VAPORIZATION OF LIQUID MIXTURES* BY J. F. KINO AND S. P. SMEDLEY
Introduction Among the different classes of physical mixtures, the simplest ones and the ones for which the relations have been most satisfactorily worked out are the gaseous mixtures. The generalizations of Raoult and van't Hoff along with the work of many investigators such as that of Morse and Frazer have cleared up to a great extent the behavior of mixtures of solids in liquids. In this class the relations are quite well understood, at least, for certain ones of solids in liquids where the mol fraction of solid is small. The properties, however, of mixtures of liquids in liquids are not so well understood and the relations which exist in this class are apt to be quite complicated. . Seldom are the properties additive or can they be calculated from the properties of the pure components. We, therefore, can classify all liquids into two groups, those which form ideal mixtures with normal properties and those whose mixtures have abnormal properties. One of the most important properties of liquid mixtures is that of vapor pressure. This property is made use of whenever it is desired to separate the liquids by fractional distillation. The abnormal liquids give the mixtures which are constant evaporating, having a maximum or minimum vapor pressure. The normal liquids can be separated from their mixtures by fractional distillation. Frequently chemists wish to know whether a given pair of liquids forms a constant evaporating mixture. The determination of the vapor pressure curve for a series of liquid mixtures gives a method for determining the existance of a constant evaporating mixture. However, once the vapor pressure curve has been constructed, it is usually difficult to determine just what the composition of the mixture is which has the maximum or minimum vapor pressure because of the flatness of the vapor pressure curve. One can use the method of Rosanoff' of finding the equation of the curve and solving for the point of inflection. Still this does not enable one to locate the composition of the mixture any more closely. Chemists usually make use of distillation methods for determining the constant evaporating mixture. The distillation method is a tedious one to carry out and quite difficult where the mixture has to be determined with any degree of accuracy. I n this paper we give three simple and rapid methods which can be used for determining the composition of the constant evaporating mixture. First Method Determination of the Vapor Pressure Curve Several static and dynamic methods for determining the vapor pressure of mixtures of liquids have been worked out. The principal objection to all these *Contribution from the Thompson Chemical Laboratory of Williams College. J. Am. Chem. SOC.36, 1993 (1914).
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J. F. KING AND 6 . P. SMEDLEY
methods is the length of time necessary to complete a determination and the difficulty of manipulation. Chemists appear to favor the dynamic methods. Among the dynamic methods are: I. The method of boiling points where the boiling points of a series of liquid mixtures are determined at different pressures. ‘Then the pressures corresponding to a certain temperature are plotted against composition. The objection t o this method is the large number of observations needed for the cofistruction of the vapor pressure curve. The air saturation method gives an accurate method for determining 2. the total vapor pressure as well as the partial pressures of the mixtures. The objection to this method is the large amount of time necessary to bring the bir t o saturation as it is drawn through the mixtures. 3. We also have the method worked out by Rosanoff, Bacon and White’ of passing a vapor of known composition through the mixture until equilibrium is reached and then analysing the mixture. This method is also slow of operation and difficult of manipulation. The “submerged bulblet” method of Smith and Menzies2 gives a very simple, rapid and accurate method for determining vapor pressures and boiling points. This method has been applied to:I. The determination of the boiling point of a pure liquid. 2. The determination of the vapor pressure or the sublimation point of a pure solid. 3. The determination of the purity of a liquid by noting the changing boiling point as vaporization takes place. 4. The determination of the vapor pressure curves of, pure substances. From the “submerged bulblet” method, Smith and Menzies developed their “static isotenoscope7’3and their “dynamic isotenoscope”4. Their static isotenoscopic method can be adapted to the determination of the vapor pressures of mixtures of liquids. Here again, however, we find the method rather difficult of manipulation. It has been our purpose to modify the very simple and rapid “submerged bulblet” method of Smith and Menzies for the determination of the vapor pressure curve of liquid mixtures. The method as worked out by Smith and Menzies cannot be so used because there is no provision for keeping the composition of the surface of the liquid mixture the same as the body of the mixture, If the mixture could be thoroughly stirred, this method could very easily be used. We have used their modified apparatus in determining the vapor pressure curve for a series of mixtures of carbon tetrachloride and ethyl alcohol. 1 2
a
J. Am. Chem. SOC. 36, J. Am. Chem. SOC.32, J. Am. Chem. SOC.32, J. Am. Chem. SOC.32,
1803 (1914). 897,907 (1910). 1412(1910). 1448 (1910).
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1267
I n Fig. I the shape of their bulblet was’changed to the shape as seen at “A?,, The tenth of a degree thermometer holding the bulblet was passed through a larger glass tube a t “a7’and fastened to it by a piece of rubber tubing a t “b”. The thermometer was tapped a t “c” by a mechanical device which consisted of a sort of cam, “d”, attached t o the extended shaft of the stirrer of the thermostat, “e”. The thermometer was held back by a rubber band a t “f” and as the cam hit the C thermometer, it rocked back and forth ! rapidly in its loose fitting in the glass tubing through the rubber stopper, “g”. The effect was to stir the liquids in the bulblet up violently from the bottom and maintain a constant composition throughout. The large neck of the bulblet allowed the splash to drain back into the bulblet. “h” is connected with a large suction bottle and ‘5’’ is connected to a mercury manometer for recording the pressure on the inside of the apparatus. With all the precautions and corrections suggested by Smith and Menzies, this method is possible of as great accuracy as they obtained for a single liquid. The ethyl alcohol was prepared by refluxing over lime. Density 2oo/4O was 0 . 7 8 9 . Merck C. P. carbon tetrachloride was allowed to stand over KOH for 12 hours. It was then distilled and treated with P z O ~ and again distilled with a Young fractionating column. Density 2oo/4O was 1.594. The mixtures were made up in % by volume. Accurate pipettes were used and the measuring was done in a constant temperature Fro. I room a t zoo C. The bulblets were filled by immersing the ends in the mixtures contained in small flasks. The air was pumped out of the bulblets and the liquid allowed to enter, filling the bulblets half full. The bulblet was fastened to the thermometer by means of rubber bands and the thermometer was fixed in its position in the apparatus. The suction was then applied and aft.er the air in the bulblet had been displaced, a reading, if constant, was taken on the manometer. The vapor pressure was then calculated according to the method
E
c
3
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J. F. KING AND S. P. SMEDLEY
of Smith and Menzies. Since we did not care to determine the vapor pressure to an accuracy greater than 4= I mm., we were able to eliminate some of the finer corrections which they pointed out. Duplicate and triplicate determinations were made. The method is very rapid and simple. The bulblets can all be filled at the start and some eight or ten points determined on the curve in less than two hours. The values which we obtained are as follows:
yo alcohol by volume
% alcohol by volume
vapor pressure of mixture at 20°C. mm. Hg.
vapor pressure of mixture a t 20%. mm. Hg.
90
50
I03
IO
I11
70
20
I12
80
87 75
30 40
I IO
90
60
108
IO0
44
0
%COMPOS/ 7fO N FIG.2 The Second Method Determination of the Constant Evaporating Mixture by Use of the Abbe Refractometer The principle of this method is as follows:A series of mixtures of two liquids is made up and the refractive indices of the mixtures are determined by use of the Abbe refractometer. The usual refractive index-composition curve as “y” in Fig. z is drawn. Then the prism is opened and a definite amount of one of the mixtures is placed inside and allowed to evaporate for a definite number of seconds. The prism is then closed and the refractive index of the residue is determined. The plotted values give the curve “z” in Fig. 2 .
VAPORIZATION O F LIQUID MIXTURES
I 269
Where the two curves cross a t ((x” we have a mixture which does not change in composition on evaporation. For an approximate determination of the composition of the constant evaporating mixture, the points “a”, “a”’, “b”, “b”’ can be determined. Then since the curves are nearly flat in this region, straight lines can be drawn connecting the points and the point of intersection can be very easily located. I n our determination of the mixture for carbon tetrachloride and alcohol, the prism was kept a t 20’ by allowing water from a thermostat to circulate through. The refract.ive indices were determined without evaporation with the prism closed and with a small stream of the mixture flowing through the side of the pripm. This prevented evaporation and gave a constant value. The refractive indices with evaporation were determined by dropping four drops of the mixture on the prism and allowing the mixt,ure to remain exposed to
%CcmPaa/r/M/ ’ FIG.3
the air for 2 0 seconds before closing the prism. Several points on both curves were determined. The points on either side of the constant evaporating mixture are as follows:Refractive indices with evaporation
% alcohol by volume without evaporation I . 4290 30 IO 1.4515
I. 4308
1.4504
The Third Method Determination of the Constant Evaporating Mixture by an Air Bubbling Method The principle of the third method is to pass a certain volume of air at a definite temperature and pressure through mixtures of the liquids and then determine the change in composition. If this change is plotted against the original composition as in Fig. 3, we are able to locate the point ‘(x” which gives us the composition of the,mixture which does not change on evaporation. Thus it would be possible by determining the points “a”, “b”, and “c”, for instance to locate with some accuracy the constant evaporating mixture a t x 11. The apparatus which we worked out for studying this method is without doubt much more elaborate than would be needed for an approximate deter(
