Polarimetry is the technique of analysis that is based on the ability of a specified compound in solution to rotate a plane polarized light to an assured direction. A normal light beam consists of waves oscillating in random directions. Each of the vibrational directions can be determined into 2 equally perpendicular directions. But the real vibrational direction is the vector sum of the two components.
If the ordinary light is collimated and passed through a crystal which allows only one vibrational orientation to be transmitted, the transmitted light is said to be plane polarized. Many natural crystals generate polarized light, but it is most conveniently attained through commercially accessible Polaroid materials.
An instrument utilized in measuring the optical rotation of a material in solution is termed a polarimeter and the phenomenon, polarimetry.
Basic Principle of Polarimetry
Several compounds in solution have the power of rotating the plane polarized light to an indeed direction. These compounds are said to be optically active and, in the case of organic compounds, are found to have one or more asymmetric carbon atoms inside the molecule. An asymmetric carbon atom is one by 4 different groups connected to it. The degree or direction of the rotation is either to the right (termed dextrorotatory, +) or to the left (described laevorotatory, -).
The extent of the degree of rotation of the plane polarized light is dependent on the:-
The wavelength of the light generally employed is that of sodium D line (589.3 nm), and the temperature is kept steady at 20 of 250C inside the instrument.
Mode of Operation of a Polarimeter
Before operation, the analyzer prism of the polarimeter is turned to build both halves of the field of examination uniformly bright; the circular scale reading for this position of the analyzer prism is reminded as its zero position. The polarimeter tube is then sealed through the experimental solution, the jacket around the tube is filled by water at a specified temperature and the solution is permitted to stand in the tube for several times to achieve that temperature. On passing the monochromatic light once more through the polarimeter, the 2 halves of the field of vision now look unequally illuminated due to the rotation of the plane polarized light via the solution in the tube. The analyzer prism is resolved until both halves look uniformly bright once more. The difference in the circular scale reading between the final position of the analyzer prism and its zero position recorded at 1st provides the angle of rotation of the polarized light.
Optical rotation is generally calculated through a polarimeter or polariscope and the phenomenon is identified Polarimetry. The polarimeter consists of a long hollow barrel carrying at its reverse ends, a fixed polarizer Nicol prism and a rotating analyzer Nicol prism correspondingly. A hollow polarimeter tube, encircled through a jacket, can be situated within the barrel between the 2 prisms. This tube of a specified length is intended for carrying the solution under investigation whilst the jacket around it might be sealed through water at a specified temperature. Between the polarizer prism and the polarimeter tube, a quartz plate is located for obscuring half the field of vision. Beyond the analyzer prism are situated the telescopic eyepiece lenses. That end of the polarimeter barrel carries a fixed circular scale regulated in degrees radian, the position of the analyzer prism might be read on this scale. A schematic diagram of a polarimeter is specified below:
Fig: A Polarimeter
There are 3 necessary parts:
Uses of Polarimeter
Below are several of utilizes of the instrument:
[α]D25 = α / l*C
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