polarimetre

A polarimeter is an optical instrument used to measure the angle by which optically active substances rotate plane-polarized light. The measurement is the basis of polarimetry, a technique used to determine concentration, identity, or purity of chiral compounds.

A typical polarimeter consists of a light source, a polarizer to produce plane-polarized light, a sample tube containing the liquid under test, an analyzer (a second polarizer), and a detector or readout scale. Light passes through the polarizer, through the sample where its plane of polarization is rotated, and then through the analyzer. The analyzer is rotated until the transmitted light is at a maximum or minimum, and the rotation angle is read. The observed rotation depends on the path length, the concentration, the wavelength of light, and the specific rotation of the substance.

Key concepts include the specific rotation [α], defined by [α] = α/(l c), where α is the observed rotation, l is the optical path length in decimeters, and c is the solution concentration in grams per milliliter. Polarimeters often rely on standard wavelengths, such as the sodium D line, and require controlled temperature and solvent conditions for accurate results.

Applications span chemistry and biochemistry, including determination of enantiomeric purity, concentration of optically active solutions (such as sugars and amino acids), and quality control in pharmaceutical and food industries. Modern instruments may be automatic or digital, offering improved precision and ease of use, while basic models remain common in teaching laboratories.