A Simple-To-Construct Density Gradient Tube - Journal of Chemical

A Simple-To-Construct Density Gradient Tube. Michael N. Quigley. J. Chem. Educ. , 1994, 71 (6), p 516. DOI: 10.1021/ed071p516. Publication Date: June ...
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A Simple-to-Construct Density Gradient Tube Michael N. Quigley Mellon Hall, Duquesne University, Pittsburgh, PA 15282 Over the " vears..this Journal has re~ortedon manvinteresting exercises concerning the deniities of solids"(1-lo), liauids (10-253. and wses (26-29). With articular reference to liquids; relagve density (specificLgravity)bottles, and hvdrometers often are used to demonstrate the ~ h v s i cal reiations between mass, density, and volume. ~ k n s i t y gradient tubes also are useful in this regard (12.19) and can be used to spectacular effect (15).The simplest type of densitv madient tube can be made from a vertical standing, ke&ring cylinder, buret or glass chromatography column, to which is added sequentially, volumes of decreasingly dense liquids. The liquids should either be run down the side of the tube with a pipet, or added with the aid of a purpose-built apparatus (19,301. Densitv " madient tubes are used bv forensic scientists for soil characterization, and because undergraduate exercises with a "whodunit" connotation oRen are found bv the majority to be interesting and thought provoking (31i, the use of similar eaui~mentin school laboratories is recommended. One cl&& forensic method involves preparation of stock solutions containine lfferent DroDortlons of bromabenzene (4' 1.494) a& bromofo-?diO 2.885). See reference 32. Because both reagents are toxic, however, their use in today's undergraduate laboratory is precluded. Another source lists a number of chemicals for use in mineral separation: tetrabrornoethane (din2.964), diiodomethane (d? 3.3251, and aqueous solutions of thallium formate (d 3.5), and thallium malonate-thallium fonnate (d 4.9). See reference 33. From a safety point of view, diiodomethane is preferred and can be used in combination with dichloromethane 1.327). All procedures are best performed in a fumehood. Add 10 mL of 0% dichloromethane1100%diiodomethane to the bottom of a sealed glass tube (cylinder, buret or chromatography column), and then 10 mL of 10% dichloromethane/90% diiodomethane, through varying proportions, to 10 mL of 100% dichloromethane/O% diiodomethane.

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Add approximately 0.5 g of accurately weighed, finely sieved soil to the top of the tube (observation is enhanced bv strateeic la cement of a ~ i e e eof white card). Organic gaterial hicollect at the tdp of the tube, and hkavyUminerals will collect at the bottom. Clavs and ouartz will occupy intermediate positions. Such ~roeedurescloselv mirror those ~erformedbv forensic shentists, when they are requirei to chara&rize soil from a shoe. for exam~le.and the site of criminal activity ~ o m ~ a r i s o nthe o f p&pokions of soil suspended at different intervals in two similarlv DreDared densitv madient tubes can provide objective e s d n i e as to whether a suspect ever visited the site, regardless of whether footprints are visible or not.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Craig, R. E. R. J Ckem. Edue. 1989,66,699. Hill, W D., Jr J. C h . Educ 1987.64.92, Harris. A.D. J. C k m . Edue, 1988, 63.718. Romen,P.;Gutierrez-ZanillaJ Cham.Edw. 1886,62,167. B w e a s , D. J. Ckm.Edue. 1984.61,242. Bergendsh1,T.J. J ChamEduc 1918.56.617. Black, H. F. J C k m . Edue. 1917.54.628. Feinstein,H. I, J. ckem. Edue 1912.49,111. Ulery. D. J. Ckem. Educ 1966,42,A308. Blank, E. W.; Willard, M. L.J. cham. Educ 19SS, 10,109. McCullough, T J. C k m . Educ 1933, 70,46. Taslirni,P M.; Kolokatmnis, D. J. Cham. Edvc. 1991.68.58, Franr.D.A. J Chem Educ. 1991.68.594.

Garman,R.P; Khoury,0.J C k m . Educ. 1985,42,A308. Cattan, RAnseker, E. W J. Chem. Educ 1960.37.36. Miller, S. P J C k m . Edue. 1949,27,135. Dunbar, R E . J Chem.Educ. 1936.13.589. B1ank.E. W J Ckm.Edue. 1933, 10,618. Heard,L. J. Ckem Edue 1930.7.1910. Hsrris,A.D. J C k m . Educ 1984,61,74. Snipp, R.: Mattson, B.: Hardy, W J. Cham. Educ 1981,58,354. Beitzel, R. E. J. Chsm. Edur 1958,35,A167. Fiekern, B.A. J Chom. Educ. 1954,31,139. Oak,G.;Yamamota,M. Chem.Rsu. 1981,W. 257. Gwig1ey.M. N. J C k m . Educ 1991,68,596. Nickolls, L. C , in Lundquist, F. (Ed.) Metho& qfFwmnsle Sclene, Vol. 1 science: Newlbrk, NY, 1962, pp 358360. 33. Weapt, R. C. 1Ed.I CRC Hondbmk afChemishy and PhysICS, 70th ed., CRC Ine.:B- Ratm, FL, 1989.1990, p E379. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.