Polarimetry

This principle can then be used to determine if a

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Unformatted text preview: s seen, the "slits" are aligned. This principle can then be used to determine if a compound rotates the plane of polarized light and the sign of that rotation. If a solution of the compound is placed between the two Nicol prisms, the plane in which the light vibrates will be changed if the compound is optically active and unchanged if the compound is not optically active. If the two prisms are aligned before the solution is placed between them, light will pass through both prisms; when the solution of an optically active compound is inserted between them, the light will dim or be completely absorbed. If the second prism is rotated, the light will gradually brighten until the second prism is aligned with the new plane of vibration. If the number of degrees of this rotation from the original position and the direction of the rotation is noted, the specific rotation of the compound can be calculated. Rotation in a clockwise direction is considered to be a positive rotation (dextrorotatory, designated as + or d), while rotation in the counterclockwise is considered to be a negative rotation (levorotatory, designated as - or l). This is the basis of the instrument called a polarimeter (A drawing of one type of polarimeter, called a Zeiss polarimeter is shown in Figure 1.). In this instrument (Figure 2), a source of monochromatic light, usually a sodium lamp emitting light at the sodium D line (wavelength = 5893 angstroms) is placed behind a N...
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