ANALYTICAL EDITION
DECEMBER 15, 1940
Literature Cited (1) Hamill, W.H.. and Alicino, J. F., ISD. EXG.CHEY.,Anal. Ed., 9, 290 (1937). (2) Kiederl, J , B,, and xiederl,Victor, 461\Zicroh.Iethods of Quantitative Organic Analysis", pp. 74-5, New Tork, John Wiley &
Sons, 193s.
777
(3) Kiederl, J. B., Trautz, O., and Saschek, W., Mikrochemie, Emich Festschrift, 219 (1930). (4) Trautz, O., Mikrochemie, 9, 300 (1931). ( 5 ) TTeyland, C., "Quantitative analytische hlikromethoden der organischen Chemie in vergleichender Darstellung", pp. 94-8, Leipzig, Akademische Verlagsgesellschaft, 1931.
Adapting Polarizing Microscope for Use as a Polarimeter .4LEX.lKDER 3IARION, Queens College, Flushing, N. Y .
T
HE polarizing microscope can be adapted for use as a polarimeter by the addition of a simple analyzer constructed from a few square centimeters of Polaroid. T h e addition of such an analyzer greatly increases the versatility of the polarizing microscope, which is a more common laboratory instrument than the polarimeter. A primary advantage of the microscopic method is the small quantity of sample which will suffice to fill the specimen tube, only 150 cu. mm. being necessary. The analyzing unit consists of a metal frame which can be attached firmly to the graduated stage hy means of a knurled machine screw, ordinarily used in fastening a mechanical or Federoff stage to the instrument. The height of the frame must be selected so as barely to clear the top of the cell. The two small pieces of Polaroid are located beneath a hole dr!lled in the metal frame concentric with the optical axis of the microscope. The sections of Polaroid are cut so that their planes of polarization include an angle of approximately 5' when the segments are mounted in place with a slight overlap. The cell is essentially a length of 2-mm. glass tubing cemented in a hard-rubber rod and then fastened to a microscope slide for easy manipulation. The height is selected so that the rack and pinion gears of the microscope adjustment are engaged and allow focusing, taking into consideration the thickness of the cover glass which is on top. The frame is bent from a strip of aluminum ( 2 . 5 em. viide), so that the distance between the microscope stage and the top of the frame is 5.25 cm. This provides sufficient clearance for the cell which is 5.15 cm. overall and has an effective cell length of 5.0 cm. B 5-mm. hole is bored in the frame and the Polaroid fastened beneath.
In use, the microscope is focused upon the slightly overlapping intersection of the two pieces of Polaroid. This junction should approximately bisect the field and when the stage is rotated the mid-point of the intersection should remain in the center of the field. Coupled with the manner of mounting of the Polaroid, this procedure gives a field roughly halved, in which the intensity of the light will be uniform only a t the zero point.
A magnification of X 100 is sufficient to make the end of the tubing cover the entire field. Higher magnification serves no useful purpose, since it complicates the adjusting of the microscope and does not produce any refinement in procedure. Best results were obtained when a compromise plane of focusing was selected midway between the Polaroid and the t o p level of the liquid in the cell. This caused the junction of the polarizing films to become indistinct b u t brought the end of the cell more closely into focus. It is for this reason t h a t it is recommended t h a t the metal frame be constructed to be a s close as possible to the top of the cell, allowing foi the cover glass, and t h a t the Polaroid be mounted on the underside of the frame.
TIBLE I. RESCLTS OBTAISEDKITEX .\PPAILIT~-S Dextrose, 2ZCc Dextrose, 16.75i Dextrose. 12.5% Dextrose, 8 . 3 % Levulose, Z5CG lIaltose, 2 5 5 llaltose, 25Yc Xaltose, 12 5% Sucrose, 2.55
Calculated 6' 30' 40 25'
;:;;:
Observed R O 32' 4; 22' o:
1:
Error, 0 /C 1.8 1.1
8.5 2.3
110 30'
110 3s'
2.6
1 7 0 15' l i o 15' S o 38'
16' 19' 16' 15' 51 46' 100 13'
5 4 5 4 1 9
100 23'
2.9
Armsement of Polproid Sections
YOW
f ram
Hicroscope r%e
The solut'ion can be made u p in a 1-ml. volumetric flask by weighing out 100 to 250 mg. of the solid on the analytical balance and dissolving in sufficient water to make the proper volume. To transfer the solution, a portion is sucked up in a capillary tube, the tip of xhich is then placed in contact with the bottom of the cell, and the liquid is carefully expelled. As the cell is filled, the capillary is slowly withdrawn, always keeping the tip below the surface of the liquid. Advantage is taken of surface tension to create a hill of liquid above the top of the cell. The cover glass is slipped on top and the excess liquid absorbed by a piece of filter paper. This draws the cover glass tightly against the top of the cell and also ensures the complete filling of the cell. I t is important to obtain the volume of liquid entirely free of air bubbles; a suitable check is to hold the cell directly a t a light, when a translucent disk of light free of any dark spots will indicate complete filling. To obtain the zero point, the cell, filled with distilled water, was manipulated until it was in the field of view and the stage rotated until both halves of the field were equally extinct. Monochromatic light was obtained from a sodium vapor lamp; the intensity of this light was regulated by the substage iris until the zero point illumination was sufficient t o provide a sharp zero point. A simplification in procedure resulted when the zero point found using the water cell was identical to that determined without any cell. This suggested the feasibility of dispensing xith the cell in locating the zero point. By this method the cell need be filled only once with the unknown solution, and dilution errors 11-ill be minimized.
