Water activities and pH for binary and ternary aqueous systems with polycar
boxylic acids (citric, malic, malonic, oxalic, succinic, and tartaric) were
measured at 298.15 K. The solute concentration in binary systems varied fr
om (5 to 50) mass % for citric, malic, malonic, and tartaric acids. For oxa
lic 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. I
n 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 specie
s which arose from the partial dissociation of weak acids. The solvation ph
enomenon between water and ions was also considered. To take into account a
ll 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 mo
del, with an average deviation of 0.4% for water activity and 1.3% for pH.