Ethylenediaminetetracetic acid (EDTA)
C10H16N2O8, mol wt 292.25
EDTA is the archetypal synthetic chelating agent. The coordinating groups in EDTA are two amine nitrogens and four carboxylic oxygens, which are capable of wrapping around a central metal ion, such as Fe(III), and satisfying the octahedral coordination requirements of Fe(III).
EDTA was the first synthetic chelating agent used for keeping Fe(III) soluble in nutrient solutions for hydroponics (1951, replacing tartaric and citric acids in earlier solutions), although it was early noted that its effectiveness was limited to pH 6.5 or lower. Soil applications of FeEDTA are likewise limited to acidic and slightly acidic soils, which restricts its usefulness since most micronutrient deficiencies that would call for chelate therapy appear in calcareous soils, with pH greater than 7.5. FeEDTA is occasionally used in foliar application of micronutrients. EDTA is permitted for use in human food and is often added to soft drinks as a preservative. EDTA is also used as a therapy for heavy metal and radionuclide poisoning in humans and has also been used experimentally and semi-commercially as a Pb-solubilizing agent as part of phytoextraction treatment for lead-contaminated fields.
Highlighting FeaturesShow central Fe and its nearest neighbors: 2 amino N and 4 carboxylate O Restore whole molecule This model shows the near-perfect octahedral coordination of the central Fe atom, with four carboxylate oxygens and two amino nitrogens as the apices of the octahedron. The symmetry of the central Fe atom with its nearest neighbors is a key factors in chelate stability. In spacefilling mode, this model show that the central Fe atom is nearly completely concealed by coordinating oxygens and nitrogens. No coordination by additional water (H2O) or hydroxide (OH-) is permitted in this structure, a situation which adds to the stability of this chelate. ethylenediamine (H2NCH2CH2NH2) acetic acid [x4] (CH2COOH) glycine [x2] (NH2CH2COOH) Restore whole molecule
Molecular coordinates were calculated by John Nash of Purdue University using HyperChem and are used here with permission.