H. M. Coleman
Chicago City College Chicago, Minois 60634
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Laboraton Demonstration of ~rattionalDistillation
A n experiment to demonstrate the principles of fractional distillation is often a part of the laboratory program. Unfortunately, experiments teaching these principles have been unduly complicated by the necessity for analyzing the distillates by indirect physical procedures such as det,ermination of specific gravities and refractive indices (see Evans),' or by chemical titration met,hods (see Edelstein).2 These Example of a Student's Fractionation of a Toluene-Acetone Mixturea using Salt-Water Separation Method
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Distillate Fraotion No.
P a l o mnle
fract. acetone toluene in 1 ml distillate present in fraction (ml) distillate
Temperature Range ('C)
a Mixture was prepared to cantsin 70% by volume of toluene (1 ml yielded 0.7 ml toluene by salt-water separation method; this corresponds to 0.4 mole fraction of acetone present). The quantitative recovery of bolwue confirmed the validity of the andysia procedure.
of salt sharpens the toluene-water interface and permits direct easy reading of the volume of toluene. Since this volume of toluene came from a 1-ml sample, simple multiplication of this volume by 100 immediately gives the percent toluene by volume in the distillate sample. Thus, the student can quickly and directly see the progress of the fractionation. An example of a representative student's results using about 30 ml in a simple distilling set-up can be seen in the table. A 50ml round-bottomed flask was employed. To it was attached a simple asbestos-insulated distilling head, mounting a thermometer in the usual manner and containing loosely packed glass wool. To the side-arm was attached a foot-long air-cooled glass tube terminating in Tygon tubing which acted as an adapter for the test tubes which collected fractions of about 2 ml each. The figure shows the liquid-vapor composition versus boiling point temperature diagram for toluene-acetone mixtures. Students should note from this diagram that the initial mixture containing the 30% by volume acetone (or approximately 0.4 mole fraction of acetone) would yield a vapor composition (or distillate) slightly greater than 0.8 mole fraction of acetone. This was confirmed by the first fraction in the table having a similar composition determined by the salt water ana!ysis procedure.
procedures are time-consuming and divert t,he student from the immediate objective. The method described here permits a quick analysis of the several distillates by direct visualization of one of the two components of a binary mixture cont,aining toluene and acetone. Inasmuch as aretone is miscible with water in all proportions arid toluene becomes more immiscible in water-acetone mixtures as more water is added, mere addition of water in sufficient amount to a given volume of distillate causes the dissolved toluene to separate quantitatively as a top layer which can be directly seen and whose volume can be directly measured. To each 1 ml of each fraction placed in a 10-ml graduated cylinder is added 10% NaC1 solution unt,il the top of the liquid is a t the 10-ml mark. The presence EVANS, G. M., J. CEEM.EDUC.,11,424 (1934). S. M., J. CAEM.EDUC.,13. 272 (1936). EDELSTEIN,
476
/ Journal of Chemicol Education
MOL FHC~/& Boiling point curve for toluene-ocetone mixtures.
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