R. W. Zuehlke'
and P. G. Cartier Lawrence Un~vers~ty Appleton, W~sconsm5491 I
An Inexpensive Vacuum and Gas Handling System for the Freshman Laboratory
I n recent years, several journal articles2 and laboratory manuals3 have appeared with descriptions of rather sophisticated experiments which can be carried out by beginning students on relatively simple vacuum lines. While these vacuum lines are conceptually simple, they are, for the most part, not inexpensive, and it is this author's feeling that the expense involved has not lead to any widespread adoption of these experiments. A quite inexpensive vacuum line has been devised a t Lawrence University, and is described below. The Vacuum Line
The vacuum line and associated components described below is shown in the figure. The frame
The arrernbled gor-hondling system.
consists of a 3 / r in. fir plywood base, 4 X 4- in. uprights of fir, and aluminum or iron rods threaded a t the ends and bolted appropriately to the wooden members. The pump guard and bases are also made of plywood and provided with rubber feet. The vacuum line has a small volume, so almost any size forepump can be used. I t is advisableto insert a protective trap between the pump and system. The vacuum line proper is built about a manifold which can be furnished by Kontes of Illinois (9943 'Present address: Department of Chemistry, University of Bridgeport, Bridgeport, Connecticut 06602. a Ko~rcs,R . J., DORFMAN, M. K., AND MATHIAS, T., J. CHEM. Eouc., 39,20 (1962). ANDREWS, D. H., AND KOKES,R. J., "L&bor&toryManual for Fundamental Chemistry," John Wiley & Sons, Inc., New York, n.d. BELL,3. A,, (Editor), "Chemical Principles in Practice," Addison-Wesley Publishing Co., Ilesding, Mass., 1967.
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West Franklin Avenue, Franklin Park, Illinois 60131). The manifold consist of 18-mm 0.d. tubing with O-ringand-groove joints provided a t the ends. The joints may be mated with similar joints to hold manometers, traps, rotating McLeod gauges, or other appendages. The joint and clamp assembly provide the useful flexibility of 360" rotation in one plane about the manifold axis. The stopcocks were designed especially for this system, and each consists of a modified ground glass joint pair. The male part of the joint is closed a t the top, and a heavy 7 mm glass rod sealed on as a handle. The wall of the female part of the joint (and the corresponding position in the male segment) is bored to a 2-mm diameter, and a 10-mm 0.d. glass tubing is sealed thereto. The base of the male part is sealed directly to the manifold, with the side lead from the stopcock oriented vertically and pointed downwards. The cost of such a stopcock, including labor, is about 2.5% less than that of the least expensive vacuum stopcock of corresponding size. Provision for connecting other components to the stopcock leads is made through use of a segment of heavy-walled pressure tubing. While other meaus of making the joint are available (i.e., standard taper and ball and socket joints, Rad-Lab fittings, etc.), none are as inexpensive or flexible as the procedure adopted, nor do they offer a better performance. The prescribed connection between the line and the atmosphere is through a pinhole orifice formed in a glass tubing attached with a rubber connection device to one of the stopcock leads. Even other gases let into the system (see below) are diverted through the pinhole to restrict the rate of gas influx. Sample bulbs and similar devices to be connected to the stopcock leads are fabricated using a 9-mm lead. The bulbs are readily fabricated from round-bottom flasks, large diameter tubing, or oversize test tubes. Individual gas reservoirs are formed inexpensively from high quality rubber balloons, rubber stoppers, glass leads, rubber tubing, and pinch clamps as shown in the figure. A means for attaching the reservoir to a compressed gas source is provided through a three-way stopcock which permits easy "purification" of the gas in the reservoir by a series of purgings. The gas source is slipped over the pinhole lead into the vacuum system, and the volume of air entrapped between the stopcock and pinch clamp is evacuated by pumping (slowly, of necessity) through the pin-hole. The gas may then be bled cleanly and at a controlled rate into the system. The pinch clamp can then be set again in place.and the gas reservoir transferred to another system, if necessary.
Experiments
It is not the purpose of this paper to introduce new experiments, for a number of them are already in print.$.? Rather, a series of experiments performed by students in the combined introductory course in chemistry and physics a t Lawrence University will be described for illustrative purposes. The primary purpose of one experiment was to introduce students to the principles of gas thermometry. I n order to do this, a large volume (in the form of a large bulb) must he provided, as well as a means for cooling (or heating) the bulb to some temperature other than room temperature. I n this case, the volume was provided in the form of a long, cylindrical bulb with a volume roughly equal to that of the manifold itself. The hulb volume was determined by students using weight-density measurements with water; then using helium (and nitrogen, air, methane, carbon dioxide, or whatever other gas students wish to experiment with), students used a series of Boyle's Law applications to determine the manifold volume. This calculation provided students with a series of data in which the effects of experimental error were clearly manifested. The gas thermometry operation itself involves equilibrating the bulb and the manifold with helium gas a t room temperature, determining the pressure, and then refrigerating the bulb with liquid nitrogen (or any other cryogenic fluid) to obtain a new pressure. The temperature of the known volume of cold gas can then be easily determined, and a set of typical student results is shown in the table. Once again, the perpetuaSvmmarv of Gar Thermometw Data
No. of values
T,,,
T,i.
18
85.6'K
64.1°K
T,. 77.0°K
Std. dm.
Treoc0w
+4.9"K
77'4°K
tion of an experimental error is clearly shown. The mathematical analysis of the propagation of these errors is not difficult, and i t illustrates, for instance, the need for minimizing the volume of the system held a t room temperature. The use of liquid nitrogen was made feasible through the development of small, inexpensive polystyrene Dewars. These were cut in 4-in. sections of appropriate length from a 4-in. sheet of commercially available material. The long blocks were bored axially to approximately a 2-in. diameter with a hot brass tube. sharpened on one end. The bottom of the hole was rounded and finished with a hot capped pipe. The entire Dewar was then impregnat~dwith a commercial water glass (sodium silicate) solution and "baked" at 110°C for several hours. Numerous modifications of the Dewar can be made, of course. The hole can be bored to just accommodate a cylindrical glass container (i.e., a large test tube) and the impregnation step omitted. This has the added advantage of
allowing a small hole for sighting to be cut through the insulating material. Following the experiment on gas thermometry, an application of Dalton's Law was studied. I n this case, a very small "mini-bulb" of glass or (preferably) copper was substituted for the gas thermometer bulb, and a mixture of helium and C02 was used in the system. The total pressure in the system is measured with the mini-bulb a t room temperature. The bulb is then cooled with liquid nitrogen, and carbon dioxide is allowed to freeze out in the hulb, thereby eliminating its pressure contribution. A composition analysis can then be readily carried out. This experiment is a fine introduction to the concept of cryo-pumping; as might be expected, there is a general disbelief among the class initially that any appreciable fraction of condensible material would actually do so. Notes
The following list of suggestions is compiled on the basis of experience with the experiments and vacuum system described above. (1) The manometers used would occssionally give rise to a broken mercury column. The column is emily rejoined by rotating the manometer 180" about the O-ring joint, applying a vacuum, and tapping the "bubble" until i t rises into the evacuated portion. ( 2 ) The error introduced by the existence of a segment of the system a t a temperature between room temperature and that of the cryogenic fluid can either be estimated or minimized by provision of glass wool insulation a t the top of the Dewar. (3) There is some advantage to providing a stopcock or glass "Y" (with rubber tubing and pinch clamp) between the fore pump and trap in order to bleed air into the pump - for shutting down. (4) The design of the stopcocks apparently requires that they be. rather heavilv ereased (occasionallv a t the start of each
nitrogen. (6) The vacuum system is unusually sturdy, and i t makes feasible the provision of two extremes in the approach to students. Very detailed directions (and explanations!) can he supplied to students so that blind, meaningless fumbling based on incnmplete directions can be avoided. On the other baud, a student could easily be asked to formulate his own procedures and experiment with thesystem on his own. (7) The system can be readily adapted for use in vacuum distillation hy attaching a Tirrill burner a t one of the stopcncks. The (reveme) air flow through the burner can be carefully regulated by its needle value. Acknowledgment
. The authors wish to thank Mr. Eric Nyberg of Kontes Glass Company for cooperation in the design, fabrication, and supply of the components for the system. Mr. Harold M. Lovdahl was instrumental in providing a workable dewar design. The cost of the project was underwritten by a National Science Foundation Undergraduate Equipment Grant.
Volume 46, Number 12, December 1969
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