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Polarimetry
Polarimetry measures the extent to which a substance interacts with plane polarized light (light which consists of waves that vibrate only in one plane); whether it rotates plane polarized light to the left, to the right, or not at all. If the substance rotates plane polarized light to the left or to the right, it is called optically active. To be optically active, a compound must have a chiral center. A chiral center is a carbon that has 4 different groups attached to it. Depending on the orientation of these four different groups about the chiral carbon, the compound may rotate plane polarized light to the left or to the right. If a compound does not have a chiral center, it will not rotate light at all. The number of degrees and the direction of rotation are measured to give the observed rotation. The observed rotation must be corrected for the length of the cell used and the solution concentration. Comparing the corrected observed rotation to literature values can aid in the identification of an unknown compound. Use the following equation to correct the observed rotation:

[a]lit = [(a)obs]/(l x c)

where: a = specific rotation (degrees) (literature value), l = path length (dm), and c = concentration (g/ml).


Procedure
Click below for a movie on how to use the polarimeter. This is best viewed with sound.