Optical rotation, 'a' is the property shown by certain substances of rotating the plane of polarisation of polarised light. Such substances are said to be optically active in the sense that they cause incident polarised light to emerge in a plane forming a measurable angle with a plane of the incident light. Where this effect is large enough for measurement, it may serve as the basis for identifying or assaying a substance. The optical rotation of a substance is the angle through which the plane of polarisation is rotated when polarised light passes through the substance, if liquid, or a solution of the substance.
Substances are described as dextro-rotatory or laevo-rotatory according to whether [a]~ the plane of polarisation is rotated clockwise or anticlockwise, respectively as determined by viewing towards the light source. Dextro-rotation is designated (+) and laevo-rotation is designated (-).The optical rotation, unless otherwise specified, is measured at the wavelength of the D line of sodium (A. = 589.3 nm) at 25°, on a layer 1 dm length. It is expressed in degrees.
The specific optical rotation, [a]~ ' of a liquid substance is the angle of rotation, 'a', of the plane of polarisation at the wavelength of the D line of sodium (A. = 589.3 nm) measured at 25°, unless otherwise specified, calculated with reference to a 1-dm thick layer of the liquid, and divided by the specific gravity at 25°.
The specific optical rotation, [a]~ ' of a solid substance is the angle of rotation, 'a', of the plane of polarisation at the wavelength of the D line of sodium (A. = 589.3 nm) measured at 25°, unless otherwise specified, calculated with reference to a 1-dm thick layer of a solution containing 1 g of the substance per ml. The specific optical rotation of a solid is always expressed with reference to a given solvent and concentration.
ApparatusA commercial instrument constructed for use with a sodium lamp and capable of giving readings to the nearest 0.02° is suitable for most purposes. For certain applications, the use of a photoelectric polarimeter capable of taking measurements at the specific wavelengths may be necessary.
The accuracy and precision of optical rotation measurements can be increased if the following precautions are taken.
(a) The instrument must be in a good condition. The optical elements must be very clean and in exact alignment. The match point should be close to the normal zero mark.
(b) The light source should be properly aligned with respect to the optical bench. It should be supplemented by a filtering system capable of isolating the D line from sodium light.
(c) Specific attention should be paid to temperature control of the solution and of the polarimeter.
(d) Differences between the initial readings or between observed and corrected optical rotation, calculated as either specific optical rotation or optical rotation, should not be more than one-fourth of the range specified in the monograph for the substance.
(e) Polarimeter tubes should be filled in such a way as to avoid air bubbles. Particular care is necessary for semimicro or micro tubes.
(f) For tubes with removable end-plates fitted with gaskets and caps, tighten the end-plates only enough to ensure a leak- proof seal between the end-plate and the body of the tube.
(g) For substances with low rotatory power, the end-plates should be loosened and tightened again after each reading, in the measurement of both the rotation and the zero point.
(h) Liquids and solutions of solids must be clear.
The apparatus may be checked by using a solution of previously dried sucrose and measuring the optical rotation in a 2-dm tube at 25° and using the concentrations indicated in the table.
For solids - Weigh accurately a suitable quantity of the substance under examination to obtain the solution of the strength specified in the individual monograph and transfer to a volumetric flask by means of water or other solvent, if specified. If a solvent is used, reserve a portion of it for the blank determination. Unless otherwise specified, adjust the contents of the flask to 25° by suspending the flask in a constant-temperature bath. Make up the volume with the solvent at 25° and mix well. Transfer the solution to the polarimeter tube within 30 minutes from the time the substance was dissolved and during this time interval maintain the solution at 25°.
Determine the zero point of the polarimeter and then make five readings of the observed rotation of the test solution at 25°. Take an equal number of readings in the same tube with the solvent in place of the test solution. The zero correction is the average of the blank readings, and is subtracted from the average observed rotation if the two figures are of the same sign or added if they are opposite in sign to obtain the corrected observed rotation.
For liquids - Unless otherwise specified, adjust the temperature of the substance under examination to 25°, transfer to a polarimeter tube and proceed as described for solids, beginning at the words "Determine the zero point...".
Calculate the specific optical rotation using the following formulae, dextro-rotation and laevo-rotation being designated by (+) and (-) respectively.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.