Chemistry Everyday for Everyone edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
CD-ROM Spectroscope: A Simple and Inexpensive Tool for Classroom Demonstrations on Chemical Spectroscopy Fumitaka Wakabayashi* Department of Science and Engineering, National Science Museum, 3-23-1 Hyakunin-cho, Shinjuku-ku, Tokyo 169-0073, Japan Kiyohito Hamada Department of Education, National Science Museum, 8-20 Ueno Park, Taito-ku, Tokyo 110-8718, Japan Kozo Sone Department of Chemistry, Faculty of Science, Josai University, Keyakidai 1-1, Sakado, Saitama 350-0248, Japan
Classroom demonstrations on color and spectroscopy are indispensable at any stage of chemical education (1). There are many proposals for easy and impressive demonstrations of emission and absorption spectra, among which those employing a handmade spectroscope with a transmission replica grating (600 lines/mm) (2–6 ) and the projection of the obtained spectra by means of an overhead projector (7 ) are of special interest. The rainbow colors appearing on the surface of a compact disk (CD) reveal that it can serve as a diffraction grating; in fact, the recording track on it has a pitch of 1.6 µm, making it a grating of 625 lines/mm. Thus, the CD is usable as a reflection grating, as contrasted to a transmission grating. Since CDs for music listening and CD-ROMs are now widely available, attempts have been made to observe atomic or emission spectra using them (8–10). We have made a simple spectroscope with a CD (11, 12). This “CD-ROM spectroscope” was found to be a very handy and inexpensive tool for classroom teaching on chemical spectroscopy.
Figure 1. A plan of CD-ROM spectroscope (scales in mm).1
Construction of the Spectroscope Figure 1 shows the plan for such a spectroscope. To make it, a sheet of cardboard is cut out in the shape and size shown in the figure, and the entrance slit, viewing window, and CD inserting port are cut open. The shape and width of the entrance slit are crucial for the quality of the observed spectra; if it is found difficult to cut the slit out exactly, it may be made somewhat wider (e.g., 2 mm) and then narrowed to 1 mm or ca. 0.4 mm by pasting or taping a rectangular cardboard piece onto it. It is recommended to paint the back of the cardboard in black in order to avoid stray light. The cardboard is then made into a box and fastened with tape or paste. A cardboard box of a similar size (e.g., a tissue box or a shoe box) can be used instead of the handmade box, with proper modifications. A 5-inch CD is then inserted through the inserting port until the top of the CD just touches the inside edge of the box, as shown in Figure 2. Use of the Spectroscope The CD spectroscope is now ready for use. Standing under the open sky or an incandescent lamp so that a good amount of light gets into the box from the entrance slit, and looking into the viewing window, one can readily observe the continuous spectrum of rainbow colors appearing on the *Corresponding author.
Figure 2. Side view of the CD-ROM spectroscope in use. The Petri dish is put on when absorption spectra of solutions are studied.
CD surface (WARNING: To protect your eyes, do not directly observe strong reflected sunlight on the CD.) One can also note that the light of a fluorescent lamp shows a quite different spectrum, which depends on the fluorescent material in it. In the case of a typical “daylight” or “white light” type lamp, sharp violet, green, and yellow lines appear on the background of a continuous spectrum. These sharp lines are due to the emission spectrum of mercury vapor in the fluorescent lamp (5) and the continuous spectrum is due to the fluo-
JChemEd.chem.wisc.edu • Vol. 75 No. 12 December 1998 • Journal of Chemical Education
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Chemistry Everyday for Everyone
rescence from halophosphate-based phosphors (13). In the case of the “three bands” type lamp, which allows good color rendition, three bright bands of blue, green, and red predominate. These bands are due to the fluorescence from rare-earthbased phosphors developed relatively recently (13). In the same way, one can easily observe and compare the emission spectra of discharge tubes of H2, He, Ne, Na, etc., and also those of the flame reactions, with results similar to those obtained by means of a replica grating spectroscope (2, 4–6, 14 ). The view can be improved with a small magnifying lens or a Fresnel lens fixed in front of the viewing window, and also with a suitable shield from direct light. Absorption spectra of colored solutions (1) can also be observed with the CD spectroscope. This can be simply done by putting a Petri dish containing the solution upon the entrance slit as shown in Figure 2, and observing the change of the continuous spectrum of an incandescent lamp caused by it. When the concentration and depth of the solution are appropriate, absorption bands appear as dark stripes on the spectrum. Changes in the concentration and depth of the solution bring about corresponding changes in their intensity (i.e., the width and darkness of the stripes). This phenomenon is a good example to explain qualitatively the Lambert– Beer law. Some examples of the results, which may be useful as effective classroom demonstrations, are given below.
Potassium Permanganate The purple aqueous solution of this salt shows an intense charge transfer (CT) band in the visible region, 450 to 580 nm, which is remarkably split into several components (1). The band appears as a broad darkening in the yellowgreen to green region (i.e., the complimentary color of purple) in the spectrum of the CD spectroscope, which gradually resolves into a number of distinct dark stripes with the decrease of the concentration and depth of the solution. The three strongest components can be recognized easily, which, together with a weak shoulder-like component at ca. 560 nm, form a quite spectacular view. Cobalt Chloride It is well known that CoC12ⴢ6H2O forms reddish-violet solutions in water and methanol, which contain octahedral solvated complexes, and deep blue solutions in ethanol and higher alcohols, which contain tetrahedral ones (1, 15, 16 ). Such a difference in color is clearly due to the difference in the position of their d–d bands; the reddish violet solutions show a weak broad band in the green region (at ca. 525 nm), as in the case of the KMnO4 solution, whereas the blue ones show a much stronger band in the orange-to-red region (at 580–720 nm) (1, 15, 16 ). This difference can be easily discerned with the CD spectroscope. It may be added that the methanol solutions are thermochromic, becoming more bluish on heating and more reddish on cooling owing to changes in solvation (16 ). The spectral changes in this interesting phenomenon can also be observed with the CD spectroscope. Fe(III)-thiocyanate Complex Aqueous solutions containing Fe3+ and NCS᎑ ions show an intense red color of the Fe(III)-thiocyanate complexes, which is familiar in qualitative analysis of metal cations. A strong and broad band in the blue region (at ca. 440 nm) is observed with the CD spectroscope, and comparison with the cases of KMnO4 and CoCl2 solutions leads to the general 1570
rules: a red solution absorbs in the blue region, a blue solution in the red, and a purple solution in the green-to-yellow.
Phenolphthalein in Alkaline Solutions The reddish purple color of this indicator in solutions of pH > 9.8 is due to a broad band in the yellow-green to green region (ca. 550 nm), which is observable with the CD spectroscope, just as expected from the general rule. Chlorophyll When the leaves of parsley are washed with water and methanol, extracted with hot methanol (60 °C) for a few minutes, and filtered, a deep-green solution containing chlorophyll a, chlorophyll b, and carotenes is obtained. Such a solution is rather unstable, but when freshly prepared it shows intense bands of the chlorophylls in the blue (ca. 460 nm) and red regions (ca. 670 nm). This leads to another general rule: a green solution absorbs in the blue and red regions, being opposite to a purple solution, which absorbs in the green region and transmits in the blue and red regions. Conclusions All these results indicate that the CD spectroscope is a useful tool in primary and secondary schools to teach what color is and what spectra are, and how the general rules given above arise. Although these are just “rules of thumb”, they will be an interesting and useful aid for the students. Moreover, this demonstration can also be used effectively in introductory courses in colleges and universities (where students are apt to think that spectra are just printed-out data supplied by computerized spectrophotometers), to teach them that spectra are really beautiful natural phenomena which can be observed simply. Note 1. A file of the paper pattern of the CD-ROM spectroscope is available on JCE Online (http://jchemed.chem.wisc.edu/Journal/Issues/1998/ Dec/abs1569.html). A PICT file or Claris Draw file is also available on request by an email to F. W. (
[email protected]). The file will be sent as an attachment of the reply email. Please specify the type of the file.
Literature Cited 1. Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin Press: Madison, 1983; Vol. 1. 2. Edwards, R. K.; Brandt, W. W.; Companion, A. L. J. Chem. Educ. 1962, 39, 147. 3. Harris, S. P. J. Chem. Educ. 1962, 39, 319. 4. Hughes, E., Jr. J. Chem. Educ. 1984, 61, 908. 5. Cortel, A.; Fernández, L. J. Chem. Educ. 1986, 63, 348. 6. Jacobs, S. F. J. Chem. Educ. 1997, 74, 1070. 7. Solomon, S.; Hur, C.; Lee, A.; Smith, K. J. Chem. Educ. 1994, 71, 250. 8. Mebane, R. C.; Rybolt, T. R. J. Chem. Educ. 1992, 69, 401. 9. Brouwer, H. J. Chem. Educ. 1992, 69, 829. 10. Cornwell, M. G. Phys. Educ. 1993, 28, 12. 11. Wakabayashi, F. Chem. Today (Gendai Kagaku) 1990, April, 23 (in Japanese). 12. Wakabayashi, F.; Hamada. K. Chem. & Educ. (Kagaku to Kyouiku) 1996, 44, 676 (in Japanese). 13. Ulmann’s Encyclopedia of Industrial Chemistry; Elvers, B.; Hawkins, S.; Hussey, W. E., Eds.; VCH: Weinheim, 1990; Vol. A15, p 139. 14. Dalby, D. K. J. Chem. Educ. 1996, 73, 80. 15. Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley: New York, 1988. 16. Sone, K.; Fukuda. Y. Inorganic Thermochromism; Springer: Heidelberg, 1987.
Journal of Chemical Education • Vol. 75 No. 12 December 1998 • JChemEd.chem.wisc.edu
