A Simple Polarimeter and Experiments Utilizing an Over head Projector H. C. Dorn, H. Bell, and T. Birkett Virginia Polytechnic Institute and State University, Blacksburg, VA 24061
Polarlmeter Construction Construction of the polarimeter begins by cutting a 4-in.diameter hole in the center of an 8-in. X 11-in. piece of dark construction paper. The circular part removed from the 8-in. X 11-in. piece is then trimmed to a diameter of 3 in. In addition, a 1-in. hole is cut in the center of this piece with a notch removed a t the top. A simple protractor can be easily used
with scale marks every 5' in a positive and negative fashion for clockwise and counterclockwise rotation about 0°, respectively. The three pieces are then taped together with transparent plastic tape. Figure 1illustrates these three assembled com~onentsas ~ h o t o m a ~ h efrom d overhead orojection. A sun&ss lens is (hen Mpeh over the center hole. ?he bottom of a tall, 16 oz., waxed soft drink CUD is next removed. In addition, a toothpick or straight pin is through the side of the cup a t -% in. from the top to serve as the scale marker for the polarimeter. Finally, the cup is inverted and d a d over the too of the center hole of the construction naner described above. The second lens is then rotated above tde ;up until a null and maximum in the light intensity are observed 0 and 90°, respectively. with the scale marker positioned a t ' This is illustrated in Figures 2 and 3. When the null and maximum are found the second lens is carefully taped to the top of the cup. Solutions are placed in a 100-mlglass graduated cylinder that has been cut off to a height of 12-15 cm. Smaller graduated cylinders (e.g., 50 ml) can also be used with reduced light beam intensity. In operation, the cylinder is filled to a height of exactly 1dm and carefully placed over the center hole. In this manner, one light beam will pass through the graduated cylinder, whereas the second beam ("notched
(left) Figure 1. PhDtograph of overhead proiectian of assembled cornponems (excluding chlral solution and polaroid elements).
(left) Figure 3. Photographof overhead projection with maximum light intensity at 90" (no chlral medium).
Polarimeters have been described previously in THIS that illustrate rotation of plane-polarized light by chiral solutions for both classroom overhead demonstrations ( 1 4 ) and individual student experiments (5,6). However, the simole r certain ad. .~olarimeterdescribed in this ~. a.o e has vantages when used in conjunction with an overhead projector lieht source. Soecificallv. onlv modest ouantities of chiral ssutions are required (-5k70 A).A second advantage is easy visual interoretation of the ~olarimeterdata bv a class (ineluding degrees of rotation). The polarimeter protides a "dual beam" light source for direct com~arisonof DkUIe-DolariZed light emerging from cbiral and achiral (e.g., air) media. The polarimeter is also easily constructed from inexpensive and readily available parts. For example, the polarizing elements are simply lenses removed from polarized sunglasses. JOURNAL
( r m ) F @ m 2. Phomgaph cd mdead projectionwim null at 0' dual source (no chiral medium).
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Journal of Chemical Education
lmbeam
(righl) Figure 4. PhOtcqraph of overhead propction at O0 ~8thtransmission d ligm Deam t h w h c h i d medium [null fa achiral medium, air).
Observed Rotations Measured with the Polarlmeter for Some Typical Sugars and Terpenes Ghirai Compound
a'
(+) pulesone [alOz0+ 22' (neat), d = 0.937
+21° (+22')
(+)-apinene [aIdz2 47.1- (neat).d = 0.656 q-)-l~ctose [aIdm-12.2'. (C = 2. H20) Lh(+tglucose [ a l p + 53'. (Gt0, H20) W+)-mannose [a], + 14.8'. (C = 4. H,O)
+44O
(+42')
neat
-63-
(-59')
0.66
+
+25" (+26') +6'
neat
0.50
0.50
Sucrose
+3S0 (+36O)
0.50
[or]?
+50° (+46O)
0.75
+ 66.S0. (C = 26, H1O)
Ill W-Bd
Flgve 5 Fimograph of overnoad profectlon fan. l of cn ral mea .mat (light transrn sston for acn ra meal-m, a r ,
c2
rotations were measwed without filter and with veliow filter I I.resoec-
--+115'
portion") passes outside the cylinder and serves as a reference beam. In operation, a concentrated solution of a c h i d compound (e.g., sucrose 0.75 g/ml of water solution) is filled to a height of 1dm. At 0" only the light emerging from the notched portion (i.e.. achiral air medium) is a t a null, whereas light emer&ngfrom the chiral solutikn is clearly visible (Fig. 4).-As the cup is rotated in a clockwise direction, the light passing through the chiral medium changes from white through various spectral colors (ORD curve). At a given angle the color transmitted is the complement of the color absorbed. Since most optical activity data is referenced to the orange sodium line (5893 A), the color of deep blue should he used as the null point. This is easily observed hy progressing to the clearly visible reddish-purple color transition and then returning (counterclockwise rotation) to the deep blue color. A second method utilizes a yellow filter1 sheet placed under the construction paper and rotating the cup in a n o d manner until a null in the light intensity is observed (Fig. 5). Results are comparable by either method with an accuracy o f f 5'. Typical Demonstrations-Optical Activity of Sugars The ~olarimeterdescribed readilv allows identification of varioui sugar solutions by measuiement of the observed rotations. For example, the table contains observed rotations measured with the polarimeter for some typical sugars and terpenes. It is convenient to treat these as unknowns duringclassroom demonstrations. For example, the students directly record the observed rotations for the various
unknown solutions and are asked to calculate the specific rotations from the familiar equation(s) reproduced below:
In the equations above, c is the concentration in grams per ml of solution, d is the density, and, 1 is the pathlength (1dm). [ o l ] x T and ol are the specific and observed rotations, respectively. The unknowns can be measured at different values of 1(ex., 0.5 dm) and/or concentration to illustrate the validitv of the equation above. The various solutions can be easiG identified by the student if the specific rotations are provided as reference data. I t is also convenient to take a second solution of sucrose a t a higher concentration (e.g., C = 0.75 glml) and add -6 ml of concentrated hydrochloric acid for every 50 ml of sugar solution. In this m i n e r , the hydrolysis of this disaccharibe sugar to a mixture of fructose and glucose can be conveniently monitored in a 2&25 min time period (2.4). Specifically, the observed rotation starts a t a value of +43O and drops by -13-15' for every 5 min to a final value of -lZO. Given the choices of mannose, glucose, and fructose as possible monosaccharides formed bv hvdrolvsis of sucrose. students will readily identify the lake; twoas the correctproducts. The mutarotation of elucose is another nossible exneriment which could be monitored with the polaiimeter (7): Literature Cited
.
111 rank. FOM J and ~idua11,shamn M J C m .Eour.46s8.19d9,. ?IHdl.Jcbn W . J . CHEM E W C .50, 574 \lW?l I! P'+mand~r..lark F ..I CHKM h>W.53. TU8 l E 6
' These solutions should be prepared at least 24 h in advance of me measurement and may require heating lo achieve a clear solution.
Answers to Trivia Quizon Page 1057 (1)p(pieo, 10-9; f(femto, 10-'s);a(atto, 10-lS). (2) Margaret Thatcher. (3) Re, Rhenium, for the Rhine. (4) Eu (Europium) and Am (Americium); In (Indium) is named for a subcontinent. (5) France: Fr (Francium) and Ga (Gallium). (6) Robert Boyle. (7) Israel (Chaim Weizmann). (8) Fermi, Mendeleev, Nobel, see elements 101, 102, and 103. (9) Tom Lehrer, a
mathematics professor and satirist. (10) Co (Cobalt) and Ni (Nickel).
Volume 61 Number 12 December 1984
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