Abstract
When measurement of the refractive index of crystals is used for the identification of a drug or polymorphic form, it is more convenient to determine the "key refractive index" than the principal refractive index. Two characteristic key refractive indices can be measured for crystals in plate, bladed, or lamellar form, but for acicular or long prismatic crystals only one key refractive index can usually be measured along the direction of elongation when the crystals show parallel extinction. Key refractive indices were measured for several drugs listed in J.P.IX. The relation between the key refractive indices and crystal symmetry, habit, and extinction is discussed. The immersion method was improved to obtain more reliable results by using a commercial kit of immersion oils with refractive indices between 1.47 and 1.73 at 0.005 intervals.
