THE VAPOR PRESSURE O F BINARY SOLUTIONS O F ISOPROPYL ALCOHOL AXD BENZENE A T 25°C. ALLEN L. OLSEN
D e p a r t m e n t oj ChemTstry, K a n s a s S t a t e College, M a n h a t t a n , K a n s a s AND
E. ROGER WASHBURN D e p a r t m e n t o j C h e m i s t r y , L'niversity o j h'ebraska, L i n c o l n , N e b r a s k a Received J u l y IS, 1936
-4s a part of a comparative study of binary and ternary systems niade u p of water, a hydrocarbon, and a lower alcohol, the following vapor pressure measurements have been niade on solutions of isopropyl alcohol in benzene. MSTERIALS
The methods of purification and the physical constants of these materials have been discussed and recorded in a preyious article (1). EXPERIMENTAL
Total pressure The difference between the vapor pressure of the isopropyl alcohol and each liquid mixture mas determined by a differential static method. The apparatus which n-as used in this measurement n-as a slight niodification of that described b y Park.. and Schwenk (3). I t is sketched in figure 2. Since considerable difficulty was encountered in closing the topi: of the manometer, mercury-sealed stoppers were employed. S o meaburable leakage through these stoppers was observed in tn-enty-eight hours when they were subjected to a difference in pressure of 1 atmosphere. The manometer was placed within a constant-temperature air bath, thermostatically controlled to &O.lO°C. The readings were observed by means of a cathetometer reading to 0.1 mm., and it was found that one could reproduce cathetometer readings to = t O . O 5 mm., which is the limiting accuracy of the inqtrument. Each value which is given in the table of data representi: the average of many independent determinations, and the agreement is of the order of kO.05 mni. -After the measurement had been completed, a portion of the mixture was removed and its composition was determined by means of a refractiye index measurement. The con457
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A L L E S L. O L S E S AND E . ROGER WASHBURN
version to mole fraction was effected by means of a curve n-hich was plotted from data in table 1.
Analysis of the vapor phase The composition of the vapor phase in equilibrium with the solutions a t 25.00"C. was determined. Thi. was accomplished by passing dry, carbon dioxide-free air through a pair of bubblers (figure l), each containing 10 cc. of the mixture under consideration. The bubblers were immersed in a constant-temperature water bath. The air thus saturated with the vapor of the mixture wa. then passed through a freezing-out tube
r
.I FIG.I
FIG.2
FIG. 1. Air saturator for vapor analysis FIG. 2 . Differential static manometer
immersed in liquid air. The alcohol-benzene mixture separated as a solid on the walls of this tube, and when about 2 cc. of distillate had been collected, it was analyzed by means of the refractometer. T o determine if there were appreciable concentration changes during the time required for the vapor to freeze, the following procedure was carried out. Four bubblers were connected in series and a 10-cc. sample, chosen successirely from nine different liquid mixtures and varying in composition from pure alcohol to pure benzene, was placed in each one. I t was found that there was no measurable change in composition of the 10-cc. sample in the first bubbler during the time required for the freezing-out process. It mas also found that the composition in the first bubbler must
459
VAPOR PRESSURE O F BINARY SOLUTIOXS
change appreciably before a noticeable difference appeared in the second. It was further noted that all the liquid must completely disappear in the first bubbler before a change took place in the third and fourth. The liquids disappeared in the order of their placement in the series. The air bubble as it left the first bubbler must have been completely saturated with the liquid in question during the first part of each determination. -1few of the values which are recorded in table 2 were obtained by means of a single unit in which 25 cc. was placed. The value' from this procedure TABLE 1 Refractive i n d e x of benzene-isopropyl alcohol solutions
,
MOLE FRACTION O F ALCOHOL
1 0 0 0 0 0 0 0
RErRACTIYE INDEX
I
I
000 823 610 536 130 330 222 000
1.37479 1.39861 1,42162 1 43444 1,44725 1,15954 1.47159 1.49800
I
I
I
I I
TABLE 2 C o m p o s i t i o n of vapor of benzene-isopropyl alcohol solutions MOLE FRACTION O F ALCOHOL I N L I Q U I D
I
0.000 0.114 0.173 0.224 0.330 0.430 0.536
i
1
MOLE FRACTION or ALCOHOL I N VAPOR
0 0 0 0 0 0 0
000 187 223 231 246 270 290
MOLE FRACTION O F ALCOHOL IN LIQEJID
-
I
I
0 0 0 0 0 1
633 820 861 910 918 000
~
I
MOLE F R A C T I O l OF ALCOHOL I N VAPOR
0 334 0 450 0 508 0 598 0.739 1 000
compared favorably with those obtained from the two-tube unit. A second bubbler, however, positively insured complete saturation. The vapor composition data are given in table 2 . The values which are recorded are representative values of thirty-four determinations. The average error of a single observation froni the mean of the mole fraction of alcohol in the vapor phase over liquid niixtures of identical mole fraction was =t0.005.
Calculation of results Parks' value of 44.0 mni. was taken as the Tapor pressure of the iscpropyl alcohol ( 2 ) . The theoretical and the observed results appear in
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A L L E S L. OLSEN AND E. ROGER WASHBURN
table 3, and the observed results are plotted in figure 3. The values for the composition of the vapors in equilibrium with these mixtures were not determined directly, but were obtained from a smooth curve plotted from values in table 2. In this sense they are really experimental, and when TABLE 3 P a r t i a l a n d total pressures ut 25.0"C. of benzene-isopropyl alcohol s o l u t i o n s MOLE FRACTION OF ALCOHOL
Liquid
Vapor
PARTIAL PRESSCRE OF ALCOHOL
Ibserved
Ideal
TOTAL PRESSURE OF MIXTURE
Observed
~~
0.000 0.059 0.146 0.362 0.521 0.700 0.836 0.924 1IO00
0.000 0.123 0.205 0.255 0.288 0.365 0.470 0.635 1,000
129 224 27 6
0 2 6 15
0 6 4 9
Ideal
_
I
PARTIAL PRESSURE OF B E N Z E N E
1
Observed
_
94 4 91 4 87 1 76 2 68 1 59 1 52 3 47 8 440 I
_
,
~
94.4
~
104.5 109.0 108.4 105.8 99.8 84.0 66 4
Ideal
91.7 86.7 80 8 75 3 63 4 44 5 24 2
88.7 80.6 60.2 45.2 28.3 15.5 7.2 0.0
4.969 1,049 3.487 1.075 1.733 1.342 1.331 1,665 1.182 2.239 1.074 2.874 1.038 3.373
FIG.3. Observed total and partial pressures of benzene-isopropyl alcohol system
multiplied by the corresponding total pressures for the various solutions, give the observed partial pressure of isopropyl alcohol. The observed partial pressure of benzene was determined in a similar manner. The ideal pressures in all cases were calculated on the assumption of Raoult's law P A = NAP::
VAPOR PRESSURE O F BINARY SOLUTIOKS
461
where p A and N - 4 are respectively the partial pressure and the mole fraction of component d in a given solution and p i is its vapor pressure in the pure state. The activity coefficients for alcohol and benzene are recorded in columns 5 and 6, respectively, in table 3. They were calculated by means of the equation
wherefis the activity coefficient, p i s the partial vapor pressure of the component above the solution, p o is the vapor pressure of the pure component, and 12’is the mole fraction of the component in the solution. DISCUSSION O F RESULTS
The difference between the vapor pressures of the pure components was taken as the criterion of accuracy of measurement. The measurement on this difference, applying many technics, was found to result in an error of 2 to 3 per cent. After having performed the usual routine of technic in removing the visible air, a tiny bubble of air would always remain above the liquids \Then the mercury reservoir was raised from its lowered position. Still further evidence of dissolved air was found in the observation of abnormal readings a t the upper end of t h e manometer. The usual procedures will never remove the dissolved air, for a t the point a t which the stoppers may be removed there will be placed the existing atmospheric pressure on the excluded gas, thereby forcing it into the liquid. The effectsof dissolved air were minimized by making pressure measurements well toward t h e bottom of the manometer. I n a study of vapor pressure data and its subsequent calculation to activity coefficients, it is observed that the alcohol-benzene system deviates in a positive manner from Raoult’s law. The wide deviation from ideal behavior took place because of the great difference in polarity of the two components. The polar molecules of alcohol have an abnormally great attraction for each other, producing greater surface tension, cohesion, etc., and tend to “squeeze” out non-polar molecules from their midst. -4s a result of the tendency of a liquid of high internal pressure to “squeeze” out a liquid of low internal pressure, it is expected t h a t the partial pressure would deviate in a positive manner from Raoult’s lam. This concept of association does not admit any definite polymers like double molecules. The polar affinities act within the liquid to form groups of molecules which become impregnable to a non-polar molecule like benzene. I n the study of the depression of the freezing point the deviations were ascribed to some combination either of alcohol molecules or alcoholbenzene molecules. There is a small range of concentration near pure
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ALLEN L. OLSEN AXD E . ROGER WASHBURN
benzene where the observed and the calculated freezing points do not deviate appreciably. There is a small range of concentration near pure benzene where the partial pressures of benzene do not deviate far from the theoretical values. The alcohol molecules are randomly and sparsely distributed to such an extent that each is without influence on the other. h decreased value for the internal pressure of the alcohol in the mixture would tend to LLsqueeze"out a lesser number of molecules of benzene. STJMMCIARI
1. The differential static method devised by Parks has been adapted for the measurement of the total and partial vapor pressures of the components of the binary solutions of kopropyl alcohol and benzene a t 25°C. The accuracy of the method has been discussed. 2. These data h a r e been used to calculate the actirity coefficientswhich indicate deviations from Raoult's law. 3. Deviations from ideal behavior hare been discussedand qualitatively explained on the basis of polarity. REFERENCES (1) OLSEN,A. L., AND WASHBURN, E. R.: J. Am. Chem. SOC.67, 303 (1935). G. S., A N D BARTON, R.: J. Am. Chem. SOC.60,24 (1928). (2) PARKS, (3) PARKS, G. S., AND SCHWESK, J. R.: J. Phys. Chem. 28,720 (1924).
