Heat of fusion measurement of a low melting polymorph of carbamazepine that undergoes multiple-phase changes during differential scanning calorimetry analysis.

Two polymorphs of carbamazepine that melt at 176 and 189 degrees C and are known to be enantiotropic have been characterized more fully than in previous reports. For the first time, a value for the heat of fusion (29.3 kJ/mol) of the lower melting form is estimated, even though single melting DSC endotherms were not obtained. A method is described for summing the heat flows of intermediate transitions that occur when low melting carbamazepine is heated to a temperature where it will remain melted. The method should have general applicability in studies of polymorphs. Other characteristics of the carbamazepine enantiotropic pair are reported, including the transition temperature (71 degrees C).