Experimental Measurement of the Solid–Liquid Equilibrium of the Systems MF₂ + H₂O (M = Mg, Ca, Zn) from 298.15 to 353.15 K

Abstract

The solubility of MgF2, CaF2, and ZnF2 in aqueous solution is seldom studied. Their solubility data available in the literature conflict with each other, and their change tendency with temperature is hard to draw. In this work, we elaborately measured their solubility over the temperature range from 298.15 to 353.15 K. To guarantee the data reliability, various methods including ethylenediamine tetraacetic acid disodium salt dihydrate (EDTA) titration and fluoride ion selective electrode (FISE) have been used for ion content analysis in the saturated solutions and X-ray diffraction (XRD) for equilibrium solid phase identification. Besides, sufficient solid–liquid contact time has been taken to guarantee the equilibrium. The determined results were compared with those reported in the literature, and their differences have been discussed. It was found that the MgF2 solubility decreases; however, the CaF2 solubility increases with increasing temperature. In the ZnF2 + H2O system, the equilibrium solid phase is ZnF2·4H2O(s) at temperatures below 333 K and ZnOHF(s) above 333 K. While the ZnF2·4H2O(s) solubility is sensitive to temperature, the ZnOHF(s) solubility is insensitive.