-
- 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
-
- © R. Paselk
- Last modified 22 July 2000
