Ferric hydroxide

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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

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.

Yellow iron oxide (CAS Template:CASREF) 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.^[1] also recently, two form of Iron oxide-hydroxides nanoparticles are identified as very good adsorbents for lead removal from aquatic media.^[2]

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.

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

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

• Rust
• Iron oxide
• Iron pillar of Delhi

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