Ferric EDTA

Synthesis and structure

Solutions of Fe(III)-EDTA are produced by combining ferrous salts and aqueous solutions of EDTA known as Jacobson's solution.

Near neutral pH, the principal complex is [Fe(EDTA)(H2O)]−, although most sources ignore the aquo ligand. The [Fe(EDTA)(H2O)]− anion has been crystallized with many cations, e.g., the trihydrate Na[Fe(EDTA)(H2O)].2H2O. The salts as well as the solutions are yellow-brown. Provided the nutrient solution in which the [Fe(EDTA)(H2O)]− complex will be used has a pH of at least 5.5, all the uncomplexed iron, as a result of incomplete synthesis reaction, will still change into the chelated ferric form.

Uses

EDTA is used to solubilize iron(III) in water. In the absence of EDTA or similar chelating agents, ferric ions form insoluble solids and are thus not bioavailable.

Together with pentetic acid (DTPA), EDTA is widely used for sequestering metal ions. Otherwise these metal ions catalyze the decomposition of hydrogen peroxide, which is used to bleach pulp in papermaking. Several million kilograms EDTA are produced for this purpose annually.

Iron chelate is commonly used for agricultural purposes to treat chlorosis, a condition in which leaves produce insufficient chlorophyll. Iron and ligand are absorbed separately by the plant roots whereby the highly stable ferric chelate is first reduced to the less stable ferrous chelate. In horticulture, iron chelate is often referred to as 'sequestered iron' and is used as a plant tonic, often mixed with other nutrients and plant foods (e.g. seaweed). It is recommended in ornamental horticulture for feeding ericaceous plants like Rhododendrons if they are growing in calcareous soils. The sequestered iron is available to the ericaceous plants, without adjusting the soil's pH, and thus, lime-induced chlorosis is prevented.

Ferric EDTA can be used as a component for the Hoagland solution or the Long Ashton Nutrient Solution. According to Jacobson (1951), the stability of ferric EDTA was tested by adding 5 ppm iron, as the complex, to Hoagland's solution at various pH values. No loss of iron occurred below pH 6.