led to the theory that a 'humification' process created 'humic substances'; most commonly 'humic acid', 'fulvic acid', and 'humin'. However, these humic substances have not been observed in soil. Although 'humification' theory is unsupported by evidence, "the underlying theory persists in the contemporary literature, including current textbooks." Attempts to redefine 'humic substances' in valid terms have resulted in a proliferation of incompatible definitions, "with far-reaching implications beyond our ability to communicate scientifically accurate soil processes and properties." Chemical characteristics In nature Since the dawn of modern chemistry, humic substances are among the most studied among the natural materials. Despite long study, their molecular structure and chemical remains elusive. The traditional view is that humic substances are heteropolycondensates, in varying associations with clay. A more recent view is that relatively small molecules also play a role. Humic substances account for 50 – 90% of cation exchange capacity. Similar to clay, char and colloidal humus hold cation nutrients. In traditional, base-soluble extracts A typical humic substance is a mixture of many molecules, some of which are based on a motif of aromatic nuclei with phenolic and carboxylic substituents, linked together; the illustration shows a typical structure. The functional groups that contribute most to surface charge and reactivity of humic substances are phenolic and carboxylic groups. Humic acids behave as mixtures of dibasic acids, with a pK1 value around 4 for protonation of carboxyl groups and around 8 for protonation of phenolate groups. There is considerable overall similarity among individual humic acids. For this reason, measured pK values for a given sample are average values relating to the constituent species. The other important characteristic is charge density. The molecules may form a supramolecular structure held together by non-covalent forces, such as van der Waals force, π-π, and CH-π bonds. The presence of carboxylate and phenolate groups gives the humic acids the ability to form complexes with ions such as Mg2+, Ca2+, Fe2+, and Fe3+. Many humic acids have two or more of these groups arranged so as to enable the formation of chelate complexes. The formation of (chelate) complexes is an important aspect of the biological role of humic acids in regulating bioavailability of metal ions. Determination of humic acids in water samples The presence of humic acid in water intended for potable or industrial use can have a significant impact on the treatability of that water and the success of chemical disinfection