Effect of Ethylene Glycol and Its Derivatives on the Solubility Behavior of CaSO 4 · 2H 2 O in the Aqueous NaCl System and Physicochemical Solution Properties at 35 °C Manjinder Kaur, K. Srinivasa Rao, Tejwant Singh, V. P. Mohandas, and Arvind Kumar* Salt and Marine Chemicals Division, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research (CSIR), G. B. Marg, Bhavnagar 364002, India The effect of addition of ethylene glycol (EG), ethylene glycol monomethyl ether (EGMME), and ethylene glycol dimethyl ether (EGDME) on the solubility behavior of CaSO 4 · 2H 2 O in aqueous NaCl solutions has been examined at 35 °C. Addition of EG/EG derivatives reduced the solubility of CaSO 4 · 2H 2 O in general. Replacement of the hydroxyl group of EG by the methoxy group decreased the solubility of CaSO 4 · 2H 2 O in the order: EGDME > EGMME > EG. We also measured precise density (F) and speed of sound (u) data for the quaternary systems CaSO 4 · 2H 2 O + NaCl + H 2 O + EG/EG derivatives at 35 °C. Measurements of speed of sound and density have been used to determine the solution isentropic compressibility (κ S ). Empirical equations describing the solubility, density, speed of sound, and isentropic compressibility in these systems are presented. Introduction Accurate and reliable data on solubility and physicochemical solution properties of aqueous multicomponent electrolyte systems are necessary for many industrial processes. Such studies are important for understanding ionic equilibria and ion-solvent and ion-ion interactions in natural waters. 1,2 We are continuing our research program on aqueous electrolyte solutions saturated with CaSO 4 · 2H 2 O, which is a predominant sparingly soluble electrolyte present in seawater and industrial water systems. It precipitates and forms scale once its saturation limit exceeds a certain threshold limit. Therefore, it is quite important to have accurate data on solubility and other physical properties of systems where CaSO 4 · 2H 2 O is an important constituent. In the past a lot of research work has been done on the solubility related problems of aqueous sodium chloride systems saturated with gypsum. 3-6 Research papers of Daniela and Wolfgang and Raju and Atkinson provide a detailed review of the crystallization, phase stability, and solubility of CaSO 4 · 2H 2 O in aqueous solution as well as in aqueous solutions of different electrolytes. 7,8 Recently we studied a number of physicochemical properties for the ternary systems (CaSO 4 · 2H 2 O + NaCl + H 2 O and CaSO 4 · 2H 2 O + CaCl 2 + H 2 O) and quaternary systems (CaSO 4 · 2H 2 O + NaCl + CaCl 2 + H 2 O and CaSO 4 · 2H 2 O + NaCl + MgCl 2 + H 2 O) and have also examined the effect of pH on the solubility of CaSO 4 · 2H 2 O in the system CaSO 4 · 2H 2 O + NaCl + H 2 O. 9-14 In continuation of the ongoing research work herein we have investigated the effect of organic solvents (ethylene glycol and its derivatives) on the solubility behavior of CaSO 4 · 2H 2 O in brines. The investigations on the solubility of inorganic salts and solution properties in mixed water-organic solvent are useful in drowning out precipitation of soluble salts at ambient temperature and obtaining crystals of high purity, desired size, and morphology. 15-19 Ethylene glycol (EG) and its derivates are widely used as solvents and solubilizing agents in many industrial applications. 20-23 EG/EG derivatives can be very effective antisolvents for crystallization of salts from aqueous solutions because of their tendency to bind the water through intermolecular hydrogen bonding. 24-26 There are hardly any investigations on the effect ethylene glycol and its derivatives on the solubility behavior of CaSO 4 · 2H 2 O in brines in the literature. 27 Therefore, the aim of the present work is to provide accurate solubility data of gypsum and to provide solution physical properties as an aid toward the assessment of the potential drowning out precipitation using ethylene glycol and its derivatives as a separation technique. Experimental Section CaSO 4 · 2H 2 O and NaCl (> 99.5 % by mol) obtained from SD Fine Chemicals (Bombay) were used after drying in an oven at 70 °C without further purification. Ethylene glycol (> 99 % by mol) and ethylene glycol monomethyl ether (> 99.5 % by mol) were obtained from Merck KGaA, Germany. Ethylene glycol dimethyl ether (> 99 mol % by mol) was procured from Merck-Schuchardt. All of the organic liquids were used after drying over the 0.4 nm molecular sieves and under vacuum at ambient conditions. Solutions were prepared by weight, using an analytical balance with a precision of ( 0.0001 g (Denver Instrument APX-200) in Millipore grade water. Stock solutions were prepared by adding oven-dried NaCl to the aqueous solutions containing fixed amounts of organic solvent. A range of solutions of different NaCl concentrations saturated with CaSO 4 · 2H 2 O were then prepared by diluting the stock solutions of sodium chloride with an aqueous solution of organic solvents and adding excess CaSO 4 · 2H 2 O. The resulting solutions were stirred in a thermostatically controlled water bath. After the solutions were stirred with an electrical paddle for about 24 h, liquid samples were withdrawn periodically and analyzed for different ions as described elsewhere. 9 In brief, Ca 2+ and Cl - concentrations were determined volumetrically using standard * Corresponding author. E-mail: mailme_arvind@yahoo.com; arvind@ csmcri.org. J. Chem. Eng. Data 2010, 55, 4704–4708 4704 10.1021/je100336q  2010 American Chemical Society Published on Web 10/07/2010