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A Simple Device for Obtaining Visible Spectra Ulrich Dammgen and Herbert Keune Department of Chemical Education, Braunschweig Technical University. Braunschweig. Federal Republic of Germany The interaction of light and matter is an important topic in the study of chemistry; however, it is often thought that spectroscopy cannot be carried out without specialized equipment. Fortunately, a standard slide projector can easily he adapted to make an effective spectroscope for this purpose. Method Light is projected through a cell containing the substance under investigation and then through a grating to form the spectra on a screen. A standard projector for 50 X 50-mm slides is used for this, but the slide is replaced hy two special items-the cell mentioned above and a masking plate having a vertical slit to form a narrow band of light. The projector is then focused on the screen in the usual manner, showing the image of the vertical slit. By attaching agrating in front of the projector, the spectra are produced on either side. It is advisable to darken the room and to place a cardboard box around the nroiector to cut out unwanted lieht. The box should have a smalioff-center hole so that only tce spectra on one side of the slit are seen, and the screen adjusted accordingly. Care should be taken to avoid overheating the projector. Figure 1shows the arrangement of the various pieces of the apparatus. The cell contains an upper and a lower compartment. The second comDartment can either he left emptv or filled with the pure soivent of the substance under it&stigation to give a reference spectrum. Figure 1 also shows the arrangement of the spectra on the screen, and how spectra in the near ultraviolet range can he obtained by fixing cloth (or paper) prepared with fluorescent brighteners on the screen in the appropriate place. To calibrate the spectra, monochromatic filters of known wavelength are used in place of the cell.
Equlprnent The Cell The cell is made by fixing two U-shaped copper wires between two cover glasses as shown in Figure 2. This provides the two compartments. one above the other in the central region where the light will shine through. Wires of between 0.5 and 2 mm diameter are suitable. and a twopnrr e p t q resin ;adhesive ( e r , Arnlditr or UHl' I'lui) is rrrommrndrd fur hundin~rhe wres t u the cover glasses. I t is a good idea to make several cells, as those that leak cannot be repaired. ~~
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The Masking Plate A rectangular hole is cut in a 0.5-mm-thick plate (aluminum or brass) as shown in Figure 3. Two halves of a razor blade are then glued to the metal plate to leave a sharp-edged slit approximately 0.3 mm wide. Before assembly, check that the combined thickness of the cell and maskine-. date will fit in the slide holder of the ~roiector . . (usually4.5 mm).
Figure 2. Preparation of the cell: A, front view. B, assembly.
Figure 1. Arrangement of the projection device: A, top view: 1, light source and condenser: 2.cell: 3. masking plate and slit 4. objective: 5. grating; 6. s a e e n i q Cardboard box: 7. original image of the slit: 8, ultraviolet spectra: 9, visible spectra. B, screen. front view: 10, imageofthe slit (color of the solution investigated): 11. visible spectrum of the solution investigated: 12, image of the slit (coiw of tungsten light or of the pure solvent): 13, reference spectrum of lungsten light or of the pure solvent: 14, sheet($ of paper prepared with fluorescent brighteners.
Figure 3. Prepsation of the masking plate and slit.
Volume 62
Number 2
February 1985
155
Experimeds T h e cell is filled as required for each experiment by using narrow ~. i. ~ e tAfter s . use. each cell should b e cleansed thoroughly by rinsing with a n appropriate solvent. T h r e e experiments t h a t demonstrate t h e use of t h e equipm e n t are set o u t below. Black Ink is not Black! Solutions: red ink, diluted approx. 1in 100 hlue ink, diluted approx. 1in 70 green ink, diluted approx. 1 in 40 hlack ink, diluted approx. 1in 80
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Bv wine" either the red.. blue.. or meen solution in one cell com" nnrtrnent and Tenvine other~comoartment emotv. it can he seen --..~ ~ ~~~ " . the~ . that the color of t h *~h l i ~ nmavhes the miming mlor in its ~ p e r l m m (compared torhe spectrum of tungsten light). However, when black ink is viewed in this way it will he found that two complementary colors (e.g., hlue and orange) are missing from its spectrum, indicating that the black ink is in fact a mixture of inks of these two colors. This can easilv be oroved h v ~ a ~chromatomaphvaf er the hlack ink (using . . . water adthe kohile dhsse). ~~~
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Colorless Substances are not Colorless! Most "colorless" substances in fact absorb ultraviolet lieht. As an example, if a cell of I -mm internal thickness has one compartment filled with nitrobmzene land the other cumpartment is left empty), it can bescen that darkness is created in the ultraviolet regLon 111the ~~~~
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nitrobenzene spectrum. As nitrobenzene is a liquid, nodiluting solvent is needed for this experiment. Light Absorption by MnO:, Croi-, a n d ~0:Solutions: KMn04: 150 mg in 100 ml water K2Cr04:1g i n 100 ml water NH4V03: saturated solution containing a few drops of dilute NaOH to form VOf ions out of the polyvanadates. Thespectra of these three solutions can be interpretedasfollows: The tetrahedral ions MnO:, GO:-, and VOQ-are isasteric and isoelectronic. MnO: absorbs green light, CrOi- blue light, and VOQultraviolet light. This change of color is obviously systematic and due to the position of the central element in the periodic table. The spectra can be interoreted hv means of MO calculations for MnO;. CrOF. and VOi-ashiseusseh in refs. (1-71.. ,(3.6-9). . , . , and (7.91.resokctiveiv: . , ., ~~,~~~ ~, ~uhse&.r d~lurionof the K M ~ O ,aoltrtion produces sprerra of higher rewlutim, a serres uf nearly equdistnnt dark lines replacing the broad absorption band Literature CHed (1) Wrobe1ski.J. T,aod Lax,J. G.,J. CHEM.EOUC.,54,15(1971). (21 ~ a e r e , ~ . ~ . , a n d ~ l d e r , ~ . , c h o mk. t~ rh y, Is I. , ~ ~ ( I V I ) . (31 Wolfabrg, M., and Hc1mhole.L.. J. Chom. Phys., 20,831 (19521. (41 Vist4,A.,aodGray,H.B.,lnorg Chem.,3.1113 (19641. (51 Fenske, R.F., and Sweeney,C. C., Inorg. Chpm., 3,1105 (19611. (6) Bsllhsuaan, C. L a n d Liehr, A. D., J. Mol. Speelroseapy, 2,342 (1958). (7) Bailey, N.. Carringtan, K. A. K., and symans. M. C. R.,J. Chpm. s o c (Londonl,290
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~ I ~L.,~B O ~ ~ ~w ,,~ . , - d wou8berg, M., J
chpmphys,23,853(1955).
(9) Carrington, A, aodJ&ensen, C. K., Mol. Phys,4,395 (19611.
