Acidity of humic acid related to its oxygen-containing functional groups

Acidic properties of polyprotic acids, such as humic acid, can be described quantitatively with potentiometric titration interpreted with the modified Henderson-Hasselbalch equation. Oxygen-containing functional groups such as carboxylic, phenolic, and alcoholic groups are the most important groups that contribute to acidity of humic acid. Those groups in isolated soil humic acid are selectively blocked by methylation with CH3I/Ag2O in dimethylformamide. The obtained values for acidic group contents, the apparent and intrinsic dissociation constants for underivatized and derivatized HAs, provide direct evidence on the oxygen-containing functional groups contribution to HA acidity, especially the contribution of alcoholic hydroxyls, which is usually neglected and ascribed to phenolic groups content.