Determination of Inorganic Salt Solubility at a Temperature above the Boiling Point of Water by Multiple Headspace Extraction Gas Chromatography

Abstract

This paper describes a novel method for the determination of the solubility of sodium carbonate and sodium sulfate in single- and bisalt aqueous solutions at 110 °C, using multiple headspace extraction gas chromatography (MHE-GC). Both benzene alcohol and methanol were used as the tracer compounds whose vapor–liquid equilibrium (VLE) partition coefficients (CV/CL) are related to the concentration of salt in the studied solution, thereby indirectly providing solubility data. During the MHE process, small amounts of water are removed stepwise by a series of headspace vapor extractions, which simulates the evaporation process, resulting in an increase in the salt concentration in the solution and a decrease in the amount of tracer in the headspace vapor. Eventually, the salt concentration reaches saturation (the limit of solubility), a transition point that can be identified experimentally via GC, because of the different behaviors of the tracer loss before and after the salt saturation point. Knowing the salt concentration in the initial solution and gathering data on the amount of water lost in reaching the transition point, we calculated the solubilities of sodium sulfate and sodium carbonate at 110 °C as 30.6 and 31.9 wt %, respectively, which are in good agreement with data reported previously, using more-complex conventional methods. The method reported here has several advantages, including its simplicity and ease of automation.