Formula Fe(OH)3 Molar mass 106.867 g/mol | IUPAC ID Iron(III) oxide-hydroxide Melting point 135 °C | |
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Appearance Vivid, dark orange, opaque crystals | ||
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.
Contents
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 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
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.
Production
It is obtained by reacting ferric chloride with sodium hydroxide:
FeCl3 + 3 NaOH → Fe(OH)3 + 3 NaClAlternately 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 H2OSafety
The risk and safety phrases for iron oxides are R36, R37, R38, S26, and S36.