Iron(III) oxide

Iron(III) oxide

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Iron oxide or ferric oxide is the inorganic compound
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

 with the formula Fe2O3. It is one of the three main oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....

s of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, the other two being iron(II) oxide
Iron(II) oxide
Iron oxide, also known as ferrous oxide, is one of the iron oxides. It is a black-colored powder with the chemical formula . It consists of the chemical element iron in the oxidation state of 2 bonded to oxygen. Its mineral form is known as wüstite. Iron oxide should not be confused with rust,...

 (FeO), which is rare, and iron(II,III) oxide
Iron(II,III) oxide
Iron oxide is the chemical compound with formula Fe3O4. It is one of a number of iron oxides, the others being iron oxide , which is rare, and iron oxide also known as hematite. It occurs in nature as the mineral magnetite. It contains both Fe2+ and Fe3+ ions and is sometimes formulated as FeO ∙...

 (Fe3O4), which also occurs naturally as the mineral magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...

. As the mineral known as hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...

, Fe2O3 is the main source of the iron for the steel industry. Fe2O3 is ferromagnetic
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...

, dark red, and readily attacked by acids. Rust
Rust
Rust is a general term for a series of iron oxides. In colloquial usage, the term is applied to red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture...

 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.

Structure


Fe2O3 can be obtained in various polymorph
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...

s. In the main ones, α and γ, iron adopts octahedral coordination geometry. That is, each Fe centre is bound to six oxygen ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

s.

Alpha phase


α-Fe2O3 has the rhombohedral, corundum
Aluminium oxide
Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...

 (α-Al2O3) structure and is the most common form. It occurs naturally as the mineral hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...

 which is mined as the main ore
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....

 of iron. It is antiferromagnetic below ~260 K (Morin transition temperature), and weak ferromagnetic between 260 K and 950 K Néel temperature
Néel temperature
The Néel temperature or magnetic ordering temperature , TN, is the temperature above which an antiferromagnetic material becomes paramagnetic—that is, the thermal energy becomes large enough to destroy the macroscopic magnetic ordering within the material....

. It is easy to prepare using both thermal decomposition
Thermal decomposition
Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes....

 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


Cubic, metastable, converts to the alpha phase at high temperatures. Occurs naturally as the mineral maghemite
Maghemite
Maghemite is a member of the family of iron oxides. It has the same structure as magnetite, that is, it is spinel ferrite and is also ferrimagnetic.Maghemite can be considered as an Fe-deficient magnetite with formula...

. Ferromagnetic. Ultrafine particles
Ultrafine particles
Ultrafine particles are nanoscale, less than 100 nanometres. Regulations do not exist for this size class of ambient air pollution particles, which are far smaller than the regulated PM10 and PM2.5 size classes and are believed to have several more aggressive health implications than those classes...

 smaller than 10 nanometers are superparamagnetic. Can be prepared by thermal dehydratation of gamma iron(III) oxide-hydroxide
Iron(III) oxide-hydroxide
A number of species are dubbed iron oxide-hydroxide. These chemicals are oxide-hydroxides of iron, and may occur in anhydrous or hydrated forms...

, careful oxidation of iron(II,III) oxide
Iron(II,III) oxide
Iron oxide is the chemical compound with formula Fe3O4. It is one of a number of iron oxides, the others being iron oxide , which is rare, and iron oxide also known as hematite. It occurs in nature as the mineral magnetite. It contains both Fe2+ and Fe3+ ions and is sometimes formulated as FeO ∙...

. The ultrafine particles can be prepared by thermal decomposition of iron(III) oxalate.

Other phases


Several other phases have been identified or claimed. The beta-phase is cubic face centered, metastable, at temperatures above 500 °C (932 °F) converts to alpha phase. It can be prepared by reduction of hematite by carbon, pyrolysis
Pyrolysis
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...

 of iron(III) chloride
Iron(III) chloride
Iron chloride, also called ferric chloride, is an industrial scale commodity chemical compound, with the formula FeCl3. The colour of iron chloride crystals depends on the viewing angle: by reflected light the crystals appear dark green, but by transmitted light they appear purple-red...

 solution, or thermal decomposition of iron(III) sulfate
Iron(III) sulfate
Iron sulfate , is the chemical compound with the formula Fe23, the sulfate of trivalent iron. Usually yellow, it is a rhombic crystalline salt and soluble in water at room temperature. It is used in dyeing as a mordant, and as a coagulant for industrial wastes. It is also used in pigments, and in...

. The epsilon phase is 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. This phase is also metastable, transforming to the alpha phase at between 500 and 750 °C (932 and 1,382 °F). Can also be prepared by oxidation of iron in an electric arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...

 or by sol-gel precipitation from iron(III) nitrate
Iron(III) nitrate
Iron nitrate, or ferric nitrate, is the chemical compound with the formula Fe3. Since it is deliquescent, it is commonly found in its nonahydrate form Fe3·9H2O in which it forms colourless to pale violet crystals.-Preparation:...

. Additionally at high pressure an amorphous form is claimed.

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
Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...

 thermite
Thermite
Thermite is a pyrotechnic composition of a metal powder and a metal oxide that produces an exothermic oxidation-reduction reaction known as a thermite reaction. If aluminium is the reducing agent it is called an aluminothermic reaction...

 reaction with aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

.
2 Al
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 + 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
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...

, 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 acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

s. It also dissolves well in solutions of the chelating agents such as EDTA
EDTA
Ethylenediaminetetraacetic acid, widely abbreviated as EDTA , is a polyamino carboxylic acid and a colourless, water-soluble solid. Its conjugate base is named ethylenediaminetetraacetate. It is widely used to dissolve limescale. Its usefulness arises because of its role as a hexadentate ligand...

 and oxalic acid
Oxalic acid
Oxalic acid is an organic compound with the formula H2C2O4. This colourless solid is a dicarboxylic acid. In terms of acid strength, it is about 3,000 times stronger than acetic acid. Oxalic acid is a reducing agent and its conjugate base, known as oxalate , is a chelating agent for metal cations...

.

Heating iron(III) oxides with other metal oxides or carbonates yields materials known as ferrate
Ferrate
In chemistry, ferrate refers either to the anion , in which iron is in the +6 oxidation state, or to a salt containing this anion. The term ferrate is often used to mean ferrate, although according to IUPAC naming conventions, it may also refer to other iron-containing oxyanions, such as ferrate...

s:
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

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
Jewellery
Jewellery or jewelry is a form of personal adornment, such as brooches, rings, necklaces, earrings, and bracelets.With some exceptions, such as medical alert bracelets or military dog tags, jewellery normally differs from other items of personal adornment in that it has no other purpose than to...

 and lenses
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

, and historically as a cosmetic
Rouge (cosmetics)
Rouge , also called blush or blusher , is a cosmetic typically used by women to redden the cheeks so as to provide a more youthful appearance, and to emphasize the cheekbones....

.

Rouge cuts more slowly than some modern polishes, such as cerium(IV) oxide
Cerium(IV) oxide
Cerium oxide, also known as ceric oxide, ceria, cerium oxide or cerium dioxide, is an oxide of the rare earth metal cerium...

, 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
Wax
thumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...

 or grease
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...

 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
Ultrasonic cleaning
thumb|300px|Ultrasonic cleaner showing the removable basket in place, and a closeup of the light and timerAn ultrasonic cleaner is a cleaning device that uses ultrasound and an appropriate cleaning solvent to clean delicate items...

.

Tool Sharpening


Products sold as stropping
Stropping
When applied to computer languages, stropping refers to the method used to mark letter sequences as having a special property such as being a keyword or certain type of variable or storage location....

 compound
are often applied to a leather strop
Razor strop
A razor strop is a flexible strip of leather or canvas used to straighten and polish the blade of a straight razor, a knife, or a woodworking tool like a chisel...

 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
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

, 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
Food and Drug Administration
The Food and Drug Administration is an agency of the United States Department of Health and Human Services, one of the United States federal executive departments...

(FDA)-approved for use in cosmetics.

Niche uses


Being inexpensive and nontoxic, ferric oxide finds many niche uses. For example, in its granular form (GFO, granular-ferric-oxide), it is used to remove phosphates in aquariums. The crystals are also used, due to their ferromagnetism, in audio tapes, as the level of magnetism records the frequency of sound to be recorded.

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