Novel Electrochemical Approach to Assess the Redox Properties of Humic Substances

Two electrochemical methods to assess the redox properties of humic substances (HS) are presented: direct electrochemical reduction (DER) on glassy carbon working electrodes (WE) and mediated electrochemical reduction (MER) and oxidation (MEO) using organic radicals to facilitate electron transfer between HS and the WE. DER allows for continuous monitoring of electron and proton transfer to HS by chronocoulometry and automated acid titration, respectively, and of changes in bulk HS redox potential Eh. Leonardite Humic Acid (LHA) showed an H+/e− ratio of unity and a decrease in potential from Eh = +0.18 to −0.23 V upon transfer of 822 μmole- gLHA−1 at pH 7, consistent with quinones as major redox-active functional moieties in LHA. MER and MEO quantitatively detected electrons in LHA samples that were prereduced by DER to different extents. MER and MEO therefore accurately quantify the redox state of HS. Cyclic DER and O2-reoxidation revealed that electron transfer to LHA was largely reversible. However, LHA contained a small pool of moieties that were not reoxidized, likely due to endergonic first electron transfer to O2. Electron accepting capacities of 13 different HS, determined by MER, strongly correlated with their C/H ratios and aromaticities and with previously published values, which, however, were a factor of 3 smaller due to methodological limitations.