Evaporation Into Vacuous Space

When two volatile liquids miscible, partially miscible or non-miscible - are placed together in a vacuous space, such as that over the mercury in a barometer tube, evaporation takes place and, as a rule, the composition of the residual liquid differs from that of the vapour. It is only when the liquids form a mixture of maximum or minimum vapour pressure - and therefore of constant boiling point - and when it is this particular mixture that is introduced into the vacuous space, that the composition of the vapour is the same as that of the liquid. In all other cases the vapour is richer in the more volatile of the two components into which the mixture tends to separate when distilled, these components being either the original substances from which the mixture was formed, or one of these substances and a mixture of the two which has a higher or lower boiling point than that of either of the original constituents.

If the volume of vapour is relatively very small, the composition of the residual liquid will differ only slightly from that of the original mixture, but if it is relatively very large, and if the boiling points of the two components into which the mixture tends to separate are not very close together, the residual liquid will be much richer in the less volatile component than the original mixture.

Methods Employed

The difficulties attending the experimental determination of the composition of liquid and vapour are, in most cases, very considerable, and, unless great care be taken, erroneous and misleading results may be obtained. The chief methods which have been employed are the following: 1. A mixture of known composition is introduced into a suitable still; a relatively very small quantity is distilled over, and the composition of the distillate - and, in some cases, of the residue also - is determined either (a) from its specific gravity (Brown),1 (b) from its refractive power (Lehfeldt),2 Zawidski,3 (c) from its boiling point

1 F. D. Brown, "Theory of Fractional Distillation," Trans. Chem. Soc, 1879, 35, 547 ; "On the Distillation of Mixtures of Carbon Disulphide and Carbon Tetrachloride," ibid., 1881, 39, 304.

2 Lehfeldt, " Properties of Liquid Mixtures, Part II.," Phil. Mag., 1898, [V], 46, 42.

3 Zawidski, "On the Vapour Pressures of Binary Mixtures of Liquids," Zeiischr. physik. Chem., 1900, 35, 129.

(Carveth),1 or (d) by quantitative analysis; but fpr organic liquids the last method is not generally suitable. The distillation may be carried out either in the ordinary manner under constant pressure, or at constant temperature.

2. A known volume of air is passed through the mixture at constant temperature 2; the total amount of evaporation is ascertained from the loss of weight of the liquid, and the weight of one component in the vapour is determined by quantitative analysis.

3. If a binary liquid whose components are in the ratio x/(l - x) is in equilibrium with a vapour containing the same components in the ratio P1/p2, then a saturated vapour of this composition will bubble through the liquid without producing or itself undergoing any change. A method, based on this principle, has been devised and employed by Rosanoff and his co-workers.3

4. A distillation is carried out with a still-head kept at a constant temperature.4 The composition of the distillate is determined in the usual manner ; that of the mixture distilled is ascertained from the temperature of the still-head, the boiling point-composition curve having been previously constructed.

First Method

Distillations under constant pressure have been carried out by Duclaux 5 and, with great care, by F. D. Brown 6 and Rosanoff, Bacon and White.7

Brown's Apparatus

The apparatus employed by Brown is shown in Fig. 24. "It consists of a copper vessel, s, shaped like an ordinary tin can, but provided with a long neck a. This neck and the upper portion of the vessel are covered with a copper jacket, c c c, which communicates with the inner vessel by means of some small holes round the upper part of a. This outer jacket is terminated below by a strip of copper placed 'obliquely to the axis of the vessel, and at its lower portion is fitted with a narrow tube, d, which serves to connect the still with the condenser. The vapour rising from the liquid in the vessel s passes through the holes at a, and then descending, passes out at d. The vapour as it rises is thus kept warm, and none of it is condensed until it has entered the outer jacket. Here a slight condensation is of no influence, as both vapour and liquid pass together into the receiver. The inclination of the bottom of the jacket serves to prevent the accumulation of any liquid at that part."

1 Carveth, " The Composition of Mixed Vapours," Journ. Phys. Chem., 1899, 3, 193.

2 Linebarger, "The Vapour Tensions of Mixtures of Volatile Liquids," Journ. Amer. Chem. Soc, 1895, 17, 615.

3 Rosanoff, Lamb and Breithut, " Measurement of the Partial Vapour Pressures of Binary Mixtures," J. Amer. Chem. Soc, 1909, 31, 448; Rosanoff and Easley, ibid., p. 953; Rosanoff and Bacon, ibid., 1915, 37, 301.

4 F. D. Brown, "Fractional Distillation with a Still-head of Uniform Temperature," Trans. Chem. Soc, 1881, 39, 517.

5 Duclaux, "Tension of the Vapour given off by a Mixture of Two Liquids," Ann. Chim. Phys., 1878, [5], 14, 305.

6 Brown, "The Comparative Value of different Methods of Fractional Distillation," Trans. Chem. Soc, 1880, 37, 49.

7 Rosanoff, Bacon and White, " Rapid Laboratory Method of Measuring the Partial Pressures of Liquid Mixtures," J. Amer. Chem. Soc, 1914, 36, 1803.

After heat has been applied, but before ebullition commences, a good deal of evaporation takes place and the mixture of warm air and vapour passes into the condenser, when most of the vapour is condensed. Brown, therefore, only made use of the data obtained from the first fraction of the distillate to ascertain the composition of the residue in the still at the moment of change from the first to the second fraction.

The form of receiver shown in Fig. 24 was used in order to avoid evaporation and consequent change in composition of the fractions, and also to allow of the distillation being carried on under reduced pressure. The quantity of liquid placed in the still was usually about 900 or 1000 grams, and in each experiment about one-fourth of the total amount was distilled over, and was collected in four fractions. The composition of the residual liquid at the end of the distillation was found in every case to agree satisfactorily with that calculated from the composition of the original mixture and of the fractions collected.

This apparatus was employed for mixtures of carbon disulphide and carbon tetrachloride; a preliminary series of determinations of the specific gravity of mixtures of these substances having been made, in order that the composition of any mixture might subsequently be ascertained from its specific gravity. A similar series of distillations was afterwards carried out under reduced pressure (about 430 mm.), the apparatus being connected with a large air reservoir and with a pump and gauge. In the earlier experiments with mixtures of carbon disulphide and benzene somewhat less satisfactory forms of still were employed.