A METHOD FOR A MORE COMPLETE EXAMINATION OF BINARY LIQUID MIXTURES W. ANDREW WRIGHT Severance Chemical Laboratory, Oberlin College, Oberlin, Ohio Received November 11, 19%
The customary examinations of liquid mixtures (either isothermal or isobaric) have been time-consuming and tedious enough, almost to prevent an exhaustive study of any particular mixture. Theoretical chemistry and the industries dealing with distillation and solvents have alike required an extension of the information available a t the present. The study of zeotropic or azeotropic mixtures and their pressure-temperature-composition equilibria, requires a far greater fund of data than the literature affords. It would be desirable to have a t hand not only a single isothermal investigation or two, but a “matte” of data from which we could construct a t will any desired isobaric or isothermal diagram. Changes in the vapor pressure-temperature equilibria a t constant composition in the liquid phase could be plotted from such data, and the heats of vaporization for a.ny desired composition, pressure, and temperature combination could be calculated. Such a matte would allow an examination of the changes in the activity coefficients with the several variables of the system, and would permit predictions for any desired conditions or point (P-T-N). These predictions are one of the prime purposes of any investigation. It is the purpose of this paper to present a method of securing sufficient quantities for any system by a more simplified and direct means. APPARATUS
Figure 1 illustrates the apparatus used by the author. The distillation flask was of somewhat new design, permitting a rapid equilibrium to be reached between the liquid and vapor phases. The flask, which contained the main volume of the sample being studied, was made from a 500-cc. round-bottomed Pyrex flask. The condenser was, as far as the author was able to ascertain, of new design. It consisted of an inner cooling tube against which the vapors impinged. This was sealed within the outer condenser wall but was left open a t the top to permit of a variety of cooling methods. It was stoppered and a continual flow of water used, or if necessary, it could be filled with dry ice and ether. Various other refrigerants may also be used. This permits distillations even a t fairly low temperatures. The condensate from the vapor phase flowed through 233
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a sampling trap and back into the liquid phase. The sampling tube for the liquid phase and the thermometer were placed in the ground glass stopper used for filling the flask. This stopper was securely held by a rubber-covered wire saddle. While no superheating was experienced in the apparatus, it can be prevented by etching the inside bottom of the flask with water glass (1). In order to prevent fractionation of the vapors before passing into the condenser, the upper half of the flask and the throat above it were insulated with asbestos rope wrapping. Control of the pressure was effected by a modified Victor Meyer mercury column regulator which could be rapidly changed in order to serve for control of pressures both above and below atmospheric. The pressure was adjusted to the
3
2. \
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FIG. 1. APPARATUSUSED IN
THE EXAMINATION OF BINARYLIQUIDMIXTURES 1, trap; 2, pressure regulator; 3, drying towers; 4, buffer bottles; 5, manometer; 6, thermometer; 7, distillation flask.
desired point by raising or lowering the depth of the glass tubing in the mercury. Two large bottles of 4-liter capacity were included to increase the effective volume, thus preventing any noticeable fluctuation in the pressure during operation. PROCEDURE
The system selected was toluene-ethyl alcohol. This selection was made partly because of previous work upon i t (2), and because of the characteristics exhibited over the temperature range used and the ease with which it permitted the use of the refractometer as a means of analysis of the samples. Eleven samples, exclusive of the pure liquids and distributed
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over the range in composition, were made up from purified liquids. Each sample contained about 300 cc. The sample was placed in the flask and the pressure adjusted to the lowest value, the solution heated to boiling and given a half hour to reach equilibrium. Ordinarily, however, equilibrium was reached in five to ten minutes. A constant thermometer reading was taken as the criterion of equilibrium. The distillation rate was regulated and noted by counting the drops falling from the bottom of the cooling tube in the condenser. The rate was regulated until the condensate in the trap changed completely in a minute's time. The pressure and temperature were noted and the distillation stopped, and the pressure in the apparatus allowed to come to atmospheric level by use of the pinchcock. (Caution must be used when the pressure within the apparatus is greater than atmospheric. In this case the solution must be given time to cool sufficiently to prevent boiling when the pressure is released.) Dried air was forced into the apparatus to blow out samples of the condensed vapor phase and the liquid phase. These were analyzed by means of an Abbe refractometer and the remainder of the samples returned to the flask. The pressure was then adjusted 8 cm. higher and the process repeated on the same sample in the flask until the desired pressure range had been covered. No trouble was experienced with changing composition of the liquid phase by removal of the samples for analysis. The relative amounts removed were too small to be of any consequence. The same procedure was followed, of course, for each succeeding mixture. This procedure gave the variations in the pressure and the temperature and the change in the composition in the vapor phase in equilibrium with a non-variant liquid phase. Variation in the liquid phase was accomplished only by changing the sample under investigation. TREATMENT O F DATA
The changes in vapor pressure with temperature for each sample were plotted as shown in figure 2, the pressure in millimeters being given on the ordinate and the temperature on the abscissa. It will be seen that this curve is similar to the vapor pressure curve for a pure liquid. The dotted line represents the composition of the vapor phase in equilibrium, a t the various temperatures, with a liquid phase of constant composition, the mole fraction being plotted on the ordinate. In the system of ethyl alcohol-toluene, the change in the vapor composition above any sample was slight, and linear in relationship to the temperature. Best line values were then read from the graph and used to construct tables 1 and 2.' While the datagiven in the tables fire given to the nearest millimeter, a more recent study of the accuracy of the apparatus indicates that vapor pressures are correct to one-tenth of a millimeter.
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From such a graph as figure 2, then, the vapor pressure and the composition of the vapor phase in equilibrium with the liquid phase for that sample can be determined a t any desired temperature. Since this plot gives any
FIQ.2. CHANGES IN VAPORPRESSURE WITH TEMPERATURE Sample No. 10; mole fraction of toluene in liquid phase = 0.096
minute variation in the pressure and the temperature, a plot such as figure 3 can be constructed. Figure 3 is a group of isotherms whose values were read from the graphs of type figure 2. The vapor phase curves in figure 3 have been omitted in all cases excepting that for the 80OC.isotherm.
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TABLE 1 Isotherms for the system toluene-eth$ alcohol MOLE FRACTION TOLUENE
Liquid
I
Vapor
-I
TOTAL PRESSURE
PARTIAL PRESSURE
75°C. 1.ooo 0.893 0.769 0.648 0.557 0.457 0.375 0.274 0.233 0.155 0.096 0.043 0.000
mm.
1,000
0.380 0.307 0.286 0.280 0.269 0.253 0.232 0.217 0,180 0.135 0.075 0.000
244 444 677 688 698 707 715 722 724 724 716 699 666.1
169 208 197 195 190 181 167 157 130 96.7 52.4 0
289.7 537 818 832 844 856 864 874 877 880 868 848 812.6
204 246 226 228 224 212 195 183 154 113 62.8 0
397.0 642 990 1005 1016 1027 1037 1047 1052 1052 1047 1026 986.3
243 291 267 264 264 245 225 211 179 132 74.9 0
80°C. 1.000 0.893 0.769 0.648 0.557 0.457 0.375 0.274 0.233 0.155 0.096 0.043 0.000
1.ooo 0,379 0.301 0.271 0.270 0.262 0.245 0.223 0.209 0.175 0.130 0.074 0.000 85°C.
1.000 0.893 0.769 0.648 0.557 0.457 0.375 0.274 0.233 0.155 0.096 0.043 0.000
1.ooo 0.379 0.294 0.266 0.260 0.257 0.236 0.215 0.200 0.169 0.126 0.073 0.000
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TABLE 2 Isotherms for the sgstem toluene-ethyl alcohol MOLE FRACTION TOLUENE
Liquid
TOTAL PRESSURE
I
PARTIAL PRESSUBE
Vapor
60°C. mm
1.ooo 0.893 0.769 0.648 0.557 0.457 0.375 0.274 0.233 0.155 0.096 0.043 0.000
1.000 0.382 0.325 0.317 0.310 0.289 0.277 0.256 0.242 0.198 0.147 0.078 0.000
.
139.5 240 367 373 382 387 390 395 397 397 388 375 352.7
91.7 119 118 118 112 108 101 96.0 78.6 57.1 30.0 0
166 301 455 466 472 477 481 486 487 488 480 466 436.9
115 145 143 141 139 129 120 114 93.8 68.7 35.9 0
65°C. 1.000 0.893 0.769 0.648 0.557 0.457 0.375 0.274 0.233 0.155 0.096 0.043 0.000
1,000 0.381 0.319 0.307 0.300 0.282 0.269 0.248 0.234 0.192 0.143 0.077 0.000 70°C.
1.000 0.893 0.769 0.648 0.557 0.457 0.375 0.274 0.233 0.155 0,096 0.043 0.000
1.000 0,380 0.314 0.297 0.290 0.276 0.261 0.240 0.226 0.186 0.139 0.076 ,
0.000
202.4 367 557 569 572 584 590 592 598 598 591 575 542.5
139 175 169 166 161
154 142 135 111 82.2 43.7 0
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239
rllOO
Id00
900
BOO
700
i
t 600
2,
@so0
400
300
3 00
Joe
FIQ.3. ISOTHERMS FOR TOLUENE-ETHYL ALCOHOL
It would be advantageous to give an example of the above use of the graphs. If the isotherm a t 80°C. is desired, we proceed as follows: On figure 2, representing sample No. 10, we see that the composition of the liquid phase in mole fraction of toluene is 0.096. At 80°C., we find on referring to the curve that the total pressure on the system is TEE JOURNAL OF PHYSICAL CHEMISTRY, VOL. XXXVII, NO.
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W. ANDREW WRIGHT
862.5 mm. This we plot on figure 3 for the isothermal diagram a t SO'C., plotting the pressure against the mole fraction of toluene. The dotted line on figure 2 indicates that at the temperature of 80°C., the composit'ion of the vapor phase in equilibrium with the liquid phase for sample No. 10 is 0.130. This, too, is then plotted on figure 3. We then proceed in a similar manner with each of the remaining samples distributed over the range of mixture, and thus determine the complete isothermal diagram for the system a t that particular temperature. DISCUSSION '
If the logarithm of the vapor pressure is plotted against 1/T for each sample as shown in figure 4, the result is a straight line similar to the plot for a pure liquid. It will be recognized immediately that this is of great value, since the data can be extended by means of the straight line to any desired temperature or used for interpolation between values. The logarithms of the partial pressures of the constituents were then plotted against 1/T as in figure 5, and the straight line was again obtained. This is to be expected, for if the total pressure plot gives a straight line, the two partial plot,s must do likewise. Data from Sameshima (3) and Cunaeus (4) for the system, acetone-ethyl ether, were plotted in a like manner and similar straight lines were found. Some data given by Schmidt (5) were also applied to such methods of plotting. I n the system of benzene-carbon tetrachloride, the straight line relationships held true. The data for the system of benzene-toluene given by Schmidt were found to contain errors in the determination of the vapor pressures. The data given for the vapor pressures of pure toluene would not give a straight line when log P was plotted against 1/T. Errors in the same direction were found to exist throughout the system a t the same temperature. In the case of toluene-ethyl alcohol, the system was azeotropic, while the system of acetone-ethyl ether and that for benzene-carbon tetrachloride were zeotropic. Evidently the principle holds for either type of liquid mixture. It becomes evident then, that the investigation of a binary liquid mixture requires only the determination of two pressure-temperature-composition diagrams to completely outline the system. This could be most rapidly accomplished by determining two isobaric diagrams. Since the ethyl alcohol-toluene mixture is azcotropic, having a maximum in vapor pressure, a plot as in figure 4 gives some lines which are located higher than the line for the pure alcohol. However, the lines are not parallel, but vary between the slopes of the lines for the pure liquids and vary as the composition of the liquid phase is varied. Examination shows that the maximum occurs a t a composition which changes with the temperature. If the straight lines were extended sufficiently, eventually all
EXAMINATION OF BINARY LIQUID MIXTURES
24 1
would lie within the lines for the pure liquids. At the temperature a t which this occurs, the system becomes zeotropic. I n this particular case,
FIQ.4. TOTAL PRESSURE ISO-COMPOSITION
the maximum, or azeotropic point, is shifting so slowly that the temperature at which this would happen would be quite high. Predictions made by the extensions of the straight lines obtained are accurate throughout the
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range where the pure liquids give a straight line when log P is plotted against 1/T.
FIG.5. PARTIAL PRESSURES Sample No. 5; mole fraction of toluene = 0.457
A liquid mixture, when outlined as described, becomes a versatile source of data. Aside from the prediction of pressure, temperature, and compo-
EXAMINATION OF BINARY LIQUID MIXTURES
243
sition equilibria, graphs of the type of figure 4 have been used to calculate the heats of vaporization a t any desired temperature and composit'ion.2 Similarly, from the graphs of the type of figure 5 the partial heats of vaporization at any particular point were calculated. Studies on activities and activity coefficients can be made, prediction of the shift of the azeotropic point, etc. SUMMARY
(1) I n the study of liquid mixtures, the desirability of data in sufficient quantities to give a matte when plotted is pointed out. (2) An apparatus is described and the procedure outlined for the determination of such data. (3) A very complete examination of the system, toluene-ethyl alcohol, has been made over a pressure range of 700 mm. and its attendant temperature changes. (4) As a consequence of the character of the data obtained, some generalizations are made on the laws of liquid mixtures, permitting equilibria predictions and calculations such as the heat of vaporization or the partial heats of vaporization a t any point. The generalizations made in the paper have been tested with data from the literature and further confirmation obtained of their value. REFERENCES (1) (2) (3) (4) (5)
SWIETOSLAWSKI: Bull. SOC. chim. 60, 1568 (1931). ROBINSON, WRIQHT, AND BENNETT: J. Phys. Chem. 36,658 (1932). SAMESHIMA: J. Am. Chem. SOC.40,1482,1503 (1918). CUNAEIJS:Z. physik. Chem. 36,232 (1901). SCHMIDT: Z. physik. Chem. 99,71 (1921); 121,221 (1926).
2 These, together with some other similar calculations, may be discussed in a later paper.
