THE PURIFICATION AND PHYSICAL PROPERTIES OF ORGANIC COMPOUNDS. I1
THE FREEZING POINTS OF SOMEOF THE THERMOMETER CALIBRATION STANDARDS FOR Low TEMPERATURES OF THE BUREAU INTERNATIONAL DES ETALONS PHYSICO-CHIMIQUES~ EVALD L. SKAUS
Department of Chemistry, Trinity College, Hartford, Connecticut Received January Q, 19%
I n the course of another investigation requiring a careful thermocouple calibration a t low temperatures, accurate freezing point determinations were made on a number of samples of organic compounds supplied “for thermometer calibration” by the Bureau International des Etalons Physico-Chimiques, University of Brussels, in order to add to the certainty of our temperature scale and also to prove that our method of determining freezing points by means of heating curves is satisfactory and has advantages over the usual methods. The freezing point values attributed to these samples were based upon measurements made by Professor Timmermans against the helium thermometer of the Laboratoire Cryog6nique de l’Universit6 de Leiden in 1922 (1) and repeated in 1928 (2), when slightly different values were assigned to chlorobenzene, carbon disulfide, and methylcyclohexane. Our freezing point measurements were made by means of a method particularly adapted to the accurate determination of this physical constant at low temperatures. We believe, therefore, that the results may help to establish definitely the true freezing points of these samples. The “accepted values” of Timmermans are identical with ours within the claimed limits of accuracy except in two cases, and in both of these cases we have been able to show that his values published in 1922 are more nearly 1 Contribution No. 258 from the Research Laboratory of Physical Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts. A part of the experimental work for this paper was completed a t Trinity College, Hartford, Connecticut, thanks t o the Cyrus M. Warren Fund of the American Academy of Arts and Sciences and to Curtis H. Veeder of Hartford, for grants t o defray the expenses, and also to the Yale University Physics Department for supplying the necessary liquid air a t cost. National Research Fellow, 1926-28, a t the Massachusetts Institute of Technology; Guggenheim Fellow, 1930-31, a t the Universities of Munich, Frankfurt a. Main, and Brussels. 609
610
EVALD L. SKAU
correct than those he assigned to them on the basis of his 1928 determination~.~ METHOD
The observations here reported were made by means of a cooling-curve and heating-curve apparatus which is being described in detail elsewhere (3). This apparatus involves a cryostat whose temperature can be very carefully regulated and in which a 0.5- 1.0 g. sample is suspended in an hermetically sealed “freezing point tube.” The latter is made of extremely thin-walled 6-mm. glass tubing (about 0.08-mm. wall thickness) in order that its heat capacity may be negligibly small with respect to that of the sample, and is so constructed that the temperature of the sample may be followed by means of a single-junction thermocouple, one element of which fits snugly into a thin-walled capillary tube extending down through the center of the tube to within 5 mm. of the bottom. The cold junction is kept a t 0°C. The readings are made to the nearest microvolt by means of a Leeds and Northrup Type K potentiometer. The freezing point is determined by running heating curves and cooling curves on the sample, always controlling the temperature of the surroundings to rise or fall, as the case may be, a t a definite recorded rate. The reproducibility of the curves so obtained, their interpretation, and the advantages of heating curves over cooling curves for the determination of freezing point and also as a criterion of purity have already been pointed out (3). Since the thermocouple calibration was only to the nearest microvolt (=kO.Q2’C.,at O’C., to 40.05”C. at liquid air temperature), and since our freezing point values are also read only to the nearest microvolt, our freezing points cannot be said to have a n accuracy greater than f0.04’C. at O”C., to f0.10’C. at -183°C. THERMOCOUPLE CALIBRATION
Five different thermocouples, made of No. 30 constantan and No. 36 copper wire, were used. Two of these were calibrated simultaneously at a
I n order to clear up a misunderstanding in regard to the samples prepared by
R. S. Taylor, and studied by Keyes, Townshend, and Young (J. Math. Phys. Mass. Inst. Tech. 1, 302 (1922)), which have been criticized by Timmermans, it should be pointed out that the sole claim made by these authors was as follows: “The freezing point found for an organic liquid depends somewhat upon the method employed and the thoroughness of the purification. The results here given will be found reproducible provided the general method of procedure is followed.” I n a letter to the author Professor Keyes states: “The use t o which these samples were put was in connection with the production of helium in Texas. Whether the samples were of the highest purity or not made little difference in this connection, since they served to transfer temperature indications obtained from my hydrogen thermometer and platinum resistance thermometers to the operators a t the Texas helium plant.”
PHYSICAL PROPERTIES O F ORGANIC COMPOUNDS. I1
611
the ice point, at the freezing point of mercury (-38.87"C.) (4), at the sublimation point of carbon dioxide (-78.51"C.) (4), and a t the boiling point of oxygen (-183.OO"C.) (4). The last two points were established by measuring the vapor pressure of the carefully purified substances in a constant temperature cryostat in which the thermocouple was immersed ( 5 ) . I n each case the purity of t,he sample was tested by evaporating to one-half volume and then redetermining the vapor pressure. The calibration a t the mercury point was carried out in the same apparatus as the freezing points and under identical conditions. The other three thermocouples were calibrated against these two, all being inserted simultaneously in a constant temperature bath of pentane, the cryostat described by Taylor and Smith ( 6 ) , and all five being read several times in rotatmionwhile the temperature of the cryostat was kept constant a t various temperatures a t intervals of about ten degrees along the scale, until at least three consecutive series of readings at each temperature gave identical values. MATERIALS
Samples of the following compounds were obtained from Professor Timmermans, Director of the Bureau International des Etalons PhysicoChimiques : carbon tetrachloride, chlorobenzene, chloroform, ethyl a,cetate, carbon disulfide, ethyl ether, and methylcyclohexane, each in a tube sealed in vucuo. Each sample was introduced without further treatment into a freezing point tube with the minimum possible exposure to the atmosphere and was sealed off by the usual procedure without evacuating. Though the samples may not have been identical with those used by him in his freezing point determinations, they were similarly purified, and had the same densities within &.0001 in all cases (7). RESULTS
A number of heating and cooling curves were run on each substance a t various intervals in the course of eighteen months, employing more than one thermocouple for each sample. The samples were always kept in a dark cupboard between measurements, With the exception of ethyl acetate and methylcyclohexane, the values accepted for the freezing points are based on both heating and cooling curves. I n all these cases all except the highest, cooling curve values were rejected, since obviously the effect of an unsatisfactory degree of supercooling, or of a failure to reach true equilibrium between the solid and liquid phases, is to give a freezing point which is too low. This was an important factor only in the case of the ethyl ether sample, where the highest cooling curve values agreed satisfactorily with the heating curve values, but where many cooling curves gave values from two to four micro-
612
EVALD L. SKAU
volts lower. It was not found possible to measure the freezing points of the ethyl acetate and methylcyclohexane by our cooling curve method; all the values so obtained were not only low but inconsistent. This was particularly true for the methylcyclohexane. The best value obtadned for the ethyl acetate, for example, was -83.84"C. Thus, the advantage of the heating curve method was particularly evident in these cases. The stable form of ethyl ether was the one most often obtained by cooling curves, though a preliminary halt of five to twenty seconds at the freezing point of the unstable form was usually noted. It seems possible TABLE 1 Freezing points of Timmermans' thermometer standards
I SUBSTANCE
I
On label
I
egrees C .
Carbon tetrachloride*.. , . -22.91
I
TIMMERMANS' VALUES
(1922)
degrees C .
-22.8941
I
(1928)
1
degrees C
-22.821
Chlorobenzene. . . . . . , . . . .
3
Chloroform, . . . . . . . . . . . . . -63.51' -63.4951 Ethyl acetate.. . . . . . . . , . . -83.6 -83.6 Carbon disulfide. . . . , . . . . -111.6 -111.613 -111.84
1
1 k-116'351
Ethyl ether (stable).. . . . , -116.3 -116.322
Accepted value (19%)
FREEZING
POINT (SKAU)
1
DIFFERENCE
FROM ACCEPTED VALUE
degrees C .
degrees C . degrees C .
-22.85
-22.@
+0.01
-45.2'
-0.13
-63.41 -83.63 -111.88
f O .03 +O ,08
-63.5 -83.6 -111.8
-0.09 (0.04)
-116,37 ( - i i 6 . 3 ) 1 - 116.29
-0.01
Ethyl ether (unstable)
-123.3 -123.301 (-123'47 -123,50 -123.3
-123.25
-0.05
Methylcyclohexane.. . .
-126.3 -126.35
-126.34
-0.51
-126.85
-126.85
* The transition point of carbon tetrachloride was also determined on this same sample. Transition point = -47.55 10.12"C.;see Skau and Meier: J. Am. Chem. SOC.61, 3517 (1929). t Private communication. that crystals of the unstable form always appear first in accordance with Ostwald's step-by-step theory (8). It should also be mentioned that our experiments showed definitely that transition from the unstable to the stable form of ether can take place completely in the solid phase. This was proved by the fact that heating curves for the stable form were obtained on samples which had solidified completely as the unstable form as shown by the complete cooling curve^.^ 4 The behavior of ether here described has a bearing on a statement made by Smits (Z. physik. Chem. 163A,287 (1931)): "Der Umwandlungspunkt der flussigen Phase liegt bei -105.4" und weiterhin kennt man zwei Schmelzpunkte bei -116' und bei -123.4". Die Modifikation mit dem niedrigsten Schmelzpunkt, die am leichtesten erhaltlich ist, hat man bisher als die metastabile Modifikation betrachtet. Da man
PHYSICAL PROPERTIES O F ORGANIC COMP0,UNDS. I1
613
I n no case was there found to be an appreciable change in freezing point with time, though the determinations were carried out over the course of about eighteen months using the original samples throughout. This brings out another advantage of our freezing point method for, as has been pointed out by Timmermans (9), pure chloroform, for example, is very sensitive on exposure to the airs6 The results have been summarized in table 1. Column 2 shows the freezing point values given on the labels of the sample tubes as received; columns 3 and 4 give Timmermans’ Leiden values of 1922 and 1928 respectively; column 7 shows the differences between our values, column 6, and the final accepted values of Timmermans, column 5. Since the accuracy claimed by Timmermans for his freezing points is only O.l”C., all of our values can be said to check his final “accepted values,” with the exception of that for methylcyclohexane and probably that for chlorobenzene. I n both of these cases, however, our values agree very closely with those determined by him6 in 1922. On the other hand, our value for carbon disulfide agrees with his more recent value, which was 0.23”C. lower than the value he reported in 1922. It is suggested that the following be accepted as the freezing points of these substances’ on the basis of the data now available: Carbon tetrachloride.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chlorobenzene.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chloroform.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ethyl acetate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-22.860C. -45.20 -63.46 -83.6O
aber nunmehr weiss, dass die Flussigkeit allotrop ist, kann auch . . . die Modifikation mit dem hoherbn Schmelzpunkt die metastabile Modifikation sein. Wir sind damit beschaftigt diese Frage experimentell zu klaren”. Our results show that for ether the form with the lower melting point is the unstable form, which is, in fact, in agreement with more recent experiments by Smits (private communication). Professor Keyes suggests the desirability of designing a freezing point tube similar in construction to the one here described but with a larger capacity so t h a t it could be used with a platinum resistance thermometer with a correspondingly modified technique. This could probably be used solely for the cooling-curve method, but would be decidedly advantageous in that (1) i t would make available for temperature comparison a number of substances which crystallize well but which become impure on exposure to air, and (2) it would obviate the necessity of handling the sample before freezing point determinations, with the resulting possibility of the introduction of impurities. The reason suggested by Professor Timmermans for the lowness of his value for the freezing point of methylcyclohexane in 1928 is that considerable air may have dissolved in the sample, for by his method the sample is stirred vigorously without excluding air (private communication). Professor Timmermans has expressed his willingness to endorse this revision of his “accepted values” in the light of the evidence now a t hand (private communication).
‘
614
EVALD L. SKAU
Carbon disulfide.. ............................................ Ethyl ether (stable). ......................................... Ethyl ether (unstable). ....................................... Methylcyclohexane.. .........................................
-111.8W. -116.30 -123.3O 126.36
-
The values for toluene and isopentane, which also belong to this series but which, unfortunately, were not included here, will be determined a t the earliest opportunity. This investigation bears out the fact that accurate freezing points can be measured by heating curves. It should be pointed out that our method has the decided advantage of being applicable t o all substances which can be caused t o crystallize within the temperature range from 250°C.down to very low temperatures, irrespective of their viscosities or other properties, such as a slow rate of crystallization, which interfere with obtaining good values by the usual cooling-curve or Beckmann method (3). It requires only a small sample, an important factor where rare substances are being dealt with, and any number of determinations can be made on the same sample. Most important, the sample can be introduced into the freezing point tube and sealed off entirely out of contact with air, and the determination made in wucuo. This is particularly important not only in the case of compounds which are hygroscopic (like ethyl alcohol), or which absorb carbon dioxide in contact with air (like polyphenols), but also in the case of any liquid freezing below O"C., for at low temperatures the condensation of moisture and of carbon dioxide, as well as the dissolving of the other constituents of the atmosphere, are a very grave source of contamination. REFERENCES (1) TIMMERMANS, VAN DER HORST,AND ONNES:Arch. n6erland. sci. IIIA, 6, 180 (1922). See also: Communications Phys. Lab. Univ. Leiden, Suppl. 51b, p. 35 (1924). (2) TIMMERMANS: Communications Phys. Lab. Univ. Leiden, Suppl. 84, p. 3 (1928). (3) SKAU:Proc. Am. Acad. Arts Sci. 67,551 (1933). (4) International Critical Tables, Vol. I, p. 53. (5) KEYES,TOWNSHEND, AND YOUNG: J. Math. Phys. Mass. Inst. Tech. 1,302 (1922), for a full description of the details of this method. See also, LOOMIS AND WALTERS:J. Am. Chem. SOC.48,3101 (1926), and HENNING: Ann. Physik 43, 282 (1914). (6) TAYLOR AND SMITH:J. Am. Chem. SOC.44,2450 (1922). KEYES,TAYLOR, A N D SMITH:J. Math. Phys. Mass. Inst. Tech. 1,211 (1922). See also, CARDOSO: J. chim. phys. 16,317 (1915).
(7) Private communication. (8) OSTWALD:Z. physik. Chem. 22,306 (1897). (9) TIMMERMANS: Helv. Chim. Acta 14,445 (1931).
