Certain substances turn the plane of polarised light. How can a polarimeter measure the angle of rotation or determine the concentration of the substance? How are polarimeters and their automated applications used for standards-compliant measurement methods?


Optical activity

Optical activity describes the ability of certain substance to rotate the plane of polarized light as it passes through. There is a distinction between clockwise – and counter-clockwise substances.



Chirality – chiral substances have molecules which can take up different spatial arrangements (configuration isomerism). Optical activity is a property unique to chiral substances, for example 2-butanol, which exists as two mirror-image isomers, or enantiomers.



Enantiomers are chiral molecules. They are non-superimposable mirror images of each other. Enatiomers are attached in the exact opposite order of each other. All compounds having such a structure are optically active.


Polarisation of an electromagnetic wave

Polarisation of an electromagnetic wave describes the direction of its oscillation. A wave packet consisting of many electromagnetic waves is normally unpolarised. If waves have only one plane of vibration, they are called polarised. The human eye cannot distinguish polarised light from unpolarised.


Optical rotation

Optical rotation is the angle through which the plane of polarisation is rotated when polarised light passes through a chiral substance. It can be determined experimentally with a polarimeter.


Specific rotation angle

The specific rotation angle is determined as follows: At known concentration c and length of the sample l, the so-called specific rotation angle

Polarimetry specific rotation angle

of a substance at the wavelength of the light λ and the temperature t can be determined. If one measures the angle of rotation α for this solution, then:

Polarimetry formula specific rotation angle

By means of this once determined specific rotation angle, the unknown concentration of a substance in a sample can now be calculated by measuring the angle of rotation in a polarimeter.



Quartz is one of the most stable optically active substances. Quartz control plates have very well-defined angles of rotation and are often used for calibration and adjustment of polarimeters.

What is a polarimeter?

Polarimetry will provide information about the molecular structure, the concentration of the substance and sometimes information about the solvent used. The optical activity is also influenced by temperature, the wavelength of the light, and the length of the optical path - the longer the path of the light through an optically active substance, the larger the angle of rotation. These variables may be used to control the sensitivity of the instrument.

How does a polarimeter actually work?

Polarimeter applications

The angle of rotation allows you to ascertain the identity and quality of substances as well as their concentration in mix tures. It can also indicate the progress of reactions and conversions. Polarimeters are used in a wide range of applications – from the determination of the purity and concentration of ngredients in pharmaceuticals to the maturity testing of agri cultural products to the measurement of the sugar con tent in beverages and candies.

Typical applications of polarimeters

Automation with polarimeters

Maximum efficiency and increased time savings through automation options - with our polarimeters P8000-TF and P8100-TF with their temperature-controllable measurement tubes, the entire measuring process can be carried out semi-automatically or fully automatically and without manual intervention, from sample supply to cleaning to drying.

More about automation

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