Iron(II) hydroxide
| Names | |
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IUPAC name
Iron(II) hydroxide
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| Other names
Ferrous hydroxide
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| Identifiers | |
18624-44-7 
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| 3D model (Jmol) | Interactive image |
| ChemSpider |
8305416
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| ECHA InfoCard | 100.038.581 |
| PubChem | 10129897 |
| UNII |
7JIM5W32UU
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| Properties | |
| Fe(OH)2 | |
| Molar mass | 89.86 g/mol |
| Appearance | green solid |
| Density | 3.4 g/cm3 |
| 0.72 g/100 mL (25 °C, pH 7) | |
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Solubility product (Ksp)
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8.0 x 10−16 |
| Hazards | |
| Flash point | Non-flammable |
| Related compounds | |
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Related compounds
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Iron(II) oxide Iron(III) hydroxide |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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| Infobox references | |
Iron(II) hydroxide or ferrous hydroxide is a compound with the formula Fe(OH)2. It is 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 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 of sorption of 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 × 10−3g/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, ...).
Under anaerobic conditions, the iron(II) hydroxide can be oxidized by the protons of water to form iron(II,III) oxide and molecular hydrogen. 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.
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).
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Wikipedia