Humboldt State University ® Department of Chemistry

Robert A. Paselk Scientific Instrument Museum

 
From: Hawk, Philip B. Practical Physiological Chemistry. P. Blakiston's Sons & Co., Philadelphia (1913) pp. 36-39.
 
© 1998 Richard A. Paselk
 

 
USE OF THE POLARISCOPE.
 
For a detailed description of the different forms of polariscopes, the method of manipulation and the principles involved, the student is referred to any standard text-book of physics. A brief description follows: An ordinary ray of light vibrates in every direction. If such a ray is caused to pass through a "polarizing" Nicol prism it is resolved into two rays, one of which vibrates in every direction as before and a second ray which vibrates in one plane only. This latter ray is said to be polarized. Many organic substances (sugars, proteins, etc.) have the power of twisting or rotating this plane of polarized light, the extent to which the plane is rotated depending upon the number of molecules which
the polarized light passes. Substances which possess this power are said to be "optically active." The specific rotation of a substance is the rotation expressed in degrees which is afforded by one gram of substance dissolved in I c.c. of water in a tube one decimeter in length. The specific rotation, , may be calculated by means of the following formula,

in which
D = sodium light.
= observed rotation in degrees.
p = grams of substance dissolved in I c.c. of liquid.
I = length of the tube in decimeters.
If the specific rotation has been determined and it is desired to ascertain the per cent of the substance in solution, this may be obtained by the use of the following formula,
The value of P multiplied by I00 will be the percentage of the substance in solution. An instrument by means of which the extent of the rotation may be determined is called a polariscope or polarimeter. Such an instru-

ment designed especially for the examination of sugar solutions is termed a saccharimeter or polarizing saccharimeter. The form of polariscope in Fig. 4, p. 37, consists essentially of a long barrel provided with a

Nicol prism at either end (Fig. 5, above). The solution under examination is contained in a tube which is placed between these two prisms. At the front end of the instrument is an adjusting eyepiece for focusing and a large recording disc which registers in degrees and fractions of a degree. The light is admitted into the far end of the instrument and is polarized by passing through a Nicol prism. This polarized ray then traverses the column of liquid within the tube mentioned above and if the substance is optically active the plane of the polarized ray is rotated to the right or left. Bodies rotating the ray to the right are called dextro-rotatory and those rotating it to the left levo-rotatory. Within the apparatus is a disc which is so arranged as to be without lines and uniformly light at zero. Upon placing the optically active substance in position, however, the plane of polarized light is rotated or turned and it is necessary to rotate the disc through a certain number of degrees in order to secure the normal conditions, i. e., "without lines and uniformly light." The difference between this reading and the zero is or the observed rotation in degrees. Polarizing saccharimeters are also constructed by which the percentage of sugar in solution is determined by making an observation and multiplying the value of each division on a horizontal sliding scale by the value of the division expressed in terms of dextrose. This factor may vary according to the instrument. "Optical" methods embracing the determination of the optical activity are being utilized in recent years in many "quantitative" connections. 1
 
1Aberhalden and Schmidt: "Determination of blood content by means of the optica method," Zeit. physiol. Chem. 66, 120, 1910; also C. Neuberg; "Determination of nucleic acid cleavage by polarization," Biochemische Zeitschrift, 30, 505, 1911
 


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Last modified 22 July 2000