Liquid-Vapor Equilibrium Edmund C. Shearer Fort Hays Kansos State College Hoys, Kansas 67601
at Constant Temperature A n experiment in physical chemistry
Liquid-vapor equilibrium is usually demonstrated in the physical chemistry laboratory by constructing a boiling point-composition diagram a t constant pressure.l.2 However, procedures for determining liquid-vapor equilibria a t constant temperature are found in few sources of experiments. There is a need for experiments a t constant temperature. Since most physical chemistry texts introduce the study of binary liquid systems when Raoult's law is presented; vapor pressure-composition diagrams would be just as interesting and possibly more informative in demonstrating the principles of liquid-vapor equilibria than boiling point-composition measurements. One such apparatus has been described by Tobey3 but it suffers from the disadvantages of being a complicated special-purpose apparatus for liquid systems having boiling points above 100°C. The experiment described below involves less complex equipment and can be applied to a wider range of liquid systems. Apparatus
The apparatus represented in Figure 1 is a portion of a portable vacuum system equipped with a mercury manometer, a sample vessel, and a gas sampling loop for direct injection of gases into a gas chromatograph. The sample vessel was constructed from a 250-ml round bottom boiling flask by attaching a male 24/40 ground glass joint for insertion into the vacuum line. A 10130 ground glass joint for receiving a standard taperware thermometer is attached so that the thermometer bulb lies near the bottom of the flask. Several sample vessels may be connected to the manifold in this manner making i t possible to determine several points in a run. The gas sampling loop has a volume of 21.02 ml in the u m e r chamber and 2.71 ml in the lower and is o ~ e r a t e d with pressure stopcocks having a 120" bore. ~ons&ucted in this manner, three different volumes may be chosen for gas analysis. Experimental Procedure
After a pair of liquids has been chosen for study, samples of the pure liquids are introduced into the gas chromatograph to optimize chromatographic conditions and to determine the retention times of liquids. Mixtures of the two liquids are prepared in approximate volume percent in 10%increments. The vapor pressure of each pure liquid and each mixture is measured by introducing into the sample vessel a volume sufficient to cover the thermometer bulb. A magnetic stirring bar is
resented at the 104th Meeting of the Kansas Academy of Science, April 14, 1972. 'Daniels, Farrington, Williams, J. W., Bender, Paul, Alberty, R. A., Cornwell, C. D., and Hamiman, John E., "Experimental Physical Chemistry," 7th ed., McGraw-Hill Book Co., New York, 1970, p. 61. Shoemaker, David P., and Garland, Carl W., "Experiments in Physical Chemistry," 2nd ed., McGraw-Hill Book Co., New York, 1967, p. 166. Tobey, Stephen W., J. CHEM. EDUC., 39,258 (1962). 446 /Journal of Chemical Education
TO
VACUUM
Figure 1. Apparatus temwrature.
for
determining liquid-vapor equilibria at constant
=h LOO
o
LIQUID
A
VAPOR
MOLE% CHLOROFORM Figure 2. Phase diagram of the acatone-chloroform system at 22'C. placed in the vessel to provide agitation. The vessel is slowly opened to vacuum to remove all traces of air from the system. When the air has been removed, the vacuum is closed and the liquid is permitted to vaporize and expand into the apparatus. The vaporization is allowed to continue until the temperature rises to a predetermined value. The temperature is just below mom temperature to prevent condensation in other areas of the system. Achieving the specified temperature is aided by immersing the sample vessel in a water bath. When equilibrium is achieved, the vapor pressure is recorded and the gas sampling loop is filled with vapor. The vapor is injected into the gas chromatograph by diverting the helium flow from the bypass through the upper and lower chambers of the LOOD. The sample vessel 1s removed from rhe vsruum system and the liquid i s onnlymd by gar chromatopaphy ualng ronvenrional liq. uid mjrrtion techriiquei. The rime required for the erprriment is approximately 20 min/solution. Results
Phase diagrams for the acetone-chloroform system a t 22°C and the acetone-diethyl ether system a t 23.5-C have been constructed from data collected by this method. The diagrams are presented in Figures 2 and 3, respectively.
