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4.8 DETERMINATION OF OPTICAL ROTATION AND SPECIFIC ROTATION

      Many drugs, in pure state or in solution, possess the inherent property of rotating the plane of incident polarized light; this property is called optical activity. The measurement of optical activity is used for Pharmacopoeial purposes mainly to establish the identity of the substance. It may also be employed to test the purity of the substance (absence of optically non-active foreign substances) and as an assay procedure.

      Optical rotation The optical rotation is the angle through which the plane of polarization is rotated when polarized light passes through a layer of a liquid. Substances are described as dextrorotatory or levorotatory according to whether the plane of polarization is rotated clockwise or counter-clockwise, respectively, as determined by viewing towards the light source. Dextrorotation is designated (+) and levorotation is designated (–).

      The optical rotation, unless otherwise specified, is measured at the wavelength of the sodium D line (589.3 nm) at 25º on a layer 1 dm thick.

      Specific rotation (specific optical rotation) The specific rotation of a liquid substance is the angle of rotation, usually measured at the wavelength of the sodium D line at 25º, unless otherwise specified, calculated with reference to a layer 1 dm thick, and divided by the specific gravity (relative density) measured at the same temperature.

      The specific rotation of a solid substance is the angle of rotation, usually measured at the wavelength of sodium D line at 25º, unless otherwise specified, and calculated with reference to a layer 1 dm thick of a solution containing 1 g of the substance per ml.

Apparatus

      Optical rotation is measured with a polarimeter. Generally, a polarimeter accurate to 0.05º of angular rotation, and capable of being read with the same precision, suffices for Pharmacopoeial purposes; in some cases, a polarimeter accurate to 0.01º of angular rotation, and read with comparable precision, may be required.

      Polarimeters for visual measurement: commercial instruments are normally constructed for use with a sodium or mercury vapour lamp or xenon.

      Photoelectric polarimeters: where it is directed in the individual monograph to determine the optical rotation photoelectrically, use a photoelectric polarimeter capable of an accuracy of at least 0.01º.

Measurement of Optical Rotation

      The accuracy and precision of optical rotatory measurements will be increased if they are carried out with due regard for the following general considerations.

      Optical elements of the instrument must be brilliantly clean and in exact alignment. The match point should lie close to the normal zero mark. The light source should be rigidly set and well aligned with respect to the optical bench. It should be supplemented by a filtering system capable of transmitting light of a sufficiently monochromatic nature. Precision polarimeters generally are designed to accommodate interchangeable discs to isolate the D line from sodium light or the 546.1-nm line from the mercury spectrum. With polarimeters not thus designed, cells containing suitably coloured liquids may be employed as filters.

      Observations should be accurate and reproducible to the extent that differences between replicates, or between observed and true values of rotation (the latter value having been established by calibration of the polarimeter scale with suitable standards), shall not exceed one-fourth of the range given in the individual monograph for the rotation of the substance being tested.

      Polarimeter tubes should be filled in such a way as to avoid creating or leaving air bubbles that interfere with the passage of the beam of light. Interference from bubbles is minimized with tubes in which the bore is expanded at one end. However, with tubes of uniform bore, such as semimicro or micro tubes, care is required for proper filling.

      In closing tubes having removable end-plates fitted with gaskets and caps, the latter should be tightened only enough to ensure a leak-proof seal between the end-plate and the body of the tube. Excessive pressure on the end-plate may set up strains that result in interference with the measurement. In determining the optical rotation of a substance of low rotatory power, it is desirable to loosen the caps and tighten them again between successive readings in the measurement of both the rotation and the zero point. Differences arising from end-plate strain thus generally will be revealed, and appropriate adjustments to eliminate the cause may be made. 

Procedure

      Determine the zero point of the polarimeter with the tube empty and closed for liquid substances and filled with the specified solvent for solutions of solid substances.

      Where the substance is a solid, accurately weigh a suitable portion and transfer to a volumetric flask by means of water, or other solvent if specified in the monograph, reserving a portion of the solvent for the blank determination. Add enough solvent to bring the meniscus close to but still below the mark, and adjust the flask contents to 25º by suspending the flask in a constant-temperature bath. Add solvent to the mark, and mix. Transfer the solution to the polarimeter tube, preferably within 30 minutes from the time the substance was dissolved, taking care to standardize the elapsed time in the case of substances known to undergo racemization or mutarotation. During the elapsed time interval, maintain the solution at a temperature of 25º.

      Where the substance is a liquid, adjust its temperature to 25º, and transfer directly to the polarimeter tube.

      When a polarimeter is used for visual measurement, make at least 6 readings, both on blank and sample, at 25º, of the observed rotation. Take half the readings in a clockwise and the other half in a counter-clockwise direction. The zero correction is the average of the blank readings, and is subtracted from the average observed rotation. It is necessary in this calculation to use the observed signs of rotation, whether positive or negative, to give the corrected observed rotation.

      When a photoelectric polarimeter is used, a smaller number of readings are required, depending on the type of instrument.