Alim U I I O ~
Saint John Institute of Technology Saint John, N. 6.. Canada
Apparatus for Vapor-Liquid Equilibria Measurements
I n our two year postrsecondary chemical technology program, two laboratory sessions of three hours each are spent in obtaining vapor-liquid equilibrium data. Due to the shortage of time and funds, i t is not possible to have standard equilibrium stills (1)set up for each individual group. The methods suggested in the texts used (2, 3) suffer from a few drawbacks. In the distillation method (2) equilibrium is never reached as the compositions of the two phases change continuously, and except for the pure components and the azeotrope, a t any instant the liquid phase temperature is different than that of the vapor phase. In the apparatus with refluxing (3) the vapor phase might carry entrainments, thereby changing the compositions. Problems might also arise in condensing the vapor a t a steady rate. The still used in our laboratories. as shown in the fieure, employs circulation of the vapor phase. It can be set up in a very short time with the common glassware generally available in any chemical laboratory. The reboiler was filled about two-thirds full with the liquid solution and it was brought to boiling. The heat was regulated by adjusting the powerstat connected to the heating jacket. When the thermometer gave a constant reading for about ten minutes, heating was stopped and about one ml of vapor sample was withdrawn into a glass-stoppered vial. After the flask cooled down, a liquid sample was also taken out. The refractive indices' of the respective samples gave their compositions.
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1 Refractometry was used as the method of analysis. Calibration plots of refractiveindex at 25'C versus mole fraction obtained earlier in the year in the Instrumental Analysis Course were referred to.
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1 lournal of Chemical Educafion
Equilibnvm still wit+ vapor phase recirculation.
Measurements on several miscible binaries for both ideal and non-ideal cases were made by different groups of students. The data were tested for thermodynamic consistency using the Gibbs-Duhem Equation (4). Considering the amount of t,ime spent, the accuracy of the data obtained is satisfactory. The procedure is not a substitute for precise vapor-liquid equilibria determination but might be of help when only approximate equilibrium data are required. Literature Cited (1) HALA,E., et ol., "Vapaur-Liquid Equilibrium," Pergamon
Press, New York, 1958, pp. 253-279. D. P., AND GARLAND, C. W., "Experiments in (2) SHOEMAKER, Physical Chemistry," McGraw-Hill, New York, 1962, pp. 170-173. (3) DANIELS,F., el al., "Experimental Physicd Chemistry," 8th ed., McGmw-Hill, New York, 1962, pp. 54-59. J. M., AND VANNESS,H. C., "Introduction to Chemi(4) SMITH, cal Engineering Thermodynamics," (%d ed.) McGrawHill, New York, 1959, p. 384.
