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

The solubility of CaSO4 center dot 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 degrees C. The solubility of CaSO4 center dot 2H(2)O increased slightly with a decrease in the pH of the solution. A nearly 12 % enhancement in the solubility maximum of CaSO4 center dot 2H(2)O was seen at pH 2.5. Surprisingly, an increase in the solubility of CaSO4 center dot 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 (rho), viscosity (eta), and speed of sound (u) for the system CaSO4 center dot 2H(2)O + NaCl + H2O at 35 degrees C and at different pH values. The isentropic compressibility (kappa(S)) of the solutions has been derived using the experimental density and speed of sound values. A relatively higher eta and kappa(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 Ca2+ and SO42- ions.