Mixtures separable into Two Components

It has been stated (p. 62) that when a mixture of two substances is heated, the vapour is richer than the liquid 1 in the more volatile of the two components into which the mixture tends to separate, whether these components are the original substances which were mixed together or a mixture of constant boiling point and one of the original substances.

If, then, we distil the mixture in the usual manner until, say, one half of the total quantity has passed over, the distillate will be richer than the residue in the more volatile component. If we were to redistil the distillate and again collect the first half, the new distillate would be still richer in the more volatile component, and by repeating the operation several times we might eventually obtain some of the more volatile component in a pure, or nearly pure, state. The amount of distillate would, however, become smaller each time, and, if a large number of distillations were required, it would be relatively very small indeed. In order to obtain a fair quantity of both components in a sufficiently pure state, systematic fractional distillation is necessary.

Let us suppose that we have 200 grams of a mixture of equal weights of benzene and toluene. The relation between the vapour pressures and the molecular composition of mixtures of these liquids is expressed very nearly by a straight line and it is probable that Brown's is very nearly true, and that c does not differ greatly from 247, the mean ratio of the vapour pressures at equal temperatures between 80° and 110°. At any rate the liquid tends to separate on distillation into the original components, benzene and toluene, no mixture of constant boiling point being formed.

Collection of Distillate in "Fractions." - The mixture should be first distilled and the distillate collected in a convenient number of fractions, the receivers being changed when the boiling point reaches certain definite temperatures to be arranged beforehand. In order to trace the course of the separation as clearly as possible we will, in the first place, make the range of temperature nearly the same for most of the fractions. The boiling point of benzene is 80.2° and of toluene, 110.6°, the difference being 304° and we might take 10 small flasks to provide for 10 fractions, 8 with a range of 3° and 2 with a range of 3.2°, but it is better to take two fractions each for the first and last 3°. It is, as a rule, convenient to have the same number of fractions above and below the middle temperature between the two boiling points. Suitable temperature ranges for the twelve fractions are given in the second column of Table 37.

1 Unless a mixture of constant boiling point, of the same composition as that of the original mixture, is formed, in which case the composition of the vapour would be the same as that of the liquid.

Table 37

The first three receivers are, however, not required for the prelimin-ary distillation.

Correction Of Temperature

Before distilling the mixture we must read the barometer, because 80.2° and 110.6° are the boiling points of benzene and toluene respectively under normal pressure, and we must find what they would be under the actual barometric pressure, and alter the temperatures accordingly. If the thermometer does not register true temperatures, the necessary corrections must be ascertained and taken into account.

Let us suppose that the height of the barometer, corrected to 0° (p. 229), is 745 mm. and that the thermometer reads 0.2° too high at 80° and 0.3° too high at 110°. Referring to p. 14 we find that the value of c (= dt/dp . 1/T) for benzene is 0.000121 and for toluene, 0.000120. The corrections will therefore be

(273 + 80) . 0.000121 - 0.6° for benzene and

(273 + 111) . 0.000120 = 0.7o for toluene, and the boiling points under a pressure of 745 mm. will therefore be 80.2°-0-6° = 79.6° and 110.6°-0-7° = 109.9° respectively. The temperatures of the fractions under 745 mm. pressure are given in the third column and the actual readings on the thermometer in the fourth column.

When it is desired to separate the components of a mixture in the purest state attainable, the above corrections must be made with the greatest possible care, and it may not be sufficient to estimate the temperatures to 04° but to 0.05° or less ; it may also be necessary to read the barometer from time to time during the course of the distillation and to recalculate the corrections if there is any change in the pressure.1