New Melting Data of the Two Polymorphs of Prednisolone

Prednisolone
is known to exist in two anhydrous solid polymorphic
forms. The substance is known to degrade upon melting, resulting in
erroneous melting data, as shown by the widely scattered results reported
in the literature. In this article, thermal analyses carried out at
different scan rates show that the onset temperature and the enthalpy
value of the signal increase with the scan rate and reach plateau
values for high scan rates. Owing to flash scanning calorimetry, the
plateau value for the temperature has been identified as the “true”
temperature of melting of both polymorphs. This consistent set of
new thermodynamic data on the two solid forms leads to the conclusion
that both forms are unambiguously enantiotropes of each other. The
solid–solid transition has been observed experimentally for
the first time and has been confirmed by calculation.