876
ANALYTICAL CHEMISTRY C A T A L Y T ~ CDEHYDROGENATION
The reaction flask for the dehydrogenation is shown in g, Figure 1. With the loaded reaction flask in place, the system is flushed with nitrogen, and the organic compound plus catalyst (palladium charcoal) is heated by means of a salt bath to a temperature a t which there is rapid gas evolution or gentle boiling. The evolved gases are collected and measured. B sample of p-cyclohexylanisole was quantitatively dehydrogenated in this apparatus. Because the nature of the evolved gas was of interest, a sample was taken by evacuating the sampling system, closing the stopcock to the pump, and slowly opening the stopcock to the buret. Mass spectrometric analysis of the gas sample showed a small quantity of methanol present, but no methane. This indicated that a portion of the evolved hydrogen was used for hydrogenolysis of the aromatic carbon-to-oxygen bond. This was confirmed by the isolation and identification of a small quantity of diphenyl in the reaction products. To keep traces of the more volatile compounds formed in the reaction flask from entering the sample bulb, i t is probably better to use liquid nitrogen in the Dewar in place of the dry ice-acetone mixture which was used in this experiment. The rate of flow through the cold trap may be reduced by placing a negative pressure in the buret before the svstem is opened to the evacuated gas-sampling bulh
The author5 have used 25-ml. and 50-ml. reaction flasks and a 500-ml. buret in these three determinations, but there is no reason why the scale of the apparatus cannot be reduced for semimicro work. The apparatus can be used, of course, for any reaction in which the measurement of gas absorption or evolution is desiiable. Thus, the quantity of osygen absorbed by coal, the determination of lithium aluminum hydride, and the extent of reduction bv this reagent can he measured i n such an apparatus. ACKNOWLEDG\ZEhT
The authors are indebted to Charles Siplc for the dratving. LITERATURE CITED
(1) Joshel, L. M., IXD.EKG.CHEM., A N ~ LED., . 15, 590 (1943).
(2) Niederl, J. B., and Niederl, V., “Organic Quantitative Microanalysis,” p. 263, New York, John Wiley & Sons, lnc., 1946. (3) Noller, C. R.,and Baruch, M. R., IND.ENG.CHEM.,ANAL.ED., 14, 907 (1942). (4) Pregl, F., and Grant, J., ”Quantitatire Organic Microanalysis,” 4t,h English ed., p. 134, Philadelphia, Rlakiston Co., 1945. REcuvb:i> Ortoher 19. 1048
All-Glass Filtration Apparatus for Radioactive Tracer Experiments JACOB SACKS
Hrookhucen i%-ationalLaboratory, Lpton, L. I . , .V. Y.
THE esteiisive use of radioactive isotopes i n tracer esperirnrnts has resulted in the development of numerous specialized techniques for preparing the sample for measurement. T h e r e the isotope can be obtained as a precipitate, this is filtered off on some device that serves as a sample holder for the radioactivit J- measurement. The apparatus described by MacKensie and Dean ( 3 ) uses a sintered-glass disk filter stick on which are mounted a piece of filter paper and a glass tube. The filter paper is then transferred to a suitable sample holder. This method is somewhat complicated and requires a high degree of technical skill. The modified Biichner funnel technique of Armstrong and Schubert ( 1 ) is simpler to use, but the preparation of the sample holder is a rather involved process. The use of the sintered-glass disk presented by a Pyrex Gooch crucible as a sample holder has many obvious advantages over such a filter paper technique, and has been adopted by many workers in the field. For use with either a thin-walled or endwindow Geiger-Muller tube, t,he usual practice is to cut off the top two thirds of the upper port,ion of the stock 15-ml. capacity crucible with a 20-mm. diameter disk. This leaves almost, 5-1111. capacit,y above the disk, and allows the sample to be placed within 1.5 mm. of the window of the counter tube. When Gooch crucibles modified in this manner are used i u the conventional way, by fitting them to filter tubes by means of Gooch tubing and filter tubes passing through rubber stoppers into suction flasks, the possibility of radioactive contamination of the rubber parts is always present. There is also considerable inconvenience in t,he frequent removal of the filter tube from the rubber stopper, which is required for the careful cleaning necessary for use with radioactive materials. ,4n all-glass apparatus has been devised which retains the advantages of the Gooch crucible technique and has the important, added one that the parts may be readily separated and cleaned. The essential unit consists of a filter tube equipped with a flat ground surface to receive the modified crucible, equipped with a standard-taper ground joint and side arm for attachment to a suction pump. The receiver is an Erlenmeyer flask fitted Jyith a standard-taper ground joint neck. Size 24/40 ground joints arc most convenient for this purpose. The lower edge of the crucible is ground to fit on the flat surface of the filter tube. The fit of the two ground surfaces is usually good enough so that no lubricant
need be used. Occasionally a water seal hab been found helpful. The filtration assembly is shown in section in Figure 1. The dimensions of the modified crucible will be determined to a vertain extent by the tvpe of Geiger-Muller tube and lead shielding available. It will probably be necessary to make changes in the sample holder of any conimercially available model so that it Ld1 accommodate the crucible. Those shown in the figure have an over-all height of 22 mm., and the distance from the top of the disk to the lower edge is 9.6 mm., with a tolerance of 0.05 nim. Such close tolerances are desirable for soft 8-emitters, but for use s i t h P3*a tolerance of 0.1 mm. is adequate. The crucibles used are sclected for relative uniformity of disk diameter and also for exactness of the plane of the disk, The distance of 15 mm. between the disk and the counter win-
OW is relatively unimportant i n the case of high energy pparticle. w r h N S those from P32. With soft p-emitters, such as CI4 or S35, the air absorption plus the reduction in solid angle resulting from this sample distance niay reduce by about 50% the “geometry” of the counting system. For counting such isotopes, the adaptation of the methane flow proportional counter described by Bernstein and Ballentine ( 2 ) is very useful, This apparatus utilizes the full length of the Gooch c r u c i b l e as t h e w i n d o w l e s s counter chamber. The distance between the sample and the end of the center wire is approximately 3 mm. Although the filter tube described was designed primarily for use in radioactive tracer experiments, it is also useful for any ordinary gravimetric determination in which the Pyrex Gooch crucible can be used. For general analytical purposes, any stock cruFigure 1. Seccihle can be used, and needs only tional View of Filtrat o have the lower end ground flat. tion Apparatus
V O L U M E 21, NO. 7, J U L Y 1 9 4 9
877
The C;ooch crucible twhriique is suitable for :ilniost any radioactive isotope which is capable of yielding a precipitatc i n such physical forni that it can be filtered off satisfactorily and forni a uniform layer of material on the surface of the disk. I n many caseq slight modifications of ytandard g r a v i m e t r i (kirocedure are necessary to obtain such uniform cliatribution of thti pribci pit ate. I11 ](,in
the particular pr(~1)for which the appa-
ratus was devised, phosphate containing P3*is prwipitated as aminoiiiurriphosphoniolybdat[,. The wsll-known tendFigure 2. Sectional View of (wcy of this prt4pitatc. .issemhl> for Solution of Preto crci'p has been overcipitate conie hy washing ,first \vith 5 C ; a m m o n i u m tritu(tt, t o ~'cinovethc nitric acid, arid t h v i i with a O.lC5 solution of .\c~rosol 11.1 in 5 5 ; ammonium nitrate. Occasioiially it is new+ sary t o use a rubher policeman to facilitate washing the last traccss of the precipitate down from the walls of the crucible onto t h t s disk. The aerosol and ammonium nitrate arc then washed out \\-itti tsthanol, and the crucible and contents arc1 dried in air. Othcr surface-active agents have bren tried, but none was as Fatisfactory as Aerosol Acetontl has liven used to wash nut t h c s salt and surface agent, but it tends to cause thc precipitatc to I)il(t up un,wi~nlyover the surface of tti(5 disk. Tho experiments in progress, \vhich coirsict i ~ f;I study of thc: ri~1:itivt~ turnover rates of thc :witl-.;oluhle phosphoi~uscompounds
of livr\r, itre such that it is niore satidiictory t o clrteriniiir~the radioactivity of the samples obtained before chemical determinations of their phosphorus content are made. The apparatus shown in Figure 2 has been devised to obtain quantitative solution of thrl phosphomolybdate precipitate.
The filter tube has been modified by making the terminal portion of tubing 4 mm. in outside diameter, to fit inside the neck of the 25-ml. volumetric flask used as receiver. The Lucite ring i i i which the flask sits serves to prevent, the flask from tipping ovt'r when tht. filter tube is removed. The suction flask consists of a 500-nil. Erlenmeyer flask with standard-taper ground joint neck. The bottom is cut off and thr edge ground flat to fit the grountlglass plate. A Pyres bottle with ground joint neck could be uscd instead of the Erlenmeyer flask. The phosphomolybdate prctcipitate is dissolved by adding 1 ml. of phosphate-free 1 S sodium hydroxide, filtering by suction, and washing with successive small portions of water. The contents of the flask are then rnade up to the mark and suitable aliquots taken for the colorimetric detixrmination of phosphorus. As a test of the completeness of the solution process, duplicati. aliquots of P32phosphate solution containing about 10,000 counts per minute were taken, carrier phosphate was added, and thv material was precipitat,ed in the usual manner. After countirrg, the precipitates were dissolved by the usual technique, and the. crucible was dried without any further washing and ag,tirr counted. One crucible now gave 6, the other, 4 counts per minuti: above background. The amounts of phosphorus encountered in the experimt,iit u i'ange from 50 to 1500 micrograms. Known amounts of pho phorus as phosphate, within this range, were precipitated as phi) phomolybdate and treated as descrihzd above. \Tith 100 to 200 micrograms, the recoreries were botween 98 and 10OC;. LITERATITRE CITED
(1) Arinstrong, \T, D.. and Schuljert, J . , .\s.AI.. CHEM.,20, BiO (1948). ( 2 ) Bernstein, W , , miti Ballentitic, 11.. Rrc. Sci'. Iustricments, 20, 347 (1949). (3) MacKeneie, A , J , , mid I ) r t i t i , L. .I.,. i s . ~ t('HLM., ., 20, ,559 (1948).
R I . . c E I ~ ~SeptrinhPr ;u 13. l!j48, lti~i,.archrarricd o i i f nt Hrookhnven National Laboratory under tlie au-pi
