Effect of pH on the Solubility of CaSO₄·2H₂O in Aqueous NaCl Solutions and Physicochemical Solution Properties at 35 °C

Abstract

The solubility of CaSO 4 ·2H 2 O has been determined in aqueous NaCl solutions up to very high salinities in the pH range of 2.5 to 8.5 at 35 °C. The solubility of CaSO 4 ·2H 2 O increased slightly with a decrease in the pH of the solution. A nearly 12 % enhancement in the solubility maximum of CaSO 4 ·2H 2 O was seen at pH 2.5. Surprisingly, an increase in the solubility of CaSO 4 ·2H 2 O was also seen at pH 8.5. Solubility data at various pH values as a function of the concentration of NaCl have been correlated using polynomial fits with a maximum standard deviation of < 0.002 mol·kg −1 . We have also measured the solution density (ρ), viscosity (η), and speed of sound ( u ) for the system CaSO 4 ·2H 2 O + NaCl + H 2 O at 35 °C and at different pH values. The isentropic compressibility (κ S ) of the solutions has been derived using the experimental density and speed of sound values. A relatively higher η and κ S have been observed for the solutions of pH 8.5. All of the experimentally determined physical properties as a function of ionic strength of the solution have been correlated using polynomial or linear fits by the least-squares method. These studies are of relevance in the production of NaCl from natural brines with low impurities of Ca 2+ and SO 4 2− ions.