ABSTRACT
Humic and fulvic acids are considered amphoteric compounds, but Stevenson (1994) assumes them to be weak acids. However, it is well known that humic substances can, in fact, react with both bases and acids, hence carry both positive and negative charges. These properties and behavior are regarded as distinctive characteristics of amphoteric substances. The negative charges are usually studied more intensively and consequently are better known than the positive charges. All these charges are developed by the ionization or dissociation of various functional groups.
