edited by
GEORGE L. GILBERT
Denison University OIanville. Ohio 43023
A Readily Made Simulated Optical Spectrum For an Overhead Projector Submitted by:
Checked by:
Colin J. R i x a n d Keith A. Phillips Royal Melbourne Institute of Technology 124 LaTrobe S t r e e t Melbourne, 3000 Victoria, Australia. Leonard C. Grotz Uniuersity of Wisconsin- Waukesha Waukesha, Wisconsin
The projection of a large image of a n optical spectrum is often useful for teaching simple concepts of color as a n introduction to visible absorption spectroscopy. A previous article in this Journal' described the production of such a spectrum using a multiple layer diffraction grating attached to an overhead projector. This produces a true optical spectrum and we have used i t for small group tutorials. However, it is somewhat restrictive since, for maximum effect, it requires an almost completely darkened room. We present below a means of producing a useful approximation to an optical spectrum, which, from our experience, is readily projected in a large lecture theatre under normal lighting conditions, and is adequate to illustrate the principles of color and visible absorption spectroscopy. Projection of a pair of such spectra readily introduces the principle of operation of a modern dual-beam saectroahotometer. T h e arraneement requires the comhination of six differently colored transparent sheets (available from artist's sundiers) and the nroiected image exhibits 15 regions of color ; k i n g from red td blue (see construction details below). The pair of spectra was prepared by cutting two rectangular holes, each 135 mm long by 25 mm wide, arranged one above the other and 30 mm apart in a cardboard frame about the same size as the top surface of the overhead projector. The layers of colored sheet were then attached with adhesive tape and the final assembly enclosed between two thin (3 mm) Plexiglas sheets for rigidity and protection. A colored filter or a 100-ml beaker containing a colored solution can then be moved across one spectrum to illustrate the principle of color absorption by comparison with the reference spectrum. Some typical examples are described below. (1) A 15 mm depth of 1M copper(I1) sulfate in water gives a hlue solution of suitable color intensity for use with this demonstration. (2) Solution (1) plus 2 M glycine in water until the total depth is 25 mm gives a deep hlue solution. 13) A 15 mm depth of 1 M cohalt(ll) sulfate in water gives a pink color. (4) Solution (3) plus concentrated hydrochloric acid until the volume is almost doubled gives such an intense color that the projected image is black. (5) Discard some of solution (4) to reduce the depth of the solution ~~~~
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to about 5 mm to show that the color is deep blue. (6) A 15 mm depth of 0.01 M iron(II1) chloride is a verypale yellow color. (7) Solution (6) plusahout 1ml of 10%ammonium thioeyanate gives a blood red color. (8) A 15 mm depth of 0.5 M sodium hydroxide plus 4 drops of 0.1% bromothymol blue solution gives a bright hlue color. (9) Solution 18)dus 0.5M hvdrochloric acid until the color changes to yellow. (10) A 15 mm depth of 0.5 M hydrochloric acid plus 10 drops of 1% phenolphthalein solution is colorless. (11) Solution (10) plus 0.5 M sodium hydroxide until the color changes to magenta. mm depth of 0.0025 M potassium permanganate dissolved (12) A in 0.1 M sulfuric acid is purple. (13) Solution (12) nlus about 1 ml of OSM sodium sulfite turns colorless. 1141 ,- ~,A 5-7 mm deoth of 0.1 M ootassium dichromate dissolved in 2 M hydrochloric acid is orange. (15) Solution (14) plus 0.5M sodium sulfite until the solution turns completely green Using these demonstrations, i t can be readily shown that the transmitted color of a solution is a result of the selective absorption of light from sections of the visible spectrum. For example, the solution of aqueous copper(I1) is blue as a result of absorption of light in the red region of the visible spectrum, and this is easily seen by comparison with the reference spectrum. Thus, the principle of colors is readily demonstrated. When the copper(I1) glycinato complex is prepared the solution assumes a deeper hlue color which can be seen to be due to more complete absorption in the red region of the spectrum. This point can be emphasized by placing the copper(I1) aquo solution over one spectrum and the copper(I1) glycinato solution over the other. T h e relationship hetween intensitv of color and extent of absoration is also shown in several of the other demonstrations described above. The demonstrations of the color changes have been chosen to assist teaching the important chemical principles of complex formation, acid-base equilibria and redox reactions. We use glycine to form the copper(I1) complex to demonstrate amino acids, and by implication proteins, as ligands in bioinorganic chemistry. We use the formation of the intense iron(II1) thiocyanato complex to demonstrate the use of ligands in trace metal analysis. Construction Details-dimensions of colored layers. Layer 1: 22 mm purple, 58 mm hlank, 10 mm yellow, 45 mm orange. 2: 12 mm purple, 30 mm blue, 58 mm blank, 35 mm orange. 3: 32 mm purple, 20 mm green, 57 mm blank, 26 mm oranre. 4: ;I1mm purple, 38 mrn green. 47 mnl hlank, 18 mm r ~ d 5: I? m m purple. 10 mm hlue, 83 mm yellow. 6: 61 mm hlue. 62 mm hlank, 12 mm red. 7: 129 mm hlank, 6 mm red.
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1 Alman, D. H.,and Billmeyer Jr., F. W., J. CHEM. EDUC., 53,166 (1976).
Volume 54, Number 9, September 1977 / 579
