3D model (JSmol)
|Appearance||Vivid, dark orange, opaque crystals|
|insoluble at pH 7|
Solubility product (Ksp)
|2.79×10−39 for Fe(OH)3|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
The compound is often encountered as one of its hydrates, FeO(OH)·nH
2O. The monohydrate FeO(OH)·H
2O (CAS , C.I. 77492) is often referred as iron(III) hydroxide Fe(OH)
3, hydrated iron oxide, yellow iron oxide, or Pigment Yellow 42.
Anhidrous ferric oxyhydroxide FeOOH occurs naturally as four different minerals (polymorphs) denoted by the Greek letters α, β, γ and δ.
- Goethite, α-FeO(OH), has been used as an ocher pigment since prehistoric times.
- Akaganeite is the β polymorph, formed by weathering and noted for its presence in some meteorites and the lunar surface. However, recently it has been determined that it must contain some chloride ions to stabilize its structure, so that its more accurate formula is FeO
- Lepidocrocite, the γ polymorph, is commonly encountered as rust on the inside of steel water pipes and tanks.
- Feroxyhyte (δ) is formed under the high pressure conditions of sea and ocean floors, being thermodynamically unstable with respect to the α polymorph (goethite) at surface conditions.
Goethite and lepidocrocite, both crystallizing in orthorhombic system, are the most common forms of iron(III) oxyhydroxide and the most important mineral carriers of iron in soils.
Iron(III) oxyhydroxide is the main component of other minerals and mineraloids:
- Limonite is a commonly occurring mixture of mainly goethite, lepidocrocite, quartz and clay minerals.
- Ferrihydrite is an amorphous or nanocrystalline hydrated mineral, officially FeOOH•1.8H
2O but with widely variable hydration.
The color of iron(III) oxyhydroxide ranges from yellow through dark-brown to black, depending on the degree of hydration, particle size and shape, and crystal structure.
The crystal structure of β-FeOOH (akaganeite) is that of hollandite or BaMn
16. The unit cell is tetragonal with a=1.048 and c=0.3023 nm, and contains eight formula units of FeOOH. Its dimensions are about 500 × 50 × 50 nm. Twinning often produces particles with the shape of hexagonal stars. 
Iron(III) oxyhydroxide precipitates from solutions of iron(III) salts at pH between 6.5 and 8.. Thus the oxyhydroxide can be obtained in the lab by reacting an iron(III) salt, such as ferric chloride or ferric nitrate, with sodium hydroxide:
3 + 3 NaOH → Fe(OH)
3 + 3 NaCl
3 + 3 NaOH → Fe(OH)
3 + 3 NaNO
3 + 2 H
2O ↔ FeOOH + 3 HCl
Therefore, the compound can also be obtained by the decomposition of acidic solutions of iron(III) chloride held near the boiling point for days or weeks:
3 + 2 H
2O → FeOOH(s) + 3 HCl(g)
The compound also readily forms when iron(II) hydroxide is exposed to air:
2 + O
2 → 4 FeOOH + 2 H
The iron(II) hydroxide can also be oxidized by hydrogen peroxide in the presence of an acid:
2 + H
2 → 2 Fe(OH)
- Iron oxide
- Yellow boy, a yellow precipitate when acidic runoff such as mine waste, is then neutralised
- "Archived copy". Archived from the original on 2015-02-26. Retrieved 2015-02-23.CS1 maint: Archived copy as title (link)
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- Iron Oxide Hydroxide (GFO) Phosphate Binders
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