LABORATORY EXPERIMENTS ON THE BOILING-POINT CURVES OF BINARY MIXTURES Fruwn J. SODAY AND Geonce W. BENNETT, GROVE C I ~ YCOLLEGE, GROVECITY, PENNA.
In a previous paper by one of us1 laboratory directions were given for plotting the boiling-point curves of non-azeotropic mixtures over the entire boiling-point range. It was pointed out in that place that the usual laboratory experiments on maximum-boiling mixtures give data for the composition of both phases over only a part of the temperature range, and that similar experiments on minimum-boiling mixtures give data for only one phase over the temperature range. It is the purpose of this paper to describe laboratory experiments which give data for the composition of both phases over the entire boiling-point range for each of the three types of binary liquid mixtures. The difficulty in devising such experiments consists mainly, of course, in utilizing systems that may be analyzed readily. In spite of some 1500 or more azeotropic systems there are only a few that are suited for student work either because they include relatively uncommon laboratory materials or because the systems themselves are unsuitable. Thus, for example, the majority of maximum-boiling systems have as one component either an aqueous solution of a gas, such as the halogen acids, or corrosive and high-boiling substances, such as formic acid, phenols, or cresols. Analysis of many of the minimum-boiling mixtures can be made only by tedious physical methods. , The analysis of some binary liquid mixtyes can be effected very rapidly and with a very small sample by the use of the Abb6 refractometer. A fair degree of accuracy, moreover, may be had if the refractive indices of the two liquids differ sufficiently. It is desirable also that relatively common materials should be used. With these limitations in mind the following systems were found suitable for laboratory experiments. A. Maximum-Boiling Mixtures.-Chloroform-acetone2 and chloroform-methyl a ~ e t a t e . ~ B. Minimum-Boiling Mixtures.-Benzene-methanol4 and carbon tetrachloride-methanol." C. Non-Azeotropic Mixtures.-Benzene-acetone,= toluene-acetone,' and acetic acid-ben~ene.~ 1
G. W. Bennett, THISJ O ~ N A6,1544 I,, (Sept., 1929). Lecat, "L'Azeotropisme-La Tension de Vapeur des Melanges de Liquides,"
Bruxelles, 1918, p. 178 for data and bibliography. Ibid., p. 180. "Ihid., p. 99. Ibid., p. 82. Ibid., p. 126. Ibid., p. 126. 8 Ibid.. p. 72.
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The materials used were carefully purified. The ordinary laboratory acetone, benzene, toluene, carbon tetrachloride, and chloroform were purified by common methods, and were then dried and fractionated. The other materials were "Eastman" grade. The methanol was acetone-free; the acetic acid was 99.9% pure and melted a t 16.6; and the methyl acetate was anhydrous. The values for the boiling points of the materials used, when corrected to 760 mm. pressure, did not vary more than O.1° from the values listed in the "International Critical Tables," and those for the refractive indices did not vary by more than three in the fourth decimal place from the standard valuesg Procedure The apparatus used for these experiments was the same as that described in the earlier paper, and the procedure also is practically the same. In every case it is desirable to start the distillation with one pure liquid and add successive amounts of the second liquid until the azeotropic point is reached. The contents of the flask are then rejected, and the same procedure is followed beginning with the other pure liquid. An excessive amount of reagent is required if one attempts to trace the curves from the azeotropic point to pure liquid. The amount of pure liquid to add after each sampling varies with the system, but in general if 10 cc. of pure liquid and 15 cc. of distillate are added the change in the boiling point is not too great. In the case of the ndn-azeotropic systems described in this paper i t is usually better to begin withthe low-boiling liquid and add the higher boiling liquid although one may follow the opposite procedure. With some non-azeotropic systems, indeed, such as acetic acidwater, i t is essential to follow this latter procedure in order to avoid using an excessive amount of acid. The samples of the liquid and vapor phases are analyzed by means of the Abb6 refractometer. The refractive indices of the samples -are determined, and the values so obtained are compared to a reference refractive index-composition curve from which the mmpositions of the samples are found. This reference curve is made by plotting the values for the refractive indices of the two pure liquids against the weight per cent composition. The refractive indices are all determined a t 20°C. Typical Experimental Details The accompanying graph and Table I give some representative details obtained by the experimental procedur&outlined above. In Table I data obtained in typical distillations are given for each of the three types of distillation mixtures. The first column in each case gives "International Critical Tables," Vol. 111, pp. 215-23; Val. I, p. 176 e t seq.; p. 276 et seq.
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1930
TABLE I Data for the Boiling-Point-Composition Curves Chloroform-Acetone Benzene-Methanol Benzene-Acetone Temp. OC.
55.2 56.0
Chloroimm Chloroform Temp. OC. m vapor in liquid
O.O'% 9.0
0.0% 16.0
78.6 68.2
?Cethanol Methanol ln vapor in liquid
0.0% 8.0
0.0% 1.0 8
12.0 10.0 40.0 60.0 69.8 79.0 86.8 95.9 100.0 Barometer 725.2 mm.
56.4 56.5 56.6 56.9 57.6 58.3 59.55 61.9 63.1
Barometer 732 mm
Temp. OC.
Acetone ~nvapor
in liquid
79.1 77.7 75.5
0.0% 4.0 10.5
0.0% 0.9 2.5
Aecrone
38.0 37.8 38.0 45.0 50.5 58.0 69.0 85.8 100.0
Barometer 732 mm.
the temperature a t which the vapor ,(column 2) and the liquid (column 3) have the composition in weight per cent of the constituent stated a t the head of the column. The barometric pres%urein each case is that a t which the distillation was performed. In the accompanying figure the data given in Table I is represented graphically. The curves marked A refer to the system benzene-methanol, those marked B to the system benzene-acetone, and those marked C to the system chloroform-acetone. The temperatures to the left of the diagram refer to the systems benzene-methanol and benzene-acetone, while those a t the right of the diagram refer to the system chloroformacetone.
Discussion The method for plotting the boiling-point curves given in this paper is not a precision method. Errors inherent in the apparatus, procedure, and analysis prevent that. The error in reading the refractometer may be as high as 1.0-1.5 per cent, but in general these errors are from 0.30.6 per cent with the liqnids uSed in these experiments. Choice of the liquids to be used depends on the cost of materials, the boiling points of the pure liquids and of the azeotropic mixtures, and on the respective refractive indices. If the range between the boiling points of the pure liqnids and the azeotropic mixture is not several degrees a t
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least, the resolution of the temperature-composition w e s is not great enough to show any appreciable difference. There must also be a considerable range in the refractive indices of a given liquid pair in order to
obtain accuracy of analysis since the instrument is not accurate to more than five in the fourth decimal place. This range in the refractive indices then should be about one in the first decimal dace. The time required for a complete determination is about three hours.
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One student can analyze the samples as fast as they are obtained by a second student who conducts the distillation. Graphs for different determinations on the same system are very much alike in general characteristics, but frequently they cannot be exactly superimposed. As a consequence, the values for the azeotropic temperature and composition are not always the same. In Table I1 typical experimental values are compared to those given by L e ~ a t . ~ System
Azeotropic Tempe~atureAzeotropic Composition Barometer
Obr'd.
Ref.
62.4' 61.95 62.25
64.5'
Chloroform-Methyl acetate
62.6 63.0
64.8
Methanol-Benzene
56.7 56.7
Methanol-Carbon tetrachloride
54.2 54.5
Acetone-Chloroform
Ob.'d.
Ref.
20.5%
732.0 mm. 716.5 719.0
56.0 50.0
77.0
717.5 726.3
58.34
38.0 38.0
39.5
725.5 737.0
55.7
21.0 23.0
20.56
718.0 734.0
20.0% 15.0 14.0
.
In column 1 the name of the system is given. In column 2 some experimentally observed azeotropic points are compared to the reference values in column 3. Column 4 gives %he azeotropic compositions in weight per cent of the first-named constituent in column 1, while the corresponding reference values are given in column 5. In the last column the barometric pressures under which the distillations were performed are given. I t has long been recognized10 that the composition of constant-boiling mixtures is dependent on a number of factors. We are, nevertheless, unable at present to account for the wide difference in the composition of the azeotropic mixture of chloroform-methyl acetate as found by us and as given by Lecat. In each of the other cases, however, the agreement of observed and reference values is fairly good. l o Briggs, I.P h y . Ckem., 28, 644 (1924).
World Mineral Bankruptcy was prophesized by Sir Thomas H. Holland, Principal of Edinburgh University, in an address t o the Mining Institute of Scotland. The total quantity of mineral consumed since 1900 exceeded that which was consumed in the whole history of the world before that year, and their destruction w a s even greater than before, because they were tending now to use only the rich deposits which satisfied their essential large-scale operations.-Cham. Age
