overhead projector
edited by DORIS KOLB Bradley University Peoria. IL 61625
Using a Projecting Voltmeter To Introduce Voltaic Cells Sally Solomon, Jeflrey Lee, Joseph Schnable, and Anthony Wlrtel Drexel University Philadelphia. PA 19104
The introduction to the tonic of voltaic cells and standard potentials is usually done bygiving definitions and performine calculations. Usine a transnarent "nroiectine" voltmeter - . and assembling a zinc versus copper cell one component a t a time allows students t o develop a more concrete notion of the nature of a voltaic cell and the potential it produces.
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"Projecting" Voltmeter
Projecting meters for dc voltage measurements in which large lecture groups can view needle deflections are commercially available? However, i t is also possible to build projecting voltmeters. One way t o do this is by modifying a Precision Panel Meter2 designed t o produce afull scale deflection of 15V dc. A 15.000-0 resistor is normallvconnected in series between the panel meter and the voltage to be measured. By reolacine this with a 1500-11 resistor it is ~ossihleto redure thk full-scale deflection to 1.5 V. The transformation of this device into a projecting voltmeter is described in Figure 1. First the front transparent plastic cover is carefully removed along with the opaque piece of plastic on the front cover thus revealing the moving coil as shown in Figure l(a). The piece of plastic can be discarded. The screw that holds the moving coil to the case is then removed and the glue holding the other end is carefully dislodged allowing the moving coil to be taken out. The "15-V" plate is easily peeled off the case leaving an opaque plastic case that must be cut away (with a small hacksaw) in order to produce a transparent "projecting" meter. The needle is revealed and the moving coil is then remounted in the bottom half of the case as shown in Figure l(h). The clear meter face is snapped into place in such a way that the needle movement is not impeded (see Fig. l(c)). T o reduce the full-scale voltage, a 1500-0 resistor is soldered to the positive terminal, and a wire is attached to the negative terminal. A projecting voltmeter can also be built from a damaged analog pH meter a t virtually no cost. The voltmeter used in our lectures was made by modifying the meter removed from the casing of a damaged Chemtrix Type 40 pH analog meter as shown in Figure %a). A p H Master meter can also be used. The voltmeter taken from the pH meter was designed to produce a half-scale deflection of 700 mV. T o increase the range t o approximately 2 V, a 20-0 resistor was placed in the Chemtrix meter as shown in Figure 2(a). A 20,000-0 resistor is needed for the pH Master model. T o convert this device into a "projection" meter the opaque plastic meter casing, which fortunately covered nothing essential for the operation of the meter, was cut away (See Fig. 2(b)). A protective transparent plastic film was substituted (to prevent needle 'Central Scientltic Company. "Teaching Materials Catalog"; Franklin Park, IL; p 259. Radio Shack. 1988 Catalog; Model Number 270-1754. Roberts, J. L.; Sawyer, D. T. Experimental Electrochemistry for Chemists; Wiiey; New York. 1974; pp 31-32. 510
Journal of Chemical Education
deflections upon exposure to air currents or slight movements). One advantage of this particular projecting voltmeter is that it can be zeroed a t center scale allowing students to see deflections in different directions depending upon the way in which leads are attached to the cell. Either of the homemade projectingvoltmeters can he calibrated with a precision voltage source and marked with pen or "transfers". Without a precision voltage source the meter can be calibrated roughly with a standard Zn versus Cu cell, or even with a flashlight battery. Salt Brldae -
An effective simple salt bridge is prepared by immersing strips of filter paper into 1 M KNOJ solution just before use. An aear salt hiidpe. that looks &st like the ones usuallv p i c t u r 2 in textboo!&'can also he irepared by filling a 6-mk U tube with a KC1-agar s ~ l u t i o nFor . ~ six salt bridges, 2.5 g of agar was added to 50 mL lukewarm water; to this was added 20 g KC1. While stirring, the warm solution was removed by pipet and transferred to the U tubes. The filled U tubes were placed upright in a beaker of hot water and heated for about 10 min until the agar began to set at the hottom of the tubes. The water remaining on top of the gelled solution was carefully drawn out, then replaced with
Figure 1.Conversion of s Precision Panel Meter to a projecting voltmeter. (a) The plastic cover and moving coil are removed. (b) The dc voltage label Is peeled off, the top half of the opaque case is cut away, and the moving coil is replaced. (c)A backview showing the replaced plastic cover and the terminals la which the 1500-0resistor and leads are anached.
sisted in the preparation of the agar salt bridges. Kurt Jung, Joseph Schoellkopff, 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 Auain Austin. TX 78712 Overhead projectors have frequently heen utilized in the chemistry 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 ~roiectionof the entire visible soectrum usine different colbr& solutions. What color is copper sulfate i n water? The resent in the incident white is absorbed to elecred lieht " tronically 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 topics such as Jahlonski 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;n d&te sodi& hydroxide, rhodamine B in dilute sodium hydroxide, and 9,lO-diphenylanthracene in cyclohexane. Concentrations can be varied to give the desired optical densities, hut 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-hlu+indig+violet and the corresponding wavelengths. This can be useful in explaining the demonstration (see the table). Fill 400-mL beakers with A
Fig. 2. Conversim of an analog pH meter to a projecting uoiimeter. (a)The vonmetar is removed fromthe casing of a damaged pH meter, and a resislor is insened to reduce the half scale deflectionto 1.5 V. (b) The top half of the opaque case is cutaway and replaced by a plastic cover ma&ed wlth voltages. more warm concentrated KC1-war solution. The filled tubes were left to cool in an uprighiposition until gelling was complete. This procedure was found to be particularly effectivein eliminating air pockets. Uslng the Voltmeter In Lecture The projecting voltmeter with leads is placed on an overhead ~roiectoron too of a blank transoarencv sheet alone te i with i w o k 0 - m ~beakkrs filled with 1~ c o ~ ~ e ; s u l f aand M zinc sulfate solutions. Strips of copper and zinc foil are used an electrodes. These can'be foldid over the side 01' the beakersandattached to the leadnfrom thevoltmeter. At this point you can note the absence of any voltmeter reading. Then introduce the salt bridge. The meter reading should be about 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 leah half ;elfand aVcopperhalf cell gives the exoected voltaee of about 0.5 V. A concentration cell notentiai can be ogserved by using 1 M and 0.001 M copper solutions. The ootential measured is about 0.09 V, a value close to that prddicted using the Nernst equation. Attaching the leads to a flashlight battery gives a potential of about 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 addi, ~ Denise Harris5 astion Shann Lin, Darren C a m e ~ o nand
'Supported by an Academy of Applied Science REAP grant Supported by the ACS Project Seed.
Transmlsslon Wavelengths abswptlon
AdnmI
transmission
emission
370 ( W ) >400 (clear) 430 (violet-blue) Fluaesceln 480 (biue-green) >540 (yellow-mange) 520 (yellow-green) 570 (wangel Rhodamine B 545 (green-yellow) >630 (pink)
DPA
approximately 100 mL of solution and, with the room lights off, place in the center of the proiector 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 ohserved. Note that this demonstration is not annronriate for large lecture classes because the students need to see the beaker clearlv from the side at eve level. Other compou~dsthat can be 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. A.
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' Rix, C. J.; Phillips, K. A. J. Chem Educ. 1977, 54, 579. Volume 66 Number 6 June 1989
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