fractions y with y = 0.20, 0.50, and 0.80. The obtained solid phases were characterized by X-ray diffraction, and showed the crystal forms, for the system {yK2SiF6 + (1-y) Na2SiF6 + H2O} as a function of ionic-strength fractions y, (y = 0): Na2SiF6(s); (y = 0.20, 0.50, 0.80):Na2SiF6(s) + KNaSiF6(s) + K2SiF6(s); and (y = 1): K2SiF6(s). However, the ternary systems {yK2SiF6 + (1-y) (NH4)2SiF6 + H2O}, {yNa2SiF6 + (1-y) (NH4)2SiF6 + H2O} present only a single crystalline form at different ionic-strength fractions y (y = 0.20, 0.50, 0.80), K2SiF6(s) or Na2SiF6(s). The solubility of mixed systems are calculated and compared with those obtained experimentally.