Ferric iron

Ferric refers to iron-containing materials or compounds. In chemistry the term is reserved for iron with an oxidation number of +3, also denoted iron(III) or Fe³⁺. On the other hand, ferrous refers to iron with oxidation number of +2, denoted iron(II) or Fe²⁺. Iron(III) is usually the most stable form of iron in air, as illustrated by the pervasiveness of rust, an insoluble iron(III)-containing material. The word ferric is derived from the Latin word ferrum for iron.

Many proteins contain ferric ions. Examples include iron-sulfur clusters, oxyhemoglobin, ferritin, and the .

The bioavailability of iron is of great interest because all known forms of life require iron and ordinary iron(III) compounds are insoluble in an aerobic environment. Iron-deficiency anemia illustrates the problems resulting from low iron intake. Many foods contain soluble iron compounds and are therefore necessary for good nutrition.

The low bioavailability of iron affects all forms of life. Bacteria secrete iron-attracting agents called siderophores that form soluble compounds of iron that can be reabsorbed into the cell and used in building iron-containing metalloproteins. The impact of increasing the bioavailability of iron was famously demonstrated by an experiment where a large area of the ocean surface was sprayed with iron(III) salts. After several days, the phytoplankton within the treated area bloomed to such an extent that the effect was visible from outer space. This fertilizing process has been proposed as a means to mitigate the carbon dioxide content of the atmosphere.

Ferric iron mitigates the eutrophication of lakes by reducing the bioavailability of phosphorus (as phosphate) in the water. Mitigation arises because ferric phosphate is insoluble. Like iron, phosphate is often a limiting nutrient, and its reduction in concentration from solution limits the growth of algae, which in turn prevents eutrophication.

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