View source for Iron(III) oxide-hydroxide

{{Short description|Hydrous ferric oxide (HFO)}}
{{Chembox
| Watchedfields = changed
| verifiedrevid = 477000862
| ImageFile = Hydroxid železitý.PNG
| ImageFile_Ref = {{chemboximage|correct|??}}
| ImageSize = 244
| ImageName = Samples of iron(III) oxide-hydroxide monohydrate in a vial, and a spoon
| IUPACName = Iron(III) oxide-hydroxide
| OtherNames = Metaferric acid<br/>Ferric oxyhydroxide<br/>[[Goethite]]
|Section1={{Chembox Identifiers
| CASNo = 1310-14-1
| CASNo_Ref = {{cascite|correct|CAS}}
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 87PZU03K0K
| CASNo1 = 20344-49-4
| CASNo1_Ref = {{cascite|correct|CAS}}
| PubChem = 91502
| ChemSpiderID = 82623
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| EINECS = 215-176-6
| MeSHName = Goethite
| SMILES = O=[Fe]O
| StdInChI = 1S/Fe.H2O.O/h;1H2;/q+1;;/p-1
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = AEIXRCIKZIZYPM-UHFFFAOYSA-M
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
}}
|Section2={{Chembox Properties
| Formula = FeO(OH)
| Appearance = Vivid, dark orange, opaque crystals
| Odor = odorless
| Density = 4.25 g/cm<sup>3</sup>
| Solubility = insoluble at pH 7
| SolubilityProduct = 2.79×10<sup>−39</sup> for Fe(OH)<sub>3</sub><ref>{{cite web |url=http://bilbo.chm.uri.edu/CHM112/tables/KspTable.htm |title=Solubility product constants at 25 oC |access-date=2015-02-23 |url-status=dead |archive-url=https://web.archive.org/web/20150226121932/http://bilbo.chm.uri.edu/CHM112/tables/KspTable.htm |archive-date=2015-02-26 }}</ref>
}}
|Section3={{Chembox Hazards
| NFPA-H = 1
| NFPA-F = 0
| NFPA-R = 0
}}
| Section6 = {{Chembox Pharmacology
| ATCCode_prefix = B03
| ATCCode_suffix = AB04
}}
}}

'''Iron(III) oxide-hydroxide''' or '''ferric oxyhydroxide'''<ref name=mack60>A. L. Mackay (1960): "β-Ferric Oxyhydroxide". ''Mineralogical Magazine'' (''Journal of the Mineralogical Society''), volume 32, issue 250, pages 545-557. {{doi|10.1180/minmag.1960.032.250.04}}</ref> is the chemical compound of [[iron]], [[oxygen]], and [[hydrogen]] with formula {{chem|FeO(OH)}}.

The compound is often encountered as one of its [[hydrate]]s, {{chem|FeO(OH)}}·''n''[[Water|{{chem|H|2|O}}]] ([[rust]]). The monohydrate {{chem|FeO(OH)}}·{{chem|H|2|O}} is often referred to as '''iron(III) hydroxide''' {{chem|Fe|(|OH|)|3}}, '''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)<sub>3</sub>·''n''H<sub>2</sub>O (''n'' = 0.0–0.25).<ref name=Mindat>{{Cite web|url=https://www.mindat.org/min-635.html|title = Bernalite}}</ref><ref name=IMA>{{Cite web|url=https://www.ima-mineralogy.org/Minlist.htm|title=List of Minerals|date=21 March 2011}}</ref> Iron oxyhydroxides, {{chem|FeOOH}}, are much more common and occur naturally as structurally different [[mineral]]s (polymorphs) denoted by the Greek letters α, β, γ and δ. 

* [[Goethite]], α-FeO(OH), has been used as an [[ochre]] pigment since prehistoric times. 

* [[Akaganeite]] is the β polymorph,<ref name=mack62>A. L. Mackay (1962): "β-Ferric oxyhydroxide—akaganéite", ''Mineralogical Magazine'' (''Journal of the Mineralogical Society''), volume 33, issue 259, pages 270-280 {{doi|10.1180/minmag.1962.033.259.02}}</ref> 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 {{chem|FeO|0.833|(OH)|1.167|Cl|0.167}} or {{chem|Fe|6|O|5|(OH)|7|Cl}}.<ref name=remaz>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. {{doi|10.1016/j.corsci.2006.06.003}}</ref>

* [[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]]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]]s:

* [[Limonite]] is a commonly occurring mixture of mainly goethite, lepidocrocite, [[quartz]] and [[clay minerals]]. 

* [[Ferrihydrite]] is an amorphous or nanocrystalline hydrated mineral, officially {{chem|FeOOH·1.8H|2|O}} 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-{{chem|FeOOH}} (akaganeite) is that of [[hollandite]] or {{chem|BaMn|8|O|16}}.  The unit cell is tetragonal with ''a'' = 1.048 and ''c'' = 0.3023 [[nanometer|nm]], and contains eight formula units of FeOOH.  Its dimensions are about 500 × 50 × 50 nm.  [[Crystal twinning|Twinning]] often produces particles with the shape of hexagonal stars. <ref name=mack60/>

===Chemistry===
On heating, β-{{chem|FeOOH}} decomposes and recrystallizes as α-{{chem|Fe|2|O|3}} ([[hematite]]).<ref name=mack60/>

==Uses==
[[Limonite]], a mixture of various hydrates and polymorphs of ferric oxyhydroxide, is one of the three major [[iron ore]]s, having been used since at least 2500 BC.<ref name=maceach>MacEachern, Scott (1996):  [http://www.panafprehistory.org/images/papers/IRON_AGE_BEGINNINGS_NORTH_OF_THE_MANDARA_MOUNTAINS_CAMEROON_AND_NIGERIA_Scott_MacEachern.pdf "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, {{ISBN|978-0-908307-55-5}}, pages 489-496. Archived [https://web.archive.org/web/20120311184136/http://www.panafprehistory.org/images/papers/IRON_AGE_BEGINNINGS_NORTH_OF_THE_MANDARA_MOUNTAINS_CAMEROON_AND_NIGERIA_Scott_MacEachern.pdf here] on 2012-03-11.</ref><ref>Diop-Maes, Louise Marie (1996): [http://www.ankhonline.com/revue/diop_lm_metallurgie_fer_afrique.htm "La question de l'Âge du fer en Afrique" ("The question of the Iron Age in Africa")]. ''Ankh'', volume4/5, pages 278-303. [https://web.archive.org/web/20080125103929/http://www.ankhonline.com/revue/diop_lm_metallurgie_fer_afrique.htm Archived] on 2008-01-25.</ref>

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]] water treatment as a [[phosphate]] binder.<ref>{{URL| http://www.reefkeeping.com/issues/2004-11/rhf/index.php |Iron Oxide Hydroxide (GFO) Phosphate Binders}}</ref>

Iron oxide-hydroxide [[nanoparticle]]s have been studied as possible [[adsorbent]]s for lead removal from aquatic media.<ref>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. {{doi|10.1016/j.jiec.2014.07.039}}</ref>

=== 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.<ref name=grund>Tim Grundl and Jim Delwiche (1993): "Kinetics of ferric oxyhydroxide precipitation". ''Journal of Contaminant Hydrology'', volume 14, issue 1, pages 71-87. {{doi|10.1016/0169-7722(93)90042-Q}}</ref>  
Thus the oxyhydroxide can be obtained in the lab by reacting an iron(III) salt, such as [[ferric chloride]] or [[ferric nitrate]], with [[sodium hydroxide]]:<ref name=gayer>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. {{doi|10.1021/j150545a021}}</ref>
:{{chem|FeCl|3}} + 3 NaOH → {{chem|Fe|(|OH|)|3}} + 3 NaCl
:{{chem|Fe(NO|3|)|3}} + 3 NaOH → {{chem|Fe|(|OH|)|3}} + 3 {{chem|NaNO|3}}

In fact, when dissolved in water, pure {{chem|FeCl|3}} will [[hydrolysis|hydrolyze]] to some extent, yielding the oxyhydroxide and making the solution acidic:<ref name=grund/>
:{{chem|FeCl|3}} + 2 {{chem|H|2|O}} ↔ {{chem|FeOOH}} + 3 {{chem|HCl}} 
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:<ref name=matij>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. {{doi|10.1016/S0021-9797(78)80011-3}}</ref>
: {{chem|FeCl|3}} + 2 {{chem|H|2|O}} → {{chem|Fe|OOH}}<sub>(s)</sub> + 3 {{chem|HCl}}<sub>(g)</sub>
(The same process applied to [[iron(III) nitrate]] {{chem|Fe|(|NO|3|)|3}} or perchlorate {{chem|Fe|(|ClO|4|)|3}} solutions yields instead particles of α-{{chem|Fe|2|O|3}}.<ref name=matij/>)  

Another similar route is the decomposition of iron(III) nitrate dissolved in [[stearic acid]] at about 120 °C.<ref name=danli1997>Dan Li, Xiaohui Wang, Gang Xiong, Lude Lu, Xujie Yang and Xin Wang (1997): "A novel technique to prepare ultrafine {{chem|Fe|2|O|3}} via hydrated iron(III) nitrate". ''Journal of Materials Science Letters'' volume 16, pages 493–495 {{doi|10.1023/A:1018528713566}}</ref>

The oxyhydroxide prepared from ferric chloride is usually the β polymorph (akaganeite), often in the form of thin needles.<ref name=matij/><ref name=whitt>Donald O.
Whittemore and Donald Langmuir (1974): "Ferric Oxyhydroxide Microparticles in Water". ''Environmental Health Perspective'', volume 9, pages 173-176. {{doi|10.1289/ehp.749173}}</ref>

The oxyhydroxide can also be produced by a solid-state transformation from [[iron(II) chloride]] tetrahydrate {{chem|FeCl|2}}·4{{chem|H|2|O}}.<ref name=mack62/>

The compound also readily forms when [[iron(II) hydroxide]] is exposed to air:
:4{{chem|Fe|(|OH|)|2}} + {{chem|O|2}} → 4 {{chem|FeOOH}} + 2 {{chem|H|2|O}}
The iron(II) hydroxide can also be oxidized by [[hydrogen peroxide]] in the presence of an acid:
:2{{chem|Fe|(|OH|)|2}} + {{chem|H|2|O|2}} → 2 {{chem|Fe|(|OH|)|3}}

==See also==
*[[Rust]]
*[[Iron oxide]]
*[[Yellow boy]], a yellow precipitate when acidic runoff such as mine waste, is then neutralised

==References==
{{reflist}}

{{Iron compounds}}
{{Antianemic preparations}}

[[Category:Iron(III) compounds]]
[[Category:Hydroxides]]
[[Category:Transition metal oxides]]