Rediscovering the Wheel
Solution D Dissolve 1 g of fluorescein [Aldrich: 16,630-81 in 1 L of ethanol Procedure In their description of the HzOz oxidation of lucigenin demonstration (51,Shakhashiri, et al., proposed two alternate procedures. The first involved mixing solutions A and B in the glass funnel of the glass-spiral apparatus described above. The second involved adding a fluorescent dve solution. such as solution C or D. to a beaker containing solution B, and then adding solution A to this beaker. We DroDose a third alternative. Pour 1 L of solutions A and ~ ' i n i othe separately funnels in the figure. Simultaneously open the stopcocks on the two separatory funnels, allowing solutions A and B to pour into the funnel a t the top of the glass-spiral apparatus until the funnel is approximately one-third full. Optimum results are obtained by adjusting the stopcocks so that the funnel remains about one-third full. The rate at which liquid flows through the spiral also can be controlled by opening or closing the inch- clam^ on the exit tube. If you find that this approach generates air bubbles within the glass spiral, i t is possible to start by filling the spiral with ethanol, and then allowing this solvent to flow through the exit tube as the pale-blue luminescent solution flows out of the glass funnel into the top of the spiral. While solutions A and B are flowinn into the elass funnel. add a small (5-mL, allquot ofsolution C. The rhodamine B dve in this solution will maduallv chanae the color of the liminescence from p a l e - h e to red. once this process has bepun. add a small aliquot of solution D. The fluorescein dye will now shift the color toward the green end of the spectrum. To shift the color back toward the red end of the spectrum, add a second aliquot of solution C. This process can be repeated until the whole spiral is a brilliant blend of colors. a t which ~ o i nvou t can clam^ off the exit tube and freeze the colors fok as long as 15 mi;. (6). Caution: If you fill the funnel too full with solutions A and B, you will reduce the concentration of the dye solution and not achieve good color separation in the spiral. Literature Cited che&d Lkm0nslmlions:AHa"dbmk
bf Education, 1965. 4. khminer, R.;Testen,M. E.: Shakhashiti, B. Dirreen,G. 0.C k m ice1 for Teachers ofch~misfry.Volume Madison. WI: University ofWismnsin Press, 1983;pp 156167. khreiner, R.: Teeten. M. E.; Shakhashiri, B. Dimen, L. G. Cham. ice1 &monslmtions:A Hondbmk for Tenchars of Chomktry. Volume 1.Madison,
5.
Z.:
E.: WiUiams,L. 1. Z.; G. E.;WUiams, WI:U n i v e m i t v o f W i ~ n s l nPress.. 1983:... 00 180-185.
6. Gundermann, K -D, and MeCapra, F. Chemiiumiwscena in Ogonic Chomlatry. Sptingsr-Verlag: Berlin, 1987;p 196.
The Flame Test Revisited Submined by Ronald 0. Ragsdale and Jerry A. Driscoll Univesity of Utah Salt Lake City, UTE4112 Checked by Deborah Whalen and David Blackman University of the District of Columbia Washington, DC 20008 With this paper we hope to establish awareness of a forgotten but useful flame test demonstration that can be easZy viewed by all students in a large or small lecture hall. In one of the typical techniques used today, a platinum wire is dipped in a test solution and then exposed to the flame of a Fisher burner or the solid chemical is put directly on the platinum wire. This produces a small frame of short duration. Spraying a concentrated chemical solution into a Fisher burner's flame is more successful. (The burner is clamped at 90' to its normal position over a sink to minimize cleanup.) However, with this approach the flame emission is brief. Also, the spray bottles are difficult to aim in the dark, and the cleanup is time-consuming. Recently, a device for another flame test demonstration was discussed in which a 2-L ~ l a s t i bottle c is Dlaced over a Tirrill burner. An atomizer is&ed to introduEe the chemical salt into the air volume of the 2-L bottle ( I ) . A Revival of an Elegant and Colorful Demonstration The flame test demonstration presented in this paper is much ouicker and more convenient than those discussed above. in fact, a dozen flame tests can be demonstrated all at the same time. Our search for a superiorly bright test was rewarded when we came across an old test with a simple arrangement that gives an intense flame for several minutes. The height of the flame is usually 10-15 cm. This demonstration was reported in the literature by Professor Paul Arthur in 1939 (2). How this simple and convenient flame t e s t was lost from t h e chemical educator's repertoire is a mystery. Perhaps i t never reached a lame audience because it was ~ v e in n a speciality book Setup To maximize the effect of the demonstration, the room should be darkened as much as possible (even though several of the flames can be seen in a classroom with normal lighting). The apparatus for the flame test is a large Petri dish, a watch glass to cover it (or a matching pair of Petri dishes), and a heat-resistant pad. Procedure 'Caution: Adequate ventilation must be used. Caution: Methanol must be handled with care. It is labeled as a health hazard. Its flammability is considered "severe". Methanol's reactivity is "slight", and its contact hazard is "moderate." Methanol may be fatal if swallowed, inhaled, or endured in prolonged contact. It may cause blindness or irritation. Methanol's flashpoint is 11 'C. Methanol should be kept enclosed in a bottle. Caution: People standing close to the demonstrationshould not stare tw intently at the flames.
/
Methanol Demonstration setup for flame tests in a Petri dish
828
Journal of Chemical Education
Place about a teas~oonof the desired chemical in the Petri dish. Add aboutb510mL of methanol, and ignite (see firmre). ARer several seconds. the full iutensitv of the color isseen. The brilliant flames are clearly visible to over 350 1 students.
Recommendations and Observations A watch glass can be used to extinguish the flame as one desires. Combustine onlv 5-10 mL of methanol a t a time reduces the hazarz of large fire and produces only a small auantitv of heat. allowinn the Petri dishes to be handled secommendedusinga polyethvlene bottle to deliver the methanol for both convenience and safety. L q e r amounts of methanol increase the length of the burn of the flame tests, a s well a s the intensity of the flame. I n most cases the comnounds can be stored in the Petri dish between denlonst;ations by covering each with a watch glass or matching Petri dish. Adding water to the methanol to increase the solubility of the salts did not improve the results. The success of this flame test is due to several factors.
a
the high temperature achieved by the methanol the clean-burning properties of methanol the greater solubilities of the salts in methanol over other alcohols After a short b u m the optimum temperature is achieved. Although some of the~ompoundshad some solubility in ethanol, it did not prove a s satisfactory as methanol because ethanol burns with a yello\r-orango flame d,.Generally the halides are the most effrctive in producing the desired results. Excellent results have heen ohtnined with the compounds listed in the table. Literature Cited 1.Gouge, E . M. J. Ckom. Ed=. 1988.65.545. 2. Arthur. PLPefvnDDmomlrotiom in Gomml Chem&fry:M&aw-Hill: NewYorkand Landon, 1939;p 311. 3. Infernoflsaol C~itLmll h b h ofNumrim1 Data, Pkwics, Chemlstq and l&hnoiogy, Vol. IY:Washburn. E. W.Ed.: Mffiraw-HiU New Yorkand London, 1928: pp 2 0 6
2w.
Line Spectra Using a CD Disc Henry Brouwer Redeemer College Ancaster, ON, Canada L9G 3N6
If you have ever viewed reflected light from a CD disc, you will have noticed that the full spectrum of the light source is resolved by the d i s c t h e disc behaves a s a diffraction grating. Such a CD disc also provides a n excellent method of viewing the emission spectrum from a gas discharge tube or from a n element heated in a flame.
Compounds Used and Colors Produced
Group IA lithium
Color / Group IIA I red
chloride sodium yellow iodide potassium faint calcium iodide purple chloride rubidium
chloride cesium chloride
faint violet faint blue
strontium chloride barium chloride
Color
orangered
Others boric acid
Color light green
copper(l1) green bromide with blue fringes
crimson yellowist
To view the spectrum, hold the CD disc behind the gas discharge tube and look a t the reflected light. In order to see the lines clearly, it is best to view the spectrum in a darkened mom. I t also may be necessary to move the disc slightly in order for others to see the lines. For example, the light from hydrogen shows three very distinct lines-violet, blue and red; whereas, helium shows four lines -blue, green, yellow, and red. The lines are clearly visible when viewed within 2-3 meters, but a t greater distances the violet, blue and green lines are more dimcult to see. To view the spectrum of an element heated i n a flame, place a slit (about 2-3 mm wide) made of paper or cardboard between the flame and the disc. Then view the light passing through this slit using the CD disc to observe the emission spedrum for a given element. [To excite the element in a flame, dip a nichrome wire having a loop a t one end into a compound or a solution of the c~mpoundcontaining the desired element and heat in the flame. I t is convenient to make a set of wires by inserting the nonlooped end of the nichrome wire into the wide end of a cork stopper, which is then used to stopper a vial containing the compound.] The line spectra are not a s distinct nor a s bright as with the gas discharge tubes and thus only students close to the apparatus will be able to view the spectra. Although replica plastic diffraction gratings can also be used for viewing the spectra, the lines are not a s bright a s with the CD disc. Unfortunately, due to the circular nature of the CD disc, the lines are curved, not parallel. Bending the disc straightens the line spectra somewhat, but i t decreases the distance between the lines.
Volume 69 Number 10 October 1992
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