T H E FREEZING-POINT DIAGRAM O F T H E SYSTEM PHENOL-WATER1 BY F. H.
RHODGS
AND A. I,. MARKLEY
Several authorities have stated that phenol forms one (or more) definite hydrates while other investigators have been unable to obtain any evidence of the existence of any definite hydrate. The first description of a definite hydrate of phenol appears in an article by F. C. Calvert2 in 1865. Calvert stated that when a mixture of four parts of phenol and one part water of water is cooled to 4 ” C rhombic crystals of phenol hydrate are formed. On further examination, the hydrate crystals were found to melt a t 16” C and have the formula 2C6HsOH.H20. At the Paris Exhibition of 1867, F. Crace Calvert and Co., and Charles Lowe and C O . ~ exhibited, among other products, a sample of this hydrate of phenol. I n the same year, Calvert4 stated that by removing from this hydrate the equivalent of water which it contains, phenol is obtained in its purest state.” I n 1874, Charles Lowe and John Gill5 patented the “separation of phenol from the cresylic acids and other liquid tar acids-by the formation and separation of hydrated carbolic acid crystals and from the mother liquor.” I n their patent they stated that the temperature a t which the hydrate crystals formed was between 1 5 ” F and 56” F. Further details of the purification process of Lowe and Gill were given by Marzell.6 Alexejeff’ tried to prepare phenol hydrate by the method described by Lowe, but was unable to obtain any indication Contribution from the Department of Chemistry of Cornel1 University,
F. C. Calvert: Jour. Chem. SOC., 18, 66 (1865). Chem. News, 16, 57 (1867). Calvert: Chem. News, 16, 297 (1867). Chem. News, 31, 52 (1875); Brit. Pat. No. 1485 (1874). e Marzell: Chem. News, 37, 105 (1878). ’ ’Alexejeff: Jour. SOC. Chem. Ind., 1, 397 (1882).
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and
A.L.Markley
of the existence of a definite compound. On the basis of his results, he concluded that the hydrate described by previous investigators was only a mixture of phenol and water. Paterna and Ampolal were also unable to obtain any evidence of the existence of a definite hydrate of phenol. Smits and Maarse2found that phenol hydrate is not easily formed when a mixture of phenol and water is cooled. Under ordinary conditions, supercooling takes place and solid anhydrous phenol is obtained. By cooling a mixture of phenol and water in a bath of solid carbon dioxide and alcohol, however, crystals of a hydrate of the formula 2CsH OH.H20 were obtained. In view of the rather meagre and contradictory nature of the previously published information concerning the existence and properties of phenol hydrate, this present investigation of the freezing-point curve of the system phenol: water was undertaken.
Experimental The phenol used in this work was prepared from a sample of refined natural phenol which was presented to this laboratory by the Chemical Department of the Barrett Company. The original material was dissolved in a twenty-five percent solution of sodium hydroxide, and steam was passed through the resulting solution to remove all traces of oil. The solution was then cooled and neutralized with a twenty percent solution of sulphuric acid. The resulting phenol layer was distilled, and the distillate was redistilled throug efficient fractionating column. Only that portion of the distillate which had a melting point of 40 .4' C or above wascollected for use in the subsequent experiments. The distillate was collected directly in large test-tubes, 8 in. x 11/4in., which had previously been dried and weighed. from nine to grams of phenol was collected in each tube. Each 1 2
Paternh and Ampola: Gazz. chim. ital., 27, 481 (1897). Smits and Maarse: Ver. Kon. Akad. Wet., 20, 100 (1911-12).
Freezing-Point Diagram of Phenol and Water
529
was then closed with a cork stopper covered with tin-foil, weighed, and stored in a desiccator over phosphorus pentoxide. I n determining the melting points of mixtures of phenol and water, the following procedure was adopted: To a tube containing a known amount of the pure phenol was added a known amount of pure water, sufficient to give the desired mixture. The tube was then suspended, by means of a cork stopper, in a larger tube which was immersed in a cooling bath (cold water, ice, or ice and salt) at a temperature slightly below the estimated freezing point of the mixture. The stopper in the sample tube was replaced by a cork carrying a thermometer and a glass stirrer, and the sample was allowed to cool slowly and with constant stirring. As the estimated melting point was approached, the mixture was “seeded” by adding small crystals of solid phenol or phenol hydrate. The highest temperature a t which permanent crystals could be obtained was taken as the freezing point of the mixture. Each determination was repeated several times to insure the elimination of errors due to supercooling or to error in obtaining the exact phenol-water ratio in the sample. The thermometer used in this work was a “Tycos” nitrogen-filled mercurial thermometer, graduated to read directly to 0 . 1 O C. The thermometer was calibrated before use and was found to read correctly. The results obtained are shown by the following tables:
TABLE I Stable Equilibria Water,
% 0 2
3 5 8 8.16 8.25 8.41 8.56 8.74
Temp., “C
40.8 33.0 1 29.3 15.9 15.8 -
I:!:
15.9 -
Melting point of phenol Solid phenol in equilibrium with liquid phenol containing phenol hydrate in solution Eutectic phenol and phenol hydrate Phenol hydrate in equilibrium with liquid hydrate containing phenol Melting point of phenol hydrate
530 9.00 10 * 11. 12. 14. 16. 18. 20 * 22. 24. 26. 27. 35.
io.
76. 88. 91. 95. 97. 98. 98.5 100.
F . H . Rhodes and A . L. Markley TABLE I (Continued) 15.75 15.45 15.15 14.40 [ 14.00 13.30 13.00 12.70 , 12.40 12.30 12.20 12.20 12.20 12 * 20 12.20 12.20,
Solid hydrate in equilibrium with liquid hydrate containing water Two liquid layers appear
C. 0.85 ) 0.85 Ice, solid hydrate, and water 0.85 containing dissolved hydrate 0.85
i
0.86-Eutectic, hydrate, ice, solution ' Ice, in equilibrium with solution 0.45 of hydrate in water 0.35 0.00-Melting point pure water
]
TABLEI1 Metastable Equilibria Water,
% 9 12 15 18 20 22 23 24 25 35 50 75 80 88 90 91 92 92.5 93 93.5
Two liquid layers appear Solid phenol in metastable equilibrium with liquid phenol containing water
C. 1.2 1. 2 1.2 1.2 1.2 1.2-
'
1
j
Ice in equilibrium with solid phenol and water containing dissolved phenol
Eutectic,
ice, solid phenol,
solution
Freezing-Point Diagram of Phenol and Water
53 1
TABLE I1 (Continued) Ice in equilibrium with solution of 94 95 1 . 0 phenol in water O.O-.- Melting point pure water 100 These results are shown graphically by the accompanying curve.
i
Fig. 1 Phenol and Water
Diseussion From these data it is apparent that we have to deal with two distinct systems: (1) The system of stable equilibria between phenol, phenol hydrate and water, and (2) The system of metastable equilibria between anhydrous phenol and water. In order to simplify the discussion, each system will be considered separately.
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F . H . Rhodes and A.L. Markley
The system of stable equilibria is represented by the curves ABDFHMG and PM. The point A represents the melting point of the purified anhydrous phenol. It is worthy of note that the melting point of the carefully purified phenol was found t o be 40.8" C. The values given in most of the chemical handbooks vary between 42" C and 43" C. I n this present investigation, as well as in previous unpublished work done by one of us, careful attempts to prepare pure phenol from both the natural and synthetic product gave a purified phenol which melted at 40.8" C. We therefore consider this to be the true melting point of phenol. The addition of small amount of water lowers the melting point of phenol very rapidly. The curve AB represents the (stable) equilibria between solid phenol and solution. A t point B (8.25% water, 15.8" C) the eutectic between phenol andphenol hydrate is reached. The further addition of water increases the melting point until point D, the melting point of the pure hydrate is attained. This hydrate contains 8.74 percent of water, corresponding to the formula 2C6H50H.Hz0, and melts at 15.9" C. The curve BD represents the equilibria between solid hydrate and solution rich in phenol. The further addition of water lowers the melting point again until point F (27% water, 12.2"C) is reached. At this point two liquid phases appear, liquid phenol saturated with water, and water saturated with phenol hydrate. The addition of water beyond this amount merely varies the relative proportions of the two liquid layers and does not affect the freezing point of the phenol hydrate layer. At point H (91% water, 12.2" C) there is present enough water to hold in solution (at 12.2' C) all of the phenol hydrate. A further increase in the amount of water prevents the formation of a separate liquid layer of phenol hydrate, and lowers the freezing point rapidly to the eutectic M. The curve HM, therefore represents the equilibria between solid phenol hydrate and solution of phenol hydrate in water. M (95yowater, -0.85" C) is the eutectic at which solid hydrate is in equilibrium with ice and solution The line
Freezing-Poiwt Diagram of Phenol awd Water
533
MG represents the temperatures at which ice is in equilibrium with solution. Thus, in thes ystem phenol-phenol have the following fields of stability:
hydrate-water,
we
Phases
Field
ABC BCYT DEB DPMHF MGV PTZV
Phenol crystals, solution, Phenol crystals, hydrate crystals, Hydrate crystals, solution, Hydrate crystals, solution of hydrate in water, Ice, solution, Ice, hydrate crystals.
In addition to this series of stable equilibria, there exists a series of metastable equilibria represented bythe curve BLKNG. The curve BL, which is the continuation of the curve AB represents the metastable equilibria between solid phenol and a solution of water in phenol. At I, (25% water, 1 . 7 O C) two liquid phases first appear: (1) a saturated solution of phenol in water, and (2) a saturated solution of water in phenol.
The further addition of water beyond this point results merely in a change in the relative amount of the two layers, the freezing point of the phenol layer remaining constant at 1. 7"
c.
At point K(90 % water, 1 . 7 " C) the amount of phenol present is just sufficient to saturate the water. The addition of more than ninety percent of water causes the phenol layer to disappear, and the freezing point drops sharply to point N (93.5% water, - 1 . 2O C). This point is the eutectic at which solid phenol and ice coexist in (metastable) equilibrium with a saturated solution of phenol in water The further addition of water causes the freezing point to rise, as indicated by line NG, representing the equilibria between ice and solution. Thus in the system phenol-water we have the following fields of equilibrium.
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F. H . Rhodes a?zd A . L. Markley
Field
ABLKNSC NGX
SXZY
Phases
Solid phenol, solution. (In this field, the area ABC represents stable equilibria. I n all of the remainder of the field the solid phenol is metastable.) Ice, solution. (Of this field, portion MGV is stable, while NMVX is metastable.) Solid phenol, ice-metastable throughout.
I n these experiments it was found that when a liquid mixture of phenol and water was cooled by any of the ordinary refrigerating agents (cold water, ice, or ice and salt) no formation of hydrate crystals occurred. I n every case the solid phase which separated was either solid phenol (from solutions containing less than 93.5y0 water) or ice (from solutions containing more than 93.5% water). No crystals of phenol hydrate could be obtained even when mixtures of phenol and water were allowed to stand overnight at -20" C. Hydrate crystals were originally obtained only by the method suggested by Smits and Maarse, i.e., by cooling a solution of fifteen percent of water in phenol for one hour to a temperature of -78" C, using as a refrigerating agent a mixture of solid carbon dioxide and alcohol. Points on the stable curve (BDEFHM) were then easily determined by seeding the proper mixtures, a t the proper temperatures, with small crystals from this lot of hydrate. By this procedure i t was found possible to avoid supercooling and suspended transformation.
Summary The complete freezing point diagram of the system phenolwater has been determined. Phenol forms a definite hydrate, 2C8Hr,0H.Hz0, melting a t 15.9" C, and forming eutectics with anhydrous phenol and with ice, respectively. Because of the tendency toward suspended transformation, the solid phase which ordinarily appears when a rnixture of phenol and water is cooled is the metastable anhydrous phenol. Hydrate crystals were obtained only by seeding with hydrate or by cooling to very low temperatures. Ithaca, New York
