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-oxidized solid is sometimes known as "green rust".
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||89.86 g/mol|
|0.72 g/100 mL (25 °C, pH 7)|
Solubility product (Ksp)
|8.0 x 10−16|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Preparation and reactions
- FeSO4 + 2NaOH → Fe(OH)2 + Na2SO4
If the solution is not deoxygenated and the iron reduced, the precipitate can vary in color 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.
Fe(OH)2 is a layer double hydroxide (LDH).
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. The natural analogue of iron(II) hydroxide compound is a very rare mineral amakinite, (Fe,Mg)(OH)2.
Under anaerobic conditions, the iron(II) hydroxide can be oxidized 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).
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.
In a basic solution iron(II) hydroxide is the electrochemically active material of the negative electrode of the nickel-iron battery.
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.
- Lide, David R., ed. (2006). CRC Handbook of Chemistry and Physics (87th ed.). Boca Raton, FL: CRC Press. ISBN 0-8493-0487-3.
- 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.
- 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.