Iron (II) hydroxide or ferrous hydroxide is an

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

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

iron(II) sulfate Iron(II) sulfate or ferrous sulfate (British English: sulphate instead of 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 ...

, 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 (LDH ...

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 solubi ...

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

. 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). Its name comes from the Ancient Greek word (), meaning "iron". A valuable iron ore, it consists of 48% iron and lacks sulfur and phosphorus. Zinc, magnesium, and manganese commonly ...

, an

iron carbonate Iron(II) carbonate, or ferrous carbonate, is a chemical compound with formula , that occurs naturally as the mineral siderite. At ordinary ambient temperatures, it is a green-brown ionic solid consisting of iron(II) cations and carbonate anions ...

(FeCO₃), if the

crystal growth Crystal growth is a major stage of a crystallization, crystallization process, and consists of the addition of new atoms, ions, or polymer strings into the characteristic arrangement of the crystalline lattice. The growth typically follows an ini ...

conditions are imperfectly controlled.

Structure

Fe(OH)₂ adopts the

brucite Brucite is the mineral form of magnesium hydroxide, with the chemical formula Magnesium, Mg(hydroxyl, OH)2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal Vein (geology), vein mineral in metamorphosed li ...

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

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

s. Each hydroxide ligand

bridges A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or railway) without blocking the path underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually somet ...

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.

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 . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...

(iron(II,III) oxide) and molecular

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

. This process is described by the Schikorr reaction: :3 Fe(OH)₂ → Fe₃O₄ + H₂ + 2 H₂O Anions such as selenite and

selenate The selenate ion is . Selenates are analogous to sulfates and have similar chemistry. They are highly soluble in aqueous solutions at ambient temperatures. Unlike sulfate, selenate is a somewhat good oxidizer; it can be reduced to selenite or ...

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 Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...

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

and selenite ions from water systems such as

wetland A wetland is a distinct semi-aquatic ecosystem whose groundcovers are flooded or saturated in water, either permanently, for years or decades, or only seasonally. Flooding results in oxygen-poor ( anoxic) processes taking place, especially ...

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

selenium Selenium is a chemical element; it has symbol (chemistry), symbol Se and atomic number 34. It has various physical appearances, including a brick-red powder, a vitreous black solid, and a grey metallic-looking form. It seldom occurs in this elem ...

, 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