Characterization of Temperature Induced Phase Transitions in the Five Polymorphic Forms of Sulfathia-zole by Terahertz Pulsed Spectroscopy and Differential Scanning Calorimetry

The far-infrared properties of all five described polymorphic forms of the drug sulfathiazole have been studied by terahertz pulsed spectroscopy and low frequency Raman spectroscopy. The spectra of the different polymorphs are distinctly different. Terahertz pulsed spectroscopy proves to be a rapid complementary alternative to other physical characterisation techniques reported in the literature for distinguishing between the five forms. Variable temperature measurements (293 K to 473 K) of all polymorphic forms have been performed. The phase transitions observed have been related to thermal analysis data. Form I is the stable high temperature form of sulfathiazole with a melting point of about 475 K. Form II melts at around 470 K and recrystallizes at higher temperatures to form I. Forms III to V all convert to form I via a solid-solid phase transition at temperatures below 450 K. The phase transitions can be monitored by terahertz pulsed spectroscopy. Common polymorphic impurities of the samples can be detected in the room temperature spectra and their effect on the phase transition behaviour can be studied.