Analytical constraints on acidic functional groups in humic substances

Four standard fulvic acids and two standard humic acids from the International Humic Substances Society (IHSS) were analyzed to quantify the in situ generation of new acidity during direct titrations, as observed by hysteresis between tandem forward and reverse titration curves. The resulting concentrations of carboxyl and phenolic groups were then corroborated by analytical constraints imposed by elemental data and 13C NMR data. Collectively, carboxyl groups in the IHSS samples, as determined by rapid forward direct titrations, consistently accounted for 69% (±6) of the maximum possible carboxyl-like structures that were estimated by 13C NMR spectra. The concentrations of carboxyl groups in the samples increased for two separate reverse titrations initiated at 30 min and 24 h after the completion of the forward titrations, representing 73% (±7) and 84% (±8) of the maximum carboxyl-like groups, respectively. Conversely, the concentrations of phenolic groups were invariant between forward and reverse titrations. Elemental constraints coupled with titration data predicted that the IHSS samples have, on average, one acidic hydroxyl group per every two phenyl rings, as constrained by 13C NMR estimates of aromatic carbon.