This page is about a red-colored oxide of iron. For other uses, see red iron.
Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one
of the three main oxides of iron, the other two being iron(II) oxide, which is rare, and
iron(II,III) oxide, which also occurs naturally as the mineral magnetite. As the mineral known
as hematite, Fe2O3 is the main source of iron for the steel industry. Fe2O3 is ferromagnetic,
dark red, and readily attacked by acids. Iron(III) oxide is often called rust, 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.
Structure Fe2O3 can be obtained in various polymorphs.
In the main ones, α and γ, iron adopts octahedral coordination geometry. That is, each Fe center
is bound to six oxygen ligands. Alpha phase
α-Fe2O3 has the rhombohedral, corundum 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, and exhibits weak ferromagnetism between 260 K and the Néel temperature, 950
K. 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. Gamma phase
γ-Fe2O3 has a cubic structure. It is metastable and converted from the alpha phase at high
temperatures. It occurs naturally as the mineral maghemite. It is ferromagnetic and finds application
in recording tapes, although ultrafine particles smaller than 10 nanometers are superparamagnetic.
It can be prepared by thermal dehydratation of gamma iron(III) oxide-hydroxide, careful
oxidation of iron(II,III) oxide. Another method involves the careful oxidation of Fe3O4. The
ultrafine particles can be prepared by thermal decomposition of iron(III) oxalate.
Other phases Several other phases have been identified
or claimed. The β-phase is cubic body centered, metastable, and 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. The epsilon
phase is rhombic, and shows properties intermediate between alpha and gamma, and may have useful
magnetic properties. Preparation of the pure epsilon phase has proven very challenging
due to contamination with alpha and gamma phases. Material with a high proportion of
epsilon phase can be prepared by thermal transformation of the gamma phase. This phase is also 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.
Additionally at high pressure an amorphous form is claimed. Recent research has revealed
epsilon iron(III) oxide in ancient Chinese Jian ceramic glazes, which may provide insight
into ways to produce that form in the lab. Hydrated iron(III) oxides
Several hydrates of Iron(III) oxide exists. When alkali is added to solutions of soluble
Fe(III) salts, a red-brown gelatinous precipitate forms. This is not Fe(OH)3, but Fe2O3·H2OOH).
Several forms of the hydrated oxide of Fe(III) exist as well. The red lepidocrocite γ-Fe(O)OH,
occurs on the outside of rusticles, and the orange goethite, which occurs internally in
rusticles. When Fe2O3·H2O is heated, it loses its water of hydration. Further heating at
1670 K converts Fe2O3 to black Fe3O4, which is known as the mineral magnetite. Fe(O)OH
is soluble in acids, giving [Fe(OH2)6]3+. In concentrated aqueous alkali, Fe2O3 gives
[Fe(OH)6]3−. Reactions
The most important reaction is its carbothermal reduction, which gives iron used in steel-making:
2 Fe2O3 + 3 C → 4 Fe + 3 CO2 Another redox reaction is the extremely exothermic
thermite reaction with aluminium. 2 Al + Fe2O3 → 2 Fe + Al2O3
This process is used to weld thick metals such as rails of train tracks by using a ceramic
container to funnel the molten iron in between two sections of rail. Thermite is also used
in weapons and making small-scale cast-iron sculptures and tools.
Partial reduction with hydrogen at about 400 °C gives magnetite, a black magnetic material
that contains both Fe(III) and Fe(II): 3 Fe2O3 + H2 → 2 Fe3O4 + H2O
Iron(III) oxide is insoluble in water but 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. Heating iron(III) oxides with other metal
oxides or carbonates yields materials known as ferrates:
ZnO + Fe2O3 → Zn(FeO2)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:
4 Fe + 3 O2 + 2 H2O → 4 FeO(OH) The resulting hydrated iron(III) oxide, written
here as Fe(O)OH, dehydrates around 200 °C. 2 FeO(OH) → Fe2O3 + H2O
It can also be prepared by the thermal decomposition of iron(III) hydroxide under temperature above
200 °C. 2 Fe(OH)3 → Fe2O3 + 3H2O
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 iron, steel, and many alloys.
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. 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. Products sold as "stropping compound"
are often applied to a leather strop to assist in getting a razor edge on knives, straight
razors, or any other edged tool. Pigment
Iron(III) oxide is also used as a pigment, under names "Pigment Brown 6", "Pigment Brown
7", and "Pigment Red 101". Some of them, e.g. Pigment Red 101 and Pigment Brown 6, are Food
and Drug Administration-approved for use in cosmetics. Iron oxides are used as pigments
in dental composites alongside titanium oxides. Hematite is the characteristic component of
the Swedish paint color Falu red. Magnetic recording
Iron(III) oxide was the most common magnetic particle used in all types of magnetic storage
and recording media, including magnetic disks and magnetic tape. However, modern magnetic
storage media - in particular, the hard disk drives - use more advanced thin film technology,
which may consist of a stack of 15 layers or more.
Photocatalyst α-Fe2O3 has been studied as a photoanode
for the water-splitting reaction for over 25 years.
Medicine A mixture of zinc oxide with about 0.5% iron(III)
oxide is called calamine, which is the active ingredient of calamine lotion.
See also Iron oxides
References
External links NIOSH Pocket Guide to Chemical Hazards