JOURNAL OF CHEMICAL EDUCATION
A DEMONSTRATION ON THE CHEMISTRY OF COLOR' STANLEY C. BUNCE and HENRY F. HAMMER Rensselaer Polytechnic Institute, Troy, New York
A
LECTURE demonstration which included a number of general principles of the chemistry of color proved to be a popular feature of an "Open House" program at Rensselaer Polytechnic Institute. The development of the concepts of the nature of light and color and of the formation and use of colored compounds in an hour program was necessarily kept simple; although conceived for an audience with a background of high-school chemistry and physics the program was enjoyed by some who had less work in science. The demonstration experiments described are not new, but the synthesis of a number of basic concepts, which could be illustrated in striking fashion, afforded an interesting and instructive program. The sequence of experiments was managed by a number of demonstrators working successively with equipment arranged along a long lecture table. Attention was focused on each experiment in its turn by lighting, while others were being prepared or equipment was being removed. As the experiments were performed, they were explained by a commentator and also by those demonst,rating the experiment. The commentator worked from aprepared script which included the basic concepts and modified this extemporaneously to fit the timing of the experiments. Calling on those performing the experiments for explanations and cornments, when appropriate, proved to be a very practical way to accommodate minor differences in timing, and
in part at the 119th Meeting of The Chemical Saciety, Boston, April, 1951.
gave an effect of spontaneity. I t also avoided rhe possible monotony of a single speaking voice, and allowed the commentator to assume the pall of the audience in asking questions. It will be realized that the experiments on dyeing, in particular, had to be scheduled carefully so that the samples would be ready, in turn, for removal from the dye bath, washing, and exhibition as t,he script moved t,o a description of the particular process. This program was developed in cooperation with a number of students who worked out the details for the most effective demonstrat.ion of the experiments and it was presented by a small group of students. It is included here in outline form, with detailed descriptions of the experiments limited to those which are not readily found in commonly available sources. Unless otherwise noted, all materials ment.ioned are available from laboratory supply companies. The explanations and presentation of the concepts by the commentator are indicated only briefly; the content of this portion of the program must depend on the time awilahle and the backcround of the audience.
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DEMONSTRATIONS
(1) Color is the effect of light of certain wave lengths on the eye. Color can be produced by physical structures which separate white light int,o selected wave lengths and send them along various paths. A prism bends the various wave lengths comprising white light to different degrees and produces a color spectrum.
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Fine or films may allow certain wave lengths to pass and reflect others. The colors of the rainbow, the blue of the sky, and the red of the setting sun are explained. (a) A spectrum, pmduced by good prism, is on a screen. (b) Colors produced by Tyndall effect are projected. This is most conveniently illustrated by passing an intense beam of white light through a tank containing dilute sodium thiosulfate to which is graduslly added small amounts of very dilute hydrochlorir mid t o form a monodisoersion of colloidal sulfur.' ~
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(2) Color produced by colored objects depends on their properties of absorbing light of some wave lengths, and of reflecting or transmitting the light of other wave lengths. white light is passed through filters, ~ are examined in white and then in red and blue light.
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(3) Many things commonly thought of as colored are colorless materials with small amounts of colored comnounds incoroorated. (a) Water is colored by a trace of a water-soluble dye, and the separation of this oolored solution by distillation is illustrated. (b) The separation of a colored solution by chromatographic adsorption is demonstrated.' An adsorption tube is prepared from a glass tube, 13 X 1.7 om., by placing a pad of cotton over a constriction near one end and pouring in a slurry of adsorhent in distilled water. The adsorbent is a suspension of a commercially available artivated alumina adsorbent mixed with 10 per cent Hyfl&upereel (Johns-Manville) in a solution 0.002 M in primary and seeonday sodium phosphate (pH 7.04). Two pieces of filter paper placed over the surface of the alumina in the column will prevent channeling. Water is passed through the column, and when the level of the water has fallen to within s. few mm. of the top of the alumina, a 2-ml. portion of a 0.5 to 2 per cent solution of mixed dyes is added and allowed to soak into the column. Following this, a 10-ml. portion of water is added to separate ("develop") the dyes. Slight suction may be necessary t o accomplish the processin areasontthle time, or tubes can he shown illustrating the various stages of the process. Mixtures of dyes may include auramine 0 and Victoria hlue (extremes in rate of adsorption) and any of the following dyes which are intermediate in rate of adsorption: malachite green, crystal violet, safranin 0, and methylene hlue.
(4) Color in chemical compounds is a result of absorption of light of selected wave lengths, and may he due to particular structural features. A change in structure may result in a change in the nature of the absorption and a consequent change in color, or a complete loss of it. (a) . . Dve . structures are illustrated by charts, with chromophoric groups indicated. (b) Indicator effects are explained by structure charts and demonstrated with solutions of some of the following dyes: methyl orange, Congo red, methyl violet, and phenolphthalein. The lztter two dyes undergo a, two-stage color change, methyl violet on acidification and phenolphthalein on addition of alkali. The color changes appear most striking if large beakers of fairly 2 BELAGAVANTAM, S., "Scattering of Light and Raman Effect," Wtaltair Andhra University, India, 1940, pp. 2-3. Directions included here are those of P. RUGGLIm P. JENSEN, Helv. Chirn. Ackz, 18, 624 (1935), including some of the modifications proposed by W. RIEMAN,111, J. CHEM.EDUC.,18, 131 (1941).
concentrated dye solutions are uaed, and if these are well illuminated and 'gitated.
(5) The use of dyes extracted from naturally occurring materials was a very old chemical art. The prepara.&tionof synthetic or &anufactured dyes, of which the preparation of mauve by Perkin and the synthesis of the valued natural dye, indigo, by van Baeyer are two early examples, was an important step in industrial progress. The 'ynthetic were cheaper, possessed more varied colors and properties, and mere purer and more uniform than the natural dyestuffs and eventually displaced them. The syntl,c-ie oi indip,, I,? a nrrtlad' which ir rapid (althouyh gisiny poor yields) i q drrnmstrated. TIw preparation may brgix. and the sequence nf with bcmaldvh\dr
