Ethylenediaminetetra-Acetic Acid

Ethylenediaminetetra-acetic acid, typically shortened to EDTA, is a chemical compound with the ability to form multiple bonds with metal ions, making it an important chemical to analytical scientists and industry alike.

The compounds used to create EDTA include ethylenediamine, formaldehyde, and sodium cyanide. When these compounds are mixed in an appropriate fashion, a series of chemical reactions take place. Formaldehyde reacts with the sodium cyanide to form formaldehyde cyanohydrin. In the presence of sulfuric acid, this compound then reacts with ethylenediamine forming an intermediate compound that eventually reacts with water to form EDTA.

Solid EDTA is readily dissolved in water where it can form multiple chemical bonds with many metal ions in a solution, in effect tying up the metal ions. A molecule such as EDTA which has at least one free pair of unbonded electrons and therefore can form chemical bonds with metal ions. These compounds are known as ligands. Ligands are typically classified by the number of available free electron pairs that they have for creating bonds. Ammonia (NH₃), for example, which has one pair of unbonded electrons, is known as a monodentate ligand (from Latin root words meaning one tooth). EDTA has six pairs of unbonded electrons and is called a hexadentate ligand. Ligands such as EDTA, which can form multiple bonds with a single metal ion, in effect surrounding the ion and caging it in, are known as chelating agents (from the Greek chele, meaning crab's claw). The EDTA molecule seizes the metal ion as if with a claw, and keeps it from reacting normally with other substances.

Chelating agents play an important roll in many products, such as food, soda, shampoo, and cleaners. All of these products contain unwanted metal ions which affect color, odor, and appearance. In addition to affecting the physical characteristics of these products, some metal ions also promote the growth of bacteria and other microorganisms. EDTA ties up metal ions in these products and prevents them from doing their damage.

EDTA is also used extensively by analytical chemists for titrations. A titration is one method for finding the amounts of certain metals in various samples based on a known chemical reaction. Since EDTA bonds with metal ions, the amount of metal in a sample can be calculated based on the amount of EDTA needed to react with it. In this way, chemists can determine the amount of such things as lead in drinking water or iron in soil.