Appendix V F. Determination of Optical Rotation and Specific Optical Rotation

Optical rotation is the property displayed by chiral substances of rotating the plane of polarisation of polarised light.

Optical rotation is considered to be positive (+) for dextrorotatory substances (i.e. those that rotate the plane of polarisation in a clockwise direction) and negative (-) for laevorotatory substances.

The specific optical rotation is the rotation, expressed in radians (rad), measured at the temperature t and at the wavelength λ given by a 1 m thickness of liquid or a solution containing 1 kg/m³ of optically active substance. For practical reasons the specific optical rotation is normally expressed in milliradians metre squared per kilogram (mrad·m²·kg^-1).

The Pharmacopoeia adopts the following conventional definitions.

The angle of optical rotation of a neat liquid is the angle of rotation α, expressed in degrees (°), of the plane of polarisation at the wavelength of the D-line of sodium (λ = 589.3 nm) measured at 20 °C using a layer of 1 dm; for a solution, the method of preparation is prescribed in the monograph.

The specific optical rotation of a liquid is the angle of rotation α, expressed in degrees (°), of the plane of polarisation at the wavelength of the D-line of sodium (λ = 589.3 nm) measured at 20 °C in the liquid substance to be examined, calculated with reference to a layer of 1 dm and divided by the density expressed in grams per cubic centimetre.

The specific optical rotation of a substance in solution is the angle of rotation α, expressed in degrees (°), of the plane of polarisation at the wavelength of the D-line of sodium (λ = 589.3 nm) measured at 20 °C in a solution of the substance to be examined and calculated with reference to a layer of 1 dm containing 1 g/mL of the substance. The specific optical rotation of a substance in solution is always expressed with reference to a given solvent and concentration.

In the conventional system adopted by the Pharmacopoeia the specific optical rotation is expressed by its value without units; the actual units, degree millilitres per decimetre gram [(°)·mL·dm^-1·g^-1] are understood.

The conversion factor from the International System to the Pharmacopoeia system is the following:

In certain cases specified in the monograph the angle of rotation may be measured at temperatures other than 20 °C and at other wavelengths.

The polarimeter must be capable of giving readings to the nearest 0.01°. The scale is usually checked by means of certified quartz plates. The linearity of the scale may be checked by means of sucrose solutions.

Method Determine the zero of the polarimeter and the angle of rotation of polarised light at the wavelength of the D-line of sodium (λ = 589.3 nm) at 20 ± 0.5 °C, unless otherwise prescribed. Measurements may be carried out at other temperatures only where the monograph indicates the temperature correction to be made to the measured optical rotation. Determine the zero of the apparatus with the tube closed; for liquids the zero is determined with the tube empty and for solids filled with the prescribed solvent.

Calculate the specific optical rotation using the following formulae.

For neat liquids:

For substances in solution:

where c is the concentration of the solution in grams per litre.

Calculate the content c in grams per litre or the content c′ in per cent m/m of a dissolved substance using the following formulae: