sisted in the preparation of the agar salt bridges. Kurt Jung, Joseph Scboellkopff, and Nicholas Flocco Jr. tested the techniques used to measure the cell potentials.
What Color Are Fluorescent Solutions? John L. Sturtevant University of Texas at Austin Austin. TX 78712
Overhead projectors have frequently heen utilized in the chemistrv classroom to demonstrate acid-base indicators, transition metal complexes, and other color-related phenomena. The concept of absorption/transmission of different wavelengths by a solution can easily be conveyed with the help of this tool. Rix and Phillips' have outlined the oroiection of the entire visible snectrum usine different colbr& solutions. What color is copper sulfate& water? The red lieht *resent in the incident white is absorhed to electronically excite the copper ions, and the remainder is transmitted as blue. But what color are fluorescent solutions? In addition to transmitting selected wavelengths, they also emit a characteristic broad hand of frequencies, the result often being an intriguing mixture of eye-satisfying colors. This process is not always well understood by students. The overhead projector, however, is ideally suited to separate the transmission and luminescence spectral properties of solutions. The true "color" of the solution is projected onto the screen while an eve-level view of the solution is dominated bv the emission. This demonstration can be used for general, organic, or physical chemistry lectures and can be accompanied by discussions of advanced topies such as Jablonski diagrams, fluorescence lifetimes, directionalitv of emission,. quantum yields, quenching processes, and Stokes shifts. The use of three solutions givine an arrav of different colon is suggested: fluorescein'in dilute sodi&n hydroxide. rhodamine H in dilute sodium hydroxide, and 9,lO-diphenylanthracene in cyclohexane. ~ o k e n t r a t i o n scan be varied-to give the desired optical densities, but should be -1-10 mgl mL. If solutions are too concentrated, it is difficult to see the emitted light due to the inner filter effect. It is useful to label the bottom of a transparency with the acronym ROYGBIV (red-orange-yellow-green-blu+indig+viole and the corresponding wavelengths. This can be useful in explaining the demonstration (see the table). Fill 400-mL beakers with
-
Fig. 2. Conversim of an analog pH meter to a projecting voiimeter. (a) The vonmeter is removed from the casing of a damaged pH meter, and a resistor is inserted to reduce the half scale deftection to 1.5 V. (b) The top half of the opaque case is cutaway and replacedby a plastic cover ma*& wlth voltages.
more warm concentrated KC1-agar solution. The filled tubes were left to cool in an uprighiposition until gelling was complete. This procedure was found to be particularly effective in eliminating air pockets. Using the Voltmeter In Lecture The projecting voltmeter with leads is placed on an overhead oroiector on too of a blank transoarencv sheet alone te i with i w o k 0 - m ~beakkrs filled with 1~ > o ~ ~ e ; s u l f aand M zinc sulfate solutions. Strips of comer and zinc foil are used as electrodes. These can'be folded over the side of the beakersandattached to the leadafrom thevoltmeter. At this point you can note the absence of any voltmeter reading. Then introduce the salt bridge. The meter reading should be ahout 1.1 V. You can do the standard reduction potential calculation in the space available below the beakers by writing directly on the transparency. Other voltaic cell measurements can also be performed convenientlv usine the oroiectine voltmeter. A cell constructed from a leLd half ;elfand :copper half cell gives the exoected voltaee of about 0.5 V. A concentration cell ootentiil can be ogseserved by using 1 M and 0.001 M copper solutions. The ootential measured is about 0.09 V, a value close to that predicted using the Nernst equation. Attaching the leads t o a flashlight battery gives a potential ofabout 1.5
v.
We have noticed that students introduced to voltaic cells in this "live" fashion develop a feeling for the topic that is difficult to convey using standard lecture techniques. Acknowledgment We wish to acknowledge the assistance of Maryann Fitzpatrick, an electronics technician who provided invaluable advice about the pH meters and their calibration. In addition Shann Lin, Darren Cameron,4 and Denise Harris5 as-
'Suppotted by an Academy of Applied Science REAP grant 5Supportedby the ACS Project Seed.
Transmlsslon Wavelengths absorption
h(nm1 transmission
emission
DPA 370 (UV) >400 (clear) 430 (violet-blue) Fluwescein 480 (blue-gem) >540 (yellow-mange) 520 (yellow-green) Rhodamine B 545 (green-yellow) >830 (pink) 570 (wangel
approximately 100 mL of solution and, with the room lights off, place in the center of the oroiector on top of the transparincy. Encourage students view the solutions from eye level to see the fluorescence and contrast with the transmitted color on the screen. Of course, some transmitted light is scattered with the luminescence, so the pure fluorescence is not observed. Note that this demonstration is not a~nronriate for large lecture classes because the students neei to-see the beaker clearlv from the side a t eve level. Other compou~dsthat can he excited by the projector lamp include perylene, eosin, Rose Bengal, and coumarins. Of course, care must be taken to avoid spilling any of the mentioned dyes in the classroom.
' Rix, C. J.: Phillips, K. A. J. Chem. ~ d u c1977, . 54, 579. Volume 66 Number 6 June 1989
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