Determination and Modeling of the Solubilities of Oleanolic Acid and Ursolic Acid in Ethanol + Sodium Hydroxide + Water Mixed Solvents from T = 283.2 to 323.2 K

The solubilities of oleanolic acid (OA) and ursolic acid (UA) in ethanol + sodium hydroxide + water mixed solvents were measured at different temperatures (283.2, 298.2, 293.2, 298.2, 303.2, 308.2, 313.2, 318.2, and 323.2 K). The solubility of OA in the mixed solvents increased with temperature. However, the solubility of UA in the mixed solvents first increased with temperature, and then decreased with temperature. The solubilities of both OA and UA increased with the initial mole fraction of ethanol in the mixed solvents. The addition of NaOH could improve the solubilities of OA and UA in the aqueous ethanol solution. The solubility of OA was correlated with temperature by the van’t Hoff model, the modified Apelblat model and the λh model, and the results indicated that the λh model had better correlation. The solubility of UA was correlated with temperature only by an empirical cubic equation.