Constant-boiling mixtures ternary

Because ethyl alcohol forms an azeotrope with water that is a constant boiling mixture, i.e. both the ethyl alcohol and the water, in a ratio of 95/5, boil together at a temperature different than either separately. Other examples in this chapter are the ternary azeotrope, ethyl alcohol - water -benzene and DIPE - isopropyl alcohol - water. An azeotrope mentioned earlier is MEK - water - toluene rafFmate used for toluene extraction. 

In contrast, certain mixtures of two (binary) or three (ternary) components form constant boiling mixtures that cannot be separated by distillation. In such cases, each component contributes a fixed amount and the boiling point of the mixture is characteristic of the components. Such a system is called an azeotrope. The boihng point of an azeotrope may be higher or lower than that of the individual components. Common binary azeotropes are listed in Table 4.7 and ternary azeotropes are listed in Table 4.8. 

Ethanol is a monohydric primary alcohol. It melts at -117.3°C and boils at 78.5°C. It is miscible (i.e., mixes without separation) with water in all proportions and is separated from water only with difficulty ethanol that is completely free of water is called absolute ethanol. Ethanol forms a constant-boiling mixture, or azeotrope, with water that contains 95% ethanol and 5% water and that boils at 78.15°C since the boiling point of this binary azeotrope is below that of pure ethanol, absolute ethanol caimot be obtained by simple distillation. However, if benzene is added to 95% ethanol, a ternary azeotrope of benzene, ethanol, and water, with boiling point 64.9°C, can form since the proportion of water to ethanol in this azeotrope is greater than that in 95% ethanol, the water can be removed from 95% ethanol by adding benzene and distilling off this azeotrope. Because small amounts of benzene may remain, absolute ethanol prepared by this process is poisonous. 

KEYES PROCESS. A distillation patcess involving the addition of benzene to a constant-boiling OS, alcohol-water solution to obtain ahsolute 1100 ) alcohol. On distillation, a ternary azeotropic mixture containing all iltree components leaves the lop of the column while anhydrous alcohol leaves the bottom. The azeotrope tvvhich separates into two layers) is redistilled separately for recovery and reuse ol the henzene and alcohol... 

When a liquid contains three components which are not closely related to each other it may happen that both a ternary and a binary mixture of constant boiling point arc formed on distillation. In that case it is only possible to separate one of the original components in a pure state. These points will be considered more fully in Chapters XIII. and XV. 

Mixtures of Constant Boiling Point.—For the sake of brevity, a mixture of constant boiling point containing two components will be referred to in this chapter simply as a binary mixture, and a mixture of constant boiling point containing three components as a ternary mixture. 

Formation of Ternary Mixture of Minimum Boiling Point.—The substance added frequently forms a ternary mixture of constant boiling point with the two components of the original mixture, but the relative weights of these components in the ternary mixture differ from those in the binary mixture of constant boiling point which they themselves form. 

Methyl alcohol and benzene cannot be separated from each other by distillation, because their properties are so dissimilar that a mixture of minimum boiling point is formed, and, as its boiling point is much lower than that of the binary benzene-water mixture, no separation can bo effected by adding water and distilling. A ternary mixture does not come over, but the first fraction still consists of the benzene-alcohol mixture of constant boiling point. 

Thus, when a mixture of ethyl alcohol, benzene and water is distilled it tends to separate into (a) the ternary mixture of constant boiling point, (6) one of the three possible binary mixtures of constant boiling point, (c) that pure component which is in excess. It may, however, happen that the quantities [in the original mixture are such that we have only the two fractions a and b, or a and c, or the fraction a alone. There are, in fact, 12 different ways in which separation may take place, or the mixture may distil unchanged (p. 217). 

Again, the substances present may be capable of forming one or two binary mixtures but no ternary mixture of constant boiling point. That would be the case, for example, with isoamyl alcohol, benzene and water, for the only mixtures of constant boiling point that, can be formed are those of benzene and water (b. p. 69 25 ) or of water and amyl alcohol (b. p. 95 15 ). There are, therefore five different ways in which separation may occur, but under no conditions can the mixture distil without change of composition. Employing the initial letters A, B and W for the components —alcohol, benzene and water—we have the following possible separations. 

There are numerous properties of materials which can be used as measures of composition, e.g. preferential adsorption of components (as in chromatography), absorption of electromagnetic waves (infra-red, ultra-violet, etc.), refractive index, pH, density, etc. In many cases, however, the property will not give a unique result if there are more than two components, e.g. there may be a number of different compositions of a particular ternary liquid mixture which will have the same refractive index or will exhibit the same infra-red radiation absorption characteristics. Other difficulties can make a particular physical property unsuitable as a measure of composition for a particular system, e.g. the dielectric constant cannot be used if water is present as the dielectric constant of water is very much greater than that of most other liquids. Instruments containing optical systems (e.g. refractometers) and/or electromechanical feedback systems (e.g. some infra-red analysers) can be sensitive to mechanical vibration. In cases where it is not practicable to measure composition directly, then indirect or inferential means of obtaining a measurement which itself is a function of composition may be employed (e.g. the use of boiling temperature in a distillation column as a measure of the liquid composition—see Section 7.3.1). 

Comparing this result with the results of Example 5.1 shows the effect of adding a higher boiling material to the still (while holding benzene distribution constant). The high boiler permits the benzene concentration to be reduced below that achieved with the binary mixture. The benzene concentration m the product receiver is also increased, but the product is now a ternary mixture instead of a binary. 

Not all liquids form ideal solutions and conform to Raoult s law. Ethanol and water are such liquids. Because of molecular interaction, a mixture of 95.5% (by weight) of ethanol and 4.5% of water boils below (78.15°C) the boiling point of pure ethanol (78.3°C). Thus, no matter how efficient the distilling apparatus, 100% ethanol cannot be obtained by distillation of a mixture of, say, 75% water and 25% ethanol. A mixture of liquids of a certain definite composition that distills at a constant temperature without change in composition is called an azeotrope 95% ethanol is such an azeotrope. The boiling point-composition curve for the ethanol-water mixture is seen in Fig. 4. To prepare 100% ethanol the water can be removed chemically (reaction with calcium oxide) or by removal of the water as an azeotrope (with still another liquid). An azeotropic mixture of 32.4% ethanol and 67.6% benzene (bp 80.1 °C) boils at 68.2°C. A ternary azeotrope (bp 64.9°C) contains 74.1% benzene, 18.5% ethanol, and 7.4% water. Absolute alcohol (100% ethanol) is made by addition of benzene to 95% alcohol and removal of the water in the volatile benzene-water-alcohol azeotrope. 

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