Chelating agents

What are chelating agents?

Chelating agents are substances that form covalent coordination compounds with metal ions or metal atoms. Their molecules or groups of atoms surround a metallic central ion or atom and, as so-called Lewis acids, give whole pairs of electrons for binding. However, the central particle does not contribute electrons to this bond. The chelating agents are also referred to as ligands and the complexes formed as chelates.

In the late nineteenth century, the systematic exploration of complex compounds began by the chemists Alfred Werner (Switzerland) and Sophus Mads Jørgensen (Denmark). The statements made by Alfred Werner in 1892 on the structure of complexes could be confirmed experimentally in the subsequent period. Coordination chemistry has made great progress with the introduction of new analytical techniques such as X-ray diffraction.

Chelating agents find wide application in many areas. For example, they are added as food additives in food processing. The binding of the metal atoms (masking) stabilizes the aroma, structure and color of the food. They also have an antioxidant and preservative effect.

A large area of ​​application of chelating agents is also the analytical chemistry.

In medicine, in turn, the anticoagulant effect of chelating agents is estimated, where they provide, inter alia, as anticoagulants in the blood for the stabilization of the blood. Because of their ability to bind heavy metals, they are also used for detoxification in heavy metal poisoning.

Since the middle of the twentieth century, phosphates have been used in washing and cleaning agents as chelating agents for calcium ions to soften the water. This significantly increased the washing and cleaning power of the surfactants used. The most important chelating agent between 1950 and 1970 was pentasodium triphosphate. However, despite its excellent secondary washing ability, there was a need to look for better alternatives due to its significant contribution to the eutrophication of waters.

Therefore, to soften the water in detergents and cleaners, pentasodium triphosphate was often replaced by EDTA (ethylenediaminetetraacetic acid) and NTA (nitrilotriacetic acid). Both substances also form complexes with the cations of calcium and magnesium, which are responsible for the water hardness. Nevertheless, these complexing agents have disadvantages, so that more ecological solutions were sought. Ecological detergents contain as substitutes for the conventional complexing agents those from renewable raw materials, which are also readily biodegradable.

Thus ethylenediaminetetraacetic acid is very poorly biodegradable under normal conditions. Their degradability works very well only at low pH values. In addition, ethylenediaminetetraacetic acid is also considered environmentally hazardous because of its ability to mobilize heavy metals from the soil. Nitrilotriacetic acid, on the other hand, is suspected of being carcinogenic.

As substitutes for nitrilotriacetic acid and ethylenediaminetetraacetic acid, in addition to zeolites, polycarboxylates or phosphonates, biobased complexing agents such as citric acid in cleaning agents and GLDA as GLDA-Na4 in detergents are increasingly being used today. Organic cleaning agents often contain citric acid in the form of cleaning agents to dissolve the lime and at the same time to soften the water by complexing. In detergents, as a green alternative, GLDA-Na4, in addition to other bio-based complexing agents, is replacing the ligands ethylenediaminetetraacetic acid and nitrilotriacetic acid. With the scientific name tetrasodium N, N-bis (carboxylatomethyl) -L-glutamate, GLDA-Na4 is the sodium salt of L-glutamic acid-N, N-diacetic acid (GDLA-H4).

The biobased chelating agent GLDA is significantly more environmentally compatible with comparable complexing properties than the petrochemically-based ligands ethylenediaminetetraacetic acid and nitrilotriacetic acid. The starting material for GLDA-Na4 is the amino acid glutamic acid, which in turn is obtained by fermentation from renewable raw materials with a high glutamic acid content. GLDA-Na4 is also characterized by a very good biodegradability.