Measurement and Correlation of Solubility of Clopidogrel Hydrogen Sulfate (Metastable Form) in Lower Alcohols

The solubility equilibrium of clopidogrel hydrogen sulfate (Form I) was set up without the nucleation of Form II, as the nucleation rate of Form II is very slow under the low supersaturation. Using the laser monitoring technique, solubilities of clopidogrel hydrogen sulfate (Form I) in ethanol, 1-propanol, 1-butanol, 1-pentanol, and 2-propanol were measured at temperatures ranging from (273.15 to 318.15) K. The experimental data were correlated by the modified Apelblat equation, and the dissolution enthalpy and entropy of Form I were determined with van't Hoff plots. Compared with those of Form II, Form I exhibits a higher solubility and a lower enthalpy of dissolution. Therefore, we can draw the conclusion that Form I was the metastable form under the conditions in this work.