5.4 : EDTA: Chemistry and Properties

Polydentate ligands are most widely used in complexometric titrations because they form more stable complexes with the metal ions than mono- or bidentate ligands due to the chelate effect. Examples of polydentate ligands are ethylenediaminetetraacetic acid (EDTA), crown ethers, and cryptands. The most important feature of optimal polydentate ligands is the ability to form 1:1 complexes in a single-step process. Amino carboxylic acid derivatives are frequently used as complexing agents. EDTA is among the most versatile ligands, and it is widely used as a chelating agent in analytical chemistry because most elements can be measured with EDTA through different titration methods.

EDTA is a hexadentate ligand consisting of six complexing groups: four carboxylate oxygens and two amine nitrogens. They coordinate with the metal ion by sharing the lone pairs with the metal. EDTA forms stable cage-like structures with most metal ions and is usually represented with octahedral geometry. However, the geometry of the different complexes depends on the size of the metal ions.

EDTA is a neutral tetraprotic acid, represented by the abbreviation H₄Y. (The disodium salt of EDTA is the preferred laboratory reagent due to its higher solubility than the parent acid.) EDTA dissociates into various species, and their relative amounts depend on the pH of the solution. At low pH, the amine nitrogens are protonated, making EDTA a hexaprotic system (H₆Y²⁺). The fully deprotonated form (Y⁴⁻) exists at high pH and can generate hexadentate complexes with metal ions. This is considered the 'active' form of EDTA.