Enter the specific rotation of a sample and the specific rotation of a pure enantiomer into the calculator to determine the optical purity (often reported as enantiomeric excess, ee). For this ratio method to be valid, the rotations must be measured under the same conditions (same solvent, temperature, wavelength, and concentration/path length conventions), and the sample should not contain other optically active components.
- Enantiomeric Excess Calculator
- Concentration from Absorbance Calculator
- Percent Transmittance to Absorbance Calculator
- Molarity Calculator
- All Chemistry Calculators
Optical Purity Formula
The following equation is used to calculate the Optical Purity (often numerically equal to enantiomeric excess, ee, for a mixture of only two enantiomers).
%OP = 100 * |SRS/SRPE|
- Where %OP is the percent optical purity (a percent value, typically reported as a positive number)
- SRS is the specific rotation of the sample, [α]sample (measured under the same conditions as the pure value)
- SRPE is the specific rotation of the pure enantiomer, [α]pure (same solvent, temperature, wavelength, etc.)
To calculate the optical purity, divide the specific rotation of the sample by the specific rotation of the pure enantiomer, take the absolute value to get a percent, then multiply by 100. The sign of the ratio (positive or negative) indicates whether the sample rotates in the same direction as the chosen pure enantiomer.
What is Optical Purity?
Definition:
Optical purity is a measure of how enriched a chiral sample is in one enantiomer relative to the other (compared to a racemic 50:50 mixture). It is commonly reported as enantiomeric excess (ee): %ee = |%major − %minor| = 100 × |(n1 − n2)/(n1 + n2)|. When only the two enantiomers contribute to optical rotation and measurements are made under the same conditions, %OP can be obtained from the ratio of specific rotations.
How to Calculate Optical Purity?
Example Problem:
The following example outlines the steps and information needed to calculate Optical Purity.
First, determine the specific rotation of the sample. In this example, the specific rotation of the sample is measured to be 50.
Next, determine the specific rotation of the pure enantiomer (measured under the same conditions). In this example the specific rotation of the pure enantiomer is 75.
Finally, calculate the optical purity using the formula above (taking the absolute value for the percent):
%OP = 100 * |SRS/SRPE|
%OP = 100 * |50/75|
%OP = 66.67%
