Distillation After The Addition Of A Third Substance

It has been pointed out that there are many cases in which the two components of a mixture cannot be separated by fractional distillation, owing to the formation of a mixture of constant boiling point. It is, however, sometimes possible to eliminate one of them by adding a third substance and then distilling the mixture.1

Formation of Binary Mixture of Minimum Boiling Point.

- Suppose, for example, that we have a mixture of isobutyl alcohol and benzene containing 10 per cent by weight of the alcohol. The components cannot be separated by fractional distillation, because a mixture of minimum boiling point (79.93°), containing 9.3 per cent,of isobutyl alcohol, is formed. Neither can they be satisfactorily separated by treatment with water, owing to the fact that the solubility of isobutyl alcohol in benzene is greater than in water.

If, however, a little water be added and the mixture be distilled, the first fraction will consist of the binary mixture of benzene and water, boiling at 69.25° and containing 91.2 per cent of benzene.

In order to remove the benzene, all that is necessary is to add water to the mixture in the ratio of 8.7 to 100 parts by weight and to distil with an efficient still-head. The mixture will then tend to separate into (a) the binary benzene-water mixture (b.p. 69.25°), and (b) pure isobutyl alcohol (b.p. 108.05°). Here the difference between the boiling points is considerable, and the separation is an easy one.

If too much water were added we should have, as an intermediate fraction, the binary alcohol-water mixture which boils at 89.8°. and contains 66.8 per cent of the alcohol.

If, on the other hand, too little water were added, only a part of the benzene would be carried over with it and the remainder would form the binary benzene-alcohol mixture; the residue in either case would consist of pure isobutyl alcohol.

The method has been employed in a number of cases by Golodetz.

1 Young, "The Preparation of Absolute Alcohol from Strong Spirit," Trans. Chem. Soc, 1902, 81, 707 ; Young and Fortey, " The Properties of Mixtures of the Lower Alcohols.with Benzene and with Benzene and Water," ibid., 1902, 81, 717.

2 Golodetz, J. Buss. Phys. Chem. Ges., 1911, 43', 1041; Chem. Zenlr., 1912, 1, 69.

Formation of Ternary Mixture of Minimum Boiling Point.

- The substance added sometimes 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.

Tertiary Butyl Alcohol And Water With Benzene

Take, for example, the case of tertiary butyl alcohol and water. This alcohol is a crystalline solid, melting at 25.53° and boiling at 82.55° ; when liquefied, it is miscible with water in all proportions and forms with it a mixture of constant boiling point (79.91°) containing 88.24 per cent of the alcohol.

The difference between the boiling point of the binary mixture and of the alcohol is only 2.64°, and the separation of the last traces of water, or rather of the binary mixture, from the alcohol by distillation is difficult. It was not, in fact, found possible to obtain the alcohol quite pure by this method, the highest melting point of the distilled alcohol observed being 25.25°, and the boiling point 82.45°.

Fractional crystallisation gave a better result, the melting point of the recrystallised alcohol being 25.43°.

Eventually, however, it was found that the last traces of water could best be removed by distillation with benzene, both the melting and boiling points (given above) of the residual alcohol being higher than those of the product purified by recrystallisation. Table 74 gives the boiling points and the composition of the binary and ternary mixtures and the boiling points of the single components.

Table 74

It will be seen that the ratio of water to alcohol in the binary mixture and in the ternary mixture so that the latter contains nearly three times as much water, relatively to the alcohol, as the binary mixture does.

Not only can we remove the last traces of water from the nearly pure alcohol by means of benzene; but it is also possible to obtain the pure alcohol from the binary alcohol-water mixture.

Suppose that we start with 100 grams of this mixture; the water in it, 11.76 grams, would require grams of benzene for conversion into the ternary mixture, and if a perfect separation could be effected by a single distillation of the mixture, 145 grams of the ternary mixture and 57 grams of alcohol would be obtained. In practice, however, we should get a rather smaller amount of the ternary mixture, a little binary alcohol-benzene mixture, and a residue of alcohol, the second and third fractions still containing a little water.

It is better to add a larger amount of benzene, say 125 grams, in the first place, when the quantity of the second fraction (alcohol-benzene) will be increased and the smaller amount of residual alcohol will be obtained free from water in a single operation.