A Demonstration of Thermal Diffusion

The Georgia School of Technology, Atlanta, Georgia. HERE has been much interest ... Chapman (2) arrived at the same conclusion also from purely theore...
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A Demonstration of Thermal Diffusion WILLIAM M . SPICER The Georgia School of Technology, Atlanta, Georgia


HERE has been much interest in thermal dsusion round-bottom flasks joined on by large ground joints. smce the announcement that this was one of the The tubes must be in a vertical position. main methods used to separate U-235 for use in the In order to make the separation visible, and thus atomic bomb. make unnecessary an analysis, an approximately eqniThe history of the process is significant in that i t molecular mixture of bromine and air is used. Within shows again how a pursuit of pure science often leads to practical results undreamed of a t the time. D. Enskog (1)in 1917 showed in his doctoral dissertation that according to the kineticiheory, a temperature gradient in a mixture of two gases gives rise to a corresponding concentration gradient. In general, the light constituent concentrates in the hotter portion of the container. A little later, and independently, Chapman (2) arrived a t the same conclusion also from purely theoretical considerations. The phenomenon was first demonstrated experimentally by Dootson (3). Into an apparatus consisting of two glass bulbs of 100-ml. capacity connected by a stopcock, he introduced a mixture of hydrogen and carbon dioxide and maintained one bulb a t 230" and the other a t 10". After four hours an analysis showed that the hot bulb contained 44.9 per cent Hz and 55.1 per cent carbon dioxide, and that the cold bulb contained 41.3 per cent Ha and 58.7 per cent carbon dioxide. Chapman suggested the use of this method for isotope separation, but Mnlliken (a), after a thorough study, concluded that in the case of isotopes, where the percentage difference in mass is relatively small, the resulting separation would be too minute to be of practical value. Little further progress was made nntfl Clusius and Dickel (5) combined this thermal diffusion process with a thermal syphoning process by carrying out the ten hours the separation is quite apparent, and w i t h i separation between concentric vertical tubes, one of a few days it appears that all of the bromine originally which is kept hot and the other cold. In such an ap- in the upper reservoir has been transferred to the lower paratus the light constituent not only concentrates one. Now if the heater is turned off, the bromine around the hot tube but, being hot, rises while the conc&tration becomes uniform throughout the apheavy constituent concentrates around the cold tube paratus in about a day. and falls. A simple but satisfactory apparatus for the demonLITERATURE CITED stration of the method is shown in the figure. It conENSKOO. D.. Doctoral dissertation. Almauist and Wiksells. sists of three concentric glass tubes of 1.2 cm., 3.5 em., upsaia, 1917. S., "On the kinetic theory of a gas. Part 11, A and 5.0 cm. diameter. The small inner tube cames a (2) CHAPMAN, composite monatomic gas: Diffusion, viscosity, and heater consisting of 12-gage nichrome wire in four-hole thermal conduction." Phil. Trans.. A 217, 181-85 (1917). insulators. The gas space is between this small tube (3) C ~ A P M ~S..N ,AND F. W. DOOTSON, "A note on thermal diffusion," Phil. Mag., 33, 248-53 (1917). and the middle tube, while tap water is circulated beMULLIKEN, R. S., "The separation of isotopes by diffusion tween the middle and the outer tubes. This main- (4) +pressure diffusion." J. Am. Chem. Soc., 44, 1034-5 (1922). tains the cold surface a t approximately 25" while a K.. AND G. DICKEL."News Verfahren zur Gasentheater current of nine amperes keeps the inner tube (5) CLUSIU~, mischung und Isotopentrennung," Natunuissmschafkn, a t about 150". The end reservoirs consist of 3-liter, 26, 546-7 (1938). 593