Abstract
The existence of four crystalline forms (forms I, II and III, and a hydrate) and an amorphous form of tulobuterol hydrochloride was confirmed by X-ray powder diffraction, infrared spectroscopy and thermal analyses (DSC and TG). The hydrate was found to be the monohydrate by elemental analysis and measurement of water content. From the DSC measurement, it was found that forms I and II melted at 163°C and 170°C, and their heats of fusion were 5.15 kcal/mol and 4.76 kcal/mol, respectively. Form III, the amorphous form and the hydrate transformed into form II at 135°C, 90°C and 75°C, respectively. Activation energy for the dehydration of the hydrate determined by Kissinger's method was 56.1 kcal/mol. No crystal changes were observed in the four crystalline forms when they were ground in a mortar or compressed at high pressure ; however, after such mechanical treatments form I transformed into form II on being heated. The investigation of phase transitions of the four crystals showed that form II was the most stable among them.
