Azeotropic Mixtures

For many years after the discovery of the formation of mixtures of constant boiling point it was thought that these azeotropic mixtures were definite chemical compounds. Thus Bineau1 described the azeotropic mixture of hydrochloric acid and water as a hydrate of the acid, and Chancel 2 regarded the azeotropic mixture of propyl alcohol and water as a hydrate of the alcohol.

Roscoe,3 however, showed that the azeotropic concentration of aqueous mineral acids varied with the pressure, and this would not be the case if definite chemical compounds were formed.

So, also, in the case of mixtures of minimum boiling point (maximum vapour pressure) the composition of the azeotropic mixtures has been found to vary with the pressure (temperature) by. Konowalow, Young and Fortey, Homfray, Wade and Merriman, Lecat and others, and in no case has the composition of a mixture of either maximum or minimum boiling point been found to be independent of the pressure.

Perhaps the most interesting case yet investigated is that of ethyl alcohol and water. Very careful determinations of the composition of the azeotropic mixtures formed by distillation of aqueous alcohol under a series of pressures were made by Wade and Merriman,4 and later by Merriman,5 who also redetermined the vapour pressures of ethyl alcohol. The results obtained by Merriman are given in the table on the following page.

1 Bineau, "Sur les combinaisons de l'eau avec les hydracides," Ann. Ch. Ph., 1843, (3), 7, 257.

2 Chancel, "Nouvelles Recherches sur l'alcool propylique de fermentation," Compt. rend., 1869, 68, 659.

3 Loc. cit.

4 Wade and Merriman, "Influence of Water on the Boiling Point of Ethyl Alcohol at Pressures above and below the Atmospheric Pressure," Trans. Chem. Soc, 1911, 99, 997.

5 Merriman, "The Vapour Pressures of the Lower Alcohols and their Azeotropic Mixtures with Water, Part I., Ethyl Alcohol," ibid., 1913, 103, 628.

Table 15

As the pressure falls the percentage of water in the azeotropic mixture diminishes, and at pressures lower than about 75 mm. no such mixture is formed.

Merriman 1 has also very carefully investigated the effect of pressure on the composition of the binary azeotropic mixtures of ethyl acetate and water, ethyl acetate and ethyl alcohol, and of the ternary azeotropic mixtures of ethyl acetate, ethyl alcohol and water. A few of the data

Table 16

Table 17. Ethyl Acetate, Ethyl Alcohol and Water

1 Merriman, "The Azeotropic Mixtures of Ethyl Acetate, Ethyl Alcohol and Water at Pressures above and below the Atmospheric Pressure," Trans. Chem. Soc, 1913, 103, 179(X

From these results and others obtained by Young and Fortey, Homfray, Thayer, Lehfeldt, Ryland and Zawidski, Merriman arrives at the conclusion that, in all cases observed except one, the percentage of that component in a binary azeotropic mixture which has the lower value of dp/dt increases as the pressure decreases. Mixtures which follow the rule are ethyl acetate and water, ethyl acetate and ethyl alcohol, propyl alcohol and water, ethyl alcohol and propionitrile, benzene and ethyl alcohol, toluene and acetic acid, carbon disulphide and acetone. The only known exception to the rule is the ethyl alcohol-water mixture.

Vrevsky1 has made careful determinations of the composition of azeotropic mixtures of alcohols and acids with water at different temperatures and gives the following general rule : - The concentration of that constituent of an azeotropic mixture which has the higher molecular heat of vaporisation increases with rise of temperature if the mixture is one of maximum vapour pressure, and diminishes if it is one of minimum vapour pressure.

The question has been treated analytically by Roozeboom,2 Kuenen,3 and by Van der Waals and Kohnstamm.4

1 Vrevsky, "Uber Zusammensetzung und Spannung des Dampfes binarer Fussigkeits-gemische," Zeitschr. physik. Chem., 1912-13, 81, 1; 1913, 83, 551.

2 Roozeboom, Die heterogenen Gleichgewichte vom Standpunkte der Phosenlehre, Braunschweig, 1904, vol. ii., Part I., p. 66.

3 Kuenen, Handbuch der angewandten physikalischen Chemie, Leipzig, 1906, p. 114.

* Van der Waals and Kohnstamm, Lehrbuch der Thermodynamik in threr Anwendung auf das Gleichgewicht von Systemen mit Gasformig-Flussigen Phasen, Zweiter Teil, Leipzig, 1912.