Polarimetry

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Examples for the specification of the rotation value α of a chiral substance including the measurement conditions, above: positive sign, 25 ° C measurement temperature, yellow sodium light ( sodium D-line ), concentration 2 g of substance dissolved in 100 ml in water, below: negative sign, 20 ° C measuring temperature, 589 nm wavelength , concentration 4 g substance dissolved in 100 ml in chloroform
Historical polarimeter

The polarimetry studies the polarization of transverse waves , mostly of light waves.

Applications

Calcium nitrate crystal in the polarizing microscope

A polarimeter determines the angle of rotation of homogeneous bodies, for example sugar solutions ( saccharimeters ). Polarimetry is a method for quantifying the optical activity of substances or solutions that are able to rotate the plane of oscillation of linearly polarized light. The rotation of the plane of polarization of linearly polarized light when passing through these substances or solutions is measured. The specific rotation, which indicates the angle of rotation at a layer depth of 10 cm of a liquid substance or a solution, calculated at a concentration of 1 g · 1 m l −1 , serves as a characteristic measured variable for the rotational ability of a sample . The specific rotation depends not only on the chirality of the molecule but also on the following quantities:

  • Wavelength of polarized light
  • Type and purity of any solvent used
  • Concentration of the solution
  • Measurement temperature (in ° C ).

When specifying the specific rotation α, i.e. the rotation value, the measurement conditions must always be given in addition to the sign (“+” or “-”); only then is the measured variable clearly defined.

Pharmacy and chemistry

The specific rotation is often used in pharmacy and chemistry for the identification and purity control of chiral substances. The specification of the specific rotation for natural substances , such as amino acids , terpenes and sugars , is particularly important , since the majority of these substances are optically active. For chiral medicinal substances , the pharmacopoeias give tolerances for the specific rotation and the measurement conditions (including wavelength and measurement temperature). Example of such a tolerance requirement for the drug penicillamine from the European Pharmacopoeia: "The specific rotation must be between - 61.0 and - 65.0 °, calculated on the dried substance ."

In some cases, polarimetry is also used in pharmacy to determine the shelf life of chiral drugs.

Sugar industry

In the sugar industry , polarimetry is often used to determine the concentration of aqueous sugar solutions; sometimes the measurement is carried out continuously with a flow polarimeter. The polarimeter is sometimes also called saccharimeter in the sugar industry .

Imaging methods are, for example, the polariscope or the polarizing microscope (see photo), which are also used in voltage optics .

Individual evidence

  1. ^ Herbert Feltkamp, ​​Peter Fuchs, Heinz Sucker (editors): Pharmaceutical Quality Control , Georg Thieme Verlag, 1983, pp. 249-251, ISBN 3-13-611501-5 .
  2. ↑ a b European Pharmacopoeia , 3rd edition, 1997, official German edition, Deutscher Apotheker Verlag Stuttgart, pp. 1450–1452, ISBN 3-7692-2186-9 .
  3. ↑ Organikum , Wiley-VCH Verlag GmbH, 23rd edition, 2009, pp. 82-83, ISBN 978-3-527-32292-3 .