Optical rotation measurement of sucrose and l-menthol

Optical rotation is the property of substances, rotating the plane of polarization when linearly-polarized light passes through such substances. This is the property which occurs specifically to optical active substances in which the refractive indices of right and left circularly-polarized light are different. Optical rotation that rotate light in a clockwise direction as viewed towards the light source is defined as dextrorotation (+) and the opposite, levorotation (-).

The angle of rotated plane of polarization is called as optical rotation and polarimeter is the instrument to measure such optical rotation. Optical rotation is proportional to cell pathlength and is related to sample concentration, measurement wavelength and temperature. The specific optical rotation [α]t x is calculated from the following formula using temperature t (ºC), wavelength x (nm), cell pathlength 1 (dm), sample concentration c (g/100 ml) and measured optical rotation α.

JP, USP and EP suggest to measure optical rotation using D-line of Na lamp. Polarimeter is used for several purposes such as purity certification of sugar, verification of pharmaceuticals and optical purity determination of optical active substances obtained from asymmetric synthesis in organic chemistry field.