Iron(III) oxide-hydroxide or ferric oxyhydroxide
[A. L. Mackay (1960): "β-Ferric Oxyhydroxide". ''Mineralogical Magazine'' (''Journal of the Mineralogical Society''), volume 32, issue 250, pages 545-557. ] is the chemical compound of
iron,
oxygen, and
hydrogen with formula .
The compound is often encountered as one of its
hydrate
In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
s, ·''n''
ust The monohydrate · is often referred to as iron(III) hydroxide ,
[CAS , C.I. 77492] hydrated iron oxide, yellow iron oxide, or Pigment Yellow 42.
Natural occurrences
Minerals
Anhydrous ferric hydroxide occurs in the nature as the exceedingly rare mineral
bernalite, Fe(OH)
3·''n''H
2O (n=0.0-0.25).
Iron oxyhydroxides, , are much more common and occur naturally as structurally different
minerals (polymorphs) denoted by the Greek letters α, β, γ and δ.
*
Goethite, α-FeO(OH), has been used as an
ochre
Ochre ( ; , ), or ocher in American English, is a natural clay earth pigment, a mixture of ferric oxide and varying amounts of clay and sand. It ranges in colour from yellow to deep orange or brown. It is also the name of the colours produced ...
pigment since prehistoric times.
*
Akaganeite is the β polymorph,
[A. L. Mackay (1962): "β-Ferric oxyhydroxide—akaganéite", ''Mineralogical Magazine'' (''Journal of the Mineralogical Society''), volume 33, issue 259, pages 270-280 ] formed by weathering and noted for its presence in some meteorites and the lunar surface. However, recently it has been determined that it must contain some
chloride ions to stabilize its structure, so that its more accurate formula is or .
[C. Rémazeilles and Ph. Refait (2007): "On the formation of β-FeOOH (akaganéite) in chloride-containing environments". ''Corrosion Science'', volume 49, issue 2, pages 844-857. ]
*
Lepidocrocite, the γ polymorph, is commonly encountered as
rust on the inside of steel water pipes and tanks.
*
Feroxyhyte (δ) is formed under the high pressure conditions of sea and ocean floors, being thermodynamically unstable with respect to the α polymorph (goethite) at surface conditions.
Non-mineral
*
Siderogel is a naturally occurring
colloid
A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...
al form of iron(III) oxide-hydroxide.
Goethite and lepidocrocite, both crystallizing in orthorhombic system, are the most common forms of iron(III) oxyhydroxide and the most important mineral carriers of iron in soils.
Mineraloids
Iron(III) oxyhydroxide is the main component of other minerals and
mineraloid
A mineraloid is a naturally occurring mineral-like substance that does not demonstrate crystallinity. Mineraloids possess chemical compositions that vary beyond the generally accepted ranges for specific minerals. For example, obsidian is an amor ...
s:
*
Limonite
Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as FeO(OH)·H2O, although this is not entirely accurate as the ratio of oxide to hydroxid ...
is a commonly occurring mixture of mainly goethite, lepidocrocite,
quartz and
clay minerals
Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.
Clay mineral ...
.
*
Ferrihydrite is an amorphous or
nanocrystalline hydrated mineral, officially •1.8 but with widely variable hydration.
Properties
The color of iron(III) oxyhydroxide ranges from yellow through dark-brown to black, depending on the degree of hydration, particle size and shape, and crystal structure.
Structure
The crystal structure of β- (akaganeite) is that of
hollandite or . The unit cell is tetragonal with ''a''=1.048 and ''c''=0.3023
nm, and contains eight formula units of FeOOH. Its dimensions are about 500 × 50 × 50 nm.
Twinning often produces particles with the shape of hexagonal stars.
[
]
Chemistry
On heating, β- decomposes and recrystallizes as α- (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 . ...
).[
]
Uses
Limonite
Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as FeO(OH)·H2O, although this is not entirely accurate as the ratio of oxide to hydroxid ...
, a mixture of various hydrates and polymorphs of ferric oxyhydroxide, is one of the three major iron ore
Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the fo ...
s, having been used since at least 2500 BC.[MacEachern, Scott (1996):]
"Iron Age beginnings north of the Mandara Mountains, Cameroon and Nigeria"
''In'' ''In'' Pwiti, Gilbert and Soper, Robert (editors) (1996) ''Aspects of African Archaeology: Proceedings of the Tenth Pan-African Congress'' University of Zimbabwe Press, Harare, Zimbabwe, , pages 489-496. Archive
here
on 2012-03-11.
Yellow iron oxide, or Pigment Yellow 42, is Food and Drug Administration (FDA) approved for use in cosmetics and is used in some tattoo inks.
Iron oxide-hydroxide is also used in aquarium
An aquarium (plural: ''aquariums'' or ''aquaria'') is a vivarium of any size having at least one transparent side in which aquatic plants or animals are kept and displayed. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, aq ...
water treatment as a phosphate binder.
Iron oxide-hydroxide nanoparticles have been studied as possible adsorbents for lead removal from aquatic media.[Safoora Rahimi, Rozita M. Moattari, Laleh Rajabi, Ali Ashraf Derakhshan, and Mohammad Keyhani (2015): "Iron oxide/hydroxide (α,γ-FeOOH) nanoparticles as high potential adsorbents for lead removal from polluted aquatic media". ''Journal of Industrial and Engineering Chemistry'', volume 23, pages 33-43. ]
Medication
Iron polymaltose is used in treatment of iron-deficiency anemia.
Production
Iron(III) oxyhydroxide precipitates from solutions of iron(III) salts at pH between 6.5 and 8.[Tim Grundl and Jim Delwiche (1993): "Kinetics of ferric oxyhydroxide precipitation". ''Journal of Contaminant Hydrology'', volume 14, issue 1, pages 71-87. ]
Thus the oxyhydroxide can be obtained in the lab by reacting an iron(III) salt, such as ferric chloride or ferric nitrate
Iron(III) nitrate, or ferric nitrate, is the name used for a series of inorganic compounds with the formula Fe(NO3)3.(H2O)n. Most common is the nonahydrate Fe(NO3)3.(H2O)9. The hydrates are all pale colored, water-soluble paramagnetic salts.
H ...
, with sodium hydroxide
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and alkali ...
:[K. H. Gayer and Leo Woontner (1956): "The Solubility of Ferrous Hydroxide and Ferric Hydroxide in Acidic and Basic Media at 25°". ''Journal of Physical Chemistry'', volume 60, issue 11, pages 1569–1571. ]
: + 3 NaOH → + 3 NaCl
: + 3 NaOH → + 3
In fact, when dissolved in water, pure will hydrolyze to some extent, yielding the oxyhydroxide and making the solution acidic:[
: + 2 ↔ + 3
Therefore, the compound can also be obtained by the decomposition of acidic solutions of iron(III) chloride held near the boiling point for days or weeks:][Egon Matijević and Paul Scheiner (1978): "Ferric hydrous oxide sols: III. Preparation of uniform particles by hydrolysis of Fe(III)-chloride, -nitrate, and -perchlorate solutions". ''Journal of Colloid and Interface Science'', volume 63, issue 3, pages 509-524. ]
: + 2 → (s) + 3 (g)
(The same process applied to iron(III) nitrate or perchlorate solutions yields instead particles of α-.[)
Another similar route is the decomposition of iron(III) nitrate dissolved in ]stearic acid
Stearic acid ( , ) is a saturated fatty acid with an 18-carbon chain. The IUPAC name is octadecanoic acid. It is a waxy solid and its chemical formula is C17H35CO2H. Its name comes from the Greek word στέαρ "''stéar''", which means tallow. ...
at about 120 °C.[Dan Li, Xiaohui Wang, Gang Xiong, Lude Lu, Xujie Yang and Xin Wang (1997): "A novel technique to prepare ultrafine via hydrated iron(III) nitrate". ''Journal of Materials Science Letters'' volume 16, pages 493–495 ]
The oxyhydroxide prepared from ferric chloride is usually the β polymorph (akaganeite), often in the form of thin needles.[Donald O.
Whittemore and Donald Langmuir (1974): "Ferric Oxyhydroxide Microparticles in Water". ''Environmental Health Perspective'', volume 9, pages 173-176. ]
The oxyhydroxide can also be produced by a solid-state transformation from iron(II) chloride tetrahydrate ·4.[
The compound also readily forms when iron(II) hydroxide is exposed to air:
:4 + → 4 + 2
The iron(II) hydroxide can also be oxidized by hydrogen peroxide in the presence of an acid:
:2 + → 2
]
See also
* Rust
*Iron oxide
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 whic ...
*Yellow boy
Acid mine drainage, acid and metalliferous drainage (AMD), or acid rock drainage (ARD) is the outflow of acidic water from metal mining, mines or Coal mining, coal mines.
Acid rock drainage occurs naturally within some environments as part of ...
, a yellow precipitate when acidic runoff such as mine waste, is then neutralised
References
{{Antianemic preparations
Iron(III) compounds
Hydroxides
Transition metal oxides