THE PARTIAL PRESSURES O F VAPOCHS O F VOLATILE L I Q U D S I N T H E PRESENCE O F I N E R T GASES'
BY
w.
G. BEARE, G . A . M C V I C A R ASD J. B. FERGUSOS?
Khen a volatile liquid is placed in a closed vessel which contains a relatively inert gas and the temperature is kept constant, the partial pressure of the vapour of this liquid is frequently regarded as approximately equal to the difference between the initial gas pressure and the final total pressure; it is considered to differ from this by a negligible amount after corrections are applied for the change in volume of the vapour phase and the loss of permanent gas by solution, provided that the gas phase is a perfect gaseous solution, and that adsorption on the walls of the vessel is not a disturbing factor. The vapour pressure is obtained from the partial pressure by making an allowance for the diminution of the partial pressure caused by the solution of gas in Lhe liquid when this is an appreciable amount. There does not appear to be any sound theoretical objection to the method outlined for the treat,ment of the case cited or for similar cases but there is the practical objection, as noted by Dornte,3 that it is quite at variance with the published resultas of Regnault4 and C a r n ~ b e l l . ~Either these results are incorrect or some additional factor must be considered in the theoretical treatment of the case. The matter is of such importance that we felt that the facts in the case should be clearly established. Accordingly, we carried out several series of experiments similar in principle to those of Campbell but essentially different in respect to the apparatus and technique used. Our results, which are given in this paper, do not confirm the results of the earlier experiments.
Materials Methyl alcohol: Kahlbaum. Dried over freshly prepared lime at room temperature, decanted off and distilled. D 2014 0 . 7 9 2 3 2 . I.C.T. value 0.7917, Estimated to be 99.8 percent alcohol. Ethyl alcohol: Squibbs. Similarly dried. D 2014 0.78913. I.C.T. value 0.78934. Estimated to be pure. Presented at the 13th Annual Canadian Chemical Convention held at Ottawa, May 26, 1930. * Measurements by Messrs. Beare and McVicar. Dornte: J. Phys. Chem., 33, 1312 (1929): 4Regnault: MBm. de Paris, 26,679 (1862). Campbell: Trans. Faraday SOC., 10, 197 (1914).
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Acetone: Kahlbaum, aus der Bisulfit Verbindung. Dried over fused calcium chloride at room temperature, decanted off and distilled. D 2 0 4 0. j9106. This was not quite dry as the material we prepared last year gave 0 . 7 9 0 7 2 . ~ Ether: Eastman Kodak. This was shaken with clean dry mercury twice, filtered through charcoal, shaken with its own volume of water and the latter operation repeated five times with one-half the quantity of water. The separated product was first dried over fused calcium chloride and then over sodium and distilled. Air: This was passed through a concentrated aqueous solution of sodium hydroxide, over soda-lime, calcium chloride and phosphorus pentoxide in the order given. Carbon dioxide : This was prepared by dropping a concentrated aqueous solution of sodium bicarbonate into concentrated sulphuric acid and dried over calcium chloride and phosphorus pentoxide.
Apparatus and Procedure The apparatus of Fergueon and Funnell’ as modified by Reare and McVicar6 together with the additional part, A’, needed for the present work, is shown in Fig. I . The part, A’, was sealed into the apparatus between the large and the small bulbs in the large air bath. The solenoids of the circulating pump are unfortunately not indicated in their proper positions. Although movable, they were placed close together in actual operation. A glass container of suitable size was filled with the liquid under investigation, sealed and placed in A’. A trace of lubricant was placed on the ground-glass joint and enough mercury was kept in the glass cap to protect this seal. The apparatus was then evacuated. The air baths were brought up to the desired temperatures, the lower air bath being about four degrees above the temperature a t which the vapour pressure was to be determined, the upper about ten degrees above this. If the manometer reading remained constant for several hours or more, the apparatus was considered gas-tight. The pas (air or carbon dioxide) was then admitted until the desired pressure was reached, approximately. Mercury was run int’o the U tube until it reached a point in the capillary tube just inside of the upper air bath. The temperatures of the air baths were again adjusted, also the height of the mercury column and the pressure reading taken. The container tip was broken with the magnetic hammer; liquid air was placed around the small bulb and the circulating pump started. A small wire heater (not shown) surrounding A’ was turned on and the sample collected in the small bulb. The heater was disconnected, the liquid air removed, the air baths brought up to temperature and cold water circulated through the water bath. When no more of the 8 7
Beare, McVicar and Ferguson: J. Phys. Chem., 34, 1310 (1930). Ferguson and Funnell: J. Phys. Chem., 33, I (1929).
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W. G. BEARE, G . A. MCVICAR AND J. B. FERGUSON
sample remained as liquid in the small bulb the water bath was heated up and maintained at the desired temperature. This temperature was read on a Beckmann thermometer which had been compared with a standard, was kept constant to i O.OI"C.and known within *o.o2'C. The temperature of the lower air bath was kept constant to o.I'C. and that of the upper to j=o.j'C. The pump was run a t full speed until the pressure became constant. With air, this required approximately 1.5 hours and with carbon dioxide 2 . 5 hours. The pressure rose gradually until it attained its maximum value. The pump was kept running at full speed for an additional
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1
I
J
f FIG.I 0.5 hours, the speed was reduced and finally the pump was stopped but observations were continued for an hour after the stoppage. Reducing the pump speed made no appreciable difference in the pressure reading but the final reading was usually 0.2-0.3 mm lower than the readings with pump operating. This difference we attributed to a heat effect. In experiments, in which air was the inert gas, the small bulb was again cooled with liquid air, the pump operated and the sample collected in this bulb. The apparatus was then evacuated, closed by the mercury column and the vapour pressure of the liquid determined. The details need not be repeated for this operation. The difference between the initial gas pressure and the final pressure was taken as the approximate partial pressure. All pressure readings were reduced to o'C. Allowance was then made for the following experimental conditions :
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(a). The initial temperature of the reaction chamber which had a volume of 1 5 cc. was not always identical with its temperature when the final pressure reading was taken. (b). The evaporation of the liquid increased the volume of the gas phase, (c). Part of the inert gas dissolved in the liquid phase, the volume of which was approximately I cc. Since the volume of the gas phase was almost 3 joo cc, the total pressure was but slightly affected by these conditions. The results were subject t o one source of error, the value of which is unknown but the maximum value of which could be calculated. This arose from the fact that the gas in the container A' was not circulated and hence may have differed somewhat in composition from the gas in the rest of the apparatus. The maximum error would have occurred if only inert gas were present in A' and would have amounted to I mm in the case of carbon dioxide and ether. The observed differences between the partial pressures and the vapour pressures in the cases of carbon dioxide-ether and carbon-dioxideacetone might have been due in part to this source of error and in part to a solubiiity effect although the differences are so small that accidental errors may have been the major factor.
Results and Discussion The partial pressures and vapour pressures determined by us are given in Table I. The values lying along the same horizontal line were obtained with the same liquid sample. The table also includes a few independent vapour pressure measurements. The vapour pressure obtained for acetone agrees almost exactly with the value given by Sameshima and is slightly lower than our previously determined value for dry acetone. Our new value supports our previous determination since the specific gravity determination indicated that the original acetone sample was probably the drier of the two. Campbell observed that the partial pressure of methyl alcohol in carbon dioxide a t 40'C. was 191 mm whereas the vapour pressure is roughly 260 mm, and offered, as a tentative explanation of this and similar discrepancies, the hypothesis that some sort of adsorption took place at the liquid surface. In our experiments the gas phase was circulated over the liquid phase so that there was ample opportunity for such a phenomenon to occur, if indeed, it were possible. Moreover the stoppage of the circulating pump did not give rise to an increase of pressure but to an almost negligible decrease. U'e must therefore conclude that such large discrepancies, as have been observed, are due to the particular experimental conditions and are not due to any factor effecting the theoretical principles which have been commonly accepted.
W. G . BEARE, G. A. MCVICAR AND J. B. FERGUSON
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Summary The partial pressures of methyl alcohol, ethyl alcohol, acetone and ether were determined in the presence of dry air and dry carbon dioxide. The determinations in the presence of air were carried out with liquid samples which were subsequently used for the determination of the vapour pressure without removal from the all-glass apparatus. The differences between the partial pressures and the vapour pres(2). sures were usually less than one millimeter. The large discrepancies which had been previously reported, were not observed. (I).
Department of Chemzslrg, Cnuerszty oj Toronto.
