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).