Refractive Index Calculation Formula

Refractive index is a basic physical property of pharmaceutical materials. Refractive index measurements of liquid pharmaceuticals, solutions, solvents, and liquid raw materials are useful for the following:

  • Identifying a pure substance
  • Establishing the purity of some substances
  • Determining the concentrations of some solutions such as alcohol solutions or sugar solutions (e.g., percentage of sugar in syrup)
  • Determining the uniformity of final compounded preparations

The refractive index (n) of a substance, or medium, is defined as:


Where, c is the velocity of light in a vacuum and v is the velocity of light in the medium.

For example, the refractive index of pure water at a measurement temperature and wavelength of 20° and 589 nm is 1.3330, meaning that light travels proportionately slower in water than in a vacuum.

The refractive index of a material is dependent on the temperature and the wavelength of light used to measure it; it is important to have precise control of both of these variables to accurately measure the refractive index.

Refractive Index Measurement Procedure

Determine the refractive index of the sample per the instrument manufacturer’s instructions. Unless otherwise indicated in the monograph, the refractive index is measured at 25°, with reference to the wavelength of the sodium d-line (at 589 nm).

Reference: USP General Chapter <831>

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