A Modern Vapor Pressure Apparatus Based on the lsoteniscope Gerald R. Van Hecke Harvey Mudd College, Claremont, CA 91 711 Perhaps no thermodynamic measurement other than temperature is as pervasive as vapor pressure. Numerous experiments and apparatus setups for vapor pressure measurements have been described in these pages (Id). Of wurse, vapor pressure measurements require a means to measure pressure. For years, introductory chemistry laboratories have relied on some version of a mercury manometer to measure either absolute pressures or pressure differences. With today's increased concern over the use of mercury in the laboratory, the use of mercury manometers is being curtailed. Recently we redesigned the experimental equipment used for vapor pressure measurements in our freshman laboratorv to eliminate the mercury manometer. Our purpose here is to share the desim of that new equipment and comment on its success. The experiment done in our introductory chemistry laboratory seeks to introduce phase equilibria by measuring equilibrium vapor pressures yre 1. Drawing and parts list for vacuum gauge vacuum rack with isoteniswpe using the isoteniscope method (6).The student is given an unknown organic liquid and asked to ap#paratus.The parts are identify the liquid from three physical properties: 1. Vent hose to PVC pipe ducted to nearby fumehood. density, refractive index, and normal boiling 2. Vacuum pump, Welch Model 1400,51300or less depending on discounts. Trap, I-L heavy-walledfilter flask held in a plastic bowl. V, pump isolation valve, ooint. To find the normal boiline noint. the stu- 3. Kcmtes 801000-004,about $35. V2 coarse adjust valve, Kontes 801000-004. V3 bent determines the vapor press& of the sample fin e adjust valve, Nupro S.S. needle valve, SS-4SA, about $40. V4 isoteniscope as a function of temnerature. fits the data to the is()lation valve, Kontes 801000-004.J,,2,3Nalgene T-joints,'/4 in. 1.d. ~lausius-~la~eymnk~uatiod, and either interpo- 4. Rubbervacuum hose, 114-in. i.d. by 118-in.wall amber latex. lates or extrapolates as necessarv to find the nor- 5. Vacuum gauge, Ashcroft 0-30 in. Hg, 4%-in. dial, 45-1279A5-02L,about $90. mal boiling point. The experimental determina- 6. Thermometer Hg, -10 to 150 'C. tion of the vapor pressure is the heart of this 7. Isoteniscope. Maonetic heater-stirrerwith a 2-L beaker water bath. experiment and introduce our new apparatus 9. ~uirtz immersion heater for auxiliary heating, requiring a Variac for control below. proven quite reliable. A needle valve is necessary for fine adjustment of pressure inside the isoteniscope aspressure Experimental Apparatus equilibrium is sought. Stainless steel was chosen IBbe the Figure 1shows a line drawing ofthe apparatus. Two feamaterial of valve construction to minimize corrosion probtures of the apparatus are most significant. First and forelems while dealing with a variety of organic liquid most is the use of a large vacuum gauge rather than a mersamples. cury manometer to measure the pressure. Second, and Several comments are pertinent to the vacuum numa . more practical than fundamental but extremely useful for Each pump is trapped wiih a heavy-walled filter flask set efficient data collection, is the joining of the various parts in a nlastic bowl to which nowdered drv ice wolant is added by rubber vacuum hose to wnstruct a small vacuum rack. (dryiceacetone has not geen used toavoid fire and vapor The rack support was constructed of two pieces of 0.5-in. hazards). Since it is the nature of the isoteniswpe method plywood joined to make a front and base by ordinary L that the sample under study is evaporated, and even shelf brackets. The hoses and joints were secured to the though the pump is trapped, some organic vapors are back of the front piece while allowing the stopcock handles mixed with the air that is evacuated from the system. The to protrude through the appropriate-sized holes drilled pump output is ducted with inexpensive PVC pipe to a through the front piece. The decision to join the parts toe accomfume hood. The connection to the W C o i ~ was gether with rubber tubing rather than glass-blowing indiplished by replacing the normal diffiser okihe pump with vidual units was made for ease of construction and repair. a hose-couoler threaded into the threaded exit nort of the The caption to Figure 1lists the materials of construction. e r made of aluminum by o& machine pump. ~ h e ' c o u ~ lwas The rubber vacuum hose joints are secured with stainless shop. A workable alternative would be a suitable rubber steel wormdrive hose clamps. Three of the valves (VI, Vz, stopper with a glass tube through it. The small amount of Vq)are 4-mm glass vacuum stopcocks with a threaded tenoil that inevitably appears whenever a great deal of sion adjustment nut. The glass stopwcks were chosen with air is evacuated from a system is allowed to wllect in the a concern for their ruggedness and those used here have PVC pipe whose diameter is large enough (about 2-in.
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i.d.1 to prevent the deposited oil from blocking its venting function. The vacuum gauge has a 4.5-in. dial that registers the difference between ambient atmospheric pressure and the vapor pressure in the isoteniscope. We use gauges that are calibrated in inches of mercury. We have found the manufacturer's stated accuracy claim of 2.5% gauge reading to be reliable. The practical reason for the particular gauge chosen was to provide a dial large enough to interpolate between the dial markings and generally provide a measurement of three significant figures. Other calibrations are possible in different sizes and costs. Larger gauges seem to cost in proportion to the area of the dial face. The total cost of the unit would depend on what equipment is on hand. The most expensive part is clearly the vacuum pump. However, because most laboratories have vacuum pumps available, probably the major required purchases would be the stainless steel needle valve, the k ~ ~ c o c kand s , the vacuum gauge. The vacuum pumps are not permanently built into the racks and thus their use is req&ed only fbr the time the experiment is being conducted. Since we have conducted this experiment with as 10 stations a t a time. beine able to tem~orarilv manv ----~-" as -~ borrow vacuum pumps for this experiment has been an imoortant loeistical To minimize the reauired c - - ~ ~ ~ - - ~ wnsideration. - ~ - ~ ~ number of pumps, in principle one large capacity pump should be able to serve several stations, though we have never tried this approach. The use of the isoteniscope for vapor pressure measurements has been described (6). However, for reference, we will present our procedure used with this apparatus. ~ e a ad barometer to determine atmospheric pressure. Fill the isoteniscope with sample and then set up the complete apparatus as shown in Figure 1. A boiling chip is added to the sample to minimize the chances of the sample superheating. To turn on the vacuum pump follow the steps below: 1. Turn the vacuum DumD isolation valve to the off oosi&n.2. lbm on the vacuum pump toevacuate the syscern includina the trap up to the isolation valve. 3. Add powdered dry ice to the plastic container around the filter flask trap. 4. Place a cloth towel loosely around the dry ice and trap. This insulates the trap well enough for an afternoon and minimizes contact with hands or other substances. 5. Proceed as instructed below. ~~~
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I n order to obtain pure vapor in the thimble, all air and dissolved gas must be forced out of the system by gently boiling the liquid sample several times. Cautiously slide the thimble gently down the inclined isoteniscope. At this point t h e vacuum isolation valve should be closed, while the coarse and fine leak valves and the isoteniscope isolation valve should be open. Using the rubber hose from the vacuum system, connect the isoteniscope to the system and place the isoteniscope in a water bath a t room temperature. The next step must be done with great care or the liquid will superheat and project the thimble to the top of the tube which usually then breaks the isoteniscope bulb when the thimhle falls back down.
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Open the vacuum pump isolation valve. The vacuum pump will cantinuously try to reduce the pressure of the system, while the coarse and fine leaks are used to adjust the pressure by leak in^ air into the system. Cautiously reduce the by closing the fine ieak valve and slowly closing the coarse leak valve until the liquid sample begins to boil. Do not close the coarse leak all of the way Boiling removes most of the air from the isotenisco~e.Allow the liauid to boil calmly for a few seconds. Then, open the fine valve slightly so that air enters the system and raises the pressure slightly. If the foreign gases have been wmpletely expelled, the wol liquid will rise to the top of the glass thimble inside the isoteniscope bulb. Repeat the boilinp and aspirating procedure as-necessary until the thimble is completely filled with liquid. With the isoteniscope in the water bath at a steady temperature, measure the vapor pressure in the following way: maintain the water bath a t a wnstant (0.5 ' C ) temperature. With the vacuum pump running and the pump isolation valve open, roughly adjust the coarse leak and carefully adjust the fme valve to bring the liquid levels in the isoteniscope to equal heights. When the liquid levels are of equal height and steady for about 30 s, equilibrium may be assumed. Isothermal conditions are necessary here. Read the temperature of the water bath, read the gauge, read the temperature, and reread the gauge. If the measurements are the same, go on to another temperature. This process is repeated for at least six temperatures. Generally a t least two readings are obtained at approximately the same temperature to check precision. As wide a ranpe of temperatures as possible is covered. Experimentally it is eas& to make amom temperature measurement first and then heat the sample to obtain pressures of -650 torr. Measurements also cHn be made while cooling the system. Even sub-ambient temperatures can be achieved by adding ice to the water bath.
Figure 2. Vapor pressure versus temperature data obtained with vacuum gauge vacuum rack using an isoteniswpe for carbon tetrachloride, tetrachloroethyiene, and binary mixtures thereof. The data was obtained by two separate investigators using different setups and on different days. The pure tetrachloroethylene data were repeated by both investigators and normalized to consistency by a wnstant difference.
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2.92 2.98 3.04 3.10 1000 KIT Figure 3. ClausiuMlapeymnplots of in(P1torr)versus fOO01Tto determine the enthabv of vaDorization of Dure carbon tetrachloride and tetrachi~roeth~lene: ~rom'theslopes the enthalpies of vaporization are CCI, 7.4 Kcal/mol; C,CI, 8.7 KcaVmol(averageof two slopes). Results
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Before discussine the s a m ~ l data e nresented in Fieure 2. several remarks pertinent to the use of apparatus are noteworthy. Students quickly learn the purpose and function of each part of the vacuum system and further appreciate the secure construction that &imizes the chanceand danger of breakaee. Furthermore the ease of readine the vacuum gauge allows students to concentrate on the-difficult part of the experiment, achieving temperature equilibrium, rather than worrying about reading manometer mercury heights. The response time of the gauge for these measurements seems virtuallv instantaneous and data takine becomes simple, qu&k, and enjoyable. Definite-improvements in data quality and laboratory morale have been maior, wonderful benefits of using these new vacuum gauge equipped vacuum racks. The new apparatus has turned a laboratom exercise once viewed bv students as a dimcult, tedious idtroduction to thermodydamic measurements into a favorite. While there may be pedagogical disadvantages using a vacuum gauge instead of a highly transparent (in terms of principle of operation) piece of equipment, the mercury manometer, the advantages of ease and speed of measurement we feel are more than offsetting. ~ A h e r m o r eusing a eauee calibrated in inches of mercurv. ". reauires additioial-practice in unit convenions. Since the gauge used registers pressure differences, the student still has to learn how to relate the gauge reading to the actual pressure inside the isoteniscope. Two weak points of the technique are (1)the loss of significantfigures when measuring vapor pressures at elevated temperatures, and (2) at high temperatures, the vacuum pump has to work very hard to evacuate the large amount of air and vapor produced.
The sample data in Figure 2 is for pure carbon tetrachloand various binary mixture ride, pure ietra~hloroe&~lene, of these components. Figure 3 presents the ClausiusClapeymn plits of the pr'e component carbon tetrachloride and tetrachloroethylene data. The heats of vaporization for these liquids agree with literature to within 3 4 % (7).The data is quite acceptable. In actual laboratory practice it would not be necessary to record as much data as is illustrated but this selected set of data does provide a sense of typical scatter that might be expected. A more elaborate experiment could be developed using the binary data. By making the assumption of ideality for this system (a good assumption (81, the vaporous surface could be calculated from the liquidus surface giving the student a complete vapor-liquid space phase diagram. An experimental caveat with respect to binary mixture measurements should be noted. Using the isoteniscope method for binarv mixtures involves the potential for sienificant loss of &e more volatile cornpone& with a subsequent change of overall system composition. While the extent of thii potential error was nit exhaustively tested, preliminary tests suggest that with care not to pump on the isoteniscope for long periods of time, losses of material could be held to below 2%which should be acceptable in an introductorv exneriment. using this nek apparatus, the greatest experimental difficulty remains temperature control and determination of when the system is actually at equilibrium. Here adequate temperature control is nmvided bv a beaker of water that is heated by a magnetie'ally stirre2 hot plate and cooled by reducine Dower and bv addine - ice.. but the temoerature control system is nextvfor improvement. Note, that since the beaker contains about 1.5 L of water. the lag time for temperature change is a blessing for thermostatting but a curse for rapid change to a new tem~erature. While wehave notyet done so, the apparatus is amenable to ready computerization.For examde, the analog signal of an inexpensive pressure transdumr could be fed & an analog-to-dig5tal converter and the di~Gtalsimal sent to a compukr for data logging and data treatment. Conclusions
The use of a simply constructed vacuum rack employing a vacuum gauge for pressure measurement has made a tremendous difference in the quality of data and acceptance of what was once generally considered a difficult experiment. The details of the apparatus have been presented and discussed. Those interested in having students make vapor pressure measurements are encouraged to replace mercury manometen with vacuum gauges. Acknowledgment
We thank our stockroom curator James Patrick for invaluable advice and the manufacture of the vacuum eauee vanrum racks. The helpful discussions with chemistry facultv wlleames W. G. Slv. P. C. Mvhre. . . and K. K. Karukstis is batefulk acknowledged. The vapor pressure data were obtained and kindlv ~rovidedbv Jenifer Curtis Monks and Elisabeth Ann Davis.
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Literature Cited
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4 . h " ~J. . W J.C k m . Edur. IBRL.59.932~ P&;. R .I Chrm E d u 1974.51 lhx 6 Shwn&rr. D P.Cnrlrnd ( ' u'.stbln. J W E r p p r r m n t o l P h ~ w dC k m f s ( r ) x h 4. M&v-HaII Scu York. 19% pp224 226 7. Hor.&.b d c h e m w c n o n d P h w lznd e d . The l'hermd Rutber PublrahuCo:
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1960, PP 2320-ziz~.
8. Fried,V.; Franreehetb,D. R.;Gallanter, A. 8. J Phya Clun 1909, 73,1476.
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