The Azeotrope -An
Introduction to Distillation
An interesting physical phenomenon, generally neglected in the college chemistry laboratory, is that of azeotropism, a subject appearing in this Journal only once in the past 30 years.' To adequately describe a n azeotrope two pieces of information are needed: the boiling point a t a given pressure, usually atmospheric, and the composition. The boiling point can be obtained quite directly with a calibrated thermometer during distillation while the composition can be determined by one or more of several different procedures. Azeotrapes containing acids, amines, or carbanyl compounds can he titrated with the appropriate standard solution. If the azeotrope consists of only one phase, its density or refractive index can be compared t o a standard curve prepared from a series of known solutions. A rapid, direct, and mcderately accurate procedure for obtaining the compasition of either one or two phase azeotropes can be carried out by simply weighing samples and observing temperatures during distillation. An accurately weighed sample of the lesser component is placed in the distilling flask along with a definite excess of the second component, which need not he weighed. The mixture is distilled slowly with careful temperature observation. A low-boiling azeotrope, the preferred kind in this case, will give a steady reading until all the minor component has been distilled aver. When the temperature begins to rise, quite rapidly if an efficient column is being used, the distillate is removed and weighed; this distillate contains all of the minor component and enaueh of the maim one to oroduee the azeotrooe.. whose comwsitian can be easilv calculated from the two weiehines. The must effective azeutroprs fur this experiment are those uhure components have sirnllar h o h g pomrs hut e x hibit n marked lowering when d!slilled together: such as ~ . n t e rplus 1-prupan~l.2-butanol, or rerr-amyl alcahalZ Regardlesfi of which anentrope is chosen. this rkpe of experiment will bring the student into meaningful contact with d:+ tillation data, material balances, and one or more analytical procedures. It also forcefully proves that a constant hoiling point does not guarantee the presence of a pure compound and that impurities do not always raise the boiling point.
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Fleer, K. B., J. CHEM. EDUC., 22,588 (1945). Lange, Norbert A,, "Handbook of Chemistry," 4th Ed., Handbook Publishers, Inc., Sandusky, Ohio, 1941, p. 1208. Thomas MeCullough C.S.C. John E. Nowicki
St. Edward's University Austin, Texas 78704
Volume 5 1 , Number 5, May 7974
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