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 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 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 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:
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.