Iron(II) hydroxide
Encyclopedia
Iron hydroxide or ferrous hydroxide is a compound
produced when iron(II) ions, from a compound such as iron(II) sulfate
, react with hydroxide
ions. Iron(II) hydroxide itself is practically white, but even traces of oxygen impart it with a greenish tinge. If the solution was not deoxygenated
and the iron reduced, the precipitate can vary in color starting from green to reddish brown depending on the iron(III) content. This precipitate is also known as "green rust" in the crystal lattice of which iron(II) ions are easily substituted by iron(III) ions produced by its progressive oxidation. In the presence of oxygen the color changes quickly. Green rust is a powerful reducing agent and also a layer double hydroxide
(LDH) capable to sorb anions because of the presence of positive electrical charges borne on its surface. The mineralogical form of green rust is a recently discovered fougerite
. 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.
Iron(II) hydroxide is poorly soluble (1.43 × 10−3 g
/L
). It precipitates from the reaction of iron(II) sulfate and hydroxide ions (from a soluble compound containing hydroxide ion):
It is also easily formed as an undesirable by-product of other reactions, a.o., in the synthesis of siderite
, an iron carbonate (FeCO3), if the crystal growth conditions are poorly controlled (reagent concentrations, addition rate, addition order, pH, pCO2, T, aging time, ...).
.
This process is described by the Schikorr reaction
:
The well crystallized iron(II,III) oxide (Fe3O4) is thermodynamically more stable than the iron(II) hydroxide.
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 wetland
s. The iron(II) hydroxide reduces
these ions to elemental selenium
, which is insoluble in water and precipitates
out.
Note: pKsp = 15.097 where p is the -log and Ksp is the Solubility Product Constant. This means it has a low tendency to dissolve, but is not entirely insoluble. An acidic solution would allow this to disassociate more because the H+ would react with the OH- in the compound.
In a basic solution iron(II) hydroxide is the electrochemically active material of the negative electrode of the nickel-iron battery
.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
produced when iron(II) ions, from a compound such as iron(II) sulfate
Iron(II) sulfate
Iron sulfate or ferrous sulfate is the chemical compound with the formula FeSO4. Known since ancient times as copperas and as green vitriol, the blue-green heptahydrate is the most common form of this material...
, react with hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
ions. Iron(II) hydroxide itself is practically white, but even traces of oxygen impart it with a greenish tinge. If the solution was not deoxygenated
Deoxygenation
Deoxygenation is a chemical reaction involving the removal of molecular oxygen from a reaction mixture or solvent, or the removal of oxygen atoms from a molecule.Classic representatives of deoxygenation are:...
and the iron reduced, the precipitate can vary in color starting from green to reddish brown depending on the iron(III) content. This precipitate is also known as "green rust" in the crystal lattice of which iron(II) ions are easily substituted by iron(III) ions produced by its progressive oxidation. In the presence of oxygen the color changes quickly. Green rust is a powerful reducing agent and also a layer double hydroxide
Layered double hydroxides
Layered double hydroxides comprise an unusual class of layered materials with positively charged layers and charge balancing anions located in the interlayer region. This is unusual in solid state chemistry: many more families of materials have negatively charged layers and cations in the...
(LDH) capable to sorb anions because of the presence of positive electrical charges borne on its surface. The mineralogical form of green rust is a recently discovered fougerite
Fougerite
Fougerite is a recently discovered representative of clay minerals, chemically and structurally related to so-called green rust, hydrotalcite-like minerals and other layered double hydroxides. The formula of the mineral is complex due to structural variabilities, as is for green rust, thus...
. 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.
Iron(II) hydroxide is poorly soluble (1.43 × 10−3 g
Gram
The gram is a metric system unit of mass....
/L
Litér
- External links :*...
). It precipitates from the reaction of iron(II) sulfate and hydroxide ions (from a soluble compound containing hydroxide ion):
- FeSO4 + 2 OH− → Fe(OH)2 + SO42−
It is also easily formed as an undesirable by-product of other reactions, a.o., in the synthesis of siderite
Siderite
Siderite is a mineral composed of iron 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...
, an iron carbonate (FeCO3), if the crystal growth conditions are poorly controlled (reagent concentrations, addition rate, addition order, pH, pCO2, T, aging time, ...).
Ageing and transformation into magnetite
Under anaerobic conditions, the iron(II) hydroxide can be oxidized by the protons of water to form iron(II,III) oxide and molecular hydrogenHydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
.
This process is described by the Schikorr reaction
Schikorr reaction
The Schikorr reaction formally describes the conversion of the iron hydroxide into iron oxide .The bases of this transformation reaction were first studied by Gerhard Schikorr, a German specialist of iron corrosion, in his early works on iron and iron hydroxides...
:
- 3 Fe(OH)2 → Fe3O4 + H2 + 2 H2O
The well crystallized iron(II,III) oxide (Fe3O4) is thermodynamically more stable than the iron(II) hydroxide.
Uses
Anions such as seleniteSelenite (ion)
The selenite anion is a selenium oxoanion with the chemical formula SeO32−.A selenite is a compound that contains this ion....
and selenate
Selenate
The selenate ion is SeO42–.Selenates are analogous to sulfates and have similar chemistry. They are highly soluble in aqueous solutions at ambient temperatures....
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
Selenate
The selenate ion is SeO42–.Selenates are analogous to sulfates and have similar chemistry. They are highly soluble in aqueous solutions at ambient temperatures....
and selenite
Selenous acid
Selenous acid is the chemical compound with the formula H2SeO3. Structurally, it is more accurately described by 2SeO. It is the principal oxoacid of selenium; the other being selenic acid.-Formation and properties:...
ions from water systems such as wetland
Wetland
A wetland is an area of land whose soil is saturated with water either permanently or seasonally. Wetlands are categorised by their characteristic vegetation, which is adapted to these unique soil conditions....
s. The iron(II) hydroxide reduces
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
these ions to elemental selenium
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...
, which is insoluble in water and precipitates
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
out.
Note: pKsp = 15.097 where p is the -log and Ksp is the Solubility Product Constant. This means it has a low tendency to dissolve, but is not entirely insoluble. An acidic solution would allow this to disassociate more because the H+ would react with the OH- in the compound.
In a basic solution iron(II) hydroxide is the electrochemically active material of the negative electrode of the nickel-iron battery
Nickel-iron battery
The nickel–iron battery is a storage battery having a nickel oxide-hydroxide cathode and an iron anode, with an electrolyte of potassium hydroxide. The active materials are held in nickel-plated steel tubes or perforated pockets...
.