n-ch2-ch2-n HOO-CH2' NCH2-COOH

This is potentially hexadentate. It coordinates as the anion, with an oxygen from each ionized carboxyl group and the two nitrogens being the donor atoms. (It may be added parenthetically that while EDTA is potentially hex-adentate, it may not be so in all its complexes. For example, a water molecule may occupy the sixth coordination site, leaving a carboxyl group free.) EDTA is widely used in analytical chemistry and for other purposes that call for the sequestration of metal ions in soluble form. A related substance, nitrilotriacetic acid (NTA),


ch2cooh was suggested as a phosphate replacement, and large-scale plans for its introduction were under way in about 1970. Again, an oxygen from each ionized carboxylate group and the nitrogen can act as the donor atoms. Because of concerns (now believed to be unfounded) that NTA could enhance the toxic effects of heavy metals (e.g., cadmium), plans for its use in the United States were canceled, although NTA is used in Canada and Finland, for example.

Sequestration of metal ions to prevent them from interfering with useful processes or materials is not limited to the laundry. Trace amounts of metals can have undesirable catalytic activity that can be prevented by the presence of complexing agents. For example, a salt of EDTA is sometimes added to canned foods to protect them from undesirable effects from trace metal components. Some forms of metal poisoning have been treated by a strong ligand (e.g., EDTA), which removes the metal ions from binding sites in the body and leads to its excretion as a soluble complex. (Such treatment is not a simple matter; the complexing agent also removes essential metals.) The unavailability of essential trace metal nutrients in soils has been overcome by addition of complexing agents that extract the needed metal ions from insoluble minerals to produce soluble forms to aid plant growth.

There is a flip side to the benefits of increased solubility of metal ions from complexing. That is the possibility that the solubility of toxic elements such as heavy metals could be increased through the formation of soluble complexes from otherwise insoluble salts.

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