POLARIMETRY - Q9

9) Define specific rotation. Does it depend on the wavelength of light used and temperature of the solution?
Ans: The specific rotation of a chemical compound [α] is defined as the observed angle of optical rotation α when plane-polarized light is passed through a sample with a path length of 1 decimeter and a sample concentration of 1 gram per 1 millilitre.

where l is the path length in decimeters, and ρ is the density of the liquid in g/mL, for a sample at a temperature T (in degrees Celsius) and wavelength λ (in nanometers).

Yes, specific rotation depends on the wavelength of light used and temperature of the solution.

• As the wavelength increases, optical activity decreases.
• As the temperature increases, optical activity decreases. Temperature dependence of specific rotation is for sugar solutions as follows:

a_(t) = a_{(20 C)}[1 – 0.000471 (t – 20.0)]