Iron(II) hydroxide or ferrous hydroxide is an

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistr ...

with the formula Fe(OH)₂. It is produced when iron(II) salts, from a compound such as

iron(II) sulfate Iron(II) sulfate (British English: iron(II) sulphate) or ferrous sulfate denotes a range of salts with the formula Fe SO4·''x''H2O. These compounds exist most commonly as the heptahydrate (''x'' = 7) but several values for x are know ...

, are treated with

hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...

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 Green rust is a generic name for various green crystalline chemical compounds containing iron(II) and iron(III) cations, the hydroxide () anion, and another anion such as carbonate (), chloride (), or sulfate (), in a layered double hydroxide s ...

Preparation and reactions

Iron(II) hydroxide is poorly

soluble In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubil ...

in water (1.43 × 10⁻³ g/ L), or 1.59 × 10⁻⁵ mol/L. It precipitates from the reaction of iron(II) and

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 Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

. It is also easily formed as a by-product of other reactions, a.o., in the synthesis of

siderite Siderite is a mineral composed of iron(II) carbonate (FeCO3). It takes its name from the Greek word σίδηρος ''sideros,'' "iron". It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus. Zinc, magnesium and ...

, an iron carbonate (FeCO₃), if the

crystal growth A crystal is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. Crystal growth is a major stage of a crystallization process, and consists of the ...

conditions are imperfectly controlled.

Structure

Fe(OH)₂ is a layer double hydroxide (LDH) easily accommodating in its

crystal lattice In geometry and crystallography, a Bravais lattice, named after , is an infinite array of discrete points generated by a set of discrete translation operations described in three dimensional space by : \mathbf = n_1 \mathbf_1 + n_2 \mathbf_2 + n_ ...

ferric ions () produced by

of ferrous ions () by the atmospheric

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

().

Related materials

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 Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...

(iron(II,III) oxide) and molecular

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxi ...

. 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 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. The natural analogue of iron(II) hydroxide compound is the very rare mineral amakinite, .

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

wetland A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free ( anoxic) processes prevailing, especially in the soils. T ...

s. The iron(II) hydroxide reduces these ions to elemental

selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...

, 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.

References

{{Hydroxides Iron(II) compounds Hydroxides Materials fr:Hydroxyde de fer