Determination of the Reducing Capacity of Humic Acids in Reactions with Different Fe(III) Complexes

The reducing capacity of humic acids (HA) was evaluated by the spectrophotometric method using Fe(III) complexes with organic and inorganic ligands as oxidizing agents, depending on the medium pH and reagent concentration. The maximum sensitivity of the method was observed at pH = 5 for Fe(III) complexes with α,α′-bipyridyl and o -phenanthroline, as well as at pH = 9 for potassium hexacyanoferrate(III). The obtained results were supported by computational modeling of the complexation and dissociation processes. The number of available surface functional groups was found to decrease with an increase in the HA concentration due to the molecular aggregation, thus resulting in a lower reducing capacity. The formal redox potential of HA, measured by redox titration, ranged from 530–to 535 mV.