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, FeCl
. 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, [Fe(C
]3− or [Fe3+][C
; and organometallic compounds, such as the cation ferrocenium, [Fe(C
]+ or [Fe3+][C

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.

Ferric oxide, commonly, though not precisely, called rust

Iron(III) and life

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 diamter) of iron(III) oxide hydroxide, inside a shell of the protein ferritin, from which it can be recovered as needed. [1]

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).)[2]

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.[3]

Pourbaix Diagram of Iron
Pourbaix diagram of aqueous iron

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.

Chemistry of iron(III)

Some iron(III) salts, like the chloride FeCl
, sulfate Fe
, and nitrate Fe(NO
are soluble in water. However, other salts like oxide Fe
(hematite) and iron(III) oxide-hydroxide FeO(OH) are extremely insoluble, at least at neutral pH, due to their polymeric structure. Therefore, those soluble iron(III) salts tend to hydrolyze when dissolved in pure water, producing iron(III) hydroxide Fe(OH)
that immediately converts to polymeric oxide-hydroxide via the process called olation and precipitates out of the solution. That reaction liberates hydrogen ions H+ to the solution, lowering the pH, until an equilibrium is reached.[4]

Fe3+ + 2H
O ⇌ FeO(OH) + 3H+

As a result, concentrated solutions of iron(III) salts are quite acidic. The easy reduction of iron(III) to iron(II) lets iron(III) salts function also as oxidizers. Iron(III) chloride solutions are used to etch copper-coated plastic sheets in the production of printed circuit boards.

This behavior of iron(III) salts contrasts with salts of cations whose hydroxides are more soluble, like sodium chloride NaCl (table salt), that dissolve in water without noticeable hydrolysis and without lowering the pH.[4]

Rust is a mixture of iron(III) oxide and oxide-hydroxide that usually forms when iron metal is exposed to humid air. Unlike the passivating oxide layers that are formed by other metals, like chromium and aluminum, rust flakes off, because it is bulkier than the metal that formed it. Therefore, unprotected iron objects will in time be completely turned into rust


Iron(III) is a d5 center, meaning that the metal has five "valence" electrons in the 3d orbital shell. These partially filled or unfilled d-orbitals can accept a large variety of ligands to form coordination complexes. The number and type of ligands is described by ligand field theory. Usually ferric ions are surrounded by six ligands arranged in octahedron; but sometimes three and sometimes as many as seven ligands are observed.

Various chelating compounds cause iron oxide-hydroxide (like rust) to dissolve even at neutral pH, by forming soluble complexes with the iron(III) ion that are more stable than it. These ligands include EDTA, which is often used to dissolve iron deposits or added to fertilizers to make iron in the soil available to plants. Citrate also solubilizes ferric ion at neutral pH, although its complexes are less stable than those of EDTA.


The magnetism of ferric compounds is mainly determined by the five d-electrons, and the ligands that connect to those orbitals.


In qualitative inorganic analysis, the presence of ferric ion can be detected by the formation of its thiocyanate complex. Addition of thiocyanate salts to the solution gives the intensely red 1:1 complex.[5][6] The reaction is a classic school experiment to demonstrate Le Chatelier's principle:

]3+ + SCN
⇌ [Fe(SCN)(H
]2+ + H

See also


  1. ^ Berg, Jeremy Mark; Lippard, Stephen J. (1994). Principles of bioinorganic chemistry. Sausalito, Calif: University Science Books. ISBN 0-935702-73-3.
  2. ^ H. Marschner and V. Römheld (1994): "Strategies of plants for acquisition of iron". Plant and Soil, volume 165, issue 2, pages 261–274. doi:10.1007/BF00008069
  3. ^ Boyd PW, Watson AJ, Law CS, et al. (October 2000). "A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization". Nature. 407 (6805): 695–702. Bibcode:2000Natur.407..695B. doi:10.1038/35037500. PMID 11048709.
  4. ^ a b Earnshaw, A.; Greenwood, N. N. (1997). Chemistry of the elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 0-7506-3365-4.
  5. ^ Lewin, Seymour A.; Wagner, Roselin Seider (1953). "The nature of iron(III) thiocyanate in solution". Journal of Chemical Education. 30 (9): 445. Bibcode:1953JChEd..30..445L. doi:10.1021/ed030p445.
  6. ^ Bent, H. E.; French, C. L. (1941). "The Structure of Ferric Thiocyanate and its Dissociation in Aqueous Solution". Journal of the American Chemical Society. 63 (2): 568–572. doi:10.1021/ja01847a059.
Ammonium ferric citrate

Ammonium ferric citrate has the formula (NH4)5[Fe(C6H4O7)2]. A distinguishing feature of this compound is that it is very soluble in water, in contrast to ferric citrate which is not very soluble.

In its crystal structure each citric acid moiety has lost four protons, and the deprotonated hydroxyl groups act as ligands together with four carboxylate groups; two carboxylate groups are not coordinated to the ferric ion.

Ferric oxalate

Ferric oxalate, also known as iron(III) oxalate, is a chemical compound composed of ferric ions and oxalate ligands; it may also be regarded as the ferric salt of oxalic acid. The anhydrous material is pale yellow; however, it may be hydrated to form several hydrates, such as Fe2(C2O4)3 · 6H2O which is bright green in colour

Hearts of Iron III

Hearts of Iron III is a grand strategy video game developed by Paradox Development Studio and published by Paradox Interactive. The Microsoft Windows version of the game was released on August 7, 2009, while the Mac OS X version was released on December 7, 2009. A grand strategy wargame that focuses on World War II, it is the sequel to 2005's Hearts of Iron II and the third main installment in the Hearts of Iron series.

Initially, the game received a mixed reception, largely because of the large number of bugs present in the game at release. After several patches, the game's reception improved. In December 2009, it had an average score of 77 on Metacritic.

A sequel, Hearts of Iron IV, was released on June 6, 2016.


Iron () is a chemical element with symbol Fe (from Latin: ferrum) and atomic number 26. It is a metal, that belongs to the first transition series and group 8 of the periodic table. It is by mass the most common element on Earth, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust.

Pure iron is very rare on the Earth's crust, basically being limited to meteorites. Iron ores are quite abundant, but extracting usable metal from them requires kilns or furnaces capable of reaching 1500 °C or higher, about 500 °C higher than what is enough to smelt copper. Humans started to dominate that process in Eurasia only about 2000 BCE, and iron began to displace copper alloys for tools and weapons, in some regions, only around 1200 BCE. That event is considered the transition from the Bronze Age to the Iron Age. Iron alloys, such as steel, inox, and special steels are now by far the most common industrial metals, because of their mechanical properties and their low cost.

Pristine and smooth pure iron surfaces are mirror-like silvery-gray. However, iron reacts readily with oxygen and water to give brown to black hydrated iron oxides, commonly known as rust. Unlike the oxides of some other metals, that form passivating layers, rust occupies more volume than the metal and thus flakes off, exposing fresh surfaces for corrosion.

The body of an adult human contains about 4 grams (0.005% body weight) of iron, mostly in hemoglobin and myoglobin. These two proteins play essential roles in vertebrate metabolism, respectively oxygen transport by blood and oxygen storage in muscles. To maintain the necessary levels, human iron metabolism requires a minimum of iron in the diet. Iron is also the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals.Chemically, the most common oxidation states of iron are iron(II) and iron(III). Iron shares many properties of other transition metals, including the other group 8 elements, ruthenium and osmium. Iron forms compounds in a wide range of oxidation states, −2 to +7. Iron also forms many coordination compounds; some of them, such as ferrocene, ferrioxalate, and Prussian blue, have substantial industrial, medical, or research applications.

Iron(II) oxide

Iron(II) oxide or ferrous oxide is the inorganic compound with the formula FeO. Its mineral form is known as wüstite. One of several iron oxides, it is a black-colored powder that is sometimes confused with rust, the latter of which consists of hydrated iron(III) oxide (ferric oxide). Iron(II) oxide also refers to a family of related non-stoichiometric compounds, which are typically iron deficient with compositions ranging from Fe0.84O to Fe0.95O.

Iron(III) bromide

Iron(III) bromide is the chemical compound with the formula FeBr3. Also known as ferric bromide, this red-brown odorless compound is used as a Lewis acid catalyst in the halogenation of aromatic compounds. It dissolves in water to give acidic solutions.

Iron(III) chloride

Iron(III) chloride (FeCl3), also called ferric chloride, is an industrial scale commodity chemical compound with iron in the +3 oxidation state. The compound also exist as a hexahydrate with the formula trans-[Fe(H2O)4Cl2]Cl · 2H2O normally written as FeCl3 · 6H2O. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The color depends on the viewing angle: by reflected light the crystals appear dark green, but by transmitted light they appear purple-red. The hexahydrate has a melting point of 37 °C and appears orange-brown in color. In nature, iron(III) chloride is known as the mineral molysite, but it is rare and mainly found from some fumaroles. It is however an industrial scale commodity.

Iron(III) chloride dissolves in water, but undergoes partial hydrolysis in an exothermic reaction, and result in a strongly acidic solution. The resulting brown, acidic, and corrosive solution is used as a flocculant in sewage treatment and drinking water production, and as an etchant for copper-based metals in printed circuit boards.

Anhydrous iron(III) chloride is deliquescent; the partial hydrolysis also occurs as it absorbs water from the air, liberating hydrogen chloride that forms mists in moist air. It is a fairly strong Lewis acid, and it is used as a catalyst in organic synthesis.

Iron(III) chromate

Iron(III) chromate is the iron(III) salt of chromic acid with the chemical formula Fe2(CrO4)3.

Iron(III) fluoride

Iron(III) fluoride, also known as ferric fluoride, are inorganic compounds with the formula FeF3(H2O)x where x = 0 or 3. They are mainly of interest by researchers, unlike the related iron(III) chlorides. Anhydrous iron(III) fluoride is white, whereas the hydrated forms are light pink.

Iron(III) nitrate

Iron(III) nitrate, or ferric nitrate, is the chemical compound with the formula Fe(NO3)3. Since it is deliquescent, it is commonly found in its nonahydrate form Fe(NO3)3·9H2O in which it forms colourless to pale violet crystals. When dissolved, it forms yellow solution due to hydrolysis.

Iron(III) oxide

Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare; and iron(II,III) oxide (Fe3O4), which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main source of iron for the steel industry. Fe2O3 is readily attacked by acids. Iron(III) oxide is often called rust, and to some extent this label is useful, because rust shares several properties and has a similar composition. To a chemist, rust is considered an ill-defined material, described as hydrated ferric oxide.

Iron(III) oxide-hydroxide

Iron(III) oxide-hydroxide or ferric oxyhydroxide is the chemical compound of iron, oxygen, and hydrogen with formula FeO(OH).

The compound is often encountered as one of its hydrates, FeO(OH)·nH2O. The monohydrate FeO(OH)·H2O (CAS 51274-00-1, C.I. 77492) is often referred as iron(III) hydroxide Fe(OH)3, hydrated iron oxide, yellow iron oxide, or Pigment Yellow 42.

Iron(III) phosphate

Iron(III) phosphate, also ferric phosphate, is the inorganic compound with the formula FePO4. Several related materials are known, including four polymorphs of FePO4 and two polymorphs of the dihydrate FePO4·(H2O)2. These materials find several technical applications as well as occurring in the mineral kingdom.

Iron(III) sulfate

Iron(III) sulfate (or ferric sulfate), is the chemical compound with the formula Fe2(SO4)3. Usually yellow, it is a salt and soluble in water. A variety of hydrates are also known. Solutions are used in dyeing as a mordant, and as a coagulant for industrial wastes. It is also used in pigments, and in pickling baths for aluminum and steel.

Iron(III) sulfide

Iron(III) sulfide, also known as ferric sulfide or sesquisulfide, is one of the three iron sulfides besides FeS and FeS2. It is a solid, black powder but decays at ambient temperature into a yellow-green powder.

This is a relatively unstable artificial product that does not occur in nature.

Iron oxide

Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely used by humans, e.g., as iron ores, pigments, catalysts, in thermite (see the diagram) and hemoglobin. Common rust is a form of iron(III) oxide. Iron oxides are widely used as inexpensive, durable pigments in paints, coatings and colored concretes. Colors commonly available are in the "earthy" end of the yellow/orange/red/brown/black range. When used as a food coloring, it has E number E172.


An oxide is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– atom. Metal oxides thus typically contain an anion of oxygen in the oxidation state of −2. Most of the Earth's crust consists of solid oxides, the result of elements being oxidized by the oxygen in air or in water. Hydrocarbon combustion affords the two principal carbon oxides: carbon monoxide and carbon dioxide. Even materials considered pure elements often develop an oxide coating. For example, aluminium foil develops a thin skin of Al2O3 (called a passivation layer) that protects the foil from further corrosion. Individual elements can often form multiple oxides, each containing different amounts of the element and oxygen. In some cases these are distinguished by specifying the number of atoms as in carbon monoxide and carbon dioxide, and in other cases by specifying the element's oxidation number, as in iron(II) oxide and iron(III) oxide. Certain elements can form many different oxides, such as those of nitrogen.

Prussian blue

Prussian blue is a dark blue pigment produced by oxidation of ferrous ferrocyanide salts. It has the idealized chemical formula Fe7(CN)18. Another name for the color is Berlin blue or, in painting, Parisian or Paris blue. Turnbull's blue is the same substance, but is made from different reagents, and its slightly different color stems from different impurities.

Prussian blue was the first modern synthetic pigment. It is prepared as a very fine colloidal dispersion, because the compound is not soluble in water. It contains variable amounts of other ions and its appearance depends sensitively on the size of the colloidal particles. The pigment is used in paints, and it is the traditional "blue" in blueprints and aizuri-e (藍摺り絵) Japanese woodblock prints.

In medicine, orally administered Prussian blue is used as an antidote for certain kinds of heavy metal poisoning, e.g., by thallium(I) and radioactive isotopes of caesium. The therapy exploits the compound's ion-exchange properties and high affinity for certain "soft" metal cations.

It is on the World Health Organization's List of Essential Medicines, the most important medications needed in a basic health system. Prussian blue lent its name to prussic acid (hydrogen cyanide) derived from it. In German, hydrogen cyanide is called Blausäure ("blue acid"). French chemist Joseph Louis Gay-Lussac gave cyanide its name, from the Ancient Greek word κύανος (kyanos, "blue"), because of the color of Prussian blue.


Tris(acetylacetonato) iron(III), often abbreviated Fe(acac)3, is a ferric coordination complex featuring acetylacetonate (acac) ligands, making it one of a family of metal acetylacetonates. It is a red air-stable solid that dissolves in nonpolar organic solvents.

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