Polarimetern

Polarimeters are instruments designed to measure the polarization properties of light, most commonly the rotation of the plane of polarization caused by optically active substances. In chemical analysis, polarimetry is used to identify and quantify chiral compounds, since many organic molecules rotate plane-polarized light by characteristic angles. Polarimeters can also be used to study the polarization state of light in physics and materials science.

Principle: A light source emits polarized light that passes through a sample contained in a tube between a polarizer (the analyser) and an analyzer. If the sample is optically active, the plane of polarization rotates by an angle α. The angle is measured by rotating the analyzer until the transmitted intensity reaches a minimum (extinction), or by detecting the intensity with a fixed analyzer. The observed rotation is related to the specific rotation [α], the sample length l, and the concentration c by α = [α] l c (for solutions at a given wavelength and temperature). Measurements are typically performed at standard wavelengths such as the sodium D line.

Types and components: A polarimeter typically includes a stable light source, a polarizer, a sample tube, an analyzer, and a detector. Simple manual devices rely on observer-dial extinction; modern instruments are automated, with photodetectors or spectrometric detectors and microprocessors. Other devices called spectropolarimeters or ellipsometers measure wavelength- or angle-dependent polarization properties using birefringent elements like Wollaston prisms, photoelastic modulators, or rotating polarizers.

Applications: Polarimeters are widely used to determine concentration and purity of optically active substances in chemistry, including sugars, amino acids, and pharmaceuticals. They are used in quality control, fermentation monitoring, and enantiomeric excess determinations, as well as in academic research on chiral media and materials science.