Electrochemical Properties of Humic Matter
Humic and fulvic acids are considered amphoteric compounds, but Stevenson (1994) assumes them to be weak acids. With the recent introduction of the supramolecular, micellar, and nanotube concepts, they are also defined as associations containing submicroscopic aggregates of amphiphilic molecules. Hence, at the current stage of knowledge, it is perhaps better to view humic substances as an association that exhibits both amphoteric and amphiphilic features. Although both these features show close relations with electrochemistry, they are defined differently with amphoterism conceptually based more on models of chemical substances that can behave as acids as well as bases, whereas on the other hand the idea of amphiphiles consists of chemical compounds with both hydrophilic and hydrophobic properties (Tan, 2011). Since amphoterism deals closely with acid-base chemistry, it is this property that is directly responsible more for the development of electrical charge characteristics of humic substances. The term amphiprotic is also used to indicate the property, whereas the chemical species exhibiting both the acidic and basic characteristics is called an ampholyte. All these terms find their roots in the Greek word amphi, meaning “both.” At present, three models are commonly used in describing acid-base relations in soil chemistry, the Arrhenius, Brønsted-Lowry, and Lewis concepts (Tan, 2011). The
fact that humic substances can act as acids or as bases makes the application of the Arrhenius concept rather untenable and, in the present author’s opinion, the last two concepts above seem to be more appropriate to describe adequately the reactions involving humic substances as ampholytes. The theory of the Brønsted-Lowry model is perhaps more applicable in relation to ion exchange reactions, whereas the Lewis theory is more adaptable for use with complex reactions and chelation processes exhibited by humic substances. The creation of both negative and positive charges due to amphoterism is an electrochemical property regarded as a distinctive feature of humic substances. The negative charges have usually been studied more intensively and, consequently, are better known than the positive charges. All these charges are developed by the ionization and dissociation of various functional groups of humic compounds. The creation of different types of charges and their relation with acid-base reactions will be explained in some detail in the following sections below.