Optical Rotation - Optika Italy POL-X Manual De Instrucciones

Optical rotation

A polarimeter is used to measure the optical rotation of plane polarized light when illuminating an optically acti-
ve substance. The principle can be explained by the following figure.
AA' (shown in fig. 4) indicates the vibration direction of the polarization analyzer and is perpendicular to the
centreline OX. OP and OP' indicate the vibration directions of the polarized lights of the two halves of the
viewing field.
When the light beam passes through the optically active substance, the plane of polarization is rotated by an
angle a, as shown by the dotted line in fig. 4.
Now, the projections of the polarized lights of the two halves on AA' are not identical, the right half is bright,
and the left half is dark. If rotating the polarization plane AA' of the analyzer by the same angle a in the same
direction, the illumination intensity of the viewing field can be made equal again.
At this time, the rotated angle of the polarization analyzer is equal to the optical rotation of the substance.
Knowing the rotated angle (i.e. the optical rotation), the length of solution column (i.e. the length of the test
tube) and the concentration, the specific optical rotation (i.e. the optical rotatory power or the specific rotation)
of the substance can be calculated in accordance with the following formula:
Where,
a - the rotation angle (optical rotation) measured by use of light, l, when the temperature is t.
L - the length of the solution column (test tube), using decimeter (dm) as the unit of measurement.
C - concentration, i.e. the gram quantity of the solute in 100 mm of solution.
From the above formula we can see that the rotation angle a is in direct proportion to the solution column (the
tube) length L and the concentration C,
i.e.
The optical rotation a is also related to the temperature. In the case of a wavelength of l= 589.3 nm (sodium
light), as used for most substances, the optical rotation will be reduced by 0.3% when the temperature is incre-
ased by 1°C. For measurements with high requirements of accuracy, it is thus recommended to work under a
temperature of 20 ± 2°C.
X
P
a
A
A
0
Fig.4
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