James E. Garvinl
Inexpensive Polarimeter
Northwestern University Medical School Chicago, Illinois
for Demonstrations and Student Use
This paper describes an easily assembled and inexpensive polarimeter and reports some results of its use in student experiments. The apparatus would appear to have three advantages: (1) all the components are easily visible making the principles of their operation almost self-evident; (2) the apparatus has a screen apparent to a group and therefore can be used to demonstrate the principles of optical rotation; and (3) measurements of optical rotation can be made simply and very rapidly and hence the apparatus may be used by students for their own experiments.
1.0-em hole drilled through the center and are rotated on a bearing made from a 100-ml beaker with the bottom cut off. In another model of this same apparatus we used an ocular and mount from a discarded microscope as the rotating analyzer assembly. The screen is made of smooth, white writing paper. A short piece of mailing tube painted black inside was glued
The Apparatus
Figure 1shows a schematic diagram of the apparatus and Figure 2 a photograph of the actual equipment. The light source is a 100-watt microscope lamp2without a filter. The flat-walled hot,tle containing dist,illed water act,s as an infrared filter. Both mirrors are flat; the lower one was taken from a microscope and mounted in a rubber stopper. The polarizer and analyzer are Polaroid mounted in green-tinted p l a s t k 3 The aperture in the diaphragm was 1.0 cm in diameter. The 100ml glass cylinder rests on a square of single strength window glass taped to a 10-cm ring. The scale and indicator are both of white cardboard. The analyzer and screen are mounted on a large cork stopper with I
United States Public Health Service Senior Research Fellow
No. 238. This contribution supported in part by RG-4734 from the National Institutes of Health. Model 370, American Optical Co., Buffalo 15, New Yark. a Polarizing Filters, American Lens and Photo Ca., 5700
Northwest Highway, Chicago 46, Illinois.
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Figure 1.
Schematic diagram of simplified polorirneter.
DIAPHRAGM
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Figure 2.
Photograph of ~ i r n p l i f i ~palarimeter. d
The app:iv;ltns \\-;is c:llil~~.atcrlhefore use with a glucose solution. This gave much more reproducible results than if wat,er alone was used. 100 ml of a 20% glucose solution was placed in a cylinder and the cylinder centered over the vertical light path above the polarizer. The poinder was now set a t the theoretically correct observed rotation for glucose. Calibration consisted simply in rotating the analyzer until t,he point of minimum illumination of the screen was reached. The apparatus was now ready for use. The theoretically correct observed rotation for glucose includes terms for the concentration of glucose and the length of the light path. For example, if the glucose solution is 0.2 gm/ml and the length of the light path is 18.5 cm, an observed rotation of 19.4' indicates that calibration is correct. White light gives more accurate results with this instrument than monochromatic light. This is due to an easily observable red-green color change which occurs as one passes through the point of minimum light intensity when white light is used. A similar color change has been observed by Spear.& SPEAR, C. S., J. CHEM.EDUC.,37, 203 (1960). Volume 37, Number 7 0, October 1960
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51 5
I t can be seen in Figure 2 that dextrorotatory measurements are indicated by a counter-clockwise movement of t>heanalyzer. This anomaly is due to the effect of int,erposing a mirror in the beam of polarized light. This is the reverse of t,he direction of rotat,ion of the analyzer for dextrorotatory measurements observed in a conventional polarimeter using paired Nicol prisms. Excluding the cost of the microscope light the entire apparatus was constructed a t a cost. of less than $1.50 using, of course, the stands, rings, and clamps from t,he st,ockroom. Student Experiments
We have used this simplified polarimeter for two years in the double role of demonstration apparatus and instrument for use by inexperienced st,udents. At the beginning of the lahoratory period a brief review of the principles of optical rotaation was given, accompanied by demonstrations using the apparatus. In a darkened room about 30 students can easily see the changes in light intenbity on the paper screen. During the laboratory period each student was required to identify two unknowns from measurements made individually on the simplified polarimeter. This involved measuring the light path and observed rotati011 for each unknown. From these data the specific rotat,ions were calculated and the unknowns identified from
516
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Journal o f Chemical Education
Table 1. Specific Rotations Obtained by Students with the Simplified Polarimeter Compared with Results Obtained with a Standard Polarimeter
Sugar Sucrose
Specific Rotations U ~ i n gsimplifipd Using standard polarimeter" polarimeter" 70 1 4 2 68 8
a posted list of known specific rotations. The specific rotations reported by GO students are shown in Table 1. The averages compare favorably with the results obtained by the author using a standard polarimeter. Triplicate readings were made by each student in about one minute so that all students in a section of 30 easily read two unknowns in three hours. Changeover from one solution to another was made by replacing the cylinder on the glass plate. The author has also used this simplified polarimeter to follow the hydrolysis of sucrose by 1 N HC1, although we have not tried such an experiment with students. Since the velocity constant varied only slightly in the range from 20-60 min., the apparatus seems adaptable to this kind of quantitative student experiment. It, is felt that the simplicity of the apparatus, its usefulness in demonstrations, and its convenience more than compensate for its relatively lower accuracy.
