37G
JULIAN C. SMITH
T H E SOLUBILITY DIAGRAM FOR THE SYSTEM BEKZENEPYRIDINE-WATER ,JULIAN U. SMI'IYI Plant Research Department, Shawinigan Chemicals, Ltd., Shawinigan Falls, Quebec, Canada Received September 10, fO4I
I n the great majority of ternary liquid systems the tie lines increase progressively in slope as the amount of the third component increases, but two systems have been investigated in which this is not the case. Miller and McPherson (5) found in 1908 that the system ethyl alcohol-ethyl ether-water is unusual, in that the direction of the tie lines changes as the amount of alcohol present increases. Later workers (3, 4) have confirmed their results. In 1925 Woodman and Corbet (9) discovered a similar anomaly in the system benzene" pyridine-water: at low concentrations of pyridine most of the pyridine was found in the benzene layer; a t high concentrations the reverse was true. Thermodynamically there is no reason why such a system should not exist. However, in 1940 Brancker, Hunter, and Nash (1) devised a method of representing the lines by means of a single straight line, which waa applied with success to all systems which they examined, except for the two noted above. These unusual systems they discounted on the basis that the anomalies were caused by inaccuracies in the available data, although it seems unlikely that different investigators would have agreed so well on the system alcohol-etherwater. Only one investigation of the system benzene-pyridine-water has been made, however, and there is a need for more accurate data on the equilibrium relationships. The following experiments were carried out to supply thie information. MATERIALS
Pyridine: Ordinary commercial pyridine undoubtedly contains a considerable amount of impurity, and especial care waa taken to obtain the pure substance. About 500 g. of Merck's reagent-grade pyridine was refluxed over 75 g. of potassium permanganate for 2 hr. The solids were then filtered off, 25 g. of fresh permanganate waa added, and the pyridine was refluxed again for half an hour. It wm then fractionally distilled from the permanganate in a 48-in. all-glass column equivalent to fifteen theoretical plates. After refluxing a few minutes the distillate was drawn off, using a reflux ratio of 2 to 1. About 30 cc. of lowboiling material wm obtained. The next 120 cc. boiled from 112.3' to 112.6"C. (uncorr.) at 753 mm.; the remainder from 112.6' to 112.7'C. The last fraction waa taken using a reflux ratio of 1 to 1. Before purification the pyridine waa a light yellow liquid having the unpleaaant sweetish odor characteristic of ordinary pyridine. Its refractive index a t 25°C. waa 1.5058. After purification it was water-clear, had a refractive index of 1.5060,and possessed a peculiar acrid odor quite different from that of the unpurified material. All the material boiling above 112.3'C. had the ssme
THE SYSTEM BENZEXE-PYRIDINE-W'ATER
377
refractive index, and both the last two cuts were used in the subsequent experiments. Benzene: Merck's C.P. thiophene-free benzene was tested for thiophene with isatin, and gave no reaction. It was then dried over metallic sodium and distilled. Water: Distilled water from the laboratory was twice redistilled, first from acid, and then from alkaline permanganate. Its refractive index a t 25'C. was 1.3323. EXPERIMENTdL PROCEDURE
The solubility diagram was constructed by the synthetic method desciibed by Taylor (6) and used more recently by Washburn and others (7, 8). The limiting solubilities were determined by titrating known mixtures of benzene and pyridine with water to a permanent turbidity. About 15 cc. each of benzene and pyridine was measured from 50-cc. burets into a 125-cc. glass-stoppered Erlenmeyer; water was then added from a 10-cc. buret graduated to read 0.05 cc. I n the great majority of cases a single drop of water sufficed to change the mixture from clear to definitely turbid. Because of the great solubility of water in the mixtures containing only a small amount of benzene, it was found easier to measure the water from a 25-cc. pipet, add the benzene, and titrate with the pyridine until the cloudiness just disappeared. The mixture was then back-titrated with water until turbidity reappeared. The titrations were made at room temperature (24.0-24.5"C.), after which the mixtures were placed in a constant-temperature bath a t 25.0'C. The change in temperature did not affevt the turbidity. After equilibrium had been established at 25.0°C., the refractive index of the mixtures was determined by an ,4bbB refractometer, the lenses of which were kept a t 25.0'C. with water siphoned from the constant-temperature bath. A 100-watt electric light was used for illumination. The refractive indexes of water saturated with benzme and benzene saturated with water were 1.3327 and 1.4963, respecti\-ely, which compare favorably with the values 1.3325 and 1.4965 obtained by Washburn under similar conditions. Known mixtures in the area of heterogeneity were then very carefully made up, thoroughly shaken, and placed in the constant-temperature bath. After settling, samples were pipetted from both layers, and their refractive indexes measured with the refractometer. The composition of each layer was then read from the graph of per cent pyridine against refractive index as previously determined. The refractive index of the benzene layer varies only a few units in the fourth decimal place over a wide range of compositions, and consequently it is not a very accurate method of analysis. On the other hand, the refractive index of the water layer changes very rapidly as the amount of pyridine present increases. Since the point representing the over-all composition was accurately known, the composition of the water layer was sufficient to fix the tie line completely. In the range where the benzene layers contained from 10 to 40 per cent pyridine,
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JULIAN C. SMITH
the analyses of these layers served as a qualitative check only; outside of this region, however, the tie line determined by the analyses of the two layers passed exactly through the point representing the over-all composition. TABLE 1 Data for the limiting curve BENZENE
wwmE
weigh1 per cml
w i s h 1 pn rcnl
86.8 74.8 67.7 62.2 49.3 42.8 36.5 20.4 17.8 11.5 7.3 4.3 1.2 0.7 0.3
12.8 24.1 30.4 34.9 44.3 48.2 51.2 56.2 56.3 54.8 50.6 43.4 26.7 20.1 3.9
1
WATER
1
.
weight per cenl
0.4 1.1 1.9 2.9 6.4 9.0 12.3 23.4 25.9 33.7 42.1 52.3 72.1 79.2 95.8
. . . .
1.4979 1.4984 1.4978 1 ,4972 1.4942 1.4900 1.4850 1.4656 1.4622 1.4506 1.4361 1.4177 1.3811 1.3685 1.3390
. . .
Benzene saturated with water.. . . . . . . . . . . . . . . . . , . . . . . . . . . . .I Water saturated with benzene. . . . , . . . . . . . . . . . . . . . . . . . . . . . . .
1.4963 1.3327
~
Data for the tie lines OVER-ALL COMPOSITION
-~
BENZENELAYEX
WAIPR LAYEP
Benzene
Pyridine
Water
Pyridine
Pyridine
per cm1
pn
cmt
par urn1
per c n l
pn cml
5.0 9.6 15.1 17.2 24.9 29.5 34.4 39.6 46.6 51,l 54.6
50.7 48.2 30.0 44.2 40.6 37.6 35.0 32.2 28.5 26.1 24.2
7.5 13.6 18.9 23.9 30.7 33.0 34.7 37.0 41.3 44.5 48.9
44.3 42.2 54.0 38.6 35.5 32.9 30.6 28.2 24.9 2!2.8
21.2
1 ,4973 1.4980 1.4980 1.4984 1 ,4980 1.4973 1.4970 1.4965 1.4948 1.4928 1.4892
2.5 5.2 8.0 11.2 18.4 26.1 34.6 41.6 50.0 54.3 56.2
1 1.3365 1.3412 1 ,3462 1.3521 1.3656 1.3802 1.3980 1.4135 1.4342 1.4481 1.4600
Densities at 25°C. (2):water = 0.9971: benzene = 0.8724;pyridine = 0.9778
The data of table 1are extremely self-consistent and indicate that the anomaly found by Woodman and Corbet actually exists. The diagram (figure 1) agrees very well with that given by the earlier investigators, although it is difficult to draw a smooth curve through the somewhat scattered points which they give. The analysis for benzene, by which they determined the points on the limiting
379
THE SYSTEM BENZEKE-PYRIDINE-W,4TER
curve, is very difficult, which probably accounts for the scattering of their experimental points. The maximum deviation between the two limiting curves, however, is only 1.5 per cent. The tie lines obtained in the two determinations agree almost exactly. The cause of the anomaly described is not known. It has been suggested that it is due to a tendency toward compound formation between benzene and pyridine, analogous to that found in the lead-zinc-tin system described by Wright (10). Py rtdine
L
'2O
6 '
FIQ.1. Solubility diagram for the system benzene-pyridine-water a t 25°C. SUMMARY
The system benzene-pyridine-water, reported to be anomalous, has been reinvestigated, using specially purified materials. The anomaly actually exists, and the experimental results agree very well with those obtained by the original workers. The author wishes to thank Dr. J. A. McCoubrey of Shawinigan Chemicals, Ltd., for his interest and advice during this investigation. REFEREKCES (1) BRANCKER, A . V., HUNTER,T.G . , AND XMH, A . W . : Ind. Eng. Chem., Anal. Ed. l a , 35 (1940). (2) Handbook of Chemistry and Phyaica,24th edition. Chemical Rubber Publishing Go. Cleveland, Ohio (1940).
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J.
n.
SIMONS AND ROBERT KINYEL SMITH
(3) HORIBA, S.: Mem. Coll. Sci. Eng. Kyoto Imp. Univ. 3, 63 (1911). (4) KONO,M.:J. Chem. SOC.Japan 44, 406 (1923). (5) MILLER, W . L., AND RICPHERSON, R . H . : J. Phys. Chem. 12, 706 (1908). (6) TAYLOR, S. F.: J . Phys. Chern. 1, 461 (1897). (7) VARTERESSIAN, K. A , , AND F ~ K S K M. E , R . : Ind. Eng. Chem. 28, 928 (1936). (8) WASHBURN, E. R., HSIZD.A,V., A N D VOLD,R. D.: J. Am. Chem. SOC.63, 3237 (1931). (9) WOODHAN, R. M.,AND CORRET, A . S.: J. Chern. SOC.,1926, 2461. (10) WRIGHT,C. R. A , : Proc. Roy. SOC. (London) 60, 372 (1892).
T H E EXTROPY OF VAPORIZATIOS ASD DESSITY OF LIQUIDS AT T H E I R BOILISG POISTS J. H . SIMOSS
AND
ROBERT KISSEL SMITH
School of Chemistry and Physics, The Pennsylvania State College, State College, Pennsylvania Received November 13, 1941
The nearly constant entropy of vaporization of liquids a t a definite pressure is a useful property and one that has been given extensive theoretical consideration. Hildebrand (4)modified Trouton’s rule (9) in order to reduce the drift with boiling temperature by considering it for constant vapor concentration. Kistyakovskii (6) formulated an expression for the entropy of vaporization which gives a reasonably good agreement with experimental data,A S = R In V
(1)
in which V is the gas volume. For a perfect gas this can be rewritten as follows:
AS = R In ( R T )
(2)
Other empirical equations have been given (1, 2, 7, 8) to express the same property. These equations use either a logarithmic function or a power series in temperature. By making use of experimental data, Huffington ( 5 ) established for a number of substances the expression
A S = 1.8 R In V,/V,
(3)
where V , and VI are the molar gaseous and liquid volumes a t the boiling temperature. This and similar equations do not and cannot be expected to hcld for all liquids. This would require the force fields surrounding the molecules in the liquid to be either the same or related in a simple manner for all liquids. By employing a picture of the vaporization process which involves two steps, we have found that it is possible to separate the total entropy of vaporization per mole a t the boiling point a t 1 atmosphere pressure into two parts. We first consider that sufficientliquid to form 1 mole of gas is converted to a perfect gas
