Determination of Refractive Index

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The refractive index (n) of a substance with reference to air is the ratio of the sin of the angle of incidence to the sin of the angle of refraction of a beam of light passing from air into the substance. It varies with the wavelength of the light used in its measurement.

Unless otherwise specified in the individual monograph, the refractive index, n ~o is measured at 20° ± 0.5° with reference to the wavelength of the D-line of sodium (A = 589.3 nm). The temperature should be carefully adjusted and maintained since the refractive index varies significantly with temperature.

Related: Calibration of Refractometer

The Abbe refractometer is convenient for most measurements of the refractive index but other refractometers of equal or greater accuracy may be used. Commercial refractometers are normally constructed for use with white light but are calibrated to give the refractive index in terms of the D line of sodium.

The apparatus is provided with a water jacket to control the temperature of measurements. The manufacturer's instructions relating to a suitable light source should be followed subject to the directions given in the Pharmacopoeia. To achieve accuracy, the apparatus should be calibrated against distilled water which has a refractive index of 1.3325 at 25° or against the reference liquids given in the following table.