LAUSR

Abstract With the isothermal dissolution equilibrium method, the solid-liquid phase equilibrium for 5-amino-3-methyl-1-phenylpyrazole in ten organic solvents including methanol, ethanol, n -propanol, isopropanol, 2-butanone, acetonitrile, n -butanol, toluene, 1,4-dioxane and isobutanol was established over a temperature range from (283.15 to 318.15) K at atmospheric pressure (101.1 kPa), and the solubility of 5-amino-3-methyl-1-phenylpyrazole in these solvents were determined by a high-performance liquid chromatography (HPLC). The mole fraction solubility of 5-amino-3-methyl-1-phenylpyrazole in the solvents increased with increasing temperature within the temperature range studied, and obeyed the following sequence from high to low in different solvents: 1,4-dioxane > acetonitrile > methanol > isobutanol > ethanol >  n -propanol > isopropanol > 2-butanone > toluene > cyclohexane. The obtained solubility values of 5-amino-3-methyl-1-phenylpyrazole were correlated with the modified Apelblat equation, λh equation, Wilson model and NRTL model. The maximum values of root-mean-square deviation ( RMSD ) and relative average deviation ( RAD ) were 4.95 × 10 − 4 and 0.62%, respectively. In total, the four thermodynamic models were all acceptable for describing the solubility behavior of 5-amino-3-methyl-1-phenylpyrazole in these solvents. Furthermore, the mixing Gibbs energy, mixing enthalpy, mixing entropy, activity coefficient at infinitesimal concentration and reduced excess enthalpy were obtained with Wilson model. The results demonstrated that the dissolution process of 5-amino-3-methyl-1-phenylpyrazole was spontaneous and favorable in the ten organic solvents. Moreover, the solute–solvent intermolecular interaction was roughly estimated based on the values of activity coefficients. The determined solubility data and thermodynamic models in this work would be helpful to optimize the purification procedure of 5-amino-3-methyl-1-phenylpyrazole.