Laurence A. Rice'
and James C. Chang University of Northern Iowa Cedar FOIIS,50613
An Apparatus for the h e ~ t i g a t i o n of Graham's Law of Effusion
A n apparatus of simple design which would give quantitative data on the rates of effusion of gases seemed remote until DavenportZsuggested the use of a hypodermic syringe. We have now modified Davenport's apparatus, avoiding the machine-shop work recommended. We find that two pieces of 8-mm o.d. glass tubing, approximately 35 mm in length, cemented end-to-end using epoxy resin, with a square of pierced aluminum, copper, or platinum foil between, give result,^ comparable to those obtained by Davenport. Apparatus
The metal foil should first be examined against a strong light for pin holes and then pierced with the tip of a sharp needle while the metal foil is pressed on a thin sheet of paper over a metal block. This method gives the hole a conical shape, thus forming knife-edge orifice walls approaching the ideal orifice suggested by Hollahan3. The diameter of the hole through which the gases effuse is critical and should be as small as possible if the leak rate of the gases is to become more nearly a function of effusive flow rather than a function of fluid-flow mechanics. The hole thus should be small enough so that it can be barely seen as a blurred spot due to diffraction of light when looking through the cqmpleted orifice tube toward a strong light.
the bottom of the bottle while the purge tube D is nearly flush with the bottom of the stopper. The syringe is connected to a short piece of 8-mm 0.d. glass tubing C by Tygon tubing, and the orifice tube B occupies the fourth hole in the stopper. The gas under investigation is passed into the bottle and the syringe through the 'gas inlet tube, and the entire system is purged with this gas for one or two minutes while letting the gas out of the system through the purge tube. When the leak rate is being determined, the purge tube is clamped off so that the gas leaks through the orifice tube. The apparatus is securely clamped to a ring stand to prevent the plunger of the syringe from vibrating as it descends during the experiment. The weight of the syringe plunger alone does not give an adequate pressure differential to give steady gas displacement when the orifice is open to the atmosphere; therefore a 60- to 100-g weight (E in the figure) is placed on the top of the piston to maintain an even leak rate. I n order to assure free movement and relative gas-tightness of the syringe plunger, dry, powdered graphite was used as the lubricant. Several lubricants including stopcock grease, mineral oils, fluorocarbons, and vacuum pump oil were tried but were found to be unsatisfactory; they assured gas-tightness but restricted the movement of the plunger and collected dust particles. When toxic or noxious gases are being investigated, a gas scrubber, filled with glass marble, similar to that of Goddard and Henry4 was connected to the gas effusion apparatus. This is also shown in the figure where F, G, and H are the gas inlet tube, the water inlet tube, and an air vent, respectively. We feel that the lower limit of effective syringe size is about 30 cc. Gases with similar molecular weights Leak Rates and Calculated Molecular Weights of Gases.
Gas Gmeffvrion opparotur and gmrcrubber.
The effusion apparatus consists of a 30- to 100-cc, gas-tight syringe and a 2-07, glass bottle fitted with a four-hole stopper (see the figure). The gas inlet tube A , a piece of 6-mm o.d. glass tubing, extends almost to 1 NSF-AYI participant, 1966-67. Present addras: Warren High School, Downey, Calif. 90241. DAVENPORT, D. A., J. CHEM.EDUC.,39,252 (1962). a HOLLAHAN, J. R., J. CHEM.EDUC.,39,23 (1962). GODDARD, C., AND HENRY,M. C., J. CHEM.EDUC.,42, 136 (1965).
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Journal o f Chemical Education
Oxygen Hydrogen Deuterium Helium Methane Acetylenec Nitrogen Ethylene Et,hane Argon Csrhon dioxide( Chlorine
Av. Leak Rate, sed
Calcd. Mol. Wt,.
Theoretical Mol. Wt.
Standard 2.02 4.07 4.07 15.9 26.2 28.0 28.1 30.1 39.8 42.6 67.8
Using an aluminum-foil orifice, 30-ec syringe, 100-gm weight on the plunger, and powdered graphite as lubricant. Constant temperature and pressure. Average of 10 trials, 1 0 . 6 see. All gases used except t,hesetwo are chemically pwe.
have leak rates differing by only a few seconds, using a 30-cc syringe; therefore the use of smaller syringes would give leak rates which would make it difficult to distinguish between variations in trials on the same gas and the variations in trials between different gases of similar molecular weights. Results
Leak rate experiments were performed on eleven gases, using oxygen as a standard, at Argonne National Laboratory during the spring of 1967. The results are shown in the table. It was found that complete purging of residual gas from the apparatus is extremely important. If any residue of one gas remained in the apparatus, the first few trials for a subsequent gas could be in error. To elim-
inate this problem, the second gas to be tested was allowed to flow through the apparatus while the plunger of the syringe was worked up and down a few times, for one or two minutes; and the results of the first one or two trials were discarded. Considering the relatively low cost of the apparatus developed here, the ease in which data can be collected, 'and the accuracy of the results, it seems reasonable to conclude that this leak rate apparatus can be used to good advantage at the beginning college level or even at the high school level. Acknowledgment
The authors wish to thank Argonne National Laboratory for the use of the gases.
Volume
45, Number 10, October 1968
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677
