Iron(II) 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".

Iron(II) hydroxide
Names
IUPAC name
Iron(II) hydroxide
Other names
Ferrous hydroxide, green rust
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.038.581
UNII
  • InChI=1S/Fe.2H2O/h;2*1H2/q+2;;/p-2 checkY
    Key: NCNCGGDMXMBVIA-UHFFFAOYSA-L checkY
  • InChI=1/Fe.2H2O/h;2*1H2/q+2;;/p-2
    Key: NCNCGGDMXMBVIA-NUQVWONBAV
  • O[Fe]O
Properties
Fe(OH)2
Molar mass 89.86 g/mol
Appearance green solid
Density 3.4 g/cm3 [1]
0.000052 g/100 g water (20 °C, pH 7)[2]
8.0 x 10−16[3]
Hazards
Flash point Non-flammable
Related compounds
Related compounds
 Iron(II) oxide
Iron(III) hydroxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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:[4]

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 is a layer double hydroxide (LDH) easily accommodating in its crystal lattice ferric ions (Fe3+
) produced by oxidation of ferrous ions (Fe2+
) by the atmospheric oxygen (O2).

Green rust is a recently discovered mineralogical form. All forms of green rust (including fougerite) are more complex and variable than the ideal iron(II) hydroxide compound.

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
Iron III hydroxide staining caused by oxidation of dissolved iron II and precipitation, Perth, 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.[5] The natural analogue of iron(II) hydroxide compound is the very rare mineral amakinite, (Fe,Mg)(OH)2.[6][7]

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.[8]

In a basic solution iron(II) hydroxide is the electrochemically active material of the negative electrode of the nickel-iron battery.

See also

References
  1. Lide, David R., ed. (2006). CRC Handbook of Chemistry and Physics (87th ed.). Boca Raton, FL: CRC Press. ISBN 0-8493-0487-3.
  2. CRC Handbook of Chemistry and Physics, 84th Edition, CRC Press, 2004, pg 4-62
  3. Stumm, Werner; Lee, G. F. (February 1961). "Oxygenation of Ferrous Iron" (PDF). Industrial & Engineering Chemistry. 53 (2): 143–146. doi:10.1021/ie50614a030. Retrieved 17 November 2022.
  4. 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.
  5. lenntech.com
  6. "Amakinite".
  7. "List of Minerals". 21 March 2011.
  8. Zingaro, Ralph A.; et al. (1997). "Reduction of oxoselenium anions by iron(II) hydroxide". Environment International. 23 (3): 299–304. doi:10.1016/S0160-4120(97)00032-9.
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