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Iron(III) oxide
other names Ferric oxide, Hematite, ferric oxide, ferric iron, red iron oxide, synthetic maghemite, colcothar
Identifiers
CAS number	1309-37-1
PubChem	518696
RTECS number	NO7400000
Properties
Molecular formula	Fe2O3
Molar mass	159.69 g/mol
Appearance	red-brown solid
Odor	odorless
Density	5.242 g/cm3, solid
Melting point	1566 °C (1838 K) decomp.
Solubility in water	insoluble
Structure
Crystal structure	rhombohedral
Thermochemistry
Std enthalpy of formation ΔfHo298	−825.50 kJ/mol
Hazards
EU classification	not listed
Flash point	non-flammable
Related compounds
Other anions	iron(III) fluoride
Other cations	manganese(III) oxide, cobalt(III) oxide
Related compounds	iron(II) oxide, iron(II,III) oxide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Iron(III) oxide is the inorganic compound with the formula Fe₂O₃. It is of one of the three main oxides of iron, the other two being FeO, which is rare, and Fe₃O₄, which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe₂O₃ is the main source of the iron for the steel industry. Fe₂O₃ is paramagnetic, reddish brown, and readily attacked by acids. Rust is often called iron(III) oxide, and to some extent, this label is useful, because rust shares several properties and has a similar composition. To a chemist, rust is considered an ill-defined material, described as hydrated ferric oxide.

Different forms

Alpha phase

α-Fe₂O₃ has the rhombohedral, corundum (α-Al₂O₃) structure and is the most common form. It occurs naturally as the mineral hematite which is mined as the main ore of iron. It is antiferromagnetic below ~260 K (Morin transition temperature), and weak ferromagnetic between 260 K and 950 K (Neel temperature).^[1] It is easy to prepare using both thermal decomposition and precipitation in the liquid phase. Its magnetic properties are dependent on many factors, e.g. pressure, particle size, and magnetic field intensity.

Beta phase

Cubic face centered, metastable, at temperatures above 500 °C converts to alpha phase. It can be prepared by reduction of hematite by carbon, pyrolysis of iron(III) chloride solution, or thermal decomposition of iron(III) sulfate.

Gamma phase

Cubic, metastable, converts to the alpha phase at high temperatures. Occurs naturally as the mineral maghemite. Ferrimagnetic. Ultrafine particles smaller than 10 nanometers are superparamagnetic. Can be prepared by thermal dehydratation of gamma iron(III) oxide-hydroxide, careful oxidation of iron(II,III) oxide. The ultrafine particles can be prepared by thermal decomposition of iron(III) oxalate.

Epsilon phase

Rhombic, shows properties intermediate between alpha and gamma. So far has not been prepared in pure form; it is always mixed with the alpha phase or gamma phases. Material with a high proportion of epsilon phase can be prepared by thermal transformation of the gamma phase. The epsilon phase is metastable, transforming to the alpha phase at between 500 and 750 °C. Can also be prepared by oxidation of iron in an electric arc or by sol-gel precipitation from iron(III) nitrate.

Other phases

High pressure, amorphous.^[2]

Reactions

Iron(III) oxide dissolves readily in strong acid, e.g. hydrochloric and sulfuric acids. It also dissolves well in solutions of the chelating agents such as EDTA and oxalic acid.

It also used in an extremely exothermic reaction called a thermite reaction.^[3]

2 Al + Fe₂O₃ → 2 Fe + Al₂O₃

This process was once used in welding.

Uses

Iron industry

The overwhelming application of Iron(III) oxide is as the feedstock of the steel and iron industries, e.g. the production of pure iron, steel, and many alloys.^[4]

Polishing

A very fine powder of ferric oxide is known as jeweler's rouge, red rouge, or simply rouge. It is used to put the final polish on metallic jewelry and lenses, and historically as a cosmetic.

Rouge cuts more slowly than some modern polishes, such as cerium(IV) oxide, but is still used in optics fabrication and by jewelers for the superior finish it can produce. When polishing gold, the rouge slightly stains the gold, which contributes to the appearance of the finished piece. Rouge is sold as a powder, paste, laced on polishing cloths, or solid bar (with a wax or grease binder). Other polishing compounds are also often called "rouge", even when they do not contain iron oxide. Jewelers remove the residual rouge on jewelry by use of ultrasonic cleaning.

Pigment

Iron(III) oxide is also used as a pigment, under names "Pigment Brown 6", "Pigment Brown 7", and "Pigment Red 101"^[5]. Some of them, e.g. Pigment Red 101 and Pigment Brown 6, are Food and Drug Administration (FDA)-approved for use in cosmetics.

Biomedical

Nanoparticles of iron(III) oxide are biocompatible, non-toxic, are chemically active on their surface, and are paramagnetic at particle sizes above a critical limit of about 5 nanometers^{[citation needed]} . They find wide use in biomedical applications. Can be used as contrast agents in magnetic resonance imaging, in labeling of cancerous tissues, magnetically controlled transport of pharmaceuticals, localized thermotherapy (where the tissue is labeled by iron oxide nanoparticles, then heated by application of AC field to particles), and preparation of ferrofluids.^[2]

Preparation

Iron (III) oxide is a product of the oxidation of iron. It can be prepared in the laboratory by electrolyzing a solution of sodium bicarbonate, an inert electrolyte, with an iron anode:

2 Fe + 3/2 O₂ + H₂O → 2 FeO(OH)

2 FeO(OH) → Fe₂O₃ + H₂O

At about 200 °C, the iron(III) hydroxide converts into Fe₂O₃.^[4]

See also

• Iron oxides

References

1. ^ J.E Greedon, (1994), Magnetic oxides in Encyclopedia of Inorganic chemistry Ed. R. Bruce King, John Wiley & Sons ISBN 0471936200
2. ^ ^{a b} "Oxid železitý, Fe2O3" (in Czech). http://atmilab.upol.cz/vys/fe2o3.html. Retrieved 20. JUN 2009. 
3. ^ Adlam & Price, Higher School Certificate Inorganic Chemistry, Leslie Slater Price, 1945.
4. ^ ^{a b} Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
5. ^ Paint and Surface Coatings: Theory and Practice William Andrew Inc. ISBN 1884207731

External links

• NIOSH Pocket Guide to Chemical Hazards