Polarimetry

Chiral substances may rotate the plane of polarized light, when it is transmitted. In which way do measurement technology use this property and what are the underlying physical fundamentals? And how does a polarimeter detect this angle of rotation and determine the concentration or purity of a substance? Here you can learn more: In which ways do polarimeters use automated applications for measurement procedures in compliance with standards? And what kind of samples can be measured? Which instrument fits to the required area of application?

Polarimetry

Optical activity / Chirality / Enantiomers

Optical activity describes the ability of certain substances 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.

Polarimetry

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.

Polarimetry

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.

Polarimetry

Specific rotation angle

The “specific rotation” is a substance specific property. At a given wavelength of light λ and temperature t, that is a constant for each optically active substance. With known concentration c and length of the sample l, then the so-called specific rotation angle of a substance at the wavelength of light λ and temperature t, can be determined.

Polarimetry specific rotation angle

The specific rotation results, in a very simple mathematical way, from the reference values of the optical rotation – meaning the actual measured value – and the tube length and the concentration of the sample. If the angle of rotation α is measured for a solution, then is:

Polarimetry formula specific rotation angle

The specific rotation angle of a sample should not be mistaken or put in the same way as the optical rotation angle. For better comparability, specific rotation is always given in the literature with reference to the standard conditions for the wavelength of light (λ = 589nm) and temperature (t = 20.0°C).

Polarimetry

Quartz

Quartz crystals are very stable optically active substances. Different to many liquids and mixtures, their rotating properties do not change much over time.
This makes them to be used as ideal reference materials. The so-called quartz control plates – i.e. tubes with a fine quartz disc in their inside – have precisely determined rotation angles and therefore they are unsurpassed tools for the 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?

Standards and directives

Polarimeters are designed and used in accordance with standards. Those who use polarimeters according to normative specifications can guarantee that measurements are correct and work according to reproducible procedures. We have compiled an overview of standards and guidelines that refer to polarimeters.

Polarimeter standards and directives

Samples and measurement values

In many areas polarimeters have become an essential measuring device. Their measurement results are used for quality control or to determine the identity and quality of substances and the concentration in mixtures. But which substances can be detected at all? We have compiled a comprehensive overview of samples with values of the specific angle of rotation.

Samples and measurement values

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

Any Questions?

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