Enigma of Assessing Redox Properties of Humic Substances.

Redox properties of humic substances (HS) are known to be linked with important biogeochemical processes in the environment which control nutrient cycling and contaminant fate.  Therefore, accurate measurement of redox properties of HS is important.  However, a consensus based on the available measurements methods of the redox properties of HS has not yet been reached.  In this study, we compared common redox property measurement methods of HS using anthraquinone-2,6-disulfonate (AQDS) as a model compound and standard Elliot soil humic acid (1S102H, ESHA), reference Pahokee peat (1R103H, PPHA), and Suwannee River natural organic matter (1R101N, SRNOM) as HS.  We found, among commonly used methods, that H₂/Pd reduction method followed by incubation with ferric citrate (FeCit) reagent was incomplete, H₂/Pd reduction method followed by incubation with potassium ferricyanide (K₃Fe(CN)₆) was insensitive, and reduction of either AQDS or HS by stannous chloride (SnCl₂) followed by titration of excess stannous (Sn²⁺) by potassium dichromate (K₂Cr₂O₇) was most accurate.  Our research will pave the way for future investigations on detailed characterizations of functional groups of HS responsible for oxidation/reduction reactions.