Calorimetric Acid-Base Titrations of Aquatic and Peat-Derived Fulvic and Humic Acids

Titration calorimetry was used to determine the protonation and ionization enthalpies of peat-derived and aquatic humic and fulvic acids from pH 3.6 to 10.4 and for 0.001–0.1 M ionic strength [NaC] and tetraethylammonium chloride (TEAC1)]. Protonation and ionization enthalpies were not completely equivalent, and protonation enthalpies were concluded to more accurately reflect the enthalpies associated with proton binding and release. Below pH7, protonation enthalpies were similar to simple carboxylic acids (6 to −5 kJ/mol), and no ionic strength effects were apparent. Above pH9, enthalpies were similar to those of simple phenolic acids (−20 to −36 kJ/mol), and less exothermic values were observed at I = 0.1 M. For NaCl, this was attributed to disruption of H-bonds between phenolic hydrogens and adjacent oxygen-containing groups by Na⁺. For TEAC1, hydrophobic interactions may be responsible. Thus, protonation enthalpies as a function of pH as well as counterion type and concentration reveal information about the nature and location of acidic functional groups in humic substances.