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 {yH 2 [SiF 6 ] + (1 − y)M 2 [SiF 6 ]}(aq) with Na + , K + , and NH 4 + 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, Na 2 [SiF 6 ], K 2 [SiF 6 ], or (NH 4 ) 2 [SiF 6 ], 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-interaction parameters θ (H,M) and ψ (H,M,[SiF 6 ]) , with M = Na + , K + , or NH 4 + , of the ternary systems were evaluated and employed with the interaction binary parameters to estimate the solute activity and the osmotic coefficients of water for these aqueous solutions at T = 353.15 K. The calculated solubility of mixed systems is compared with that obtained experimentally.

Solubility Measurement and Correlation of Potassium Hexafluorosilicate (K2SiF6) in H2O, HCl, and H3PO4 Solutions at a Temperature Range of 5–70 °C

“…12−15 The solubility of potassium (sodium) fluorosilicate in phosphoric−sulfuric acid was measured, and it was found that the solubility decreases with the reduction of temperature and increase of fluorosilicic acid concentration. 16 Skafi et al 17 and Frayret et al, 18 1. All of the chemicals were used without further purification, and deionized water (18.2 MΩ cm) was used in all of the experimental runs.…”

“…For instance, literature data on K 2 SiF 6 were available only in water. − The solubility of potassium (sodium) fluorosilicate in phosphoric–sulfuric acid was measured, and it was found that the solubility decreases with the reduction of temperature and increase of fluorosilicic acid concentration . Skafi et al and Frayret et al., respectively, measured the solubilities of three alkaline salts of hexafluorosilicate (K 2 SiF 6 , Na 2 SiF 6 , and (NH 4 ) 2 SiF 6 ) in different acidic (hydrofluoric, hydrochloric, phosphoric, and nitric acids and hexafluoridosilicic acid) solutions. Besides, Wang et al found that the solubility values of K 2 SiF 6 increased with increasing temperature, whether in water, pure fluoride solution (H 2 SiF 6 +HF+H 2 O), or phosphoric acid digestion liquor.…”