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.
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