Contributions of the Non Quinone Moieties to the Reducing Ability of Humic Substances.

Despite the broad consensus that non quinone moieties (NQ) in humic substances (HS) can also contribute to its redox activity, reasonable quantification and contribution of NQ in reduction ability of HS are still lacking.  A very simple methodology based on chemical reduction of Q using stannous chloride (SnCl₂) and subsequent determination of excess SnCl₂ by potentiometric titration with potassium dichromate (K₂Cr₂O₇) is proposed to quantify quinone (Q) content in HS, which can be used for quantification of NQ in HS by subtracting the Q contents from the respective chemical reducing capacity (CRC) values of HS. The ability of HS to transfer electron charge is termed their CRC (as defined in earlier work from this group).  One model compound (anthraquinone-2,6-disulfonate, AQDS) and three HS (Elliot soil humic acid, ESHA, Pahokee Peat humic acid, PPHA and Suwannee River natural organic matter, SRNOM) were selected for determination of Q contents. Although estimated Q content for AQDS was similar to the theoretical value, Q contents determined for HS were higher than that of the CRC content of the respective HS indicating overestimation of Q contents.  There are two possible reasons for these unexpected higher values of Q for HS.  One, reduced HS is reacting with dichromate along with the excess SnCl₂ and two, SnCl₂ is reacting with functional groups of HS that is otherwise not reducible by H₂/pd reduction methods.  Corrections for the determination of Q content were measured by control experiments where H₂/Pd reduced HS was added in the reaction mixture of SnCl₂ in alkaline medium and titrated potentiometrically with K₂Cr₂O₇.  Although the variability of these correction factors were high for HS, corrected Q contents remained lower than the CRC of HS indicating contributions of NQ in the electron shuttling.