Polarimetry ( Mr. Shaise Jacob )
POLARIMETRY
The term Polarimetry may be defined as the study of the rotation of polarised light by transparent substance.
Optical rotatory power is useful for »
Qualitative analysis
Quantitative analysis
Elucidation of chemical structures
An ordinary light usually vibrates in all the planes → UNPOLARISED LIGHT
The vibrations may all be restricted to one direction only, in the perpendicular plane → PLANE POLARIZED LIGHT
Unpolarised light gets converted into a plane-polarized light by simply passing it through a lens called a NICOL Prism. (after William Nicol – the inventor)
NICOL Prism – Iceland Spar
Calcite ( CaCO3 form )
(or) Polaroid
A plane polarized light which consists of two components of fixed magnitude rotating in opposite directions to one another;
The right circulatory polarized light
The left circulatory polarized light
Plane polarized light is the vector sum of these two components
THEORY
An optically active substance is one that rotates the plane of polarized light
(1) Lactic acid
Latin : Dexter = right
designated by ‘d’
Latin : Laevus = left
designated by ‘l’
OPTICAL ACTIVITY
When a certain organic liquids, solutions (sugar) or quartz crystals are placed in the path of plane polarized light, the plane of polarization is rotated
Magnitude of rotation depend upon
Nature of the substance
Length of liquid column
Conc. Of the solution
Nature of the solvent
Temperature of the solution
WL of the light used
The rotatory power of a given solution is generally expressed as Specific rotation
a – observed angle of rotation
L – length in decimeters
C – grams of substance in 100ml of solution
INSTRUMENTATION
Optical activity may be detected & measured by an instrument → Polarimeter
Light source – Sodium vapor lamp
Analyzer – another Nicol prism aligned to intercept the linearly polarized ray as it emerges from the sample solution
Determination of optical activity of Pharmaceutical substances
Optical rotation
Specific optical rotation
noting the angle through which the plane of polarization is rotated.
Applications
Identification
Determination of O.A compounds
Quantitative – sugar industry (sucrose)
O.A is the only one parameter, for distinguishing b/w D & L isomeric forms
SACCHARIMETRY
» important practical application of polarimetry
» determination of high conc. of sugars
» visual saccharimeters called POLAROSCOPES
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