Study of crystalline drugs by means of a polarizing microscope. VI. Polarizing microscopy of drugs acting on the nervous system and on individual organs listed in the Japanese pharmacopoeia X.

Twenty-six drugs acting on the nervous system and 30 drugs acting on the individual organs, all listed in the JPX, were investigated by polarizing microscopy using the improved immersion method. For 30 drugs having the form of plates, lamellar or scales, two key refractive indices were measured ; these drugs were classified as group A. For 8 drugs having the form of elongated prisms, needles or rods showing always parallel extinction, one key refractive index along the direction of elongation was measured (classified as group B). For 18 other anisotropic crystalline drugs, two refractive indices were measured approximately at the most frequently observable positions ; these drugs were classified as group C. It was found that drugs of groups A and B could be conveniently identified or analyzed by polarizing microscopy by measuring their key refractive indices, and these drugs accounted for 68% of the 56 drugs tested. The correlation between refractive index and birefringence was plotted on rectangular coordinates ; such a plot was also useful for the analysis of drugs by polarizing microscopy. Furthermore, the thicknesses of crystal sections might be easily estimated from the birefringence and interference colors, so polarizing microscopy could be useful for the quality control of slightly soluble drugs by estimating their specific surface areas.