Polarimetry is a widely used method of chemical analysis that is concerned with the extent to which a beam of linearly polarized light is rotated during its transmission through a medium containing an optically active species.(1) Helpful discussions regarding polarized light may be found elsewhere.(2,3) In general, a compound is optically active if it has no plane of symmetry and is not superimposable on its mirror image. Such compounds are referred to as being "chiral". Sucrose, nicotine, and the amino acids are only a few of these substances that exhibit an optical rotary power.
The simplest polarimeter might consist of:
1). a light source -- typically set to 589 nm (the sodium "D" line)
2). a primary fixed linear polarizing lens (customarily called the "polarizer")
3). a glass sample cell (in the form of a long tube)
4). a secondary linear polarizing lens (customarily called the "analyzer") and
5). a photodetector.(4)
Biot is credited with the determination of the basic equation of polarimetry.(5,6) The specific rotation of a substance (for a given wavelength and temperature) is equivalent to the observed rotation (in degrees) divided by the pathlength of the sample cell (in decimeters) multiplied by the concentration of the sample (for a pure liquid, -density- replaces concentration). Influences of temperature, concentration, and wavelength must always be taken into consideration. If necessary, it is possible to apply corrections for each of these variables.(7) A few early contributors to our understanding of optical activity and polarimetry include: Malus, Arago, Biot, Drude, Herschel, Fresnel, and Pasteur.