Iron(II) hydroxide

Iron(II) hydroxide
Names
IUPAC name
Iron(II) hydroxide
Other names
Ferrous hydroxide
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.038.581
UNII
Properties
Fe(OH)2
Molar mass 89.86 g/mol
Appearance green solid
Density 3.4 g/cm3 [1]
0.72 g/100 mL (25 °C, pH 7)
8.0 x 10−16[2]
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

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".

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:[3]

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.

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).

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.

Reactions

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.[4]

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. ↑ http://www.gfredlee.com/SurfaceWQ/StummOxygenFerrous.pdf
  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.
  4. ↑ 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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