Simultaneous determination of enthalpies of dilution and dissociation of protocatechuic acid in aqueous salt solutions by calorimetric measurements

Abstract

Protocatechuic acid, a major metabolite of antioxidant polyphenols found in green tea, has been shown to prevent carcinogenesis or antitumor growth in vitro and in vivo studies. The dilution enthalpies ΔdilHm, dissociation enthalpies ΔdisHm and thermodynamic dissociation constants Ka for protocatechuic acid in aqueous NaCl or KCl solutions were simultaneously determined by mixing-flow microcalorimetry at T=298.15K. In order to verify the reliability of the fitted dissociation parameters, the values of dissociation enthalpies and thermodynamic dissociation constants were also determined by isothermal titration calorimetry and electrical conductivity methods. The Ka values obtained through the proposed method agree well with those reported in literatures and obtained by other techniques. Enthalpic interaction coefficients (h2, h3 and h4) were computed according to the McMillan–Mayer model. The trends of h2 and ΔdisHm for protocatechuic acid with increasing salt molality in both the salt solutions were obtained. The different influence of KCl and NaCl on the values of h2 and ΔdisHm were discussed in terms of (solute+solute) and (solute+solvent) interactions. The results showed that it is possible to perform in a single calorimetric experiment the simultaneous determination of dilution enthalpies, dissociation enthalpies and constants in a given solvent.