Iron(II,III) oxide is the chemical compound with formula Fe
3O
4. It occurs in nature as the mineral
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With ...
. It is one of a number of
iron oxides
Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of w ...
, the others being
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 ...
(FeO), which is rare, and
iron(III) oxide (Fe
2O
3) which also occurs naturally as the mineral
hematite
Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
. It contains both Fe
2+ and Fe
3+ ions and is sometimes formulated as FeO ∙ Fe
2O
3. This iron oxide is encountered in the laboratory as a black powder. It exhibits permanent magnetism and is
ferrimagnetic, but is sometimes incorrectly described as
ferromagnetic.
Its most extensive use is as a black pigment. For this purpose, it is synthesized rather than being extracted from the naturally occurring mineral as the particle size and shape can be varied by the method of production.
Preparation
Heated iron metal interacts with steam to form iron oxide and hydrogen gas.
3Fe + 4H2O->Fe3O4 + 4H2
Under
anaerobic conditions,
ferrous hydroxide (Fe(OH)
2) can be oxidized by water to form magnetite and molecular
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
. This process is described by the
Schikorr reaction
The Schikorr reaction formally describes the conversion of the iron(II) hydroxide (Fe(OH)2) into iron(II,III) oxide (Fe3O4). This transformation reaction was first studied by Gerhard Schikorr. The global reaction follows:
:\underset -> \underse ...
:
:
\underset -> \underset + \underset + \underset
This works because crystalline magnetite (Fe
3O
4) is thermodynamically more stable than amorphous ferrous hydroxide (Fe(OH)
2 ).
The
Massart method of preparation of magnetite as a
ferrofluid, is convenient in the laboratory: mix
iron(II) chloride
Iron(II) chloride, also known as ferrous chloride, is the chemical compound of formula FeCl2. It is a paramagnetic solid with a high melting point. The compound is white, but typical samples are often off-white. FeCl2 crystallizes from water a ...
and
iron(III) chloride
Iron(III) chloride is the inorganic compound with the formula . Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The col ...
in the presence of
sodium hydroxide.
A more efficient method of preparing magnetite without troublesome residues of sodium, is to use ammonia to promote chemical co-precipitation from the iron chlorides: first mix solutions of 0.1 M FeCl
3·6H
2O and FeCl
2·4H
2O with vigorous stirring at about 2000 rpm. The molar ratio of the FeCl
3:FeCl
2 should be about 2:1. Heat the mix to 70 °C, then raise the speed of stirring to about 7500 rpm and quickly add a solution of NH
4OH (10 volume %). A dark precipitate of nanoparticles of magnetite forms immediately.
In both methods, the precipitation reaction relies on rapid transformation of acidic iron ions into the spinel iron oxide structure at pH 10 or higher.
Controlling the formation of magnetite nanoparticles presents challenges: the reactions and phase transformations necessary for the creation of the magnetite spinel structure are complex. The subject is of practical importance because magnetite particles are of interest in bioscience applications such as
magnetic resonance imaging
Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio wave ...
(MRI), in which iron oxide magnetite nanoparticles potentially present a non-toxic alternative to the gadolinium-based
contrast agents
A contrast agent (or contrast medium) is a substance used to increase the contrast of structures or fluids within the body in medical imaging. Contrast agents absorb or alter external electromagnetism or ultrasound, which is different from radioph ...
currently in use. However, difficulties in controlling the formation of the particles, still frustrate the preparation of superparamagnetic magnetite particles, that is to say: magnetite nanoparticles with a coercivity of 0 A/m, meaning that they completely lose their permanent magnetisation in the absence of an external magnetic field. The smallest values currently reported for nanosized magnetite particles is ''Hc'' = 8.5 A m
−1, whereas the largest reported magnetization value is 87 Am
2 kg
−1 for synthetic magnetite.
Pigment quality Fe
3O
4, so called synthetic magnetite, can be prepared using processes that use industrial wastes, scrap iron or solutions containing iron salts (e.g. those produced as by-products in industrial processes such as the acid vat treatment (
pickling) of steel):
*Oxidation of Fe metal in the Laux process where
nitrobenzene is treated with iron metal using FeCl
2 as a catalyst to produce
aniline
Aniline is an organic compound with the formula C6 H5 NH2. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine. It is an industrially significant commodity chemical, as well as a versatile starti ...
:
:C
6H
5NO
2 + 3 Fe + 2 H
2O → C
6H
5NH
2 + Fe
3O
4
*Oxidation of Fe
II compounds, e.g. the precipitation of iron(II) salts as hydroxides followed by oxidation by aeration where careful control of the pH determines the oxide produced.
Reduction of Fe
2O
3 with hydrogen:
:3Fe
2O
3 + H
2 → 2Fe
3O
4 +H
2O
Reduction of Fe
2O
3 with CO:
:3Fe
2O
3 + CO → 2Fe
3O
4 + CO
2
Production of nano-particles can be performed chemically by taking for example mixtures of Fe
II and Fe
III salts and mixing them with alkali to precipitate colloidal Fe
3O
4. The reaction conditions are critical to the process and determine the particle size.
Iron(II) carbonate
Iron(II) carbonate, or ferrous carbonate, is a chemical compound with formula , that occurs naturally as the mineral siderite. At ordinary ambient temperatures, it is a green-brown ionic solid consisting of iron(II) cations and carbonate anions ...
can also be thermally decomposed into Iron(II,III):
:
Reactions
Reduction of magnetite ore by
CO in a
blast furnace is used to produce iron as part of steel production process:
:
+ 4CO -> + 4CO2
Controlled oxidation of Fe
3O
4 is used to produce brown pigment quality
γ-Fe2O3 (
maghemite):
[Gunter Buxbaum, Gerhard Pfaff (2005) ''Industrial Inorganic Pigments'' 3d edition Wiley-VCH ]
:
More vigorous calcining (roasting in air) gives red pigment quality
α-Fe2O3 (
hematite
Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
):
:
Structure
Fe
3O
4 has a cubic inverse
spinel group structure which consists of a cubic close packed array of oxide ions where all of the Fe
2+ ions occupy half of the octahedral sites and the Fe
3+ are split evenly across the remaining octahedral sites and the tetrahedral sites.
Both
FeO
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 ...
and
γ-Fe2O3 have a similar cubic close packed array of oxide ions and this accounts for the ready interchangeability between the three compounds on oxidation and reduction as these reactions entail a relatively small change to the overall structure.
Fe
3O
4 samples can be
non-stoichiometric.
The
ferrimagnetism of Fe
3O
4 arises because the electron spins of the Fe
II and Fe
III ions in the octahedral sites are coupled and the spins of the Fe
III ions in the tetrahedral sites are coupled but anti-parallel to the former. The net effect is that the magnetic contributions of both sets are not balanced and there is a permanent magnetism.
In the molten state, experimentally constrained models show that the iron ions are coordinated to 5 oxygen ions on average.
There is a distribution of coordination sites in the liquid state, with the majority of both Fe
II and Fe
III being 5-coordinated to oxygen and minority populations of both 4- and 6-fold coordinated iron.
Properties
Fe
3O
4 is
ferrimagnetic with a
Curie temperature of . There is a phase transition at , called
Verwey transition where there is a discontinuity in the structure, conductivity and magnetic properties. This effect has been extensively investigated and whilst various explanations have been proposed, it does not appear to be fully understood.
While it has much higher
electrical resistivity than iron metal (96.1 nΩ m), Fe
3O
4's electrical resistivity (0.3 mΩ m ) is significantly lower than that of
Fe2O3 (approx kΩ m). This is ascribed to electron exchange between the Fe
II and Fe
III centres in Fe
3O
4.
Uses
Fe
3O
4 is used as a black pigment and is known as ''C.I pigment black 11'' (C.I. No.77499) or
Mars Black.
Fe
3O
4 is used as a catalyst in the
Haber process and in the
water-gas shift reaction.
[Sunggyu Lee (2006) Encyclopedia of Chemical Processing CRC Press ] The latter uses an HTS (high temperature shift catalyst) of iron oxide stabilised by
chromium oxide.
This iron–chrome catalyst is reduced at reactor start up to generate Fe
3O
4 from α-Fe
2O
3 and Cr
2O
3 to CrO
3.
Bluing is a
passivation process that produces a layer of Fe
3O
4 on the surface of steel to protect it from rust. Along with sulfur and aluminium, it is an ingredient in steel-cutting
thermite.
Medical uses
Nano particles of Fe
3O
4 are used as contrast agents in
MRI scanning.
Ferumoxytol, sold under the brand names Feraheme and Rienso, is an
intravenous Fe
3O
4 preparation for treatment of
anemia
Anemia or anaemia (British English) is a blood disorder in which the blood has a reduced ability to carry oxygen due to a lower than normal number of red blood cells, or a reduction in the amount of hemoglobin. When anemia comes on slowly, t ...
resulting from
chronic kidney disease.
Ferumoxytol is manufactured and globally distributed by
AMAG Pharmaceuticals.
Biological occurrence
Magnetite has been found as nano-crystals in
magnetotactic bacteria
Magnetotactic bacteria (or MTB) are a polyphyletic group of bacteria that orient themselves along the magnetic field lines of Earth's magnetic field. Discovered in 1963 by Salvatore Bellini and rediscovered in 1975 by Richard Blakemore, this ...
(42–45 nm)
and in the beak tissue of
homing pigeons.
References
External links
*
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Iron(II,III) compounds
Semiconductor materials
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