Ferric

In chemistry, iron(III) refers to the element iron in its +3 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe3+. In chemistry, iron(III) refers to the element iron in its +3 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe3+. The adjective ferric or the prefix ferri- is often used to specify such compounds — as in 'ferric chloride' for iron(III) chloride, FeCl3. The adjective 'ferrous' is used instead for iron(II) salts, containing the cation or Fe2+. The word ferric is derived from the Latin word ferrum for iron. Iron(III) atoms may also occur as coordination complexes, such as the anion ferrioxalate, 3− or 3; and organometallic compounds, such as the cation ferrocenium, + or 2. Iron is almost always encountered in the oxidation states 0 (as in the metal), +2, or +3. Iron(III) is usually the most stable form in air, as illustrated by the pervasiveness of rust, an insoluble iron(III)-containing material. All known forms of life require iron. Many proteins in living beings contain bound iron(III) ions; those are an important subclass of the metalloproteins. Examples include oxyhemoglobin, ferredoxin, and the cytochromes. Almost all living organisms, from bacteria to humans, store iron as microscopic crystals (3 to 8 nm in diameter) of iron(III) oxide hydroxide, inside a shell of the protein ferritin, from which it can be recovered as needed. Insufficient iron in the human diet causes anemia. Animals and humans can obtain the necessary iron from foods that contain it in assimilable form, such as meat. Other organisms must obtain their iron from the environment. However, iron tends to form highly insoluble iron(III) oxides/hydroxides in aerobic (oxygenated) environment, especially in calcareous soils. Bacteria and grasses can thrive in such environments by secreting compounds called siderophores that form soluble complexes with iron(III), that can be reabsorbed into the cell. (The other plants instead encourage the growth around their roots of certain bacteria that reduce iron(III) to the more soluble iron(II).) The formation of insoluble iron(III) compounds is also responsible for the low levels of iron in seawater, which is often the limiting factor for the growth of the microscopic plants (phytoplankton) that are the basis of the marine food web. The insolubility of iron(III) compounds be exploited to remedy eutrophication (excessive growth of algae) in lakes contaminated by excess soluble phosphates from farm runoff. Iron(III) combines with the phosphates to form insoluble iron(III) phosphate, thus reducing the bioavailability of phosphorus — another essential element that may also be a limiting nitrient.