Carleton S. Spear
Continental Classroom National Broadcasting Company
A Demonstration Polarimeter
In the course of televised lectures for Continental Classroom-Modern Chemistry it was found desirable to demonstrate both the phenomenon of optical activity of organic compounds and the instrument by which it is measured. The polarimeter described here was constmeted within three hours from materials such as heavy art cardboard, brass bolts and washers, and two squares of Polaroid sheet. The tools used were a sharp cutting knife, file, saw, and screw driver. The instrument is versatile. Several solutions in separate containers may be placed together and compared one at a time. The purpose and function of the instrument can be explained while describing and assembling the completed component parts on the lecture table. The apparatus is shown in the figure. The stator and rotor of the dial assembly (analyzer) are made of two pieces of art cardboard. Concentric holes, 5 in. in diameter, are cut with a sharp knife in each. The stator may be one foot square, and the rotor a ten-in. diameter circle. The outer edge of the rotor is smoothed by using a flat file in order to provide smooth, even contact with the three bearings. The surface and edges of the rotor are sprayed with clear lacquer before the Polaroid square is fastened to it. Corks, fastened with adhesive, are added for convenience of operation. Construction of the bearings is indicated in the Section Detail. Index marks for stator and rotor are small triangles cut from black adhesive masking tape. The markers can easily be repositioned for calibration. The finished assembly becomes the analyzer. It stands erect in the slots in two wooden supports. Small wedges hold the supports in place. Where shop facilities are available, more durable materials than smooth cardboard can be utilized. Bigger squares of Polaroid sheet can be used if necessary. The apparatus is relatively simple and inexpensive and can be constructed by students for use in laboratory experiments. The rectangular glass containers used in the polarimeter are of extremely fine quality, but are correspondingly expensive. They measure 2 X 5 X 10 cm (inside dimensions) and were obtained from Klett Manufacturing Company, 179 East 87th Street, New York 28, N. Y. Square containers such as battery jars or square bottles can be used. Good results are obtained when using beakers and ordinary round bottles by making observations a t the vertical axis of the containers. The rotor (analyzer) may be graduated in degrees if quantitative measurements of rotatiou are desired.
L reprerents a light box (photographic printed with opal glass. H derignater a foot-square cardboard holder for a 6 X 6 in. square of Polaroid sheet. [The l m e r is fastened by Ssokh t o p e tabs in front of the 5-in. diameter hole.) This assembly becomes the polorizer. B i s a block support for the solution contoinerr. S and R represent stator and rotor of the dial osembly mode d cardboard.
Since the light source was not monochromatic, trials were made with optically active solutions to determine a common effect of rotation. A sharp transition in color, blue to red, was selected as the "end point" when the analyzer is turned. This effect can be observed by a class. The polarizer and analyzer are first crossed to shut out light transmission. Corresponding black tabs are positioned on the rotor and stator to mark this point. A container of water is placed on the block with no change in light transmission. A two-molar solution of levulose is placed on the block to the left of the water, and a similar solution of dextrose is located on the right of the water container. Light is transmitted through both solutions. The dial is rotated counterclockwise until the sharp blue-red transition auuears for levulose. When the dial is rotated clockwise, the sharp transition from blue to red occurs in the light transmitted by the dextrose in the right hand container. The liquids may be removed and unknowns substituted. Cane sugar solutions, syrups, honey, as well as a number of other organic compounds may be used. The magnitude of the angles of rotation observed from the solutions used for demonstration varies between twent,y and seventy degrees. If suitable containers, whose inside measurement can be determined, are available, perhaps good relative values for optically active solntions can be obtained.
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Volume 37, Number 4, April 1960
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203
