0
A SIMPLIFIED ISOTENISCOPE FOR VAPOR PRESSURE MEASUREMENTS JAMES C. STERNBERG Michigan State University, East Lansing, Michigan
Tm
vapor pressure-temperature relationship of a pure liquid is a property which can be extremely useful in the characterization of volatile compounds. It can also be used to estimate the heat of vaporization, and it finds considerable practical application in chemistry and in chemical engineering. Despite their potential usefulness, however, vapor pressure-temperature data have not been as widely measured as might be expected. Thomson,' in an excellent review of the subject, states that vapor pressure measurements have been reported on less than 2000 organic compounds, with probably less than 200 accurately determined. The paucity of data on this useful property can
largely be attributed to the difficulty in its measurement by the usual methods. The special techniques and equipment required tend to discourage its widespread measurement for new compounds as a routine adjunct of preparative work, and usually only one point on the vapor pressure-temperature curve (the boiling point) is reported. Because of its usefulness in illustrating physicochemical principles and techniques, vapor pressure is a property often measured in the elementary physical chemistry l a b o r a t ~ r y . ~The techniques employed are essentially two: the dynamic method of Ramsay and Y ~ u n g and , ~ the static isoteniscope method of Smith and M e n ~ i e s . ~Both techniques suffer from serious disadvantages. The Ramsay-Young method, in inexperienced hands, does not readily yield accurate results, the equipment is rather special and inconvenient, and the theoretical basis offers certain conceptual difficulties. The conventional isoteniscope method, while conceptually attractive, involves an often exasperating manipulation with a fragile special item of equipment. A modified isoteniscope method has been developed, using an isoteniscope that the undergraduate student can, if desired, readily make for himself. The apparatus is simpler to operate than the conventional isoteniscope, and is capable of furnishing results limited in accuracy only by the manometer and thermostatting used. APPARATUS
The apparatus is shown in the figure. The vacuum can be supplied by means of a mechanical pump or a water aspirator. The ballast tank and manometer
' THOMSON,G. W.,
-
in A. WE~SSBERGER (editor), "Physical Methods of Organic Chemistry," 2nd ed., Vol. I, Pt. I, Interscience Publishers, Inc., New York-London, 1949. ZSee. for examnle. DANIELSF.. J . H. MATHEWS.J . W.
MCG~~LW- ill ~ o d Co.; k Inc., New ~ & k 1949, , pp. 5 k l . RAMSAY W., AND S. YOUNG,J . Chen. Soc. (London), 47, 42 (1885). SMITHA,, AND A. W. C . MENZIES,J. A n . Chem. Soc., 32, 1412~(1910). ~ ~ ed.,
Appurtu. for Obtainins Vapor ~ . e ~ ~ - - T e r n w n t ~ Date e i n Und&; -duet. P ~ Y S ~ C . chemistry ~ ~ ~ b ~ ~ .
t
JOURNAL OF CHEMICAL EDUCATION
portion might, if desired, be made identical with that shown in Daniels, Mathews and William~.~The isoteniscope itself is made from 8-mm. i.d. glass tubing bent a t a 20' angle with one end sealed; it can be connected to the rest of the system by means of flexible tubing. The thermostat bath (which, with the manometer, determines the accuracy of the method) can be as simple or as complicated as desired. PROCEDURE
The liquid to be studied should f i s t be boiled to eliminate dissolved air. The sample is then poured into the detached isoteniscope tube, which has been cleaned and dried. The tube is tipped to displace all the air from the closed end (the angle of end of the tube is chosen to facilitate manipulations of bubbles in the tube). The isoteniscope can then be connected to the vacuum line and evacuated. In theory, the liquid should break loose from the closed end of the tube as the vapor pressure a t the temperature of the closed end is approached; in practice, a "sticking vacuum" is obtained, the liquid remaining in the closed end even when the system is evacuated to a pressure of a few millimeters of mercury. This phenomenon, which indicates a high vacuum when obtained in a mercuryfilled McLeod gauge, occurs far more readily with less dense liquids. The situation illustrates a metastable state-a nonequilibrium state which persists for a considerable time, even surviving such treatment as heating the closed end of the isoteniscope, or suhjecting it to the discharge of a Tesla coi1"leak tester." In essence the situation is the same as that frequently encountered in attempting to crystallize a substance from solution; although separation of the new phase should occur (thermodynamically), the new phase does not appear unless the system is"seeded" to give a starting point for the nucleation (or cavitation) proems, which is essential to the attainment of equilibrium. In crystallization the required "seed" is a small crystal of the desired substance; in vaporization the required "seed" is a small bubble of air or vapor. The introduction of the seed is achieved by the following simple manipulations. After the air has been displaced from the closed end of the tube, but before evacuation of the open end, a small bubble of air is readmitted into the closed end of the tube. By tilting the tube and gently tapping or flicking it with a finger, the readmitted hubble can he fragmented into smaller bubbles. All of the smaller bubbles but one are caused to escape around the bend into the open end of the tube. There should then be just a very small air bubble in the closed end. The tube is then evacuated, observing the instant that the bubble starts to grow significantly in size. The pumping is stopped a t this point and the manometric pressure noted. It is advisable here to readmit enough air to the system to increase the pressure by
VOLUME 34, NO. 9, SEPTEMBER, 1951
1-3 cm. of mercury. The buhble at the dosed end of the tube is again fragmented, with the system at the reduced pressure (which is now only slightly above the vapor pressure of the liquid a t room temperature). The bubble behavior will now be considerably different from that observed a t atmospheric pressure--as a large bubble is fragmented, myriads of tiny bubbles appear, and the total bubble volume decidedly increases. The bubbles are manipulated into the open end of the tube, retaining only a single tiny bubble in the closed end. This buhble will grow with slight warming (as by touching the end of the tube with the fingers), and the fragmentation and manipulation process should be repeated several times. The repetition insures romplete outgassing, so that after several times the bubble must certainly he vapor, not air. The validity of this assumption must be established by inserting the isoteniscope in the thermostatting liquid, halancing the liquid levels in the isoteniscope, reading the manometric pressure, increasing the pressure by 1-3 cm. of mercury, and repeating the fragmentation, manipulation, thermostatting, balancing, and pressure reading until a reproducible pressure is obtained. This is the vapor pressure of the liquid a t the thermostat temperature. The reading of the liquid levels in the isoteniscope is accurately done by viewing one arm of the apparatus through the other, and aligning the two arms with a line around the outside of the thermostat. Any small error in balancing or aligning the isoteniscope is diminished still further by the density ratio between the liquid studied and the mercury used as the manometric fluid in the external manometer. The thermostat temperature can then he set to any desired value and the vapor pressure obtained by adjusting the system pressure until the isoteniscope liquid levels are identical. It is interesting to note that the condensation process, as well as the nucleation (or cavitation) process, does not take place instantaneously. A bubble of vapor trapped in the closed end of the tube requires considerable time and agitation before it will "dissolve" (or condense), even when atmospheric pressure is applied a t the closed end a t a temperature well below the boiling point of the liquid. The simplified isoteniscope presented here should be capable of as accurate results as any of the more complicated designs in the literature. Its ease of operation and the elimination of a dependence on fragile special equipment should make vapor pressure-temperature work more appealing and accessible to the synthetic chemist for characterization of new compounds. The use of vapor pressure-temperature data in identification work might also be considerably extended. The accuracy of results possible with simple and rngged equipment which is readily fabricated, coupled with the conceptual attractiveness of the isoteniscope method, makes the experiment ideal for an elementary physical chemistry laboratory program.
