Enter the angle through which plane-polarized light is rotated, the mass concentration of the solutions, and the path length.

Specific Rotation Formula

The following formula is used to calculate the specific rotation.

SR = a/(y*l)

  • Where SR is the specific rotation (deg*mL/(g*dm))
  • a is the angle through which plane-polarized light is rotated (degrees)
  • y is the mass concentration (g/mL).
  • l is the path length (dm)

Specific Rotation Definition

What is specific rotation? A specific rotation is defined as “the change in orientation of monochromatic plane-polarized light, per unit distance-concentration product, as the light passes through the sample of a compound in solution.” (Source)

Example Problem

How to calculate specific rotation?

  1. First, determine the angle.

    For this problem, the angle through which the light is rotated is 25 degrees.

  2. Next, determine the mass concentration.

    In this example, the mass concentration of the compound in the solutions is 50g/mL.

  3. Next, determine the path length.

    In this example, the total length of the path is 3dm.

  4. Finally, calculate the specific rotation.

    Using the formula above, the specific rotation is found to be 25/(50*3) = .1666

  5. (deg*mL*g^-1*dm^-1)

About Specific Rotation

Is specific rotation concentration dependent? A specific rotation is dependent on the mass concentration of the compound in the solution.

Can specific rotation be negative? A specific rotation can be negative if the rotation counterclockwise is considered negative. The absolute specific rotation will always be positive.