Solubility of a New Polymorph of Phenobarbital obtained by Crystallization in the Presence of Phenytoin

Physicochemical properties of some polymorphs of phenobarbital were studied. A new polymorph, form III-Ph, with a slightly changed crystal habit was obtained by addition of phenytoin at the time of recrystallization. The transition temperature and the heat of transition between form III-Ph and form III, and between form III and form II were determined to be 46°C and 0.201 kcal/mol, and 77°C and 0.404 kcal/mol, respectively, by solubility measurement. As the dissolution rates of form III-Ph and form III were higher than that of form II, they are expected to show better bioavailability than form II.