Richard Shavitz University of California, San Diego La Jolla.92093
I
Several recent articles in this Journal (1-6) indicate a continuing interest in demonstrations and experiments illustrating the rotation of plane-polarized light by optically active comnounds. Desoite this interest, no experiments have been described that can he performed by an entire laboratory class without the use of special equipment or elaborate construction of apparatus. ~ C l e a sone t s"ch experiment, in which optical activity is observed and measured, deserves to he included in the organic laboratory. This article describes a polarimeter that can be easily constructed from inexpensive, readily availahle materials. Less than fifteen minutes is required for its construction. One three-hour lab period therefore provides sufficient time for each member of a class to build the polarimeter and to make several measurements of rotation with it. (:mstruction uf the polarimetw requires only a tall 50-ml graduated cylinder (1:,-'20 cm in length), twosmall polarizing f'iltera,' somt. tape, cardboard, and gniph paper, and a cylindrical can large enough to h d d the graduated cylinder inside it. The can m i s t have a plastic topthat can he rotated while attached to the can. Containers of the type that tennis halls arr sold in w n he used. In addition, several prudurt;; that nre availahle in supermarkets-ior example, Pringle's's potato chips-are packaged in suitahle containers. The polarimeter is constructed in the following steps 1) Remove the metal bottom of the container with an ordinary can-
opener.
2) Stand the container on a piece of corrugated cardboard. Draw a circle around the container on the cardboard,and cut out this circle
with scissors. This will be the bottom piece of the palarimeter. 3) Cut holes for the polarizing filters in the centers of both the
cardboard bottom piece and the plastic tap piece. The holes should be slightly smaller than the filters. Plastic polarizing filters are mrily rut, and van be iksued to R C I ~ J m e - i n c h squaws. 11 Tnpp n polarizing tiltrr over each of rhe holes. 51 T n w the c~rdlx~ard ~.ircIrS ~ C U I C I S I C Ithe tmttom of rhr contninrr. ~ e t u r nthe plastic cap to the todaf the container. 6) Cut a thin strip of graph paper and tape it so that it runs completely around the container. Tape the strip to the container parallel to and immediately below the top piece. 'I'he strip or graph paper is 11ser1as a scale to measure d r grres of rotation. Tu calihrato the scale, mark one of thr lines on the graph paper as "llO". Any line on the graph can be used, hut it is bertrrtomakethe mark far from thetsped-together ends i ~ thr f graph paper. I'iew any light source through the pt~larimeterand rotate the top of the umtainer until the puint of maximum liaht cancelliltion is reached. With thr too in this position, mark the point on the top of the container that is directlv ahove the "0'" line on the -eraoh . oaoer. . . Rotation of pdarired light in the conmner will cause cancellation to occur u,irh this mark a h r a wint other than 0". Each spaceon the graph paper represent; a rotation of (360/n)0, with n equal to the total number of spaces on the graph paper needed t o completely circle the container. Samples t o he measured are held in a graduated cylinder placed inside the container. The tallest availahle graduated cylinder that will fit inside the container should he chosen. The taller the cylinder, the greater the path length and the rotation of the sample can he. Most containers that will he used are close t o 20 cm in length. Some 50-ml graduated cylinders fit comfortably in these containers, and provide a v
682 / Journal of Chemical Education
An Easily Constructed Student Polarimeter maximum sample path length greater than 17 cm. For most samples, a path length of at least 10 cm is desirable. The samole is first added to the cvlinder. the heieht of the sample in t i e cylinder is measured, and the'cylindeFis placed in the container bottom. As long as the container is held in an upright position, the cylinder is reasonably stable inside it. If necessary, the cylinder can he stabilized by surrounding i t it with wadded-up paper, or by cutting a cardboard circle to fit around the cylinder and inside the container. To use the polarimeter, the light source is positioned directly below it. Light enters the oolarimeter through the filter a t the bottom of tbe container, travels through the sample in the cylinder, and leaves a t the top. The emerging light is viewed from ahove, and the top is rotated from 0' towards maximum cancellation. A clockwise rotation of the top is ohserved with dextrorotatorv s a m ~ l e,sand . a counterclockwise rotation with levorotatory samples. For any sample, a complete rotation of the top will oroduce two ooints of maximum cancellation, 180' apart. If a white light source is used, then the light transmitted by the polarimeter will appear to change in color as the point of maximum cancellation is aoproached. As has been pointed out rl), this is due to the f i n that different wavelengths of liyht are rotated hy different nnrles. (:ancellation of unr area ofthe spectrum re&s in transmission of the complementary color. The color effects found with a white light source can he avoided hy using the traditional monochromatic light source for polarimetry, a sodium lamp. If this is not available, a yellow "Bug-Lite" makes an excellent, inexpensive substitute. Even an ordinary white light source is satisfactory if the point of darkest purple is measured as the cancellation ooint. This easiiy constructed polarimeter is suitaile for many experiments involving.. optical activitv. One important limi. tation tn its use is that accurate rwdts~arehest ohtained when relatively l a ~ rotations e can he ohsened. Exoerimen~sshould he designed to give rotations of at least 15', since it is difficult to use any simple polarimeter of this type to accurately measure small rotations of only a few degrees. This limitation is important because it means that at least 10 g of optically active material is usually needed for each experiment. If classes are at all large, inexpensive materials should he chosen. For this reason, sugars are probably the most suitahle compounds for experiments with this polarimeter. Sugars such as sucrose, glucose, and fructose are inexpensive, have fairly large specific rotations, and are highly soluble in water. Possible experiments using sugars include determination of specific rotations, inversion of sucrose, and observation of mutarotation. In addition to sugars, several optically active terpenes are convenient as a basis for rotation experiments. These include limonene, pinene, camphor, and menthol.
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Literature Cited
'These are available from Edmund Scientific Co., Edscorp Bldg., Barrington, N.J. 08807. We have used their 2 X 2-in. plastic filters (catalog no. P-41,168),cut into 1-in.squares.
