A Polarimeter Experiment for Introductory Courses Because of the eaoense of the instruments and the time reouired for most exoeriments. exoerienee with ootical rotation is eenerallv restricted.to advanced chemistrv laboratories. TGSnote descrihesk exoerikent which can h;done with an
a-D-glucose
-
P-D-glucose'
To do this the students must determine the rotation of the equilibrium mixture by plotting rotation in degrees versus concentration in glml for a series of prepared, equilibrated, aqueous solutions of D-glucose. The line is extrapolated to 1 glml, and the rotation a t that concentration determined graphically. The extrapolated value of the equilihrium mixture can then be used t o solve for the concentrations, C, and Ce, of aD-glumse and 8-D-glucose,respectively. The calculations assume that the a and 6-D-glucosedo not interact, and that amounts of other forms of glucose are negligible. The specific rotation of a-D-glucose is 112.2' and of 8-D-glucose is 18.1°; thusZ
+
(a) 0, = 112.2' C, 18.7 (1 - C,) where (a)D, = rotation ofequilibrium mixture a t 1glml and (1 - C,) = Ce. The equilibrium constant can then be calculated
The experiment was performed by a class of 59 students in an introductory organic-biochemistry course using a single
12R polarimeter. The solutions were prepared and equilibrated in advance and the students signed up for half-hour apoointments with the instrument. They were able to work as individuals and still make the half dozen measured values usuallv in less than 20 min. Inclusion of indvidual unknowns could ensure that students do individual work. The average value obtained by the students for (a)Deqwas 53', which is reasonably close t o the accepted value of 52.F0 z. The major source of error was inaccuracy in filling the tube t o a path length of 10 cm. The experiment provided experience with chemical equilihrium, optical rotation, and carbohydrate chemistry. It gave good results a t a low cost hoth in terms of time and money. Details of the experimental procedure may he obtained from the Chemistry Department, Mount Marty College, Yankton, South Dakota 57078.
' Tanret, C., Compt. Rend, 120,1060, (1895)
Bates. F. J.. et. al.. "Palarimetw. .. Saecharinetw and the Suears." - . National Bureau of Standards Circular 440. Washineton. " . 0. c., 1942, p. 128 Present address: Indiana University-Purdue University a t Fort Wayne, Fort Wayne, Indiana 46805. M u r r a y A. Gibas3 Mount Marty College Yankton, South Dakota 57078
462 / Journal of Chemical Education
