ARTICLE pubs.acs.org/IECR
Determination of Inorganic Salt Solubility at a Temperature above the Boiling Point of Water by Multiple Headspace Extraction Gas Chromatography Xin-Sheng Chai*,†,§ and Christopher L. Verrill‡,§ †
State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China ‡ International Paper Company, Loveland, Ohio, United States § Institute of Paper Science and Technology, Georgia Institute of Technology, 500 10th Street, N.W., Atlanta, Georgia 30332, United States 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 vaporliquid 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.
’ INTRODUCTION The solubility of salts in aqueous solutions is an important parameter in many chemical engineering processes. The conventional method for determining such solubility involves measuring the salt concentration in a saturated state in which the solid salt is in equilibrium with the surrounding solution.13 When sampling saturated solutions, any solids are generally removed by filtration. However, filtration procedures for highly viscous liquids, such as concentrated spent liquor in wood pulping processes, are very difficult. Temperature control during filtration is also difficult, especially in solubility studies at elevated temperatures. Furthermore, there may be additional problems because of the presence of multiple species in the solution that may affect the accuracy of determination of the analyte of particular interest. In a previous study,4 we reported that the vaporliquid equilibrium (VLE) partition coefficient of methanol (i.e., CV/CL, where CV is the concentration of methanol in the vapor at equilibrium with the concentration of methanol in the liquid) is a function of the amount of salt in solution at a given temperature and, therefore, will not change after the salt concentration reaches saturation. Similar effects also have been observed with various ketones.4,5 This phenomenon has been used in a headspace gas chromatographic method (HS-GC) to indirectly determine the solubility of salt in aqueous solutions and weak black liquors (i.e., pulping spent liquor) at temperatures of 60 and 70 °C.6 The method involves adding methanol to a series of aqueous solutions that contain carefully measured amounts of salt and r 2011 American Chemical Society
measuring the methanol vapor concentration in the headspace of the differently spiked samples, covering the range from less than saturated to more than saturated. The salt solubility is determined from the observed transition point in a plot of equilibrated vapor methanol signal (measured by GC), versus the amount of salt in the solution. The HS-GC system provides a precisely controlled temperature environment and is amenable to multiple analyses to improve precision; cf., a relative standard deviation of
