Boiling Points Of Components

For mixtures of closely related sub-stances, or others which behave normally on distillation, the greater the difference between the boiling points the more readily can the components be separated by distillation. Thus, it has been shown (p. 125) that a mixture of normal and isopentane (b.p. 36.3° and 27.95° respectively) would require a much larger number of fractions than one of benzene and toluene if a still-head of the same efficiency were employed for both.

Boiling Point-Composition Curve

When the form of the boiling point-molecular composition curve is normal it is steeper in the region of high temperature than of low, and in general the temperature ranges above the middle point may be somewhat greater than below ; in other words, the number of fractions may be somewhat smaller. When the actual boiling points of mixtures of the two substances are lower than those given by the normal curve, this difference above and below the middle point becomes accentuated ; and when the curve is very flat near its lower extremity, the number of fractions in the low temperature region must be considerably increased while those above the middle point may be diminished in number. In most cases, however, data for the construction of the boiling point-molecular composition curve are wanting, and, unless we can judge from the nature of the substances in the mixture whether the deviation of the actual from the normal curve is likely to be large or not, we cannot decide on the number of fractions that may ultimately be required until after the fractionation has made some progress.

Three Components

If there are three substances present and the mixture behaves normally on distillation, the least volatile component will be the easiest to separate and the substance of intermediate boiling point the most difficult.

If, however, the components are not closely related to each other, one or more mixtures of constant boiling point may be formed and the problem becomes more complicated. 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, (b) one of the three possible binary azeotropic mixtures, (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. 179).

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 azeotropic mixtures 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: -

If only a single binary mixture of constant boiling point can be formed, as would be the case, for example, with a mixture of ethyl alcohol (A1), benzene, and isoamyl alcohol (A2), the following three separations would be possible: -

A detailed description of the separation of the three closely related liquids, methyl acetate, ethyl acetate, and propyl acetate (both with an ordinary and an improved still-head) is given in Chapters VII and XIV.

3. The use of Fractional Distillation as a Method of Quantitative Analysis has been fully discussed in Chapter XVI (Determination Of Composition Of Mixture).

4. A General Study of the Qualitative and Quantitative Composition of a Complex Mixture when only the General Nature of the Chief Components is known, or when it is definitely known that certain substances are present, but it is not known what others there may be.

Rough Estimate Of Composition

When a complex mixture, the qualitative composition of which is only partly known, is distilled through an efficient still-head and the distillate is collected in fractions of either equal temperature range or of approximately equal weight, a rough estimate of the boiling points and of the amounts of the components may frequently be obtained from the quantities of distillate in the first case, or from the temperature ranges in the second, or, generally, from the values of