LAUSR

Abstract The solubility of m-phthalic acid has been measured in co-solvent mixtures conformed by acetone and ethanol over the entire solvent composition and in the temperature range of (278–323) K. Experimental results showed that the solubility always increased with increasing temperature in the binary solvent mixtures with constant solvent composition, indicating the endothermic character of the dissolution process. The solubility data was well correlated by semi-empirical regression models of the van't Hoff and the λh equations with average relative deviation (ARD) of 1.02% and 2.79%, respectively. Apparent deviation from the van't Hoff plot on solubility was observed but no polymorphic change of m-phthalic acid was found, implying the decisive influence of the change in the solvent molecular structure and the solvent-solvent interactions with temperature. On a solvent composition basis, the solubility in the co-solvent mixtures is clearly higher than that in both pure ethanol and acetone with the maximum appearing at acetone mole fraction in between 0.3 and 0.4, depending on the dipolarity/polarizability of the mixed solvents. On the other hand, m-phthalic acid exhibited reduced solubility when small amount of ethanol is added into acetone at temperatures above 315 K. The Simplified Modified Wilson (SMW) model was adopted to reproduce the isothermal solubility data over the entire acetone composition and the obtained overall ARD was 3.50%, with which the exact maximum solubility and the corresponding solvent composition at different temperatures were calculated.