Solubility pattern of CaSO4 center dot 2H(2)O in the system NaCl+CaCl2+H2O and solution densities at 35 degrees C: Non-ideality and ion pairing

摘要

The solubility of CaSO4.2H(2)O was determined in aqueous NaCl solutions up to very high salinities and constant CaCl2 concentrations at 35 degrees C. Addition of CaCl2 into the aqueous NaCl system reduces the solubility of CaSO4.2H(2)O quite dramatically while maintaining the basic pattern of the solubility curve. Mean ionic activity coefficients gamma(+/-) of CaSO4.2H(2)O derived using the extended Debye-HUckel law with quadratic terms decrease with an increase in ionic strength. A comparison of the value of the thermodynamic solubility product constant, K-sp(th) of CaSO4.2H(2)O to the solubility product K-sp obtained from the observed solubility data indicate sizable differences. This has been explained through a combination of the extended Debye-HUckel law and ion association theory. We also measured accurate densities for the quaternary system CaSO4.2H(2)O + NaCl + CaCl2 + H2O at 35 degrees C. The density of the solution increases linearly with an increase in concentration. Solutions containing higher amounts of CaCl2 were found to be less dense when compared at the same ionic strengths. Solubility and density data as a function of concentration have been correlated using polynomial and linear fits by the method of least squares. These studies are of relevance in the production of salt with low impurities of Ca2+ and SO42- ions.