**2. Antecedents**

#### **2.1 What is a chelating agent?**

The word chelation is derived from Greek, meaning "claw." The ligands lie around the central atom like the claws of a lobster.

The IUPAC definition of chelation is the formation or presence of two or more separate bindings between a polydentate (multiple bonded) ligand and a single central atom. Usually these ligands are organic compounds and are called chelants, chelators, chelating agents, or sequestering agents. (IUPAC)

The ligand forms a chelate complex with the substrate. Chelate complexes are contrasted with coordination complexes composed of monodentate ligands, which form only one bond with the central atom. (Morgan & Drew, 1920)

The terms bidentate (or didentate), tridentate, tetradentate,... multidentate are used to indicate the number of potential binding sites of the ligand, at least two of which must be used by the ligand in forming a "chelate". For example, the bidentate ethylenediamine forms a chelate with **Cu (I)** in which both nitrogen atoms of ethylenediamine are bonded to copper. (The use of the term is often restricted to metallic central atoms). (Kramer, Cotter-Howells, Charnock, Baker & Smith. 1996*)* 

Chelants, according to ASTM-A-380, are "chemicals that form soluble, complex molecules with certain metal ions, inactivating the ions so that they cannot normally react with other elements or ions to produce precipitates or scale".
