A direct potentiometric titration study of the dissociation of humic acid with selectively blocked functional groups

Abstract

A direct potentiometric titration method was applied to commercial and soil humic acids in
order to determine their carboxyl and phenol group concentrations and apparent and intrinsic pK. In
that context, acid-base properties of humic acids are interpreted by selective blocking of carboxylic and
phenolic groups by esterification and acetylation. Differences in underivatized and derivatized HA’s
acid-base properties are ascribed to carboxyl and phenol groups influence on total humic acidity.
Potentiometric data were treated with the modified Henderson-Hasselbalch equation. Infra red results,
the acidic group contents and the average values of apparent and intrinsic pK for underivatized and
derivatized HAs confirmed the selectivity of esterification derivatization method. After blocking of the
functional groups, the values of acidic group contents decreased, while the value of apparent pK
increased after derivatization. Phenol groups cannot be specifically identified by the acetylation
method, due to low selectivity of the acetylation method.