A Novel Method for Examination of Vapor-Liquid Equilibria P. F. Knewstubb
Lensfield Road, Cambridge CB2 1EW, UK The nonideal nature of mixed liquids and measurements of their vapor pressures are topics of considerable technical relevance but not easy for students to explore in their practical studies. The reasons for positive or negative deviations from Raoult's Law are surely worth bringing forward, even for qualitative demonstration. A development of the theory of regular solutions (I,2) is a n interesting and useful further stage. This paper describes a method of obtaining reasonably accurate vapor-liquid equilibrium data that can be offered a t university-level teaching in physical chemistry and chemical eneineerine. I t has been on student trial for over a year, andsome qiite good results have been obtained. The method reauires care and uatience, but it shows definlte adwnta& in tht: ease of ;orking compared with the ~ouilihriumstill mrthud, whlch operates essentially ahuve room temperature and yields good data only very slowly. Thermal equilibrium is obtained between a small volume of liquid and a large volume of vapor, and the total vapor pressure is measured. The liquid phase is then separated mechanically from the vapor and collected. The vapor phase is compressed isothermally to form another liquid phase of essentially the same composition a s the vapor, which is also collected. Analyses of the liquid compositions are by refractive index measurement.
to scale. A is taD A: Fiaure 1. Sketch of the amaratus.. aDDroxirnateiv ,, B 7s tao B: X is vacuum Aduo~ina: . e is vacuum-skaled end cover:' f ib connecl on lo flexlole 1-oe, s lmmerslon leve lp s q~ d phase p s p slon r sax s of rolal on d
Apparatus
Construction of the Vacuum Apparatus
Procedure
The System A diagram of the apparatus, approximately to scale, is shown in Figurel. The envelope is constructed in borosilicate -elass., and the onlv other material in contact with organic vapors is poly(tetrafluoroethylene) (PTFE). The most obvious features are a niston of PTFE. sealed with a PTFE-sheathed neoprene 0-ring, which slides inside a .,elass cvlinder. The diameter of the cvlinder is 100 mm. and the full stroke movement range, of the pisron is 162 mm. For the most reliable ouerntiun. the cvlindt,r ihould l ~ vrer cision-bore tubing; thebne used in this work was madeusing tubing carefully selected from standard stock (for economy). I t is successful, but the chance for failure of the sliding seal is higher. The greaseless screw-action. taps a t A, B and screw coupling a t X are of a pattern using PTFE components. Facilities not shown in Figure 1 include two flexible tubes (lined with PTFE) connected a t X and to the lid, each of which allows for pumping to
t
174.6 172.9 169.0 165.4 150.9 134.0
In regular solution theory the entropy of mixing of the liquids is taken a s the ideal value: The mixing of the two species is assumed to be random. The enthalpy of mixing of a regular solution is nonzero, differing thus from the ideal, and is derived in the theory as
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Journal of Chemical Education
0.460 0.510 0.634 0.683 0.840 1.00
0.466 0.537 0.716 0.772 0.915 1.00
0.252 0.203 0.110 0.088 0.033 0.0
0.156 0.191 0.366 0.420 0.631
8
497 490. 453 406 209
0.806 0.796 0.900 0.930 1.081
The quantifies 1istedare:Total vapor pressure p; mole fraction of iodoethane y~(vapor),liquid); activity coefficientsyI(iodoethane), ~(hexane); A G ~ X C ~= S S~ ~ ( x ryli n + xzln y2) J mo~"; P is A G ~ X ...J(RTxix2). Estimated errors are: f12 J mol-' in AGexcersand i0.02in P at XI = 0.5,rising to t0.06 in P at XI, x2 = 0.085,except where marked ' (see text).
The regular solution theory might be considered to apply reasonably well over this range. Conclusion The experiment offers a good examination of features of liquid-vapor equilibria and a fairly searching test of careful and methodical working with vacuum apparatus. The vapor pressures of the pure components can be found with this method, but the figures are quoted to students to speed the work. I n one day of about 5 h of practical work, about five pairs of points might be found (allowing time for learning the procedure). With this time limit, students are directed to explore the hexane-rich half of the composition range. Those with two or three days available are encouraged to investigate the whole range. The apparatus would he suitable for other systems. The principal restriction i s the amount of liquid resulting from the compression step. With the dimensions used here the amount is 10.3 mmol for hexane, or 1.34 mL (by ideal gas
estimation). Although there is some retention of liquid in the apparatus, enough is collected for duplicate measurements of refractive index. The precise shape of the base of the cylinder (i.e., very slightly but uniformly conical) is the most helpful feature in this collection of liquid. If the components do not differ markedly in refractive index, some other mode of analysis obviously must be used. Acknowledgment
Among those helping with the construction of the apparatus it is especially relevant to acknowledge the skill of Derek Edwards i n shaping the base of the cylinder with minimal distortion of its diameter. Literature Cited 1. Atkins. E W Physical Chemistry, 4th ed.; Oxford Uniu, 1990:p165 2. Moore. W J.PhyPicd Chemistry, 5th ed.: l o n p a n , 1972:p260.
Volume 72 Number 3 March 1995
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