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 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:[1]
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.
Fe(OH)2 adopts the brucite structure, i.e. the arrangement of the atoms in the crystal are the same as the arrangement of the atoms in Mg(OH)2. The Fe(II) centers are bonded to six hydroxide ligands. Each hydroxide ligand bridges to three Fe(II) sites. The O-H bonds are perpendicular to the planes defined by the oxygen atoms, projecting above and below these layers.[2]
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).
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.[3] The natural analogue of iron(II) hydroxide compound is the very rare mineral amakinite, .[4] [5]
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.[6]
In a basic solution iron(II) hydroxide is the electrochemically active material of the negative electrode of the nickel-iron battery.