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| Names | |
|---|---|
| IUPAC name Iron(II) hydroxide | |
| Other names Ferrous hydroxide, green rust | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider |
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| ECHA InfoCard | 100.038.581 |
| UNII | |
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| Properties | |
| Fe(OH)2 | |
| Molar mass | 89.86 g/mol |
| Appearance | green solid |
| Density | 3.4 g/cm3[1] |
| 0.000052 g/100 g water (20 °C, pH 7)[2] | |
Solubility product (Ksp) | 8.0 x 10−16[3] |
| Acidity (pKa) | 17[4] |
| Hazards | |
| Flash point | Non-flammable |
| Related compounds | |
Related compounds | Iron(II) oxide Iron(III) hydroxide |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Iron (II) hydroxide orferrous hydroxide is aninorganic compound with the formula Fe(OH)2. It is produced when iron (II) salts, from a compound such asiron(II) sulfate, are treated withhydroxide 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 poorlysoluble in water (1.43 × 10−3g/L), or 1.59 × 10−5 mol/L. Itprecipitates from the reaction of iron(II) andhydroxide salts:[5]
If the solution is notdeoxygenated 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 progressiveoxidation.
It is also easily formed as a by-product of other reactions, a.o., in the synthesis ofsiderite, aniron carbonate (FeCO3), if thecrystal growth conditions are imperfectly controlled.
Fe(OH)2 adopts thebrucite 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 hydroxideligands. Each hydroxide ligandbridges 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.[6]
Under anaerobic conditions, the iron(II) hydroxide can be oxidised by the protons of water to formmagnetite (iron(II,III) oxide) and molecularhydrogen.This process is described by theSchikorr reaction:
Anions such asselenite andselenate 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 ingroundwater is in the reduced iron II form. If this groundwater comes in contact with oxygen at the surface, e.g. in naturalsprings, iron II is oxidised to iron III and forms insoluble hydroxides in water.[7]The natural analogue of iron(II) hydroxide compound is the very rare mineralamakinite,(Fe,Mg)(OH)2.[8][9]
Iron(II) hydroxide has also been investigated as an agent for the removal of toxicselenate andselenite ions from water systems such aswetlands. The iron(II) hydroxidereduces these ions to elementalselenium, which is insoluble in water andprecipitates out.[10]
In a basic solution iron(II) hydroxide is the electrochemically active material of the negative electrode of thenickel-iron battery.
