Correlation and prediction of the solubility of the racemic tartaric acid–ethanol–water system with the NRTL model

The solubility data on solid–liquid equilibria (SLE) of racemic tartaric acid in ethanol   +   water mixtures were measured in the temperature range of (293.15 to 318.15) K at 0.1   MPa by acid–base titration method. In the binary solvent mixtures the solubility of racemic tartaric acid increases with rising temperature and decreases with increasing ethanol content. The modified Apelblat equation, λ h equation and CNIBS/R–K equations were applied to describe and predict the change tendency of solubility. Computational results show that the modified Apelblat equation is superior to the others. Furthermore, a predictive version of the NRTL model was proposed for the description of the non-ideality of the racemic tartaric acid–ethanol–water system. The predicted values were compared with the experimental data. The agreements are good. By using the van't Hoff equation the molar enthalpy, entropy and Gibbs free energy changes of solution were calculated. A linear ∆ sol H versus ∆ sol S compensation plot with a positive slope was obtained by means of enthalpy–entropy compensation effect.