20.2 Polarimetry properties and terminology
Description
This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
20.2 Polarimetry properties and terminology
Supplied by: Vince Hamner <vinny@vt.edu>
Polarimetry is a method of chemical analysis that is concerned
with the extent to which a beam of linearly polarised light is rotated
during its transmission through a medium containing an optically active
species.[2] Helpful discussions regarding polarised light may be found
elsewhere.[3,4] 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.
A simple polarimeter instrument would consist of:
1). a light source -- typically set to 589 nm (the sodium "D" line)
2). a primary fixed linear polarising lens (customarily called the
"polariser")
3). a glass sample cell (in the form of a long tube)
4). a secondary linear polarising lens (customarily called the
"analyser") and
5). a photodetector.[5]
Biot is credited with the determination of the basic equation
of polarimetry.[6,7] The specific rotation of a substance (at a given
wavelength and temperature) is equivalent to the observed rotation (in
degrees) divided by the path length 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.[8] A few early contributors to our understanding of optical
activity and polarimetry include: Malus, Arago, Biot, Drude, Herschel,
Fresnel, and Pasteur.
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