Iron(III) oxide-hydroxide

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Iron(III) oxide-hydroxide
Samples of iron(III) oxide-hydroxide monohydrate in a vial, and a spoon
Identifiers
CAS number 1310-14-1 YesY, 20344-49-4 YesY
PubChem 91502
ChemSpider 82623 YesY
EC number 215-176-6
MeSH Goethite
Jmol-3D images Image 1
Properties
Appearance Vivid, dark orange, opaque crystals
Odor odorless
Density 4.25 g/cm3
Solubility in water 0.115 g/100 mL (20 °C)
Solubility product, Ksp 2.79×10-39 for Fe(OH)3[1]
Hazards
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentine Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Except where noted otherwise, 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(OHH2O) 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[edit]

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 a pigment since prehistoric times. Akaganéite is the β polymorph, formed by weathering and noted for its presence in some meteorites and the 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[edit]

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.[2]

Chemistry[edit]

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

Fe3+ + 3OH- → Fe(OH)3

Ferrous hydroxide precipitates between pH 7 to 9:[citation needed]

Fe2+ + 2OH- → Fe(OH)2

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

Production[edit]

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[edit]

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

See also[edit]

References[edit]

External links[edit]