Azeotropic Mixtures

Azeotropic Mixtures

Alcohol forms azcotropic mixtures with water -i.e., mixtures which distil at a constant temperature, when the pressure is kept constant, without change of composition. At the standard pressure, 760 mm., the "mixture of constant boiling point' distils at 78.15°, and contains 443 per cent. of water (by weight).2 Since this temperature is lower than the boiling point of either of the two components, it is impossible at ordinary pressures to obtain pure alcohol from a dilute aqueous solution by fractional distillation only; some dehydrating agent is necessary.

The composition of the azeotropic mixture, as well as the boiling point, depends upon the pressure employed. The following values were found by R. W. Merriman: - 3

Pressure in mm.	B.p. of mixture.	Water, per cent.	B.p. of ethyl alcohol
1451.3	95.35°	4.75	96.68°
1075.4	87.12	4.65	87.34
760.0	78.15	4.4	78.30
404.6	63.04	3.75	63.13
198.4	47.63	2.7	47.66
129.7	39.20	1.3	39.24
94.9	33.35	0.5	33.38
70.0	....	0.0	27.96

The ratio of water to alcohol is thus diminished as the pressure is lowered. Eventually a point is reached below which an azeotropic mixture (binary mixture) is not formed; this occurs at pressures below about 80 mm. Under these conditions, anhydrous alcohol is the more volatile phase present; and it is then possible to separate absolute alcohol from the water in ordinary strong spirit by systematic fractional distillation, as shown in the last line of the table.1

1 J. Amer. Chem. Soc, 1901, 23, 467.

2 Young, Trans. Chem. Soc, 1902, 81, 710.

3 Ibid., 1913, 103, 635.

For the same reason, although alcohol, contrary to the general belief, is not specially hygroscopic, the percentage of water in moist alcohol is slowly increased by exposing it to air, or even by passing dry air through it.

M. Wrewski2 finds that the azeotropic mixture of alcohol and water has the following composition at the temperatures shown: -

Temp.	Alcohol, per cent.
39.76°......	97.6
54.81°......	96.5
74.79o ................................	95.7

Ethyl alcohol forms with benzene also a mixture of constant boiling point, 6825°, at normal pressure, and with water these two liquids give a ternary azeotropic mixture boiling at a still lower temperature (64.85°). Based on these properties, an ingenious method of dehydrating alcohol has been described by Young.3 If benzene be added to a mixture of alcohol and water, and the whole distilled, the ternary mixture of all three liquids will come over first, since it has the lowest boiling point (64.85°). If there is more than sufficient benzene to carry over the whole of the water, and if the alcohol is present in excess, the ternary mixture will be followed by the binary mixture of alcohol and benzene boiling at 68.25°, and the last substance to come over will be the alcohol, which should now, theoretically, be free from water. In practice, however, a single operation does not suffice completely to eliminate the water even when a dephlegmating column is used; but by redistilling the partly dehydrated alcohol once or twice with a further quantity of benzene, the water can be finally removed, and "absolute" alcohol obtained. Equal weights of benzene and alcohol of 93 per cent, strength (by weight) may be employed, and the distillate collected in fractions corresponding with the points 675°, 73°, and 783° for the recovery of weak alcohol and benzene; the remainder in the still is the more or less completely dehydrated alcohol.

W. R. G. Atkins 4 points out that this method can be applied to the dehydration of organic tissues, thus avoiding risk of oxidation; and also to the preparation of solid chemical compounds in the anhydrous state for analysis. The substance is placed in, the distillation flask with alcohol and a suitable quantity of benzene. When all the turbid ternary mixture of constant boiling point has been removed by distillation from a water-bath, the remaining mixture of alcohol and benzene may be rapidly distilled away, and the last traces can be eliminated in a vacuum desiccator. By adjusting the quantities, either alcohol or benzene can be obtained as the final liquid.

1 Wade and Merriman, Trans. Chem. Soc, 1911, 99, 997.

2 J. Russ. Phys. Chem. Soc, 19.10, 42, 1-35. 3 Loc cit.

4 Nature, 1915, 95, 118; Trans. Chem. Soc, 1915, 107, 916.

The two isomeric acetylene dichlorides also form binary mixtures with alcohol, and ternary mixtures with water and alcohol, boiling at low temperatures. They can therefore be used in the same way as benzene to obtain absolute alcohol. For the isomer of b.p. 4835°, the binary mixture boils at 465° and the ternary at 444°; for the other isomer, boiling at 6025°, the corresponding mixtures have the boiling points 57.7° and 53.8° respectively.1

With chloroform, alcohol forms a binary mixture distilling at 594°, the composition of the mixture being 93 per cent. of chloroform and 7 per cent. of alcohol. A ternary mixture boiling at 555° has the composition: chloroform, 925, alcohol 40, and water 35 per cent.2

As noted by Atkins,3 azeotropic mixtures may sometimes serve to identify an unknown liquid. The identification of a liquid by its boiling point is possible only when the liquid can be purified previously, and in dealing with small quantities such purification is often a difficult matter. It may happen, however, that another liquid can be added, with which the unknown substance will form a mixture of constant boiling point, so that the unknown liquid may be identified in this way. For example, benzene was added to a liquid smelling of alcohol, and the mixture was distilled. The temperature remained constant for a short time at 58.35° (the boiling point of the binary mixture of methyl alcohol and benzene), thus indicating the presence of methyl alcohol, and there were other halts at 648°, 6825°, and 783°, these being respectively the boiling points of the mixtures ethyl alcohol-water-benzene, alcohol-benzene, and of alcohol. Hence the liquid consisted of a mixture of methyl alcohol, ethyl alcohol, and a trace of water.

Distillation for azeotropic mixture

Distillation is the most widely used separation technique in the chemical and petroleum industry. However, not all liquid mixture are amenable to ordinary fractional distillation.

When the components of the system have low relative volatilities (1.00 < a <>Modified Raoult’s Law:

When g_i = 1, the mixture is said to be ideal Equation 1 simplifies to Raoult’s Law. Non-ideal mixtures (g_i ¹ 1) can exhibit either positive (g_i > 1) or negative deviations (g_i <>Raoult’s Law. In many highly non-ideal mixtures these deviations become so large that the pressure-composition (P-x, y) and temperature-composition (T-x, y) phase diagrams exhibit a minimum or maximum azeotrope point.

In the context of the T-x, y phase diagram, these points are called the minimum boiling azeotrope (where the boiling temperature of the azeotrope is less than that of the pure component) or maximum boiling azeotrope (the boiling temperature of the azeotrope is higher than that of the pure components). About 90% of the known azeotropes are of the minimum variety. At these minimum and maximum boiling azeotrope, the liquid phase and its equilibrium vapour phase have the same composition, i.e.,

x_i = y_i for i = 1, …, c (2)

Two main types of azeotropes exist, i.e. the homogeneous azeotrope, where a single liquid phase is in the equilibrium with a vapour phase; and the heterogeneous azeotropes, where the overall liquid composition which form two liquid phases, is identical to the vapour composition.