Joseph A. Feighan Saint Joseph's College Philadelphia, Pennsylvania
Measurement of Vapor Pressure A gas saturation method
Although most elementary and advanced texts in physical chemistry give the theoretical discussion of the measurement of the vapor pressure of liquids by gas saturation methods, there is a noticeable absence of experimental procedures in the current laboratory texts. This experiment. is appropriate for teaching ex~erimentaltecbniaues and can give satisfactorv resuits in the hands of the typical unzergraduate phjsical chemistry student in a three-hour session. The Experiment. The apparatus as used is shown in the figure. Tank nitrogen was metered slowly through a wet test gas meter a t a known tem~eratureto measure its volum'and then dried upon eGit to remove water that was vaporized in the meter. The flow of gas was controlled by a Hoke needle valve and could be varied to cover a range of flow rates. The exit gas was directed into a saturator (sintered glass type gas dispersion unit) containing the liquid under study, and the gas bubbles allowed to escape, and then was directed into a bypass by means of a three-way stopcock, B. After thoroughly saturating the tubing in the system with the vapor of the liquid to he measured, the stopcock was turned and the vapors directed into a collection system, C . The method of collection of vapors was of two types. Either a solid desiccant, in previously weighed drying tubes was used for materials such as water, alcohols, or amines, or a cold trap for materials more difficult to remove by adsorption. The exit nitrogen, freed of its vapors, was passed out through the drying tubes, C.
wmmEu Yn0-
m.
I
Gar saturation apporatvs for "apor pressure meorurementr. A, drying tuber for carrier gas; B, three-way stopcock; and C, two drying tubes, containing d i d adsorbent, witable for odrorption of vapor under study.
Calculations of the vapor pressure were carried out by use of the ideal gas law in cases where the volume of Presented before the Division of Chemical Education at the 134th ti^^ ,,f the *mefiean chemical society, chicago, Illinois, September, 1958.
vapor was negligible compared with the volume of inert gas. The more accurate equation, PV
=
(*.>
Pi
where .. ..... p, = vapor pressure of liquid under study = number of moles of liquid adsorbed n, = number of moles of carrier gas used as inert gas sweep and, P I= barometric pressure
n,
is used where the volume of vapor carried over is appreciable when compared with the volume of carrier gas used. Under carefully established conditions, it has been poeeible to obtain results showing 1-2Yo deviation from accepted values for many liquids, such as water, methyl, ethyl, propyl and isopropyl alcohols, and acetone. More important than these results, however, the method of demonstrating saturation of the carrier gas and the need for an equilibrium between carrier and vapor of the liquid being studied are seen by comparison of results at two different flow rates. The following results for water are indicative of this effect. Deviation from Literature 3.5% 1.5% 3.3% 20.5%
Flow Rate 0.80 liter/min 0.53 literlmin 0.44 liter/min 1.40 liter/min
The vapor pressure for water showed acceptable results in the first three runs, but was far too low (20.5% deviation) on reaching a 1.40 l/min flow rate, preventing attainment of equilibrium between nitrogen and water vapor in the saturator. Selective adsorption of gases in a mixture and the basis of the adsorption method of analysis can be emphasized by suitable choice of liquid and adsorbent. Under approximately identical conditions of flow, anhydrous CaSO4 has demonstrated 10% deviation from accepted literature values for vapor pressures of water, whereas anhydrous calcium chloride as adsorbent gave approximately 1 3 % deviations. In adsorption of methyl alcohol on CaSOa, with flow rates 0.31 and 0.90 39% and 49% deviations, respectively, were obtained, when silicaigel used as adsorbent, a t 0.27, 0.71, and 1.20 l/min flow rates, results were 0.5%, 2.5Y0, and 5.0% deviation?, respectively. The high heat of adsorption in this set of runs made it possible to conveniently follow the adsorption Volume
37, Number 3,
Morch 1960
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149
process down the length of the drying tube. The present studies have indicated that the experiment is satisfactorily performed a t flow rates varying from 0.25 l/miu to 1.5 l/min if suitable choice of gas saturator is made and if the method of removal is carefully chosen. The student accuracy is sufficient to give the student confidence in his laboratory technique, and the time of the experiment (three hours) has been found sufficient for even the slower student pairs to complete four determinations. The method can be conveniently extended to the study of the variation of the vapor pressure with temperature and calculation of the heat of vaporization, by immersion of the saturator and connecting tubing in a constant temperature bath. The drying tubes or traps
would in this case be left outside the constant temperature device without introduction of serious error. The heat of vaporization of methyl alcohol and of water were determined by this approach using temperature ranges between 15 and 25°C and gave reproducible accuracy of about 1% of literature values. I t is well to emphasize again that no claim is made that this simple apparatus will duplicate results of more elaborate types such as several saturators in series or rigid flow control that might be required in research studies of vapor pressure. The principal benefit to the student chemistry major a t this point in his training is the familiarization obtained with the new apparatus and methods of the physical chemistry laboratory.
