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

The solubility of potassium hexafluorosilicate (K2SiF6) in pure water and acid solutions (HCl and H3PO4) was conducted within a temperature range of 5–70 °C by the isothermal method. The stability of K2SiF6 in the systems above was also investigated by analyzing the equilibrium solids obtained in different conditions using X-ray diffraction (XRD), energy dispersive X-ray fluorescence (XRF), scanning electron microscope (SEM), and Fourier transform infrared (FT-IR). The results revealed that the solubility always increases with increasing temperature in all investigated systems. In the presence of hydrochloric acid and phosphoric acid, the solubility first increases and then decreases with the increase of acid concentration due to the increase of H+ concentration in hydrochloric acid, the effect of the stepwise dissociation of phosphoric acid, and the changed average ionic activity in solutions. K2SiF6 is stable in pure water and acid solutions. In addition, the experimental data were successfully correlated by the Apelblat empirical equation.