Solubility of Mixed Ternary Systems of Hexafluoridosilicate Salts (M = Na+, K+, or NH4+) in Hexafluoridosilicic Acid Aqueous Solutions at T = 353.15 K

Abstract

The solubility of hexafluoridosilicate salts and acid aqueous solutions {yH2[SiF6] + (1 – y)M2[SiF6]}(aq) with Na+, K+, and NH4+ was experimentally determined at various ionic strength fractions (y) with y = 0.20, 0.50, and 0.80 at the temperature 353.15 K. The precipitated solid phases obtained by filtration were characterized by X-ray diffraction, which show only the presence of a single crystalline form for each ternary system at various (y) values, Na2[SiF6], K2[SiF6], or (NH4)2[SiF6], respectively. To find the solubility, the thermodynamic properties of these ternary systems are required, based on the measured activity of water. Then, the water activity measurements for the mixed systems were done at various (y) values with y = 0.20, 0.50, and 0.80 from diluted to the saturated solutions, using the hygrometric method at T = 353.15 K. The related thermodynamic properties of hexafluoridosilicate salts and acid aqueous solutions were also determined using Pitzer’s ion-interaction approach. The new ion-i...