INTRODUCTION

Many pharmaceutical substances are optically active in the sense that they rotate an incident plane of polarized light so that the transmitted light emerges at a measurable angle to the plane of the incident light. This property is characteristic of some crystals and of many pharmaceutical liquids or solutions of solids. Where the property is possessed by a liquid or by a solute in solution, it is generally the result of the presence of one or more asymmetric centers, usually a carbon atom with four different substituents. The number of optical isomers is 2ⁿ, where n is the number of asymmetric centers. Polarimetry, the measurement of optical rotation, of a pharmaceutical substance may be the only convenient means for distinguishing optically active isomers from each other and thus is an important criterion of identity and purity.