Experimental Verification

The boiling points of three mixtures of chlorobenzene and bromobenzene were determined at a series of pressures between about 690 and 800 mm.1 The logarithms of the pressures were plotted against the temperatures, and the boiling points under normal pressure were read from the curve with the following results: -

The points representing these values are indicated in Fig. 22 by circles, and it will be seen that they fall very well indeed on the theoretical curve.

1 Young, " The Vapour Pressures and Boiling Points of Mixed Liquids, Part I.," Trans. Chem. Soc., 1902, 81, 768.

Determination By Dynamical Method

In determining the boiling points of mixed liquids by the dynamical method, it is of great importance that the vapour phase should be as small as possible, because if a relatively large amount of liquid were converted into vapour the composition of the residual liquid would, as a rule, differ sensibly from that of the original mixture. A reflux condenser must be used, and it is advisable that the thermometer should be shielded from any possible cooling effect of the returning liquid. The temperature of the vapour should be read, that of a boiling liquid, even when pure, being higher than the "boiling point " (p. 21); but as the difference between the temperatures of liquid and vapour is practically constant for a given mixture, in a given apparatus with a steady supply of heat, it is a good plan to read both temperatures at each pressure, to subtract the average difference between them from the temperature of the liquid, and to take the mean of the read temperature of the vapour and the reduced temperature of the liquid at each pressure as the true boiling point.

Apparatus

A suitable apparatus1 is shown in Fig. 23. It consists of a bulb of about 155 c.c. capacity with a wide vertical tube, to which is sealed a narrow side tube cooled by water to act as a reflux condenser. The upper end of the side tube is connected with an exhaust and compression pump and a differential gauge. The wide vertical tube is provided with a well-fitting cork, through which passes a rather narrower thin-walled tube, which has a hole blown in it just below the cork. This narrower tube is also fitted with a cork, through which passes the thermometer.

The quantity of each mixture placed in the bulb was such that its volume at the boiling point was about 125 c.c. ; the volume of vapour in the bulb and vertical tubes was about 75 c.c. The thin-walled tube was pushed down until the bottom of it was about 3 mm. above the surface of the liquid when cold, and the bottom of the thermometer bulb was about level with that of the tube.

This arrangement possesses the following advantages: (a) The liquid that returns from the reflux condenser cannot come near the thermometer, and the amount of liquid that condenses on the thermometer, and on the inner walls of the thin-walled tube, is exceedingly small; on the other hand, with the large quantity of liquid present and the small flame that is required, there is no fear of the vapour being superheated.

(b) It is possible to take readings of the temperature both of the vapour and of the liquid without altering the position of the thermometer, for when the burner is directly below the centre of the bulb, the liquid boils up into the thin-walled tube well above the thermometer bulb, but when the burner is moved a little to one side, the surface of the liquid immediately below that tube remains undisturbed and the liquid does not come in contact with the thermometer bulb.

1 Loc. cit.

Fig. 23. - Boiling point apparatus for mixtures.

Calculated Boiling Point-Molecular Composition Curves

- The relation between the boiling points and the molecular composition of mixtures of chlorobenzene and bromobenzene is represented by a curve (Fig. 22), the temperatures being lower than if the line were straight, and that is the case for any other pair of liquids for which the formula holds good. It is found that the curve approximates more and more closely to a straight line as the difference, A, between the boiling points of the components diminishes.

Liquids Not Closely Related

here is evidence that the actual boiling points of mixtures of liquids which are not closely related are, as a rule, considerably lower than those calculated from the vapour pressures, though occasionally higher, and that the formation of a mixture of constant boiling point is by no means an uncommon occurrence. The convenient term "azeotropic" was proposed by Wade and Merriman1 and adopted by Lecat2 for such mixtures, and will be employed in this book in preference to the term "hylotropic " suggested by Ostwald.

Azeotropic Mixtures Mixtures Of Minimum Boiling Point

In the great majority of cases where the formation of mixtures of minimum boiling point has been observed, one of the two liquids is a hydroxyl compound - an alcohol, an acid or water - and water also forms such mixtures with all the lower alcohols, except methyl alcohol. It is well known that the molecules of these liquids are more or less associated in the liquid state, and we may therefore conclude that mixtures of minimum boiling point (maximum vapour pressure) are most readily formed when one or both of the liquids exhibit molecular association.

It is probable that the molecules of acetone, and also of the lower aliphatic esters, are associated to a slight extent in the liquid state, and Ryland 3 found in 1899 that the following pairs of liquids form mixtures of minimum boiling point: carbon disulphide and acetone; carbon disulphide and methyl acetate; carbon disulphide and ethyl acetate; acetone and methyl acetate; acetone and ethyl iodide; ethyl iodide and ethyl acetate.

In the following year Zawidski4 showed that the first and last of these pairs of liquids form mixtures of maximum vapour pressure.

Lecat 5 has observed the formation of a considerable number of mixtures of minimum boiling point one of the components of which is an ester or one of the lower ketones.

1 Trans. Chem. Soc, 1911, 99, 1004.

2 Le Tension de vapeur des melanges de liquides, VAzeotropisme (Brussels), 1918.

3 Ryland, "Liquid Mixtures of Constant Boiling Point," Amer. Chem. Journal, 1899,22,384.

4 Zawidski, " On the Vapour Pressures of Binary Mixtures of Liquids," Zeitschr. physik. Chem., 1900, 35, 129. 5 hoc. cit.

It was thought by Speyers 1 that a mixture of minimum boiling point cannot be formed when both constituents have normal molecular weight at all concentrations, but this conclusion does not appear to be borne out by the facts, and it is clear that when is very small, a comparatively slight deviation from the normal form of the boiling point-molecular composition curve would be sufficient to account for the formation of a mixture of minimum boiling point. It is certain that benzene and carbon tetrachloride =344°) form such a mixture and most probable that benzene and n-hexane =11.3°) do so. Ryland (loc. cit.) states that carbon disulphide and ethyl bromide

= about 7.6°) form a mixture of minimum boiling point, and Zawidski (loc. cit.) finds that methylal and carbon disulphide =4.15°) form a mixture of maximum vapour pressure. These results have been confirmed and a considerable number of other cases observed by Lecat, who has made an exhaustive study of the literature on this subject and has himself examined more than two thousand mixtures and discovered a great number of azeotropic mixtures.