polarisimetre

A polarisimetre is an instrument used to measure the angle by which polarized light is rotated when it passes through an optically active substance. The measurement, called optical rotation, is a common method for characterizing chiral compounds and for determining the concentration or enantiomeric composition of solutions.

Principle and operation: A light source, often emitting a single wavelength such as the sodium D line (589 nm), produces plane-polarized light with a polarizer. This light passes through a sample contained in a transparent tube. An analyzer, another polarizer, is rotated until the transmitted light reaches a maximum or minimum intensity. The resulting rotation angle is recorded as the observed rotation. By performing measurements at a specified wavelength, temperature, and solvent, the sample’s optical activity can be quantified.

Specific rotation and calculations: The property used to compare substances is the specific rotation [α], which depends on wavelength, temperature, and solvent. It is related to the observed rotation by [α] = α / (l · c), where α is the observed rotation in degrees, l is the path length in decimeters, and c is the concentration in grams per milliliter. Standard conditions often specify the wavelength and temperature for reporting [α].

Applications and history: Polarimetry is widely used in chemistry, pharmacology, and the food industry to determine enantiomeric purity, assess concentration, or confirm compound identity. The method traces back to early 19th-century studies of optical activity by Biot, with later developments in polarimetry contributing to enantioselective analysis.

Limitations: Accurate results require transparent samples, known concentration, and careful control of wavelength and temperature. Mixtures containing more than one optically active species can complicate interpretation.

Modern polarimeters include digital readouts and automated controls, enabling rapid, routine analysis in labs and process streams.