Determination of Refractive Index of Pharmaceutical Substances

DETERMINATION OF REFRACTIVE INDEX OF PHARMACEUTICAL SUBSTANCES

A large number of pharmaceutical substances such as volatile oils, namely : peppermint oil, lemon oil, aniseed oil have a definite range of refractive index. Based on this physical characteristic it is possible to ascertain the purity of this volatile oil precisely and accurately.

Materials Required : 

Abbe’ refractometer, volatile oil, xylene, capillary tubes ;

Procedure : 

In order to obtain precise and accurate measurements the prism case of Abbe’ refractometer is attached to a thermostat bath whose temperature is previously maintained at 25°C. Open the prism box gently and place a few drops of pure volatile oil on the lower prism with the help of a capillary tube and finally close the box. The mirrors are duly adjusted so as to obtain a bright illumination of the field of view. The knurled knob is turned gradually until the field of view displays a dark and light zone. In case, a col-oured-fringe is observed between the two zones it becomes necessary to adjust the Amici prisms carefully to achieve a sharp and black boundary. It is important to adjust this on the cross hair and finally the reading of refractive index is noted. After use, the prism box is opened and cleaned thoroughly with a lens cleansing tissue moistened with xylene/acetone. Thus, the refractive index, n_D²⁵ for certain volatile oils as per BP (1993) are as follows :

Peppermint oil     :         1.460—1.467

Lemon oil   :         1.474—1.476

Aniseed oil  :         1.553—1.560

Clove oil     :         1.528—1.537

Dill oil        :         1.481—1492

Eucalyptus oil      :         1.458—1.470