Closely Related Substances

By far the simplest cases are those in which the substances present are chemically closely related to each other, for the form of the boiling-point molecular composition curve must then be normal or nearly so (Chap. IV.); there is no possibility of the formation of mixtures of constant boiling point, and the only points to be considered are the actual boiling points of the components and the difference between them.

Boiling Points Of Components

On the actual boiling points will depend the nature of the still-head that can be used ; the greater the difference between them, the more easily can the separation be effected.

Substances not Closely Related ; Mixtures of Constant Boiling Point.

- If the substances are not closely related to each other, mixtures of constant boiling point may be formed and the list of such mixtures which are at present known (p. 49) may be consulted. It will be sufficient to remark here that water forms mixtures of minimum boiling point with the majority of organic compounds ; that, generally, such mixtures are most frequently met with when the molecular weight of one of the components is abnormal in the liquid state, and that this is usually the case with compounds which contain a hydroxyl group.

It should also be remembered that when the boiling points of any two substances are near together, a small deviation from the normal boiling point-molecular composition curve is sufficient to give rise to the formation of a mixture of constant boiling point, or, at any rate, to make the curve nearly horizontal over a large part of its course starting from one extremity, usually that corresponding to the lowest temperature. It has been pointed out (p. 181) that when the curve is of this form it is very difficult, and may be impossible, to separate that component which is in excess where the curve is nearly horizontal.

When a mixture which distils without change of composition is formed, and its boiling point is far below that of the more volatile component, as in the case of normal propyl alcohol and water, or of methyl alcohol and benzene, or far above that of the less volatile component, as with nitric acid and water, it is usually possible to separate in a pure state both the mixture of constant boiling point and that component which is in excess. But, if the boiling point of the mixture is very near that of one of the two components, as in the case of ethyl alcohol and water (Fig. 81), or of normal hexane and benzene, it is practically impossible to separate that component in a pure state, and it may be impossible to separate the mixture of constant boiling point even when the component which boils at a widely different temperature is in excess. Thus, in the cases referred to, whatever the composition of the original mixture, it is impossible to obtain normal hexane, ethyl alcohol, the binary hexane-benzene mixture or the alcohol-water mixture in a pure state by fractional distillation, and it is only the less volatile component, benzene or water, which can be so obtained.

The composition of a mixture of constant boiling point, however, depends on the pressure, and it has been shown by Wade and Merriman 1 in the case of ethyl alcohol and water that as the pressure falls the percentage of water in the azeotropic mixture diminishes, and that at pressures lower than about 75 mm. no such mixture is formed. It would, therefore, theoretically be possible to separate both ethyl alcohol and water from a mixture of these substances by distillation under sufficiently reduced pressure. In practice, however, the separation of pure alcohol under low pressures would be as difficult as that of the azeotropic mixture under atmospheric pressure.

1 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; also Merriman, " The Vapour Pressures of the Lower Alcohols and their Azeotropic Mixtures with Water," ibid., 1913, 103, 628.

Separation Of Components

The manner in which the composition of the distillate is related to the total amount collected has been discussed in Chap. VIII., and full details of the separation of benzene and toluene by fractional distillation both with an ordinary and an improved still-head have been given (pp. 100 and 158).

Fig. 81. - Boiling points of mixtures of ethyl alcohol and water.

Number of Fractions required and Choice of Still-head.It is difficult to formulate any definite rules regarding the number of fractions in which the distillate should be collected, so many points have to be taken into consideration. The most important of these are the following: (a) The efficiency of the still-head employed.

(b) The approximate quantity of each component.

(c) The difference between the boiling points of the components.

(d) The form of the boiling point-molecular composition curve.

Efficiency Of Still-Head

It may be stated quite generally that, for a given mixture, the more efficient the still-head the smaller is the number of fractions that will be required.

Amount Of Each Component Present

The approximate quantity of each component must be taken into consideration, not only in deciding upon the number of fractions but also in choosing the still-head to be used.

Suppose, for example, that it was desired to separate the benzene as completely as possible from a mixture consisting of 30 grams of that hydrocarbon with 270 grams of toluene.

The best plan would be to employ for the first distillation the most efficient still-head available, and if the recovery of the toluene need not be regarded as of special importance, the complete return of the liquid from the still-head to the still at the end of the distillation would not be essential, and a "bubbling " still-head of many sections might be used. A very slow distillation would be advantageous. It would probably be best to collect all the distillate that came over below 110.0° in one fraction and that from 110.0° to 110.6° in a second. The large residue would consist of pure toluene. The same still-head might be used for the second distillation and the distillate might be collected in three or four fractions, the temperature ranges of which would depend on the efficiency of the still-head. With an 18 section Young and Thomas still-head it is probable that nearly 20 grams of distillate would come over below 81.2°. The fractions would now be small, and it would be necessary to employ a still-head of fewer sections for the remaining distillations. It would be important that the amount of condensed liquid in the still-head should be as small as possible, and an evaporator still-head of 5 sections would probably be the most convenient.

If the original mixture contained 270 grams of benzene and 30 grams of toluene, and it was desired to recover the toluene, it would again be best to employ a very efficient still-head, and as, in this case, a most essential point would be that the liquid should return very completely from the still-head to the still at the end of the distillation, an evaporator still-head of many sections would be the best for the purpose.

A mixture of this composition, distilled through a very efficient still-head, would give a considerable amount of nearly pure benzene ; so far as the recovery of toluene is concerned, the first 100 grams, or even more, would not be worth redistilling. After rejecting this, fractions might be collected below 80.5°, from 80.5° to 95.6°, and above 95.6°. If the temperature reached 110.6° before the end of the distillation the residue would consist of pure toluene ; if not, it would require to be redistilled. In the second fractionation, the distillate from the first fraction of I. would consist of nearly pure benzene and might be rejected. The remaining fractions would now be small, and an evaporator still-head of not more than 5 sections would have to be used for the remaining distillations.