NOTES
72 hr of conta,ct with pyridine and water vapor following evacuation at 500” may be a function of this slow site rearrangement. Total acidity alters little after 16 hr of dosing, and the value obtained after this time is regarded as a correct final measure. Acknowledgment. Permission to publish this paper has been given by The British Petroleum Co., Ltd.
A n Apparatus for Degassing Liquids by Vacuum Sublimation
by T. N. Bell, E. L. Cussler, K. R. Harris, C. N. Pepela, and Peter J. Dunlop’
4693 heptane by keeping the distilling liquid at about 0”, but in addition it was found necessary to degas the mixtures two or three times to obtain reproducible results. The hydrocarbons were always degassed in the presence of sodium wire. The results of static measurements of the vapor pressures of water, benzene, n-hexane, n-heptane, n-octane, and cyclohexane samples degassed in this manner are given in Table I, together with literature values. Table I : Vapor Pressures a t 25’ of Several Hydrocarbons and Water Vapor pressure, torr This work Lit.
Benzene
95.15 I t 0 . 0 3
Department of Physical and Inorganic Chemistry, University of Adelaide, Adelaide, South Australia 6001 (Received June 18,1068)
Cyclohexane
The precision of vapor pressure and virial coefficient measurements depends in part upon the efficiency with which the liquid systems are degassed. Most methods described in the literature are repetitive and consequently often tedious and time consuming. A common procedure2 is that of freezing the liquid, pumping away the noncondensable gases, melting, and refreezing, the process being continued until some experimental parameter is constant. Experience in this laboratory has shown that up to 20 such cycles are necessary with aqueous system^.^ Hermsen and Prausnitz4 have described a method whereby their materials were “refluxed for several days. . . at low pressure,” and Cruickshank and Cutler6 described another involving “several vacuum redistillations.” The method reported in this paper is similar to the technique of “freeze drying,”6 and usually one cycle is sufficient to completely degas aqueous systems and pure hydrocarbons, The apparatus (see Figure 1) consists of a Pyrex glass dewar, the vacuum chamber being the vapor pressure cell. The material to be degassed is frozen, either in the cell itself or in the detachable sampling bulb. (This is also used to sample mixtures for analysis at the end of a run without breaking the vacuum in the system.) The cell is then evacuated and a suitable freezing mixture is placed in the central compartment of the dewar. The frozen liquid slowly sublimes onto the cold glass surface, while noncondensable gases are continuously pumped away (less than 5% of the sample is lost in this process). These gases do not seem to redissolve unless the sublimate thaws. This was found to occur with n-heptane and its mixtures with benzene when the deposit was thick; in these cases anomalous vapor pressure measurements were noted with only one degassing (up to 0.2 torr too high). The effect was prevented in the case of m
97.59 f O . 0 3
n-Hexane
151.16 f 0 . 0 2
n-Heptane
45.79 I t o . 0 2
n-Octane Water
13.97 f 0 . 0 2 23.755k0.003
95.18k0.04 95.15fO.05 95.17 95.04k0.08 95.03f0.04 95.14 i 0 . 0 3 97.583=0.04 97.81i0.04 97.41=!=0.03 151.26f0.04 151.05“ 45.72 k 0.04 45. 64n 13.98” 23.753
Ref
6 C
d e
f g
b h
f b
i 6 i b j
C. B. Willinga Extrapolated using the Antoine equation. ham, W. J. Taylor, J. N. Pignocco, and F. D. Rossini, J . Res. Nat. Bur. Stand., 35, 219 (1945); F. D. Rossini, K. S. Pitser, R. L. Arnett, and G. C. Pimental, “Selected Values of Physical and Thermodynamic Properties of Hydrocarbons,” Carnegie Press, Pittsburgh, Pa., 1953. Reference 4. R. H. Stokes, Department of Physical and Inorganic Chemistry, University of New England, Armidale, New South Wales, private communication. e Reference 2b. D. H. Everett and F. L. Swinton, Trans. Faraday Soc., 59,2476 (1963). W. J. Gaw and F. L. Swinton, Reference 5. * I. Brown, Aust. J . Sci. ibid., 64, 637 (1968). Res., A5, 530 (1952). F. G. Keyes, J . Chem. Phys., 15, 602 (1947).
’
Samples which were degassed more than the minimum number of times stated above gave identical results within the experimental error. A sample of 40 ml of liquid takes from 1 to 2 hr to be degassed. The alkanes were allowed to melt in the sampling cell during the process, but, as mentioned above, it was only (1) To whom all correspondence should be addressed. (2) E.g., (a) J. B. Gilmour, J. 0. Zwieker, J. Katz, and R. L. Scott, J. Phys. Chem., 71, 3259 (1967); (b) G. Korttim and W. Vogel, 2. Elektrochem., 62, 40 (1958). (3) H. D. Ellerton, Ph.D. Thesis, University of Adelaide, 1966. (4) R. W. Hermsen and J. M. Prausnitz, Chem. Eng. Sci., 18, 485 (1963). ( 5 ) A. J. B. Cruikshank and A. J. B. Cutler, J. Chem. Eng. Data, 12,
326 (1967). (6) M. Dixon and E. C. Webb, “Enzymes,” Longmans, Green and Co., Ltd., London, 1958, p 13.
Volume 72,Number 13 December 1068
NOTES
4694
Freezing
and again with water. The dried product was distilled t,hrough a dry, 1-m column with an evacuated jacket. The column was packed with stainless steel helices approximately 4 X 4 mm and was fitted with a reflux head. That fraction which showed negligible impurity on analysis by gas phase chromatography was collected and stored over sodium wire. The alkanes were similarly treated using chlorosulfonic acids as the sulfonating agent. The densities of the pure hydrocarbons together with literature values are given in Table 11. These were measured, at 25.000 rt. 0.002", in triplicate, in single-stem pycnometers with volumes of approximately 30 em3. The precision of these density measurements is believed to be rt. g cm+. Table 11: Density Data a t 25' for Several Hydrocarbons Density,
7
cm-b------Mol % Lit.Q impurity
This work
Benzene Cyclohexane %-Hexane %-Heptane n-Octane
Figure 1. Diagram of the cell used for degassing organic liquids and solutions. The cell used for aqueous systems (e.g., a sodium chloride solution) did not have the bottom tap and sampling cell.
necessary to slow the rate of distillation of n-heptane. The sublimation of water and benzene was accelerated by warming the containing vessel. All measurements were made at 25.000 f 0.002'. The method has also been used to degas aqueous solutions7 containing nonvolatile solutes, e.g., sodium chloride, and to provide degassed liquid samples for second virial coefficient measurements.8
Experimental Section Materials. The hydrocarbons, with the exception of n-octane, were obtained from the Ajax Chemical Co., Ltd., Auburn, New South Wales. The n-octane was supplied by the Aldrich Chemical Co., Inc., Milwaukee, Wis. Benzene and cyclohexane were purified by treatment with concentrated sulfuric acid, followed by washing with water, bicarbonate solution, The Journal of Physical Chemktru
0.87368 0.77389 0.65479 0.67949 0.69854
g
0.87368 0.77389 0.65479 0.67949 0 69846 I
