Title: Determination of Optical Rotation and Specific Optical Rotation
- Objective: To determine the Optical Rotation and Specific Optical Rotation of the solutions and solvents.
- Principle: There are 2 types of plane polarized light [PPL], 1) Right circularly plane polarized light (RCPL), 2) Left circularly plane polarized light (LCPL). They are equal and opposite direction. When a PPL is pass through the optical active compound, due to its circular birefringence results unequal rate of propagation of left and right circularly polarized rays. This unequal rate of propagation of both RCPL and LCPL deviates the PPL from its original direction and it is called to be ‘Optical Rotation’.
Optical Activity: The compounds which are having the ability to rotate the plane of polarised light are called optically active compounds .They rotate plane of polarized light due to their chirality (asymmetric character) this phenomenon is known as ‘optical activity’. When a ray of monochromatic polarized light strikes a solution, several phenomenon’s occurs like:-1. Reflection on the surface. 2. Refraction. 3. Rotation of plane polarization 4. Absorption.
3. Procedure:
- Definitions:
- Optical Rotation: 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, [α]D25 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 (-).
- Specific Optical Rotation of Liquid: The specific optical rotation,[α]D25 of a liquid substance is the angle of rotation α , of the plane of polarisation at the wavelength of the D line of sodium (l = 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 250
- Specific Optical Rotation of Solid: The specific optical rotation, [α]D25,of a solid substance is the angle of rotation, α , of the plane of polarisation at the wavelength of the D line of sodium (l = 589.3 nm) measured at 250C, 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.
- Apparatus:
A 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 measurement 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 semi-micro 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.
- Calibration:
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 Table 1.
Table 1
| Concentration (g/100 ml) | Angle of Rotation (+) at 25o |
| 10.0 | 13.33 |
| 20.0 | 26.61 |
| 30.0 | 39.86 |
| 40.0 | 53.06 |
- Method:
The optical rotation, unless otherwise specified, is measured at the wavelength of the D line of sodium (l = 589.3 nm) at 25°, on a layer 1 dm thick. It is expressed in degrees.
- For Solids:
Weigh accurately a suitable quantity of the substance being examined 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°C by suspending the flask in a constant-temperature bath. Make up to volume with the solvent at 25°C 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°C. Determine the zero point of the polarimeter and then make five readings of the observed rotation of the test solution at 25°C. 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 being examined to 25°C, transfer to a polarimeter tube and proceed as described for solids, beginning at the words “Determine the zero point…”
- Calculations:
Calculate the specific optical rotation using the following formulae, dextro-rotation and laevo-rotation being designated by (+) and (-) respectively.
For liquids [α]D25 = α/ld 25
For solids [α]D25 = 100α /lc
Where α = corrected observed rotation, in degrees, at 25°C.
D = D line of sodium light (l = 589.3 nm)
l = length of the polarimeter tube in dm
d25 25 = specific gravity of the liquid or solution at 25°
c = concentration of the substance in % w/v
NOTE: The requirement for optical rotation and specific optical rotation in the Pharmacopoeia apply to a dried, anhydrous or solvent-free material in all those monographs in which standards for loss on drying, water, or solvent content respectively are given. In calculating the result, the loss on drying, water or solvent content determined by the method specified in the monograph is taken into account.