Structure in Chemistry


SC7. The polarimetry experiment

In measuring optical rotation, plane-polarized light travels down a long tube containing the sample.  If it is a liquid, the sample may be placed in the tube as a pure liquid (its is sometimes called a neat sample).  Usually, the sample is dissolved in a solvent and the resulting solution is placed in the tube.

There are important factors affecting the outcome of the experiment.


FigureSC7.1.  The effect of concentration on optical rotation.

FigureSC7.2.  The effect of path length on optical rotation.

In summary:

[a] = a / (c x l)

Problem SC7.1.  A pure sample of the naturally-occurring, chiral compound A (0.250 g) is dissolved in acetone (2.0 mL) and the solution is placed in a 0.5 dm cell.  Three polarimetry readings are recorded with the sample:  0.775o, 0.806o, 0.682o.

a) What is [a]?

b) What would be the [a] value of the opposite enantiomer?

Problem SC7.2.  A pure sample of the (+) enantiomer of compound B shows [a] = 32o.  What would be the observed a if  a solution of the sample was made by dissolving 0.150 g in 1.0 mL of dichloromethane and was then placed in a 0.5 dm cell?


This site is written and maintained by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's University (with contributions from other authors as noted).  It is freely available for educational use.

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