Iron(III) oxide-hydroxide

Iron(III) oxide-hydroxide

Names
IUPAC name Iron(III) oxide-hydroxide
Other names Ferric acid
Identifiers
CAS Number	1310-14-1 Y 20344-49-4 Y
ChemSpider	82623 Y
EC Number	215-176-6
InChI InChI=1S/Fe.H2O.O/h;1H2;/q+1;;/p-1 Y Key: AEIXRCIKZIZYPM-UHFFFAOYSA-M Y
Jmol interactive 3D	Image
MeSH	Goethite
PubChem	91502
SMILES [O][Fe]O
Properties
Appearance	Vivid, dark orange, opaque crystals
Odor	odorless
Density	4.25 g/cm3
Solubility in water	insoluble at pH 7
Solubility product (Ksp)	2.79×10−39 for Fe(OH)3
Hazards
NFPA 704	0 1 0
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

A number of chemicals are dubbed iron(III) oxide-hydroxide. These chemicals are oxide-hydroxides of iron, and may occur in anhydrous (FeO(OH)) or hydrated (FeO(OH)·nH₂O) forms. The monohydrate (FeO(OH)·H₂O) might otherwise be described as iron(III) hydroxide (Fe(OH)₃), and is also known as hydrated iron oxide or yellow iron oxide.

Natural occurrences

Iron(III) oxide-hydroxide occurs naturally as four minerals, the polymorphs denoted by the Greek letters α, β, γ and δ: Goethite, α-FeO(OH), has been used as a pigment since prehistoric times. Akaganéite is the β polymorph, formed by weathering and noted for its presence in some meteorhe lunar surface. The γ polymorph lepidocrocite 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. It also occurs as siderogel - a colloid - and limonite, which is a commonly found mixture of mainly goethite, lepidocrocite, quartz and clay minerals. Goethite and lepidocrocite, both crystallizing in orthorhombic system, are the most common forms of iron(III) oxide-hydroxide and the most important mineral carriers of iron in soils. The mineral ferrihydrite, also a soil constituent, is a related compound.

Uses

Yellow iron oxide (CAS 51274-00-1) is used as a pigment, e.g. Pigment Yellow 42 or C.I. 77492. Pigment Yellow 42 is Food and Drug Administration (FDA) approved for use in cosmetics and is used in some tattoo inks. Solid material color ranges from yellow through dark-brown to black. Iron oxide-hydroxide is also used in aquarium water treatment as a phosphate binder. Recently, two forms of Iron oxide-hydroxides nanoparticles were identified as very good adsorbents for lead removal from aquatic media.

Chemistry

In a solution containing both iron(II) and iron(III), ferric hydroxide precipitates between pH 2 to 4:

Fe³⁺ + 3OH⁻ → Fe(OH)₃

Ferrous hydroxide precipitates between pH 7 to 9:

Fe²⁺ + 2OH⁻ → Fe(OH)₂

This property of iron cations is exploited for the fractional precipitation of the two hydroxides in chemical analysis.

Production

It is obtained by reacting ferric chloride with sodium hydroxide:

FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl

Alternately iron(II) may be oxidized to iron(III) by hydrogen peroxide in the presence of an acid:

Fe²⁺ → Fe³⁺ + e⁻

H₂O₂ + 2 e⁻ → 2 OH⁻

Overall equation:

2 Fe²⁺ + H₂O₂ + 2 H⁺ → 2 Fe³⁺ + 2 H₂O

Safety

The risk and safety phrases for iron oxides are R36, R37, R38, S26, and S36.

See also

• Rust
• Iron oxide
• Iron pillar of Delhi