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10
2025
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07
Optical rotation determination method 0621 of the 2025 Edition of the Chinese Pharmacopoeia
When plane-polarized light passes through liquids or solutions containing certain optically active compounds, it can cause optical rotation, causing the plane of the polarized light to rotate to the left or right. The degree of rotation is called the optical rotation. At a certain wavelength and temperature, when polarized light passes through a solution containing 1g of optically active substances per 1ml and the optical path is 1dm long, the measured optical rotation is called specific rotation. Recommended brand: Shanghai Jiahang: AIpol-P630 Digital optical rotation meter. Specific rotation (or optical rotation) can be used to identify or check the purity and impurity level of optically active drugs, and can also be used to determine the content of optically active drugs.
Stereoisomers that cannot overlap in space and are mirror images of each other are called enantiomers. The enantiomers of chiral substances have the same physicochemical properties in a non-chiral environment, except that they cause the plane polarized light to deflect to the same extent but in opposite directions. Biological macromolecules such as enzymes and biological receptors are usually chiral substances and always exhibit stereoselectivity for one enantiomer. Therefore, enantiomers can differ in pharmacology and toxicology. Substances derived from nature, such as amino acids, proteins, alkaloids, antibodies, glycosides, and sugars, mostly exist in the form of enantiomers. Racemic bodies are generally composed of equal amounts of enantiomers, with a net optical rotation of zero, and their physical properties may also differ from those of their enantiomers.
The most commonly used light source is a sodium lamp in the D-line of the visible light region (589.3nm), but shorter wavelengths are also used. For instance, a photoelectric polarizer uses a filter to obtain monochromatic light with the maximum transmittance at approximately 578nm, 546nm, 436nm, 405nm, and 365nm from a mercury lamp, which has higher sensitivity. It can determine the concentration of the compound to be tested at a lower level. There are also some other light sources, such as xenon lamps or halogen tungsten lamps with appropriate filters.
Unless otherwise specified, this method uses the D-line (589.3nm) of the sodium spectrum to measure the optical rotation. The length of the measurement tube is 1dm (if other tube lengths are used, conversion should be made), and the measurement temperature is 20℃. Use a calibrated polarimeter with a reading of 0.01°.
Optical rotation determination should generally be carried out within 30 minutes after the solution is prepared. When measuring the optical rotation, rinse the test tube several times with the test solution, slowly inject an appropriate amount of the test solution (be careful not to cause bubbles), place it in the polarimeter and measure the reading. The optical rotation of the test solution is obtained. The one that rotates polarized light to the right (clockwise direction) is called right-rotating, which is indicated by the "+" symbol. The one that rotates polarized light to the left (counterclockwise direction) is left-handed, represented by the symbol "-". Read the optical rotation three times in the same way, take the average of the three times, and calculate it according to the following formula to obtain the specific rotation of the test sample.
For liquid test samples
For solid test samples
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In the formula, [α] represents specific curl;
D is the D line of the sodium spectrum.
t represents the temperature at the time of measurement, in ℃.
l represents the length of the measuring tube, dm.
α is the measured optical rotation;
d is the relative density of the liquid;
c represents the weight of the substance to be tested contained in each 100ml of the solution (calculated as dry or anhydrous substance), in grams.
The verification of a polarimeter can be carried out using a standard quartz polarimeter tube, and the reading error should comply with the regulations.
【 Notes
(1) Before each measurement, a blank correction should be made with a solvent. After the measurement, the correction should be conducted once again to determine if there is any change in the zero point during the measurement. If the optical rotation difference exceeds ±0.01 during the second calibration, it indicates that the zero point has changed, and the optical rotation should be remeasured.
(2) When preparing solutions and conducting measurements, the temperature should be adjusted to 20.0℃±0.5℃ (or the temperature specified under each variety item).
(3) The solution of the liquid or solid substance being tested should be fully dissolved, and the test solution should be clear.
(4) The optical rotation of a substance is related to factors such as the measurement light source, measurement wavelength, solvent, concentration and temperature. Therefore, when indicating the optical rotation of a substance, the measurement conditions should be specified.
(5) When it is known that the test sample has racemic effect or optical rotation conversion phenomenon, corresponding measures should be taken to specify the time for sample preparation and the interval time for filling the solution into the optical rotation tube for measurement.
Keyword:
Optical rotation
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