Binary Liquid Systems Systems in which two liquids form an azeotrope with a boiling point lower than that of either component are numerous. The author describes here a system which is eminently suitable for demonstration, in part because of an uncommonly large positive deviation from Raoult's Law. One of the components of this azeotropic system also provides an illustration of refractive index differences in two-phaseliquid systems. Methyl formate (b.p. 32'C) and n-pentane (b.p. 36") form an azeotrope' a t 47 wt% n-pentane, boiling at 22% Hence if the r w m temperature is about 25%. merely mixing the components in the correct proportions gives a mixture which boils immediately. (If the room temperature is much lower than 25OC, the reagents may be warmed in a water bath.) Volume proportions are 55 ml of methyl formate to 76 ml of n-pentane. For best effect, display samples of the components along with the azeotrope. Add boiling stones to each sample. Visibility may be improved by using graduated cylinders and by adding a dye such as methylene blue or rhodamine B to the methyl formate before mixing. (The latter gives a bright orange luminescence under near-ultraviolet excitation.) The system methyl formate (refractive ihder n = 1.343 at 2OoC)-water (n = 1.333,20PC) shows the importance of refractive-index differences to the visibility of the boundary between phases in a two-phase liquid system. This demonstration is perhaps better done by individual students. Add 0.5-ml portions of methyl formate to 2 ml of water in a test tube, shaking well and carefully observing the mixture after each addition of methyl formate. Add up to 3 ml of the methyl formate. For improved visibility, repeat the experiment, tinting the water with a trace of strongly colored unreactive inorganic salt such as sodium chromate. For comparison, run further experiments as above, substituting 2-butanone (n = 1.381) or l-butanol (n = 1.399) for methyl formate. (Each of these three systems has appreciable but incomplete mutual solubility a t room temperature.) CAUTION: Methvl -~ . f o n n a t e i s extremelv flammable a n d is a respiratory a n d eye irritant. I t does not possess a pronounced odor. These demonatrstiona should be performed in a hood. Methyl furmate 19%. Aldrieh, is sativfer!ory for hoth demonstration- ns receiwd. Arknnwledgrnrnl is mndr to the Department of Chemirtry of the University of Virgmia for pnwiding I he reagents and fachties. ~
Harsley, L. H., "Azeotropic Data-111," Advances in Chemistry Series, No. 116, American Chemical Society, Washington, D.C., 1973, p. 115 (system No. 3281). University of Virginia Charlottesville, VA 22901
Charles M. Flynn, Jr.
Volume 52, Number 10, October 1975 / 641
