TOTAL AND PARTIAL PRESSURES OF BINARY SOLUTIONS OF THE BUTYL ALCOHOLS IN BENZENE AT 25OC.I BRUCE B. ALLEN
AND
S. POWELL LINGO WITH W. A. FELSING
Department of Chemistty and Chemical Enginem'ng, The University of Tezas, Auetin, Texas Received July 80, 1938
The differential static method of measuring vapor pressures, as devised by Parks and Schwenck (€9,has been modified recently by Olsen and Washburn (6) and has been applied by them to the investigation of the system benzene-isopropyl alcohol. The latter authors claim an accuracy to within 2 to 3 per cent for their method when their results are compared with recognized recorded values. In this laboratory it was decided to employ the method in modified form. The systems chosen were the binary solutions of the four butyl alcohols in benzene. It was hoped that the results would yield information concerning the effect of the branching of the chain or of the position of the hydroxyl group upon the degree of deviation from ideality of these solutions of the non-polar-polar type. EXPERIMENTAL
The method in essentials was that of Olsen and Washburn (6) and consisted of the comparison of the vapor pressure of the pure alcohol with the total pressure of a solution of that alcohol in benzene. This comparison waa effected by placing the pure alcohol in one arm of a glass manometer containing mercury and the solution in the other arm, also over mercury. The technique was essentially that of Olsen and Washburn; certain refinements were introduced and will be described later. The partial pressures of the constituents of the solutions were determined by passing dry, carbon dioxide-free air through bubblers containing the solutions, condensing the vaporized constituents at about -65"C., and analyzing the condensate. Purification of
matffiak
Benzene of reagent grade was refluxed with mercuric acetate and then distilled onto freshly pressed sodium wire. It was allowed to remain in Contribution No. 137 from the Department of Chemistry and Chemical Enpineering of the University of Texas. 426
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426
B. B. ALLEN AND 8. P. LINGO WITH W. A. FELSING
contact with the sodium for 36 hr.; after distilling from the sodium-sodium oxide residue, the benzene was carefully fractionated twice through an efficient column. The final product, roughly 50 per cent of the original sample, boiled at 79.6"C. at 746 mm. It melted at 5.5OC. and had at 25°C. a refractive index of 1.4980 and a density of 0.8732 g. per cubic centimeter. The butyl alcohols were ,products of high quality obtained from the Eastman Company. They were purified by a preliminary refluxing with lime, followed by distillation onto calcium metal turnings. After a 12-hr. contact with the metal, the alcohols were distilled slowly from the calcium alccholate residue and fractionally distilled thrice through a long insulated column of the indented type. The final product represented from 50 to 60 per cent of the original sample. The observed physical constants of these purified samples are presented in table 1. TABLE 1 Physical constants of the butyl alcohols
I
BOILIN0 POINT
'C.
grama par cubic eentimetu
OC.
Normal, . . . . 117.3 (746 mm.) 117.71 (760 mm.) 1.39741.39740.80570.8057 (2) 180. . . . . . . . . . 107.6 (749 mm.) 107.89 (760 mm.) 1.3936 1.39390.79820.7976 (2, 1) Secondary., 99.2(750 mm.) 99.53 (760 mm.) 1.39461.39500.80220.8023(1) 1.3845 0.78&.7806 (3) Tertiary*. . . 82.3 (752 mm.) 82.55
* Observed melting point, 25.4"C.;literature value, 25.50'C.
(7).
Preparation and anulysk of solutions All solutions were prepared by weight, using weight burets. Weighing8 were made directly into volumetric flasks equipped with special seals to exclude moisture. For each alcohol there were prepared five solutions in benzene; the ezad concentrations were approximately 20, 35, 50, 65, and 80 mole per cent alcohol. In addition, approximately 10 mole per cent solutions of the secondary and the tertiary alcohols were prepared. The refractive indices at 25°C. f 0.1' were determined for these solutions by means of an Abbe refractometer, using daylight as the light source. The following relations represent the dependence of the index of refraction on the mole fraction, 2, of the alcohol (in the alcohol-benzene solution): *Butyl alcohol. ............................... Isobutyl alcohol. .............................. Secondary butyl alcohol.. Tertiary butyl alcohol
..................... .........................
n=
1.4980
- 0.11222 + 0.01162' + + -
n = 1.4980 - 0.11842 0.0140~' n 5 1.4980 - 0.12142 0.01802' n = 1.4980 0.13742 f 0.0240~'
VAPOR PRESSURES OF ALCOHOLS IN BENZENE
427
These relations reproduce the experimentally determined values to better than 1 part in 5000. In the determination of the composition of the condensate resulting in the air-bubbling procedure of determining the partial pressures, a determination of the index yielded the molar composition immediately. The densities of the alcohol-benzene solutions were also determined at 25'C. f 0.05' by means of a Weld precision specific gravity bottle. The following relations were obtained, the densities in grams p a cubic centimeter being reported as functions of the mole fraction, 2, of the alcohol in the benzene solution: n-Butyl alcohol. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Isobutyl alcohol. . . . . . . . . . . . . . . . . . . . . . . . . . . . Secondary butyl alcohol. . . . . . . . . . . . . . . . . . . . . Tertiary butyl alcohol., . . . . . . . . . . . . . . . . . . . . . .
d = 0.8732 - 0.07732 d = 0.8732 - 0.089172 d = 0.8732 - 0.09182
d = 0.8732 - 0.12402
+ 0.00982*
+ 0 .014172* + 0.O2O8z1 + 0.03122*
These densities were necessary to make the proper corrections for the height of the alcohol-benzene solutions above the mercury in the arm of the manometer in which total pressures are determined.
The apparatus All measurements of vapor pressure were made in a water thermostat with glass windows. The temperature was kept at 25°C. f0.05' by means of a mercury column actuating a sensitive relay. All temperatures were determined by means of a long mercury thermometer of limited range calibrated by the National Bureau of Standards. Total pressures were determined by means of the manometer described by Olsen and Washburn (6), the arms being about 40 cm. long and ending in glass-mercury seals. All readings were made with a cathetometer reading to 0.1 mm. The pure alcohols used in one arm of the manometer had the following vapor pressures a t 25°C. : n-butyl, 6.44 mm. (4); isobutyl, 12.6 mm.; secondary butyl, 18.4 mm.; and tertiary butyl, 42.0 mm. (7). The values for the isobutyl and secondary butyl alcohols were determined in this laboratory, no values being available in the literature. The aerator used in the partial pressure determinations waa essentially that of Olsen and Washburn (6) and was maintained in a constanttemperature bath at 25°C. f 0.05'. The condenser was kept in a dry ice-acetone bath during a run, assuring complete condensation of the vapors. At least 2 cc. of condensate was collected during each run;the composition of the condensate was read from a large-scale plot of the index of refraction uersus the mole fraction of alcohol. Accuracy of the method An examination of the Olsen and Washburn total pressure method reveals that errors are due to ( a ) impurities in the substances investigated,
428
B. B. ALLEN AND 5. P. LINGO WITH W. A. FELSINQ
TABLE 2 Total pressures at 86'C. o j benzene-butyl alcohol solutions
Z
I
P
o.oO0
94.4(5)
0.200
89.4 84.9 77.8 67.3 50.5 6.4(4)
0.322
0.500 0.650 0.803 1.OOO
Pa?
P -2
0.202 0.343 0.498 0.640 0.805 1.oO0
Normal . . . . . . . .
2
0.095 0.197 0.353
0.500 0.646 0.797 1.om
96.4 95.1 91.0 85.4 76.9 80.7 18.4
0.100 0,198 0.351 0.499 0.652 0.802 1.ooO
104.7 106.5 104.3 100.2 92.8 80.5 42.0(7)
TABLE 3 wessures at 66°C.of benzene-butyl alcohol solutions FRACFIOj
or ALCOHOL
SOLE FBACFIOI OF AICOHOL
IN LIQUID
IN VAPOB
1BWYL ALCOHOL
93.2 89.3 83.7 75.3 56.8 12.6
~P - P
2
0.197 0.313
PABTTAL PEEMUBB
Alcohol
Bemne
0.650 0.798
1.4927 1.4910 1.4897 1.4886 1.4850
0.045 0.059 0.070 0.081 0.113
4.0 5.1 5.5 5.5 5.8
85.3 80.7 72.4 61.9 45.8
Is0 . . . . . . . . . . . .
0.202 0.343 0.498 0.604 0.805
1.4892 1.4874 1.4848 1.4827 1.4759
0.072 0.087 0.099 0.128 0.187
6.7 7.8 8.3 9.6 10.6
86.4 81.5 75.4 65.7 46.2
Secondary. . . . .
0.197 0.353 0.500 0.646 0.797
1.4845 1.4821 1.4796 1.4756 1.4669
0.112 0.133 0.150 0.185 0.262
10.7 12.1 12.8 14.2 15.9
84.5 78.9 72.6 62.7 44.8
Tertiary . . . . . . .
0.100 0.198 0.351 0.499 0.652 0.802
1,4798
0.130 0.196 0.245 0.291 0.352 0.455
13.6 20.9 25.6 29.2 32.7 3R.6
91.1 85.7 78.7 71.O 80.1 43.9
0.500
1.4712 1,4650 1.4593 1.4520 1.4400
(a) errors made in making up the solutions, (c) errors in the temperature control, (d) errors in the cathetometer readings, and (e) the presence of a m a l l bubble of permanent gas in the manometer. In the present investi-
VAPOR PRE88URES OF ALCOHOLS IN BENZENE
429
gation the over-all accuracy of the method is believed to be within ctl per cent. This was checked by measuring the vapor pressure of benzene of highest purity; the measured value and the “best” value recorded in the literature agree to within 1 per cent.
FIG.1. Partial and total pressures (in millimeters) a t 25OC. of benzene-butyl alcohol solutions. 0 , n-butyl alcohol-benzene solutions; A , isobutyl alcoholbenzene solutions; 0 , secondary butyl alcohol-benzene solutions; x , tertiary butyl alcohol-benzene solutions. THE DATA OBTAINED
The total and partial vapor pressures are presented in tables 2 and 3 and graphically in figure 1. Pressures are given in millimeters of mercury (1/760of standard atmosphere); the mole fraction of the alcohol is designated by x.
430
B. B. ALLEN AND 8 . P. LINGO WITH W. A. FELBING DISCUSSION OF RESULTS
An inspection of the curves of the figure reveals that there is a marked deviation from ideality (Le., Raoult’s law) in each of the four cases studied. This deviation was to be expected, since these solutions are composed of non-polar and polar (or semi-polar) constituents. Most striking is the comparison of the structural effects of the isomers on ideality in solution; the figure shows that vapor pressures of solutions of n-butyl alcohol in benzene most nearly approximate the ideal, while those of tertiary butyl alcohol in the same solvent vary most. This may indicate that a substitution of the hydrogen of the carbinol carbon by an alkyl group causes an increased deviation from ideality; Le., that the polarity of the alcohol increases as the hydrogens of the carbinol carbon are substituted by alkyls, thereby increasing effects due to differences in the polarity of the constituents. This latter observation is in accord with accepted electronic structures of these alcohols. SUMbfARY
1. Total pressures of binary solutions of the four butyl alcohols in benzene have been measured with an accuracy within 1 per cent. 2. Partial pressures of these solutions have been determined with an accuracy better than 1 per cent. 3. The observed data are presented graphically. Such plots show a marked difference in the degree of deviation from Raoult’s law for the four isomeric alcohols. An increasing polarity with change in structure of the alcohols in mixtures with the non-polar substance benzene seems to account for the increase in deviation. REFERENCES (1) BRUNEL:J. Am. Chem. SOC.46, 1334 (1923). (2) BRUNEL,CRENSHAW, AND TOBEN:J. Am. Chem. SOC.43, 561 (1921). (3) DOROSHEVSKI: J. Russ. Phys. Chem. SOC.43, 66 (1911); Chem. Abstracts 6, 10 (1912). (4) International Critical Tables, Vol. 111, p. 219. McGraw-Hill Book Co., New York (1928). (5) Reference 4, p. 221. (6) OLSENAND WASHBURN: J. Phys. Chem. 41, 457 (1937). (7) PARKSAND BARTON:J. Am. Chem. SOC. 60,24 (1928). (8) PARKSAND SCHWENCK: J. Phys. Chem. 28, 720 (1924).
