:Iron(II) hydroxide
{{chembox
| Watchedfields = changed
| verifiedrevid = 415522853
| Name = Iron(II) hydroxide
| ImageFile =
| ImageFile1 = Mg(OH)2Xray.jpg
| ImageName = Iron(II) hydroxide
| IUPACName = Iron(II) hydroxide
| OtherNames = Ferrous hydroxide, green rust
| SystematicName =
| Section1 = {{Chembox Identifiers
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 8305416
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 7JIM5W32UU
| InChI = 1/Fe.2H2O/h;2*1H2/q+2;;/p-2
| InChIKey = NCNCGGDMXMBVIA-NUQVWONBAV
| SMILES = O[Fe]O
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/Fe.2H2O/h;2*1H2/q+2;;/p-2
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = NCNCGGDMXMBVIA-UHFFFAOYSA-L
| CASNo = 18624-44-7
| CASNo_Ref = {{cascite|correct|CAS}}
| PubChem = 10129897
}}
| Section2 = {{Chembox Properties
| Formula = Fe(OH)2
| MolarMass = 89.86 g/mol
| Appearance = green solid
| Density = 3.4 g/cm3 {{RubberBible87th}}
| Solubility = 0.000052 g/100 g water (20 °C, pH 7)CRC Handbook of Chemistry and Physics, 84th Edition, CRC Press, 2004, pg 4-62
| pKa = 17{{cite book|title=Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution|editor-first=D. D.|editor-last=Perrin|edition=2nd|series=IUPAC Chemical Data|issue=29|publisher=Pergamon|location=Oxford|year=1982|publication-date=1984|orig-date=1969|lccn=82-16524|isbn=0-08-029214-3|at=Entry 130}}
}}
| Section3 = {{Chembox Hazards
| FlashPt = Non-flammable
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| Section4 =
| Section5 =
| Section6 =
| Section8 = {{Chembox Related
| OtherCompounds = Iron(II) oxide
Iron(III) hydroxide
}}
}}
Iron (II) hydroxide or ferrous hydroxide is an inorganic compound with the formula Fe(OH)2. It is produced when iron (II) salts, from a compound such as iron(II) sulfate, are treated with hydroxide ions. Iron(II) hydroxide is a white solid, but even traces of oxygen impart a greenish tinge. The air-oxidised solid is sometimes known as "green rust".
Preparation and reactions
Iron(II) hydroxide is poorly soluble in water (1.43 × 10−3 g/L), or 1.59 × 10−5 mol/L. It precipitates from the reaction of iron(II) and hydroxide salts:H. Lux "Iron(II) Hydroxide" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 1498.
:FeSO4 + 2 NaOH → Fe(OH)2 + Na2SO4
If the solution is not deoxygenated and iron not totally reduced in Fe(II), the precipitate can vary in colour starting from green to reddish brown depending on the iron(III) content. Iron(II) ions are easily substituted by iron(III) ions produced by its progressive oxidation.
It is also easily formed as a by-product of other reactions, a.o., in the synthesis of siderite, an iron carbonate (FeCO3), if the crystal growth conditions are imperfectly controlled.
Structure
Fe(OH)2 adopts the brucite structure, i.e. the arrangement of the atoms in the crystal are the same as the arrangement of the atoms in Mg(OH)2. The Fe(II) centers are bonded to six hydroxide ligands. Each hydroxide ligand bridges to three Fe(II) sites. The O-H bonds are perpendicular to the planes defined by the oxygen atoms, projecting above and below these layers.{{cite journal |doi=10.1016/0022-2860(94)08355-x |title=Lattice vibration spectra. Part LXXXII. Brucite-type hydroxides M(OH)2 (M = Ca, Mn, Co, Fe, Cd) — IR and Raman spectra, neutron diffraction of Fe(OH)2 |date=1994 |last1=Lutz |first1=H.D. |last2=Möller |first2=H. |last3=Schmidt |first3=M. |journal=Journal of Molecular Structure |volume=328 |pages=121–132 }}
Reactions
Under anaerobic conditions, the iron(II) hydroxide can be oxidised by the protons of water to form magnetite (iron(II,III) oxide) and molecular hydrogen.
This process is described by the Schikorr reaction:
:3 Fe(OH)2 → Fe3O4 + H2 + 2 H2O
Anions such as selenite and selenate can be easily adsorbed on the positively charged surface of iron(II) hydroxide, where they are subsequently reduced by Fe2+. The resulting products are poorly soluble (Se0, FeSe, or FeSe2).
Natural occurrence
Image:Limestone building with pollution.jpg, Western Australia.]]
Iron dissolved in groundwater is in the reduced iron II form. If this groundwater comes in contact with oxygen at the surface, e.g. in natural springs, iron II is oxidised to iron III and forms insoluble hydroxides in water.[https://www.lenntech.com/groundwater/iron.htm lenntech.com]
The natural analogue of iron(II) hydroxide compound is the very rare mineral amakinite, {{chem2|(Fe,Mg)(OH)2}}.{{Cite web|url=https://www.mindat.org/min-180.html|title = Amakinite}}{{Cite web|url=https://www.ima-mineralogy.org/Minlist.htm|title=List of Minerals|date=21 March 2011}}
Application
Iron(II) hydroxide has also been investigated as an agent for the removal of toxic selenate and selenite ions from water systems such as wetlands. The iron(II) hydroxide reduces these ions to elemental selenium, which is insoluble in water and precipitates out.{{cite journal |last=Zingaro |first=Ralph A. |display-authors=etal |date=1997 |title=Reduction of oxoselenium anions by iron(II) hydroxide |journal=Environment International |volume=23 |issue=3 |pages=299–304 |doi=10.1016/S0160-4120(97)00032-9}}
In a basic solution iron(II) hydroxide is the electrochemically active material of the negative electrode of the nickel-iron battery.
See also
References
{{Iron compounds}}
{{Hydroxides}}