Mary S. Vennor Russell Sage College Troy, New York 12180
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Construction and U S ~ S of an inexpensive Polarimeter
T h e experiment as performed by our students (15 non-science majors) was designed to illustrate the design of a polarimeter and its use in analytical chemistry. The simple and very inexpensive polarimeter made by the students permits more accurate measurements than the one designed by Shaw.' The demonstration apparatus described by Garvin2 does not involve a permanent set up and would thus not be as useful for student experiments as ours. Each student made her own instrument (Fig. 1) from a 1-qt cardboard food containera 18 em X 9 cm. 1.5-cm circular coaxial holes were cut in the lid and bottom and 2.5-cm squares of Polaroid sheet4were fixed in position over these holes with transparent tape to act as polarizer and analyzer. A circular strip of cardboard with a narrow ring of polar coordinategraph paper glued to it and marked a t 5' intervals was slid over the container and anchored close to the top with transparent tape. This was to serve as a scale. A pin was taped onto the lid so that as the lid was rotated, the end of the pin could indicate its position on the scale. Thus any angle through which the lid was rotated could be measured. One solution of known concentration (6-20% sucrose) was prepared by each student and its angle of rotation measured using standard 10.5 cm polarimeter tubes5 (three of these tubes were shared by the class). The tubes were centered in the polarimeters using pads of cotton wool. It was found that daylight or any artificial light source gave adequate results. The data obtained by all the students were used to plot a graph (Fig. 2) to test the relationship
Figure 1. Cross section of polarimekr. A, food container bottom;'B, cardboard ring; C, ring of polar coordinate graph paper; D, pin; E, lid of food container; F, Pobroid square.
(*1")an excellent graph was obtained from which individual students could tell how well their results fell in line with those of the rest of the class. The value of the specific rotation of sucrose at room temperature (22'C) was calculated from the graph to be 67" as compared with the literature value6 of 66.4" at 20°C for the sodium D lines. The students were also expected to find the concentration of an unknown sucrose solution. It is suggested that a polarimeter such as the ones used in this experiment -would be perfectly adequate to perform experiments such as "The Rate of Inversion of Sucrose"' in elementary physical chemistry or biochemistry and for identification purposes in the organic laboratory.
Where A is the angle of rotation; A', the specific rotation of sucrose; L, the length of observation tube in decimeters:. C,. the concentration of sucrose in -g/100 ml. Considering the accuracy with which A can be read
' SHAW,W ~ L L I .H. ~ MR., J. CHEM. EDUC.,32, 10 (1955).
GARVIN, JAMES E., J. CHEM.EDUC.,37, 516 (1960). Obtainable from supermarkets or delicatessens. 'Welch Scientific Co. supply 6in. squares of Polaroid sheet at $7..50 per pair. 6 If standard polarimeter tubes are not available, Shaw (see footnote 1) describes one constructed from glsss tubing, microscope cover glasses, and rubber stoppers. 6 "Handbook of Chemistry and Physics (47th ed.), Chemical Rubber Publishing Co., 1%7. ' STEINRICH, O m F., AND KING,CECILV., "Experiments in Physical Chemistry," American Book Co., 1950, p. 217. a
Yz 4
CONCENTRATION OF SUCROSE ( g p l ) Figure 2.
Dependence of angle of rotation on *veroro concentration.
Volume 46, Number
7, July 1969
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459
