New polymorphic forms of phenobarbital.

Two new polymorphic forms, form II-Ba and form III-Cy, were obtained by recrystallization of phenobarbital in the presence of a small quantity of barbital or cyclobarbital, respectively. The two forms could be discriminated by X-ray diffraction or infrared (IR) absorption measurements. The transition temperature and the heat of transition between form II-Ba and form II, and between form III-Cy and form II were determined to be 84°C and 0.23 kcal/mol, and 44°C and 0.68 kcal/mol, respectively, by solubility measurements. The dissolution rate of form II-Ba was higher than that of form II, whereas the dissolution rate of form III-Cy was lower than that or form II.