In the Laboratory edited by
Arden P. Zipp
The Microscale Laboratory
SUNY-Cortland Cortland, NY 13045
A Useful System for Microscale and Semi-microscale Fractional Distillation of Air-Sensitive Substances with High Boiling Points J. Alfredo Gutiérrez† Departamento de Química, Centro de Investigación y de Estudios Avanzados (CINVESTAV-I.P.N.) Apdo. Postal 14-740, México D.F. 07000, México;
[email protected] Purification of liquid substances by distillation is a recurrent task in research and teaching laboratories (1). Distillation of high-boiling substances has attracted special attention because these samples often decompose at the high temperatures required to evaporate them, especially when many impurities are present. The situation is improved with reduced-pressure distillations, especially under high vacuum. The Kugelrohr system (2, 3) often gives good results but is not well suited to distilling air-sensitive compounds or to separating several components. Moreover, it is rather costly to use in teaching laboratories. The horizontal distillation device described here eliminates extreme refluxing but allows fractionation of the vapors by a gentle reflux caused by the cold finger that serves as a thermometer holder. The best results can be obtained by the procedure described below. All glassware is dried in an oven. The apparatus is assembled using a silicone high-vacuum grease in the standard 14/23 or 14/29 ground-glass joints. The system is connected to a manifold high-vacuum inert-gas line through the V/IG connection and is properly evacuated and then filled with inert gas (Fig. 1). The liquid mixture is introduced with the help of a double-tipped cannula by puncturing the rubber septum of the hole in vessel A; the septum is replaced by a new one after this operation. We tried sample volumes in the range of 0.5–10 mL in the distillation flask. To avoid the formation of large bubbles that project liquid to the curved neck B, the sample is first degasified by increasing the vacuum slowly without heating, while the magnetic stirring bar is turning slowly. Once the foaming has ceased and with the vacuum at its highest level (we work with pressures in the range of 0.01 to 0.05 mm Hg) one can turn on the heating mantle on flask A, maintain a low heat input, and observe the behavior of the vapors. As the temperature rises, the vapors reflux at increasing speed from the cold finger that contains the external thermometer with its bulb immersed in some drops of mineral oil, until liquid begins to drop into the cow receiver C. Notice that the bottom of the cold finger is at the same level as the exit of the vapors from vessel A. If needed, one can cool the curve of the distilling neck by affixing a strip of wet paper or †
Present address: Universidad de Guanajuato, Facultad de Química, Departamento de Posgrado en Química, Noria Alta s/n, Guanajuato, Gto., 36050, México.
Figure 1. The distillation apparatus. A: distillation flask; B: curved neck; C: cow receiver; D: receiving flask; sb: magnetic stirring bar.
by molding a Styrofoam cup and putting some pieces of ice inside. It is not necessary that the liquid show boiling bubbles. In fact, the magnetic stirrer and the heating must be maintained at the minimal levels just to keep a uniform temperature in the liquid. Faster stirring or higher heating could create large bubbles that might project liquid into the curved neck B, contaminating the distillates. When the distillation is over, the system is flushed with inert gas and the fractions are transferred with the help of double-tipped cannulas or syringes by puncturing the rubber septum in each receiving flask D. Acknowledgments I thank the glassblower, David Andrés Pérez, who made the glassware. I also thank CONACYT and Universidad de Guanajuato for fellowships. Literature Cited 1. JCE Index Online Search; http://jchemed.chem.wisc.edu/Journal/ Search/index.html. An electronic search of this Journal through 1999 for articles with the word “distillation” in the title gives 92 references, of which about a half are related to new apparatus or devices to improve this technique. 2. Graeve, R.; Wahl, G. H. Jr. J. Chem. Educ. 1964, 41, 279. 3. Aldrich Catalog Handbook of Fine Chemicals; Aldrich Chemical Company: Milwaukee, WI, 1996; p T222.
JChemEd.chem.wisc.edu • Vol. 78 No. 5 May 2001 • Journal of Chemical Education
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