Relative stabilities of the five polymorphs of sulfathiazole

The relative stabilities of the five polymorphs of sulfathiazole have been investigated using solution-based and solid-state methods. In the lower temperature range, the stability order is proposed to be FI < FV < FIV < FII < FIII. FI and FV were identified as the least stable polymorphs below 50 °C using a combination of solubility measurements and isothermal suspension equilibration, with FII, FIII, and FIV displaying very similar stabilities. Between 30 and 50 °C, the stability order was established as FIV < FII < FIII. At 10 °C, FII is still more stable than FIV, but it was not possible to place FIII in relation to these two forms. Above 100 °C, the results from DSC and high-temperature XRD measurements suggest that the stability order changes completely as a result of several enantiotropic transitions. In this upper temperature range, FII and FIII are the least stable forms, with FII being less stable than FIII. The stability order among the remaining three forms is FI < FV < FIV initially, but this reverses with increasing temperature, and as the transition into a melt is approached, a stability order of FII < FIII < FIV < FV < FI is suggested.