Water activity and pH in aqueous polycarboxylic acid systems

Water activities and pH for binary and ternary aqueous systems with polycarboxylic acids (citric, malic, malonic, oxalic, succinic, and tartaric) were measured at 298.15 K. The solute concentration in binary systems varied from (5 to 50) mass % for citric, malic, malonic, and tartaric acids. For oxalic and succinic acids, which present low solubilities in water, the solute concentration varied from (2 to 10 and 1.5 to 5.5) mass %, respectively. In ternary mixtures the total acid concentration varied from (5 to 50) mass %. A modified UNIFAC group contribution model was used for predicting water activity and pH data. In the model formulation, a Pitzer-Debye-Huckel term was added to represent the electrostatic interactions between ionic species which arose from the partial dissociation of weak acids. The solvation phenomenon between water and ions was also considered. To take into account all species present in the mixture, a numerical procedure based on chemical equilibrium constants, the electroneutrality condition, and stoichiometric balances was associated with the modified UNIFAC model. The close agreement between measured and calculated data showed the high performance of the model, with an average deviation of 0.4% for water activity and 1.3% for pH.