DETERMINATION OF SOME REDOX PROPERTIES OF HUMIC-ACID BY ALKALINE FERRICYANIDE TITRATION

A method for the direct potentiometric oxidation titration of aqueous phase humic substances with potassium ferricyanide (K,Fe(CN),) has been developed. Oxidative titrations of humic acid were carried out at varying analyte concentrations, and at values of pH ranging from 5.0 to 11.0. Model synthetic mixtures of phenols and hydroquinones were examined in the same pH range. Ferricyanide titrations of humic acid at pH 9.0 yielded single discrete endpoints, and generally stable potential measurements in the pre-equivalence point region of the titration curve. Cyclic voltammetry of alkaline solutions of humic acid provides evidence for direct interaction between phenolic components in the humic acid and the electrode surface, and that for titrations conducted at pH 9.0 and above, humic acid can contribute to the poising of the potential measurements in the region of the half-titration point.