Measurement of Optical Activity

(i.e. the angle of rotation). The instrument used for the measurement of the amount of rotation (in degrees) caused by an optically active substance the plane of the polarized light is called a Polari meter*. The principle of this instrument has already been discussed. It consists essentially of two Nicola prisms one called the polarizer and the other called the analyzer. Both of these are fitted not he opposite ends of the same metal tube. However, the polarizer is fixed but the analyzer can be rotated about the axis of the tube. In between there polarizer and analyzer, there is space for keeping the glass tube containing the solution. The glass tube is usually 5cm, 10cm, 20cm or 25cm and has glass widows on both ends. A circular scale is fixed on the metal tube near the analyzer, as shown in fig.

        
In the procedure, first of all monochromatic light from a suitable source (usually sodium vapor lamp) is passed through the instrument, without placing the tube containing the liquid or the solution. The light emerging out of the analyzer is viewed thought he eye-piece. The analyzer is then rotated slowly till the field of view is completely dark. This gives the initial reading of the instrument. Now the tube containing the liquid or the solution (of known concentration) is placed and the shutter closed (not shown in fig.).On sealing through the eye-piece he field of view is no linger found to be dark. The alter is rotated slowly till the field of view is completely dark again. This gives the final reading. The difference between eh final reading and the initial reading gives the angle of rotation (). If it comes out to be positive the substance is dextrorotatory and if negative, it is laevorotatory.

Modern instruments have numerous improvements over the simple type .However; the basic principle remains the same.

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