Determination of Specific optical rotation

 Determination of Specific optical rotation

Equipment : Polarimeter


( ά  ) is the property shown by certain substances
of rotating the plain of polarization of polarized light. Such substances are
said to be optically active in the sense that they cause incident polarized
light to emerge in a plane forming a measurable angle with a plane of the
incident light.

The optical rotation of a substance is the angle through
which a plane of polarization is rotated when polarized light passes through
the substance.

Substances are described as Dextro-rotatory or
Leavo-rotatory according to the plane of polarization, clockwise or

The optical rotation,
unless otherwise specified, is measured as the wave length of D line of sodium
(λ=589.3nm) measured at 250C., on a layer of 1-dm thick. It is
expressed in degrees.                                                                       


Weigh accurately a
suitable quantity of the substance being examined to obtain the solution of the
strength specified in the individual monograph and transfer to a volumetric
flask by means of water or other solvent, if specified. If a solvent is used,
reserve a portion of it for the blank determination. Unless otherwise
specified, adjust the contents of the flask to 250 by suspending the
flask in a constant temperature bath. Make up to the volume with the solvent at
250 and mix well. Transfer the solution to the polarimeter tube with
in thirty minutes from the time the substance was dissolved and during this
time interval, maintain the temp. at 250C.

Determine the zero point of the polarimeter and then
make five readings of the observed rotation of the test solution at 250.
Take equal number of readings in the same tube with the solvent in place of the
test solution. The zero correction is the average of the blank readings, and is
subtracted from the average observed rotation if the two figures are of the
same sign or added if they are opposite in sign to obtain the corrected
observed rotation.

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