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= Iron =
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Introduction
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Iron is a chemical element; it has symbol Fe () 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 abundant element in the Earth's crust, being mainly deposited by
meteorites in its metallic state.
Extracting usable metal from iron ores requires kilns or furnaces
capable of reaching 1500 C, about 500 °C (900 °F) higher than that
required to smelt copper. Humans started to master that process in
Eurasia during the 2nd millennium BC and the use of iron tools and
weapons began to displace copper alloys - in some regions, only around
1200 BC. That event is considered the transition from the Bronze
Age to the Iron Age. In the modern world, iron alloys, such as steel,
stainless steel, cast iron and special steels, are by far the most
common industrial metals, due to their mechanical properties and low
cost. The iron and steel industry is thus very important economically,
and iron is the cheapest metal, with a price of a few dollars per
kilogram or pound.
Pristine and smooth pure iron surfaces are a mirror-like silvery-gray.
Iron reacts readily with oxygen and water to produce 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 more fresh
surfaces for corrosion. 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, −4 to +7. Iron also forms many coordination
complexes; some of them, such as ferrocene, ferrioxalate, and Prussian
blue have substantial industrial, medical, or research applications.
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 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.
Allotropes
============
At least four allotropes of iron (differing atom arrangements in the
solid) are known, conventionally denoted α, γ, δ, and ε.
The first three forms are observed at ordinary pressures. As molten
iron cools past its freezing point of 1538 °C, it crystallizes
into its δ allotrope, which has a body-centered cubic (bcc) crystal
structure. As it cools further to 1394 °C, it changes to its
γ-iron allotrope, a face-centered cubic (fcc) crystal structure, or
austenite. At 912 °C and below, the crystal structure again
becomes the bcc α-iron allotrope.
The physical properties of iron at very high pressures and
temperatures have also been studied extensively, because of their
relevance to theories about the cores of the Earth and other planets.
Above approximately 10 GPa and temperatures of a few hundred
kelvin or less, α-iron changes into another hexagonal close-packed
(hcp) structure, which is also known as ε-iron. The higher-temperature
γ-phase also changes into ε-iron, but does so at higher pressure.
Some controversial experimental evidence exists for a stable β phase
at pressures above 50 GPa and temperatures of at least
1500 K. It is supposed to have an orthorhombic or a double hcp
structure. (Confusingly, the term "β-iron" is sometimes also used to
refer to α-iron above its Curie point, when it changes from being
ferromagnetic to paramagnetic, even though its crystal structure has
not changed.)
The Earth's inner core is generally presumed to consist of an
iron-nickel alloy with ε (or β) structure.
Melting and boiling points
============================
The melting and boiling points of iron, along with its enthalpy of
atomization, are lower than those of the earlier 3d elements from
scandium to chromium, showing the lessened contribution of the 3d
electrons to metallic bonding as they are attracted more and more into
the inert core by the nucleus; however, they are higher than the
values for the previous element manganese because that element has a
half-filled 3d sub-shell and consequently its d-electrons are not
easily delocalized. This same trend appears for ruthenium but not
osmium.
The melting point of iron is experimentally well defined for pressures
less than 50 GPa. For greater pressures, published data (as of
2007) still varies by tens of gigapascals and over a thousand kelvin.
Magnetic properties
=====================
Below its Curie point of , α-iron changes from paramagnetic to
ferromagnetic: the spins of the two unpaired electrons in each atom
generally align with the spins of its neighbors, creating an overall
magnetic field. This happens because the orbitals of those two
electrons (d'z'2 and d'x'2 − 'y'2) do not point toward neighboring
atoms in the lattice, and therefore are not involved in metallic
bonding.
In the absence of an external source of magnetic field, the atoms get
spontaneously partitioned into magnetic domains, about
10 micrometers across, such that the atoms in each domain have
parallel spins, but some domains have other orientations. Thus a
macroscopic piece of iron will have a nearly zero overall magnetic
field.
Application of an external magnetic field causes the domains that are
magnetized in the same general direction to grow at the expense of
adjacent ones that point in other directions, reinforcing the external
field. This effect is exploited in devices that need to channel
magnetic fields to fulfill design function, such as electrical
transformers, magnetic recording heads, and electric motors.
Impurities, lattice defects, or grain and particle boundaries can
"pin" the domains in the new positions, so that the effect persists
even after the external field is removed - thus turning the iron
object into a (permanent) magnet.
Similar behavior is exhibited by some iron compounds, such as the
ferrites including the mineral magnetite, a crystalline form of the
mixed iron(II,III) oxide (although the atomic-scale mechanism,
ferrimagnetism, is somewhat different). Pieces of magnetite with
natural permanent magnetization (lodestones) provided the earliest
compasses for navigation. Particles of magnetite were extensively used
in magnetic recording media such as core memories, magnetic tapes,
floppies, and disks, until they were replaced by cobalt-based
materials.
Isotopes
==========
Iron has four stable isotopes: 54Fe (5.845% of natural iron), 56Fe
(91.754%), 57Fe (2.119%) and 58Fe (0.282%). Twenty-four artificial
isotopes have also been created. Of these stable isotopes, only 57Fe
has a nuclear spin (−). The nuclide 54Fe theoretically can undergo
double electron capture to 54Cr, but the process has never been
observed and only a lower limit on the half-life of 4.4×1020 years has
been established.
60Fe is an extinct radionuclide of long half-life (2.6 million
years). It is not found on Earth, but its ultimate decay product is
its granddaughter, the stable nuclide 60Ni. Much of the past work on
isotopic composition of iron has focused on the nucleosynthesis of
60Fe through studies of meteorites and ore formation. In the last
decade, advances in mass spectrometry have allowed the detection and
quantification of minute, naturally occurring variations in the ratios
of the stable isotopes of iron. Much of this work is driven by the
Earth and planetary science communities, although applications to
biological and industrial systems are emerging.
In phases of the meteorites 'Semarkona' and 'Chervony Kut,' a
correlation between the concentration of 60Ni, the granddaughter of
60Fe, and the abundance of the stable iron isotopes provided evidence
for the existence of 60Fe at the time of formation of the Solar
System. Possibly the energy released by the decay of 60Fe, along with
that released by 26Al, contributed to the remelting and
differentiation of asteroids after their formation 4.6 billion
years ago. The abundance of 60Ni present in extraterrestrial material
may bring further insight into the origin and early history of the
Solar System.
The most abundant iron isotope 56Fe is of particular interest to
nuclear scientists because it represents the most common endpoint of
nucleosynthesis. Since 56Ni (14 alpha particles) is easily produced
from lighter nuclei in the alpha process in nuclear reactions in
supernovae (see silicon burning process), it is the endpoint of fusion
chains inside extremely massive stars. Although adding more alpha
particles is possible, but nonetheless the sequence does effectively
end at 56Ni because conditions in stellar interiors cause the
competition between photodisintegration and the alpha process to favor
photodisintegration around 56Ni. This 56Ni, which has a half-life of
about 6 days, is created in quantity in these stars, but soon
decays by two successive positron emissions within supernova decay
products in the supernova remnant gas cloud, first to radioactive
56Co, and then to stable 56Fe. As such, iron is the most abundant
element in the core of red giants, and is the most abundant metal in
iron meteorites and in the dense metal cores of planets such as Earth.
It is also very common in the universe, relative to other stable
metals of approximately the same atomic weight. Iron is the sixth most
abundant element in the universe, and the most common refractory
element.
Although a further tiny energy gain could be extracted by synthesizing
62Ni, which has a marginally higher binding energy than 56Fe,
conditions in stars are unsuitable for this process. Element
production in supernovas greatly favor iron over nickel, and in any
case, 56Fe still has a lower mass per nucleon than 62Ni due to its
higher fraction of lighter protons. Hence, elements heavier than iron
require a supernova for their formation, involving rapid neutron
capture by starting 56Fe nuclei.
In the far future of the universe, assuming that proton decay does not
occur, cold fusion occurring via quantum tunnelling would cause the
light nuclei in ordinary matter to fuse into 56Fe nuclei. Fission and
alpha-particle emission would then make heavy nuclei decay into iron,
converting all stellar-mass objects to cold spheres of pure iron.
Cosmogenesis
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Iron's abundance in rocky planets like Earth is due to its abundant
production during the runaway fusion and explosion of type Ia
supernovae, which scatters the iron into space.
Metallic iron
===============
Metallic or native iron is rarely found on the surface of the Earth
because it tends to oxidize. However, both the Earth's inner and outer
core, which together account for 35% of the mass of the whole Earth,
are believed to consist largely of an iron alloy, possibly with
nickel. Electric currents in the liquid outer core are believed to be
the origin of the Earth's magnetic field. The other terrestrial
planets (Mercury, Venus, and Mars) as well as the Moon are believed to
have a metallic core consisting mostly of iron. The M-type asteroids
are also believed to be partly or mostly made of metallic iron alloy.
The rare iron meteorites are the main form of natural metallic iron on
the Earth's surface. Items made of cold-worked meteoritic iron have
been found in various archaeological sites dating from a time when
iron smelting had not yet been developed; and the Inuit in Greenland
have been reported to use iron from the Cape York meteorite for tools
and hunting weapons. About 1 in 20 meteorites consist of the unique
iron-nickel minerals taenite (35-80% iron) and kamacite (90-95% iron).
Native iron is also rarely found in basalts that have formed from
magmas that have come into contact with carbon-rich sedimentary rocks,
which have reduced the oxygen fugacity sufficiently for iron to
crystallize. This is known as telluric iron and is described from a
few localities, such as Disko Island in West Greenland, Yakutia in
Russia and Bühl in Germany.
Mantle minerals
=================
Ferropericlase , a solid solution of periclase (MgO) and wüstite
(FeO), makes up about 20% of the volume of the lower mantle of the
Earth, which makes it the second most abundant mineral phase in that
region after silicate perovskite ; it also is the major host for iron
in the lower mantle. At the bottom of the transition zone of the
mantle, the reaction γ- transforms γ-olivine into a mixture of
silicate perovskite and ferropericlase and vice versa. In the
literature, this mineral phase of the lower mantle is also often
called magnesiowüstite. Silicate perovskite may form up to 93% of the
lower mantle, and the magnesium iron form, , is considered to be the
most abundant mineral in the Earth, making up 38% of its volume.
Earth's crust
===============
While iron is the most abundant element on Earth, most of this iron is
concentrated in the inner and outer cores. The fraction of iron that
is in Earth's crust only amounts to about 5% of the overall mass of
the crust and is thus only the fourth most abundant element in that
layer (after oxygen, silicon, and aluminium).
Most of the iron in the crust is combined with various other elements
to form many iron minerals. An important class is the iron oxide
minerals such as hematite (Fe2O3), magnetite (Fe3O4), and siderite
(FeCO3), which are the major ores of iron. Many igneous rocks also
contain the sulfide minerals pyrrhotite and pentlandite. During
weathering, iron tends to leach from sulfide deposits as the sulfate
and from silicate deposits as the bicarbonate. Both of these are
oxidized in aqueous solution and precipitate in even mildly elevated
pH as iron(III) oxide.
Large deposits of iron are banded iron formations, a type of rock
consisting of repeated thin layers of iron oxides alternating with
bands of iron-poor shale and chert. The banded iron formations were
laid down in the time between and .
Materials containing finely ground iron(III) oxides or
oxide-hydroxides, such as ochre, have been used as yellow, red, and
brown pigments since pre-historical times. They contribute as well to
the color of various rocks and clays, including entire geological
formations like the Painted Hills in Oregon and the Buntsandstein
("colored sandstone", British Bunter). Through 'Eisensandstein' (a
jurassic 'iron sandstone', e.g. from Donzdorf in Germany) and Bath
stone in the UK, iron compounds are responsible for the yellowish
color of many historical buildings and sculptures. The proverbial red
color of the surface of Mars is derived from an iron oxide-rich
regolith.
Significant amounts of iron occur in the iron sulfide mineral pyrite
(FeS2), but it is difficult to extract iron from it and it is
therefore not exploited. In fact, iron is so common that production
generally focuses only on ores with very high quantities of it.
According to the International Resource Panel's Metal Stocks in
Society report, the global stock of iron in use in society is
2,200 kg per capita. More-developed countries differ in this
respect from less-developed countries (7,000-14,000 vs 2,000 kg
per capita).
Oceans
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Ocean science demonstrated the role of the iron in the ancient seas in
both marine biota and climate.
Chemistry and compounds
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Oxidation state !! Representative compound
−2 (d10) Disodium tetracarbonylferrate (Collman's reagent)
−1 (d9)
0 (d8) Iron pentacarbonyl
1 (d7) Cyclopentadienyliron dicarbonyl dimer ("Fp2")
2 (d6) Ferrous sulfate, Ferrocene
3 (d5) Ferric chloride, Ferrocenium tetrafluoroborate
4 (d4) , FeO(BF4)2
5 (d3)
6 (d2) Potassium ferrate
|7 (d1) |[FeO4]- (matrix isolation, 4K)
Iron shows the characteristic chemical properties of the transition
metals, namely the ability to form variable oxidation states differing
by steps of one and a very large coordination and organometallic
chemistry: indeed, it was the discovery of an iron compound,
ferrocene, that revolutionalized the latter field in the 1950s. Iron
is sometimes considered as a prototype for the entire block of
transition metals, due to its abundance and the immense role it has
played in the technological progress of humanity. Its 26 electrons are
arranged in the configuration [Ar]3d64s2, of which the 3d and 4s
electrons are relatively close in energy, and thus a number of
electrons can be ionized.
Iron forms compounds mainly in the oxidation states +2 (iron(II),
"ferrous") and +3 (iron(III), "ferric"). Iron also occurs in higher
oxidation states, e.g., the purple potassium ferrate (K2FeO4), which
contains iron in its +6 oxidation state. The anion [FeO4]- with iron
in its +7 oxidation state, along with an iron(V)-peroxo isomer, has
been detected by infrared spectroscopy at 4 K after
cocondensation of laser-ablated Fe atoms with a mixture of O2/Ar.
Iron(IV) is a common intermediate in many biochemical oxidation
reactions. Numerous organoiron compounds contain formal oxidation
states of +1, 0, −1, or even −2. The oxidation states and other
bonding properties are often assessed using the technique of Mössbauer
spectroscopy. Many mixed valence compounds contain both iron(II) and
iron(III) centers, such as magnetite and Prussian blue (). The latter
is used as the traditional "blue" in blueprints.
Iron is the first of the transition metals that cannot reach its group
oxidation state of +8, although its heavier congeners ruthenium and
osmium can, with ruthenium having more difficulty than osmium.
Ruthenium exhibits an aqueous cationic chemistry in its low oxidation
states similar to that of iron, but osmium does not, favoring high
oxidation states in which it forms anionic complexes. In the second
half of the 3d transition series, vertical similarities down the
groups compete with the horizontal similarities of iron with its
neighbors cobalt and nickel in the periodic table, which are also
ferromagnetic at room temperature and share similar chemistry. As
such, iron, cobalt, and nickel are sometimes grouped together as the
iron triad.
Unlike many other metals, iron does not form amalgams with mercury. As
a result, mercury is traded in standardized 76 pound flasks
(34 kg) made of iron.
Iron is by far the most reactive element in its group; it is
pyrophoric when finely divided and dissolves easily in dilute acids,
giving Fe2+. However, it does not react with concentrated nitric acid
and other oxidizing acids due to the formation of an impervious oxide
layer, which can nevertheless react with hydrochloric acid.
High-purity iron, called electrolytic iron, is considered to be
resistant to rust, due to its oxide layer.
Oxides and sulfides
=====================
Iron forms various oxide and hydroxide compounds; the most common are
iron(II,III) oxide (Fe3O4), and iron(III) oxide (Fe2O3). Iron(II)
oxide also exists, though it is unstable at room temperature. Despite
their names, they are actually all non-stoichiometric compounds whose
compositions may vary. These oxides are the principal ores for the
production of iron (see bloomery and blast furnace). They are also
used in the production of ferrites, useful magnetic storage media in
computers, and pigments. The best known sulfide is iron pyrite (FeS2),
also known as fool's gold owing to its golden luster. It is not an
iron(IV) compound, but is actually an iron(II) polysulfide containing
Fe2+ and ions in a distorted sodium chloride structure.
Halides
=========
The binary ferrous and ferric halides are well-known. The ferrous
halides typically arise from treating iron metal with the
corresponding hydrohalic acid to give the corresponding hydrated
salts.
:Fe + 2 HX → FeX2 + H2 (X = F, Cl, Br, I)
Iron reacts with fluorine, chlorine, and bromine to give the
corresponding ferric halides, ferric chloride being the most common.
:2 Fe + 3 X2 → 2 FeX3 (X = F, Cl, Br)
Ferric iodide is an exception, being thermodynamically unstable due to
the oxidizing power of Fe3+ and the high reducing power of I−:
:2 I− + 2 Fe3+ → I2 + 2 Fe2+ (E0 = +0.23 V)
Ferric iodide, a black solid, is not stable in ordinary conditions,
but can be prepared through the reaction of iron pentacarbonyl with
iodine and carbon monoxide in the presence of hexane and light at the
temperature of −20 °C, with oxygen and water excluded. Complexes
of ferric iodide with some soft bases are known to be stable
compounds.
Solution chemistry
====================
The standard reduction potentials in acidic aqueous solution for some
common iron ions are given below:
: [Fe(H2O)6]2+ + 2 e− Fe E0 = −0.447 V
[Fe(H2O)6]3+ + e− [Fe(H2O)6]2+ E0 = +0.77 V
+ 8 H3O+ + 3 e− [Fe(H2O)6]3+ + 6 H2O E0 = +2.20 V
The red-purple tetrahedral ferrate(VI) anion is such a strong
oxidizing agent that it oxidizes ammonia to nitrogen (N2) and water to
oxygen:
:4 + 34 → 4 + 20 + 3 O2
The pale-violet hexaquo complex is an acid such that above pH 0 it is
fully hydrolyzed:
: 'K' = 10−3.05 mol dm−3
'K' = 10−3.26 mol dm−3
'K' = 10−2.91 mol dm−3
As pH rises above 0 the above yellow hydrolyzed species form and as it
rises above 2-3, reddish-brown hydrous iron(III) oxide precipitates
out of solution. Although Fe3+ has a d5 configuration, its absorption
spectrum is not like that of Mn2+ with its weak, spin-forbidden d-d
bands, because Fe3+ has higher positive charge and is more polarizing,
lowering the energy of its ligand-to-metal charge transfer
absorptions. Thus, all the above complexes are rather strongly
colored, with the single exception of the hexaquo ion - and even that
has a spectrum dominated by charge transfer in the near ultraviolet
region. On the other hand, the pale green iron(II) hexaquo ion does
not undergo appreciable hydrolysis. Carbon dioxide is not evolved when
carbonate anions are added, which instead results in white iron(II)
carbonate being precipitated out. In excess carbon dioxide this forms
the slightly soluble bicarbonate, which occurs commonly in
groundwater, but it oxidises quickly in air to form iron(III) oxide
that accounts for the brown deposits present in a sizeable number of
streams.
Coordination compounds
========================
Due to its electronic structure, iron has a very large coordination
and organometallic chemistry.
Many coordination compounds of iron are known. A typical
six-coordinate anion is hexachloroferrate(III), [FeCl6]3−, found in
the mixed salt tetrakis(methylammonium) hexachloroferrate(III)
chloride. Complexes with multiple bidentate ligands have geometric
isomers. For example, the
'trans'-chlorohydridobis(bis-1,2-(diphenylphosphino)ethane)iron(II)
complex is used as a starting material for compounds with the moiety.
The ferrioxalate ion with three oxalate ligands displays helical
chirality with its two non-superposable geometries labelled 'Λ'
(lambda) for the left-handed screw axis and 'Δ' (delta) for the
right-handed screw axis, in line with IUPAC conventions. Potassium
ferrioxalate is used in chemical actinometry and along with its sodium
salt undergoes photoreduction applied in old-style photographic
processes. The dihydrate of iron(II) oxalate has a polymeric structure
with co-planar oxalate ions bridging between iron centres with the
water of crystallisation located forming the caps of each octahedron,
as illustrated below.
Iron(III) complexes are quite similar to those of chromium(III) with
the exception of iron(III)'s preference for 'O'-donor instead of
'N'-donor ligands. The latter tend to be rather more unstable than
iron(II) complexes and often dissociate in water. Many Fe-O complexes
show intense colors and are used as tests for phenols or enols. For
example, in the ferric chloride test, used to determine the presence
of phenols, iron(III) chloride reacts with a phenol to form a deep
violet complex:
:3 ArOH + FeCl3 → Fe(OAr)3 + 3 HCl (Ar = aryl)
Among the halide and pseudohalide complexes, fluoro complexes of
iron(III) are the most stable, with the colorless [FeF5(H2O)]2− being
the most stable in aqueous solution. Chloro complexes are less stable
and favor tetrahedral coordination as in [FeCl4]−; [FeBr4]− and
[FeI4]− are reduced easily to iron(II). Thiocyanate is a common test
for the presence of iron(III) as it forms the blood-red
[Fe(SCN)(H2O)5]2+. Like manganese(II), most iron(III) complexes are
high-spin, the exceptions being those with ligands that are high in
the spectrochemical series such as cyanide. An example of a low-spin
iron(III) complex is [Fe(CN)6]3−. Iron shows a great variety of
electronic spin states, including every possible spin quantum number
value for a d-block element from 0 (diamagnetic) to (5 unpaired
electrons). This value is always half the number of unpaired
electrons. Complexes with zero to two unpaired electrons are
considered low-spin and those with four or five are considered
high-spin.
Iron(II) complexes are less stable than iron(III) complexes but the
preference for 'O'-donor ligands is less marked, so that for example
is known while is not. They have a tendency to be oxidized to
iron(III) but this can be moderated by low pH and the specific ligands
used.
Organometallic compounds
==========================
Organoiron chemistry is the study of organometallic compounds of iron,
where carbon atoms are covalently bound to the metal atom. They are
many and varied, including cyanide complexes, carbonyl complexes,
sandwich and half-sandwich compounds.
Prussian blue or "ferric ferrocyanide", Fe4[Fe(CN)6]3, is an old and
well-known iron-cyanide complex, extensively used as pigment and in
several other applications. Its formation can be used as a simple wet
chemistry test to distinguish between aqueous solutions of Fe2+ and
Fe3+ as they react (respectively) with potassium ferricyanide and
potassium ferrocyanide to form Prussian blue.
Another old example of an organoiron compound is iron pentacarbonyl,
Fe(CO)5, in which a neutral iron atom is bound to the carbon atoms of
five carbon monoxide molecules. The compound can be used to make
carbonyl iron powder, a highly reactive form of metallic iron.
Thermolysis of iron pentacarbonyl gives triiron dodecacarbonyl, , a
complex with a cluster of three iron atoms at its core. Collman's
reagent, disodium tetracarbonylferrate, is a useful reagent for
organic chemistry; it contains iron in the −2 oxidation state.
Cyclopentadienyliron dicarbonyl dimer contains iron in the rare +1
oxidation state.
A landmark in this field was the discovery in 1951 of the remarkably
stable sandwich compound ferrocene , by Pauson and Kealy and
independently by Miller and colleagues, whose surprising molecular
structure was determined only a year later by Woodward and Wilkinson
and Fischer.
Ferrocene is still one of the most important tools and models in this
class.
Iron-centered organometallic species are used as catalysts. The
Knölker complex, for example, is a transfer hydrogenation catalyst for
ketones.
Industrial uses
=================
The iron compounds produced on the largest scale in industry are
iron(II) sulfate (FeSO4·7H2O) and iron(III) chloride (FeCl3). The
former is one of the most readily available sources of iron(II), but
is less stable to aerial oxidation than Mohr's salt (). Iron(II)
compounds tend to be oxidized to iron(III) compounds in the air.
Development of iron metallurgy
================================
Iron is one of the elements undoubtedly known to the ancient world. It
has been worked, or wrought, for millennia. However, iron artefacts of
great age are much rarer than objects made of gold or silver due to
the ease with which iron corrodes. The technology developed slowly,
and even after the discovery of smelting it took many centuries for
iron to replace bronze as the metal of choice for tools and weapons.
Meteoritic iron
=================
Beads made from meteoric iron in 3500 BC or earlier were found in
Gerzeh, Egypt by G. A. Wainwright. The beads contain 7.5% nickel,
which is a signature of meteoric origin since iron found in the
Earth's crust generally has only minuscule nickel impurities.
Meteoric iron was highly regarded due to its origin in the heavens and
was often used to forge weapons and tools. For example, a dagger made
of meteoric iron was found in the tomb of Tutankhamun, containing
similar proportions of iron, cobalt, and nickel to a meteorite
discovered in the area, deposited by an ancient meteor shower. Items
that were likely made of iron by Egyptians date from 3000 to
2500 BC.
Meteoritic iron is comparably soft and ductile and easily cold forged
but may get brittle when heated because of the nickel content.
Wrought iron
==============
The first iron production started in the Middle Bronze Age, but it
took several centuries before iron displaced bronze. Samples of
smelted iron from Asmar, Mesopotamia and Tall Chagar Bazaar in
northern Syria were made sometime between 3000 and 2700 BC. The
Hittites established an empire in north-central Anatolia around
1600 BC. They appear to be the first to understand the production
of iron from its ores and regard it highly in their society. The
Hittites began to smelt iron between 1500 and 1200 BC and the
practice spread to the rest of the Near East after their empire fell
in 1180 BC. The subsequent period is called the Iron Age.
Artifacts of smelted iron are found in India dating from 1800 to
1200 BC, and in the Levant from about 1500 BC (suggesting
smelting in Anatolia or the Caucasus). Alleged references (compare
history of metallurgy in South Asia) to iron in the Indian Vedas have
been used for claims of a very early usage of iron in India
respectively to date the texts as such. The rigveda term 'ayas'
(metal) refers to copper, while iron which is called as 'śyāma ayas',
literally "black copper", first is mentioned in the post-rigvedic
Atharvaveda.
Some archaeological evidence suggests iron was smelted in Zimbabwe and
southeast Africa as early as the eighth century BC. Iron working was
introduced to Greece in the late 11th century BC, from which it
spread quickly throughout Europe.
The spread of ironworking in Central and Western Europe is associated
with Celtic expansion. According to Pliny the Elder, iron use was
common in the Roman era. In the lands of what is now considered China,
iron appears approximately 700-500 BC. Iron smelting may have
been introduced into China through Central Asia. The earliest evidence
of the use of a blast furnace in China dates to the 1st century AD,
and cupola furnaces were used as early as the Warring States period
(403-221 BC). Usage of the blast and cupola furnace remained
widespread during the Tang and Song dynasties.
During the Industrial Revolution in Britain, Henry Cort began refining
iron from pig iron to wrought iron (or bar iron) using innovative
production systems. In 1783 he patented the puddling process for
refining iron ore. It was later improved by others, including Joseph
Hall.
Cast iron
===========
Cast iron was first produced in China during 5th century BC, but was
hardly in Europe until the medieval period. The earliest cast iron
artifacts were discovered by archaeologists in what is now modern Luhe
County, Jiangsu in China. Cast iron was used in ancient China for
warfare, agriculture, and architecture. During the medieval period,
means were found in Europe of producing wrought iron from cast iron
(in this context known as pig iron) using finery forges. For all these
processes, charcoal was required as fuel.
Medieval blast furnaces were about 10 ft tall and made of fireproof
brick; forced air was usually provided by hand-operated bellows.
Modern blast furnaces have grown much bigger, with hearths fourteen
meters in diameter that allow them to produce thousands of tons of
iron each day, but essentially operate in much the same way as they
did during medieval times.
In 1709, Abraham Darby I established a coke-fired blast furnace to
produce cast iron, replacing charcoal, although continuing to use
blast furnaces. The ensuing availability of inexpensive iron was one
of the factors leading to the Industrial Revolution. Toward the end of
the 18th century, cast iron began to replace wrought iron for certain
purposes, because it was cheaper. Carbon content in iron was not
implicated as the reason for the differences in properties of wrought
iron, cast iron, and steel until the 18th century.
Since iron was becoming cheaper and more plentiful, it also became a
major structural material following the building of the innovative
first iron bridge in 1778. This bridge still stands today as a
monument to the role iron played in the Industrial Revolution.
Following this, iron was used in rails, boats, ships, aqueducts, and
buildings, as well as in iron cylinders in steam engines. Railways
have been central to the formation of modernity and ideas of progress
and various languages refer to railways as 'iron road' (e.g. French ,
German , Turkish , Russian , Chinese, Japanese, and Korean 鐵道,
Vietnamese ').
Steel
=======
Steel (with smaller carbon content than pig iron but more than wrought
iron) was first produced in antiquity by using a bloomery. Blacksmiths
in Luristan in western Persia were making good steel by 1000 BC.
Then improved versions, Wootz steel by India and Damascus steel were
developed around 300 BC and AD 500 respectively. These
methods were specialized, and so steel did not become a major
commodity until the 1850s.
New methods of producing it by carburizing bars of iron in the
cementation process were devised in the 17th century. In the
Industrial Revolution, new methods of producing bar iron without
charcoal were devised and these were later applied to produce steel.
In the late 1850s, Henry Bessemer invented a new steelmaking process,
involving blowing air through molten pig iron, to produce mild steel.
This made steel much more economical, thereby leading to wrought iron
no longer being produced in large quantities.
Foundations of modern chemistry
=================================
In 1774, Antoine Lavoisier used the reaction of water steam with
metallic iron inside an incandescent iron tube to produce hydrogen in
his experiments leading to the demonstration of the conservation of
mass, which was instrumental in changing chemistry from a qualitative
science to a quantitative one.
Recent discoveries
====================
* discovery of Mössbauer effect
* many enzymes use iron in the catalytic center
* Nickel-56 is the natural end product of silicon burning in massive
stars. However, nickel-56 decays to cobalt-56 and then to stable
iron-56, ultimately making iron the most abundant heavy element
produced by that nucleosynthesis.
* superconductivity?
* magnetic effect
* ferrocene -->
Symbolic role
======================================================================
Iron plays a certain role in mythology and has found various usage as
a metaphor and in folklore. The Greek poet Hesiod's 'Works and Days'
(lines 109-201) lists different ages of man named after metals like
gold, silver, bronze and iron to account for successive ages of
humanity. The Iron Age was closely related with Rome, and in Ovid's
'Metamorphoses'
An example of the importance of iron's symbolic role may be found in
the German Campaign of 1813. Frederick William III commissioned then
the first Iron Cross as military decoration. Berlin iron jewellery
reached its peak production between 1813 and 1815, when the Prussian
royal family urged citizens to donate gold and silver jewellery for
military funding. The inscription 'Ich gab Gold für Eisen' (I gave
gold for iron) was used as well in later war efforts.
Laboratory routes
===================
For a few limited purposes when it is needed, pure iron is produced in
the laboratory in small quantities by reducing the pure oxide or
hydroxide with hydrogen, or forming iron pentacarbonyl and heating it
to 250 °C so that it decomposes to form pure iron powder. Another
method is electrolysis of ferrous chloride onto an iron cathode.
Main industrial route
=======================
Iron production 2009 (million tonnes)
!Country!!Iron ore!!Pig iron!!Direct iron!!Steel
| 1,114.9 549.4 573.6
| 393.9 4.4 5.2
| 305.0 25.1 0.011 26.5
| 66.9 87.5
| 257.4 38.2 23.4 63.5
| 92.1 43.9 4.7 60.0
| 65.8 25.7 29.9
| 0.1 27.3 48.6
| 0.4 20.1 0.38 32.7
!World!! 1,594.9!!914.0!! 64.5!! 1,232.4
Nowadays, the industrial production of iron or steel consists of two
main stages. In the first stage, iron ore is reduced with coke in a
blast furnace, and the molten metal is separated from gross impurities
such as silicate minerals. This stage yields an alloy - pig iron -
that contains relatively large amounts of carbon. In the second stage,
the amount of carbon in the pig iron is lowered by oxidation to yield
wrought iron, steel, or cast iron. Other metals can be added at this
stage to form alloy steels.
Blast furnace processing
==========================
The blast furnace is loaded with iron ores, usually hematite or
magnetite , along with coke (coal that has been separately baked to
remove volatile components) and flux (limestone or dolomite). "Blasts"
of air pre-heated to 900 °C (sometimes with oxygen enrichment) is
blown through the mixture, in sufficient amount to turn the carbon
into carbon monoxide:
:
This reaction raises the temperature to about 2000 °C. The carbon
monoxide reduces the iron ore to metallic iron:
:
Some iron in the high-temperature lower region of the furnace reacts
directly with the coke:
:
The flux removes silicaceous minerals in the ore, which would
otherwise clog the furnace: The heat of the furnace decomposes the
carbonates to calcium oxide, which reacts with any excess silica to
form a slag composed of calcium silicate or other products. At the
furnace's temperature, the metal and the slag are both molten. They
collect at the bottom as two immiscible liquid layers (with the slag
on top), that are then easily separated. The slag can be used as a
material in road construction or to improve mineral-poor soils for
agriculture.
Steelmaking thus remains one of the largest industrial contributors of
CO2 emissions in the world.
File:Chinese Fining and Blast Furnace.jpg|17th century Chinese
illustration of workers at a blast furnace, making wrought iron from
pig iron
File:Iron-Making.jpg|How iron was extracted in the 19th century
File:Geography of Ohio - DPLA - aaba7b3295ff6973b6fd1e23e33cde14 (page
111) (cropped).jpg|Iron furnace in Columbus, Ohio, 1922
Steelmaking
=============
The pig iron produced by the blast furnace process contains up to 4-5%
carbon (by mass), with small amounts of other impurities like sulfur,
magnesium, phosphorus, and manganese. This high level of carbon makes
it relatively weak and brittle. Reducing the amount of carbon to
0.002-2.1% produces steel, which may be up to 1000 times harder than
pure iron. A great variety of steel articles can then be made by cold
working, hot rolling, forging, machining, etc. Removing the impurities
from pig iron, but leaving 2-4% carbon, results in cast iron, which is
cast by foundries into articles such as stoves, pipes, radiators,
lamp-posts, and rails.
Steel products often undergo various heat treatments after they are
forged to shape. Annealing consists of heating them to 700-800 °C
for several hours and then gradual cooling. It makes the steel softer
and more workable.
File:LightningVolt Iron Ore Pellets.jpg|This heap of iron ore pellets
will be used in steel production.
File:Melted raw-iron.jpg|A pot of molten iron being used to make steel
Direct iron reduction
=======================
Owing to environmental concerns, alternative methods of processing
iron have been developed. "Direct iron reduction" reduces iron ore to
a ferrous lump called "sponge" iron or "direct" iron that is suitable
for steelmaking. Two main reactions comprise the direct reduction
process:
Natural gas is partially oxidized (with heat and a catalyst):
:
Iron ore is then treated with these gases in a furnace, producing
solid sponge iron:
:
Silica is removed by adding a limestone flux as described above.
Thermite process
==================
Ignition of a mixture of aluminium powder and iron oxide yields
metallic iron via the thermite reaction:
:
Alternatively pig iron may be made into steel (with up to about 2%
carbon) or wrought iron (commercially pure iron). Various processes
have been used for this, including finery forges, puddling furnaces,
Bessemer converters, open hearth furnaces, basic oxygen furnaces, and
electric arc furnaces. In all cases, the objective is to oxidize some
or all of the carbon, together with other impurities. On the other
hand, other metals may be added to make alloy steels.
Molten oxide electrolysis
===========================
Molten oxide electrolysis (MOE) uses electrolysis of molten iron oxide
to yield metallic iron. It is studied in laboratory-scale experiments
and is proposed as a method for industrial iron production that has no
direct emissions of carbon dioxide. It uses a liquid iron cathode, an
anode formed from an alloy of chromium, aluminium and iron, and the
electrolyte is a mixture of molten metal oxides into which iron ore is
dissolved. The current keeps the electrolyte molten and reduces the
iron oxide. Oxygen gas is produced in addition to liquid iron. The
only carbon dioxide emissions come from any fossil fuel-generated
electricity used to heat and reduce the metal.
Applications
======================================================================
Characteristic values of tensile strength (TS) and Brinell hardness
(BH) of various forms of iron.
!Material !TS (MPa) !BH (Brinell)
|Iron whiskers |11000
|Ausformed (hardened) steel |2930 |850-1200
|Martensitic steel |2070 |600
|Bainitic steel |1380 |400
|Pearlitic steel |1200 |350
|Cold-worked iron |690 |200
|Small-grain iron |340 |100
|Carbon-containing iron |140 |40
|Pure, single-crystal iron |10 |3
As structural material
========================
Iron is the most widely used of all the metals, accounting for over
90% of worldwide metal production. Its low cost and high strength
often make it the material of choice to withstand stress or transmit
forces, such as the construction of machinery and machine tools,
rails, automobiles, ship hulls, concrete reinforcing bars, and the
load-carrying framework of buildings. Since pure iron is quite soft,
it is most commonly combined with alloying elements to make steel.
Mechanical properties
=======================
The mechanical properties of iron and its alloys are extremely
relevant to their structural applications. Those properties can be
evaluated in various ways, including the Brinell test, the Rockwell
test and the Vickers hardness test.
The properties of pure iron are often used to calibrate measurements
or to compare tests. However, the mechanical properties of iron are
significantly affected by the sample's purity: pure, single crystals
of iron are actually softer than aluminium, and the purest
industrially produced iron (99.99%) has a hardness of
20-30 Brinell. The pure iron (99.9%～99.999%), especially called
electrolytic iron, is industrially produced by electrolytic refining.
An increase in the carbon content will cause a significant increase in
the hardness and tensile strength of iron. Maximum hardness of 65 Rc
is achieved with a 0.6% carbon content, although the alloy has low
tensile strength. Because of the softness of iron, it is much easier
to work with than its heavier congeners ruthenium and osmium.
Types of steels and alloys
============================
α-Iron is a fairly soft metal that can dissolve only a small
concentration of carbon (no more than 0.021% by mass at 910 °C).
Austenite (γ-iron) is similarly soft and metallic but can dissolve
considerably more carbon (as much as 2.04% by mass at 1146 °C).
This form of iron is used in the type of stainless steel used for
making cutlery, and hospital and food-service equipment.
Commercially available iron is classified based on purity and the
abundance of additives. Pig iron has 3.5-4.5% carbon and contains
varying amounts of contaminants such as sulfur, silicon and
phosphorus. Pig iron is not a saleable product, but rather an
intermediate step in the production of cast iron and steel. The
reduction of contaminants in pig iron that negatively affect material
properties, such as sulfur and phosphorus, yields cast iron containing
2-4% carbon, 1-6% silicon, and small amounts of manganese. Pig iron
has a melting point in the range of 1420-1470 K, which is lower
than either of its two main components, and makes it the first product
to be melted when carbon and iron are heated together. Its mechanical
properties vary greatly and depend on the form the carbon takes in the
alloy.
"White" cast irons contain their carbon in the form of cementite, or
iron carbide (Fe3C). This hard, brittle compound dominates the
mechanical properties of white cast irons, rendering them hard, but
unresistant to shock. The broken surface of a white cast iron is full
of fine facets of the broken iron carbide, a very pale, silvery, shiny
material, hence the appellation. Cooling a mixture of iron with 0.8%
carbon slowly below 723 °C to room temperature results in
separate, alternating layers of cementite and α-iron, which is soft
and malleable and is called pearlite for its appearance. Rapid
cooling, on the other hand, does not allow time for this separation
and creates hard and brittle martensite. The steel can then be
tempered by reheating to a temperature in between, changing the
proportions of pearlite and martensite. The end product below 0.8%
carbon content is a pearlite-αFe mixture, and that above 0.8% carbon
content is a pearlite-cementite mixture.
In gray iron the carbon exists as separate, fine flakes of graphite,
and also renders the material brittle due to the sharp edged flakes of
graphite that produce stress concentration sites within the material.
A newer variant of gray iron, referred to as ductile iron, is
specially treated with trace amounts of magnesium to alter the shape
of graphite to spheroids, or nodules, reducing the stress
concentrations and vastly increasing the toughness and strength of the
material.
Wrought iron contains less than 0.25% carbon but large amounts of slag
that give it a fibrous characteristic. Wrought iron is more corrosion
resistant than steel. It has been almost completely replaced by mild
steel, which corrodes more readily than wrought iron, but is cheaper
and more widely available. Carbon steel contains 2.0% carbon or less,
with small amounts of manganese, sulfur, phosphorus, and silicon.
Alloy steels contain varying amounts of carbon as well as other
metals, such as chromium, vanadium, molybdenum, nickel, tungsten, etc.
Their alloy content raises their cost, and so they are usually only
employed for specialist uses. One common alloy steel, though, is
stainless steel. Recent developments in ferrous metallurgy have
produced a growing range of microalloyed steels, also termed 'HSLA' or
high-strength, low alloy steels, containing tiny additions to produce
high strengths and often spectacular toughness at minimal cost.
Alloys with high purity elemental makeups (such as alloys of
electrolytic iron) have specifically enhanced properties such as
ductility, tensile strength, toughness, fatigue strength, heat
resistance, and corrosion resistance.
Apart from traditional applications, iron is also used for protection
from ionizing radiation. Although it is lighter than another
traditional protection material, lead, it is much stronger
mechanically.
The main disadvantage of iron and steel is that pure iron, and most of
its alloys, suffer badly from rust if not protected in some way, a
cost amounting to over 1% of the world's economy. Painting,
galvanization, passivation, plastic coating and bluing are all used to
protect iron from rust by excluding water and oxygen or by cathodic
protection. The mechanism of the rusting of iron is as follows:
:Cathode: 3 O2 + 6 H2O + 12 e− → 12 OH−
:Anode: 4 Fe → 4 Fe2+ + 8 e−; 4 Fe2+ → 4 Fe3+ + 4 e−
:Overall: 4 Fe + 3 O2 + 6 H2O → 4 Fe3+ + 12 OH− → 4 Fe(OH)3 or 4
FeO(OH) + 4 H2O
The electrolyte is usually iron(II) sulfate in urban areas (formed
when atmospheric sulfur dioxide attacks iron), and salt particles in
the atmosphere in seaside areas.
Catalysts and reagents
========================
Because Fe is inexpensive and nontoxic, much effort has been devoted
to the development of Fe-based catalysts and reagents. Iron is
however less common as a catalyst in commercial processes than more
expensive metals. In biology, Fe-containing enzymes are pervasive.
Iron catalysts are traditionally used in the Haber-Bosch process for
the production of ammonia and the Fischer-Tropsch process for
conversion of carbon monoxide to hydrocarbons for fuels and
lubricants. Powdered iron in an acidic medium is used in the Bechamp
reduction, the conversion of nitrobenzene to aniline.
Iron compounds
================
Iron(III) oxide mixed with aluminium powder can be ignited to create a
thermite reaction, used in welding large iron parts (like rails) and
purifying ores. Iron(III) oxide and oxyhydroxide are used as reddish
and ocher pigments.
Iron(III) chloride finds use in water purification and sewage
treatment, in the dyeing of cloth, as a coloring agent in paints, as
an additive in animal feed, and as an etchant for copper in the
manufacture of printed circuit boards. It can also be dissolved in
alcohol to form tincture of iron, which is used as a medicine to stop
bleeding in canaries.
Iron(II) sulfate is used as a precursor to other iron compounds. It is
also used to reduce chromate in cement. It is used to fortify foods
and treat iron deficiency anemia. Iron(III) sulfate is used in
settling minute sewage particles in tank water. Iron(II) chloride is
used as a reducing flocculating agent, in the formation of iron
complexes and magnetic iron oxides, and as a reducing agent in organic
synthesis.
Sodium nitroprusside is a drug used as a vasodilator. It is on the
World Health Organization's List of Essential Medicines.
Biological and pathological role
======================================================================
Iron is required for life.
The iron-sulfur clusters are pervasive and include nitrogenase, the
enzymes responsible for biological nitrogen fixation. Iron-containing
proteins participate in transport, storage and use of oxygen. Iron
proteins are involved in electron transfer.
Examples of iron-containing proteins in higher organisms include
hemoglobin, cytochrome (see high-valent iron), and catalase. The
average adult human contains about 0.005% body weight of iron, or
about four grams, of which three quarters is in hemoglobin--a level
that remains constant despite only about one milligram of iron being
absorbed each day, because the human body recycles its hemoglobin for
the iron content.
Microbial growth may be assisted by oxidation of iron(II) or by
reduction of iron(III).
Biochemistry
==============
Iron acquisition poses a problem for aerobic organisms because ferric
iron is poorly soluble near neutral pH. Thus, these organisms have
developed means to absorb iron as complexes, sometimes taking up
ferrous iron before oxidising it back to ferric iron. In particular,
bacteria have evolved very high-affinity sequestering agents called
siderophores.
After uptake in human cells, iron storage is precisely regulated. A
major component of this regulation is the protein transferrin, which
binds iron ions absorbed from the duodenum and carries it in the blood
to cells. Transferrin contains Fe3+ in the middle of a distorted
octahedron, bonded to one nitrogen, three oxygens and a chelating
carbonate anion that traps the Fe3+ ion: it has such a high stability
constant that it is very effective at taking up Fe3+ ions even from
the most stable complexes. At the bone marrow, transferrin is reduced
from Fe3+ to Fe2+ and stored as ferritin to be incorporated into
hemoglobin.
The most commonly known and studied bioinorganic iron compounds
(biological iron molecules) are the heme proteins: examples are
hemoglobin, myoglobin, and cytochrome P450. These compounds
participate in transporting gases, building enzymes, and transferring
electrons. Metalloproteins are a group of proteins with metal ion
cofactors. Some examples of iron metalloproteins are ferritin and
rubredoxin. Many enzymes vital to life contain iron, such as catalase,
lipoxygenases, and IRE-BP.
Hemoglobin is an oxygen carrier that occurs in red blood cells and
contributes their color, transporting oxygen in the arteries from the
lungs to the muscles where it is transferred to myoglobin, which
stores it until it is needed for the metabolic oxidation of glucose,
generating energy. Here the hemoglobin binds to carbon dioxide,
produced when glucose is oxidized, which is transported through the
veins by hemoglobin (predominantly as bicarbonate anions) back to the
lungs where it is exhaled. In hemoglobin, the iron is in one of four
heme groups and has six possible coordination sites; four are occupied
by nitrogen atoms in a porphyrin ring, the fifth by an imidazole
nitrogen in a histidine residue of one of the protein chains attached
to the heme group, and the sixth is reserved for the oxygen molecule
it can reversibly bind to. When hemoglobin is not attached to oxygen
(and is then called deoxyhemoglobin), the Fe2+ ion at the center of
the heme group (in the hydrophobic protein interior) is in a high-spin
configuration. It is thus too large to fit inside the porphyrin ring,
which bends instead into a dome with the Fe2+ ion about
55 picometers above it. In this configuration, the sixth
coordination site reserved for the oxygen is blocked by another
histidine residue.
When deoxyhemoglobin picks up an oxygen molecule, this histidine
residue moves away and returns once the oxygen is securely attached to
form a hydrogen bond with it. This results in the Fe2+ ion switching
to a low-spin configuration, resulting in a 20% decrease in ionic
radius so that now it can fit into the porphyrin ring, which becomes
planar. Additionally, this hydrogen bonding results in the tilting of
the oxygen molecule, resulting in a Fe-O-O bond angle of around 120°
that avoids the formation of Fe-O-Fe or Fe-O2-Fe bridges that would
lead to electron transfer, the oxidation of Fe2+ to Fe3+, and the
destruction of hemoglobin. This results in a movement of all the
protein chains that leads to the other subunits of hemoglobin changing
shape to a form with larger oxygen affinity. Thus, when
deoxyhemoglobin takes up oxygen, its affinity for more oxygen
increases, and vice versa. Myoglobin, on the other hand, contains only
one heme group and hence this cooperative effect cannot occur. Thus,
while hemoglobin is almost saturated with oxygen in the high partial
pressures of oxygen found in the lungs, its affinity for oxygen is
much lower than that of myoglobin, which oxygenates even at low
partial pressures of oxygen found in muscle tissue. As described by
the Bohr effect (named after Christian Bohr, the father of Niels
Bohr), the oxygen affinity of hemoglobin diminishes in the presence of
carbon dioxide.
Carbon monoxide and phosphorus trifluoride are poisonous to humans
because they bind to hemoglobin similarly to oxygen, but with much
more strength, so that oxygen can no longer be transported throughout
the body. Hemoglobin bound to carbon monoxide is known as
carboxyhemoglobin. This effect also plays a minor role in the toxicity
of cyanide, but there the major effect is by far its interference with
the proper functioning of the electron transport protein cytochrome a.
The cytochrome proteins also involve heme groups and are involved in
the metabolic oxidation of glucose by oxygen. The sixth coordination
site is then occupied by either another imidazole nitrogen or a
methionine sulfur, so that these proteins are largely inert to
oxygen--with the exception of cytochrome a, which bonds directly to
oxygen and thus is very easily poisoned by cyanide. Here, the electron
transfer takes place as the iron remains in low spin but changes
between the +2 and +3 oxidation states. Since the reduction potential
of each step is slightly greater than the previous one, the energy is
released step-by-step and can thus be stored in adenosine
triphosphate. Cytochrome a is slightly distinct, as it occurs at the
mitochondrial membrane, binds directly to oxygen, and transports
protons as well as electrons, as follows:
:4 Cytc2+ + O2 + 8H → 4 Cytc3+ + 2 H2O + 4H
Although the heme proteins are the most important class of
iron-containing proteins, the iron-sulfur proteins are also very
important, being involved in electron transfer, which is possible
since iron can exist stably in either the +2 or +3 oxidation states.
These have one, two, four, or eight iron atoms that are each
approximately tetrahedrally coordinated to four sulfur atoms; because
of this tetrahedral coordination, they always have high-spin iron. The
simplest of such compounds is rubredoxin, which has only one iron atom
coordinated to four sulfur atoms from cysteine residues in the
surrounding peptide chains. Another important class of iron-sulfur
proteins is the ferredoxins, which have multiple iron atoms.
Transferrin does not belong to either of these classes.
The ability of sea mussels to maintain their grip on rocks in the
ocean is facilitated by their use of organometallic iron-based bonds
in their protein-rich cuticles. Based on synthetic replicas, the
presence of iron in these structures increased elastic modulus 770
times, tensile strength 58 times, and toughness 92 times. The amount
of stress required to permanently damage them increased 76 times.
Diet
======
Iron is pervasive, but particularly rich sources of dietary iron
include red meat, oysters, beans, poultry, fish, leaf vegetables,
watercress, tofu, and blackstrap molasses. Bread and breakfast cereals
are sometimes specifically fortified with iron.
Iron provided by dietary supplements is often found as iron(II)
fumarate, although iron(II) sulfate is cheaper and is absorbed equally
well. Elemental iron, or reduced iron, despite being absorbed at only
one-third to two-thirds the efficiency (relative to iron sulfate), is
often added to foods such as breakfast cereals or enriched wheat
flour. Iron is most available to the body when chelated to amino acids
and is also available for use as a common iron supplement. Glycine,
the least expensive amino acid, is most often used to produce iron
glycinate supplements.
Dietary recommendations
=========================
The U.S. Institute of Medicine (IOM) updated Estimated Average
Requirements (EARs) and Recommended Dietary Allowances (RDAs) for iron
in 2001. The current EAR for iron for women ages 1418 is
7.9 mg/day, 8.1 mg/day for ages 1950 and 5.0 mg/day
thereafter (postmenopause). For men, the EAR is 6.0 mg/day for
ages 19 and up. The RDA is 15.0 mg/day for women ages 1518,
18.0 mg/day for ages 1950 and 8.0 mg/day thereafter. For
men, 8.0 mg/day for ages 19 and up. RDAs are higher than EARs so
as to identify amounts that will cover people with higher-than-average
requirements. RDA for pregnancy is 27 mg/day and, for lactation,
9 mg/day. For children ages 13 years 7 mg/day,
10 mg/day for ages 4-8 and 8 mg/day for ages 913. As for
safety, the IOM also sets Tolerable upper intake levels (ULs) for
vitamins and minerals when evidence is sufficient. In the case of
iron, the UL is set at 45 mg/day. Collectively the EARs, RDAs and
ULs are referred to as Dietary Reference Intakes.
The European Food Safety Authority (EFSA) refers to the collective set
of information as Dietary Reference Values, with Population Reference
Intake (PRI) instead of RDA, and Average Requirement instead of EAR.
AI and UL are defined the same as in the United States. For women the
PRI is 13 mg/day ages 1517 years, 16 mg/day for women ages
18 and up who are premenopausal and 11 mg/day postmenopausal. For
pregnancy and lactation, 16 mg/day. For men the PRI is
11 mg/day ages 15 and older. For children ages 1 to 14, the PRI
increases from 7 to 11 mg/day. The PRIs are higher than the U.S.
RDAs, with the exception of pregnancy. The EFSA reviewed the same
safety question did not establish a UL.
Infants may require iron supplements if they are bottle-fed cow's
milk. Frequent blood donors are at risk of low iron levels and are
often advised to supplement their iron intake.
For U.S. food and dietary supplement labeling purposes, the amount in
a serving is expressed as a percent of Daily Value (%DV). For iron
labeling purposes, 100% of the Daily Value was 18 mg, and
remained unchanged at 18 mg. A table of the old and new adult
daily values is provided at Reference Daily Intake.
Deficiency
============
Iron deficiency is the most common nutritional deficiency in the
world. When loss of iron is not adequately compensated by adequate
dietary iron intake, a state of latent iron deficiency occurs, which
over time leads to iron-deficiency anemia if left untreated, which is
characterised by an insufficient number of red blood cells and an
insufficient amount of hemoglobin. Children, pre-menopausal women
(women of child-bearing age), and people with poor diet are most
susceptible to the disease. Most cases of iron-deficiency anemia are
mild, but if not treated can cause problems like fast or irregular
heartbeat, complications during pregnancy, and delayed growth in
infants and children.
The brain is resistant to acute iron deficiency due to the slow
transport of iron through the blood brain barrier. Acute fluctuations
in iron status (marked by serum ferritin levels) do not reflect brain
iron status, but prolonged nutritional iron deficiency is suspected to
reduce brain iron concentrations over time. In the brain, iron plays a
role in oxygen transport, myelin synthesis, mitochondrial respiration,
and as a cofactor for neurotransmitter synthesis and metabolism.
Animal models of nutritional iron deficiency report biomolecular
changes resembling those seen in Parkinson's and Huntington's disease.
However, age-related accumulation of iron in the brain has also been
linked to the development of Parkinson's.
Excess
========
Iron uptake is tightly regulated by the human body, which has no
regulated physiological means of excreting iron. Only small amounts of
iron are lost daily due to mucosal and skin epithelial cell sloughing,
so control of iron levels is primarily accomplished by regulating
uptake. Regulation of iron uptake is impaired in some people as a
result of a genetic defect that maps to the HLA-H gene region on
chromosome 6 and leads to abnormally low levels of hepcidin, a key
regulator of the entry of iron into the circulatory system in mammals.
In these people, excessive iron intake can result in iron overload
disorders, known medically as hemochromatosis. Many people have an
undiagnosed genetic susceptibility to iron overload, and are not aware
of a family history of the problem. For this reason, people should not
take iron supplements unless they suffer from iron deficiency and have
consulted a doctor. Hemochromatosis is estimated to be the cause of
0.3-0.8% of all metabolic diseases of Caucasians.
Overdoses of ingested iron can cause excessive levels of free iron in
the blood. High blood levels of free ferrous iron react with peroxides
to produce highly reactive free radicals that can damage DNA,
proteins, lipids, and other cellular components. Iron toxicity occurs
when the cell contains free iron, which generally occurs when iron
levels exceed the availability of transferrin to bind the iron. Damage
to the cells of the gastrointestinal tract can also prevent them from
regulating iron absorption, leading to further increases in blood
levels. Iron typically damages cells in the heart, liver and
elsewhere, causing adverse effects that include coma, metabolic
acidosis, shock, liver failure, coagulopathy, long-term organ damage,
and even death. Humans experience iron toxicity when the iron exceeds
20 milligrams for every kilogram of body mass; 60 milligrams
per kilogram is considered a lethal dose. Overconsumption of iron,
often the result of children eating large quantities of ferrous
sulfate tablets intended for adult consumption, is one of the most
common toxicological causes of death in children under six. The
Dietary Reference Intake (DRI) sets the Tolerable Upper Intake Level
(UL) for adults at 45 mg/day. For children under fourteen years
old the UL is 40 mg/day.
The medical management of iron toxicity is complicated, and can
include use of a specific chelating agent called deferoxamine to bind
and expel excess iron from the body.
ADHD
======
Some research has suggested that low thalamic iron levels may play a
role in the pathophysiology of ADHD. Some researchers have found that
iron supplementation can be effective especially in the inattentive
subtype of the disorder.
Some researchers in the 2000s suggested a link between low levels of
iron in the blood and ADHD. A 2012 study found no such correlation.
Cancer
========
The role of iron in cancer defense can be described as a "double-edged
sword" because of its pervasive presence in non-pathological
processes.
People having chemotherapy may develop iron deficiency and anemia,
for which intravenous iron therapy is used to restore iron levels.
Iron overload, which may occur from high consumption of red meat, may
initiate tumor growth and increase susceptibility to cancer onset,
particularly for colorectal cancer.
Bioremediation
================
Iron-eating bacteria live in the hulls of sunken ships such as the
'Titanic'. The acidophile bacteria 'Acidithiobacillus ferrooxidans',
'Leptospirillum ferrooxidans', 'Sulfolobus' spp., 'Acidianus
brierleyi' and 'Sulfobacillus thermosulfidooxidans' can oxidize
ferrous iron enzymically. A sample of the fungus 'Aspergillus niger'
was found growing from gold mining solution, and was found to contain
cyano metal complexes such as gold, silver, copper iron and zinc. The
fungus also plays a role in the solubilization of heavy metal
sulfides.-->
Marine systems
================
Iron plays an essential role in marine systems and can act as a
limiting nutrient for planktonic activity. Because of this, too much
of a decrease in iron may lead to a decrease in growth rates in
phytoplanktonic organisms such as diatoms. Iron can also be oxidized
by marine microbes under conditions that are high in iron and low in
oxygen.
Iron can enter marine systems through adjoining rivers and directly
from the atmosphere. Once iron enters the ocean, it can be distributed
throughout the water column through ocean mixing and through recycling
on the cellular level. In the arctic, sea ice plays a major role in
the store and distribution of iron in the ocean, depleting oceanic
iron as it freezes in the winter and releasing it back into the water
when thawing occurs in the summer. The iron cycle can fluctuate the
forms of iron from aqueous to particle forms altering the availability
of iron to primary producers. Increased light and warmth increases the
amount of iron that is in forms that are usable by primary producers.
See also
======================================================================
* Economically important iron deposits include:
** Carajás Mine in the state of Pará, Brazil, is thought to be the
largest iron deposit in the world.
** El Mutún in Bolivia, where 10% of the world's accessible iron ore
is located.
** Hamersley Basin is the largest iron ore deposit in Australia.
** Kiirunavaara in Sweden, where one of the world's largest deposits
of iron ore is located
** The Mesabi Iron Range is the chief iron ore mining district in the
United States.
* Iron and steel industry
* Iron cycle
* Iron nanoparticle
* Iron-platinum nanoparticle
* Iron fertilization - proposed fertilization of oceans to stimulate
phytoplankton growth
* Iron-oxidizing bacteria
* List of countries by iron production
* Pelletising - process of creation of iron ore pellets
* Rustproof iron
* Steel
Further reading
======================================================================
* H.R. Schubert, 'History of the British Iron and Steel
Industry ... to 1775 AD' (Routledge, London, 1957)
* R.F. Tylecote, 'History of Metallurgy' (Institute of Materials,
London 1992).
* R.F. Tylecote, "Iron in the Industrial Revolution" in J. Day and
R.F. Tylecote, 'The Industrial Revolution in Metals' (Institute of
Materials 1991), 200-60.
External links
======================================================================
* [
https://education.jlab.org/itselemental/ele026.html It's Elemental
- Iron]
* [
https://www.periodicvideos.com/videos/026.htm Iron] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
https://books.google.com/books?id=brpx-LtdCLYC&pg=frontcover
Metallurgy for the non-Metallurgist]
* [
https://mysite.du.edu/~jcalvert/phys/iron.htm Iron] by J. B.
Calvert
License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Iron
