HOWARD NECHAMKIN Pratt Institute, Brooklyn, New York
P o L m r M m w as an analytical technique is, unfortunately, rarely, if ever, used in courses prior to junioror senior-year physical chemistry. The fundamental manipulations and the theoretical background required for successful use of the polarimeter are not the basis for the existence of this situation. Colorimetric procedures, which are nomore involved than polarimetricones, are commonly employed. It appears that the reason for ignoring and neglecting polarimetry in analytical or even freshman courses is that the instruments are so expensive. The purpose of this paper is t o describe the construction and use of a crude polarimeter that would be suitable for use in these earlier courses. Construction. The apparatus uses pieces of polaroid film as the polarizer and analyzer. The material dis-
trihuted by theaters exhibiting 3-D motion pictures is well suited for this application. An ordmary microscope slide is cut in half and each half is sealed t o the ends of a condenser jacket with rubber cement (Figure 1). This serves as the polarimeter sample tube. At end A, which is the eyepiece end, a slip of polaroid film is fastened with a paper clip to filter
JOURNAL OF CHEMICAL EDUCATION
580
any light passing through A from the other end of the tube. 4 sheet of filter paper (11.5 cm.) is then mounted with rubber cement to the glass cover a t B, first having a circle cut out of it from the exact center, using a cork borer (Figure 2).
The tube is now mounted by cradling it horizontally across two buret clamps each attached to a small ring stand and pointing the setup toward a window or other convenient souree of light. A third ring stand, which carries ancther piece of polaroid film, is set up so that this second strip of polaroid film is in the direct line of sight leading from A, through the polarimeter tube, and through B to the light source. A pencil is also h e d to this ring stand so that its point just touches the edge of the filter paper on the tuhe at B. Manipulation. With the empty tube as originally assembled, the entire tube is rotated about its long axis while the operator looks through the apparatus a t the light source. Figure 3 shows the equipment ready for use. As the tube is rotated about the point of maximum extinction a chromatic phenomenon appears. The extinction point is taken as the position of the tuhe when the deepest blue shade is observed. At this time the filter paper attached to B is pressed against the pencil point to leave a mark on the paper. The entire tuhe is now removed from the supports
rigure3
carefully, so as not t o move the ring stands or to disturb the position of the filter paper or either piece of polaroid. The solution to he measured for optical rotatory power is now introduced into the tube. This is conveniently done by attaching short lengths of rubber tubing to the water inlet and outlet tubes of the condenser jacket and sucking on one tube while the other dips into the solution. When the tuhe is full, the inlet tube is pinched. The sucking tube is removed and is replaced by a rubber bulb from a medicine dropper. Another such bulb is used to replace the other length of rubber tubing. Care must be taken, of course, to exclude any air bubbles from the polarimeter tuhe. The filled tube is now replaced in the original position on the supports. -4new reading is taken by rotating the tube while looking through the eyepiece and a new pencil mark made on the filter paper. -4second sheet of filter paper, the record sheet, is now superimposed upon the one attached a t Band the pencil marks are transferred to it.
Chords
for b d ingeenter
Calculation. The center of the record sheet is found by constructing the intersection point of perpendicular bisectors of two chords drawn on the DaDer. Lines joining the center and the two recorded'points are drawn and the angle of rotation between themis measured with a protractor. A typical record sheet is shown in Figure 4. Resalts. When a tuhe length of 312 cm. was used to measure the rotation of sucrose solutions varying from 5 per cent to 12 per cent a t room temperature (ca. 25°C.) values for specific rotation ranged within 66.2+ 1.4'. These are quite consistent results, considering the crudity of the equipment. To check the validity of the data, readings were taken on these same solutions using a Kern Polarimeter and a value of 66.0' was ohtained. The precision of the apparatus may be taken to be in the order of magnitude of one degree of rotation corresoondine to a variation in s~ecificrotation values for a 5 per cent solution of about 0.65".
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