According to the system conditions, humic acids act as oxidants or as reductants. In natural systems the electron flow is directly associated with humus content and quality. Electrode potential and oxidation capacity inform about the intensity and capacity of the redox systems. In previous studies it has been assumed that free radicals participate in humic acid redox reactions. In our study six humic acids isolated from composed organic residues were titrated with an oxidant (I2) in inert atmosphere and specified conditions. Humic acids presented similar standard formal electrode potentials, with values between 0.773 and 0.794 V, at 25 ºC. The oxidation capacity of humic acids varied from 3.88 to 4.39 mol c kg-1 at pH 5.0 and from 5.35 to 7.89 mol c kg-1 at pH 7.0. Positive and significant correlations were observed between the oxidation capacity and the concentrations of the phenolic, quinone and semiquinone functional groups in the humic acids.
soil chemistry; humic substances; redox potential; oxidation capacity; humification
