Solubility Measurement, Correlation, and Solvent Effect of 3,5-Dihydroxybenzoic Acid in Nine Pure Solvents and Binary Solvent Mixture (Ethanol + Acetonitrile) at Temperatures from 293.15 to 333.15 K

The solubility of 3,5-dihydroxybenzoic acid (3,5-DHBA) in nine pure organic solvents and in one binary mixture (ethanol + acetonitrile) was measured gravimetrically at ambient pressure over the temperature range of 298.15–333.15 K. The mole fraction solubility increased with rising temperature in all systems studied. In pure solvents, solubility followed this order: isopropanol > ethanol > n-propanol > n-butanol > isobutanol > ethyl acetate > n-propyl acetate > isopropyl acetate > acetonitrile. In the ethanol + acetonitrile mixture, solubility increased continuously with higher ethanol mass fraction.Experimental data were correlated using three semiempirical equations (van’t Hoff, modified Apelblat, and λh) and three activity coefficient models (NRTL, Wilson, and UNIQUAC). The modified Apelblat equation yielded the best correlation, with average relative deviations (ARD) below 0.60% across all systems. Dissolution in pure solvents was analyzed using the “like-dissolves-like” principle and Hansen solubility parameters. Solvent effects were quantitatively assessed via the KAT-LSER model, where multiple linear regression showed that solubility was mainly governed by π* (41.83%) and δH (30.69%), with lesser contributions from β (21.80%) and α (5.68%). These solubility data provide a valuable reference for designing and optimizing industrial crystallization processes for 3,5-DHBA.