Iron(II) hydroxide

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Iron(II) hydroxide
IUPAC name
Iron(II) hydroxide
Other names
Ferrous hydroxide, green rust
3D model (JSmol)
ECHA InfoCard 100.038.581 OOjs UI icon edit-ltr-progressive.svg
PubChem CID
  • InChI=1S/Fe.2H2O/h;2*1H2/q+2;;/p-2 Yes check.svgY
  • InChI=1/Fe.2H2O/h;2*1H2/q+2;;/p-2
  • O[Fe]O
Molar mass 89.86 g/mol
Appearancegreen solid
Density 3.4 g/cm3 [1]
0.000052 g/100 g water (20 °C, pH 7) [2]
8.0 x 10−16 [3]
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).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

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 × 103 g/L), or 1.59 × 10−5 mol/L. It precipitates from the reaction of iron(II) and hydroxide salts: [4]

FeSO4 + 2NaOH → Fe(OH)2 + Na2SO4

If the solution is not deoxygenated and the iron reduced, 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.


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.


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. Limestone building with pollution.jpg
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]


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

Related Research Articles

Oxide Chemical compound with at least one oxygen atom attached to the central atom

An oxide is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. Metal oxides thus typically contain an anion of oxygen in the oxidation state of −2. Most of the Earth's crust consists of solid oxides, the result of elements being oxidized by the oxygen in air or in water. Even materials considered pure elements often develop an oxide coating. For example, aluminium foil develops a thin skin of Al2O3 (called a passivation layer) that protects the foil from further corrosion. Certain elements can form multiple oxides, differing in the amounts of the element combining with the oxygen. Examples are carbon, iron, nitrogen (see nitrogen oxide), silicon, titanium, lithium, and aluminium. In such cases the oxides are distinguished by specifying the numbers of atoms involved, as in carbon monoxide and carbon dioxide, or by specifying the element's oxidation number, as in iron(II) oxide and iron(III) oxide.

Rust Type of iron oxide

Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3), and is typically associated with the corrosion of refined iron.

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In chemistry, iron(III) refers to the element iron in its +3 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe3+.

Iron(II) sulfate Chemical compound

Iron(II) sulfate (British English: iron(II) sulphate) or ferrous sulfate denotes a range of salts with the formula FeSO4·xH2O. These compounds exist most commonly as the heptahydrate (x = 7) but several values for x are known. The hydrated form is used medically to treat iron deficiency, and also for industrial applications. Known since ancient times as copperas and as green vitriol (vitriol is an archaic name for sulfate), the blue-green heptahydrate (hydrate with 7 molecules of water) is the most common form of this material. All the iron(II) sulfates dissolve in water to give the same aquo complex [Fe(H2O)6]2+, which has octahedral molecular geometry and is paramagnetic. The name copperas dates from times when the copper(II) sulfate was known as blue copperas, and perhaps in analogy, iron(II) and zinc sulfate were known respectively as green and white copperas.

Potassium ferricyanide Chemical compound

Potassium ferricyanide is the chemical compound with the formula K3[Fe(CN)6]. This bright red salt contains the octahedrally coordinated [Fe(CN)6]3− ion. It is soluble in water and its solution shows some green-yellow fluorescence. It was discovered in 1822 by Leopold Gmelin.

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Sodium selenite Chemical compound

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Ferrate(VI) Ion

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Cobalt(II) hydroxide Chemical compound

Cobalt(II) hydroxide or cobaltous hydroxide is the inorganic compound with the formula Co(OH)
, consisting of divalent cobalt cations Co2+
and hydroxide anions HO
. The pure compound, often called the "beta form" is a pink solid insoluble in water.

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In chemistry, iron(II) refers to the element iron in its +2 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe2+.

Green rust Generic name for various green-colored iron compounds

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) anion, and another anion such as carbonate (CO2−
), chloride (Cl
), or sulfate (SO2−
), in a layered double hydroxide structure. The most studied varieties are

Iron(II) selenate (ferrous selenate) is an inorganic compound with the formula FeSeO4. It has anhydrous and several hydrate forms. The pentahydrate has the structure, [Fe(H2O)4]SeO4•H2O, isomorphous to the corresponding iron(II) sulfate. Heptahydrate is also known, in form of unstable green crystalline solid.


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  3. [ bare URL PDF ]
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  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.