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| Names | |
|---|---|
| IUPAC name Iron(III) oxide | |
| Other names | |
| Identifiers | |
3D model (JSmol) | |
| ChEBI | |
| ChemSpider |
|
| ECHA InfoCard | 100.013.790 |
| EC Number |
|
| E number | E172(ii)(colours) |
| 11092 | |
| KEGG |
|
| RTECS number |
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| UNII | |
| |
| Properties | |
| Fe2O3 | |
| Molar mass | 159.687 g·mol−1 |
| Appearance | Red solid |
| Odor | Odorless |
| Density | 5.25 g/cm3[1] |
| Melting point | 1,539 °C (2,802 °F; 1,812 K)[1] decomposes 105 °C (221 °F; 378 K) β-dihydrate, decomposes 150 °C (302 °F; 423 K) β-monohydrate, decomposes 50 °C (122 °F; 323 K) α-dihydrate, decomposes 92 °C (198 °F; 365 K) α-monohydrate, decomposes[2] |
| Insoluble | |
| Solubility | Soluble in dilutedacids,[1] barely soluble insugar solution[2] Trihydrate slightly soluble in aq.tartaric acid,citric acid,acetic acid[2] |
| +3586.0x10−6 cm3/mol | |
Refractive index (nD) | n1 = 2.91, n2 = 3.19 (α, hematite)[3] |
| Structure | |
| Rhombohedral,hR30 (α-form)[4] Cubic bixbyite, cI80 (β-form) Cubic spinel (γ-form) Orthorhombic (ε-form)[5] | |
| R3c, No. 161 (α-form)[4] Ia3, No. 206 (β-form) Pna21, No. 33 (ε-form)[5] | |
| 3m (α-form)[4] 2/m3 (β-form) mm2 (ε-form)[5] | |
| Octahedral (Fe3+, α-form, β-form)[4] | |
| Thermochemistry[6] | |
| 103.9 J/mol·K[6] | |
Std molar entropy(S⦵298) | 87.4 J/mol·K[6] |
Std enthalpy of formation(ΔfH⦵298) | −824.2 kJ/mol[6] |
Gibbs free energy(ΔfG⦵) | −742.2 kJ/mol[6] |
| Hazards | |
| GHS labelling: | |
 [7] | |
| Warning | |
| H315,H319,H335[7] | |
| P261,P305+P351+P338[7] | |
| NFPA 704 (fire diamond) | |
Threshold limit value (TLV) | 5 mg/m3[1] (TWA) |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) | >10 g/kg (rats, oral)[9] |
| NIOSH (US health exposure limits): | |
PEL (Permissible) | TWA 10 mg/m3[8] |
REL (Recommended) | TWA 5 mg/m3[8] |
IDLH (Immediate danger) | 2500 mg/m3[8] |
| Related compounds | |
Otheranions | Iron(III) fluoride |
Othercations | Manganese(III) oxide Cobalt(III) oxide |
Relatediron oxides | Iron(II) oxide Iron(II,III) oxide |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Iron(III) oxide orferric oxide is theinorganic compound with the formulaFe2O3. It occurs in nature as the mineralhematite, which serves as the primary source of iron for the steel industry. It is also known asred iron oxide, especially when used inpigments.
It is one of the three mainoxides ofiron, the other two beingiron(II) oxide (FeO), which is rare; andiron(II,III) oxide (Fe3O4), which also occurs naturally as the mineralmagnetite.
Iron(III) oxide is often calledrust, since rust shares several properties and has a similar composition; however, in chemistry, rust is considered an ill-defined material, described as hydrous ferric oxide.[10]
Ferric oxide is readily attacked by even weakacids. It is a weakoxidising agent, most famously when reduced byaluminium in thethermite reaction.
Fe2O3 can be obtained in variouspolymorphs. In the primary polymorph, α, iron adopts octahedral coordination geometry. That is, each Fe center is bound to six oxygenligands. In the γ polymorph, some of the Fe sit on tetrahedral sites, with four oxygen ligands.
α-Fe2O3 has therhombohedral,corundum (α-Al2O3) structure and is the most common form. It occurs naturally as the mineralhematite, which is mined as the mainore of iron. It isantiferromagnetic below ~260 K (Morin transition temperature), and exhibits weakferromagnetism between 260 K and theNéel temperature, 950 K.[11] It is easy to prepare using boththermal decomposition and precipitation in the liquid phase. Its magnetic properties are dependent on many factors, e.g., pressure, particle size, and magnetic field intensity.
γ-Fe2O3 has acubic structure. It is metastable and converted from the alpha phase at high temperatures. It occurs naturally as the mineralmaghemite. It isferromagnetic and finds application in recording tapes,[12] althoughultrafine particles smaller than 10 nanometers aresuperparamagnetic. It can be prepared by thermal dehydratation of gammairon(III) oxide-hydroxide. Another method involves the careful oxidation ofiron(II,III) oxide (Fe3O4).[12] The ultrafine particles can be prepared by thermal decomposition ofiron(III) oxalate.
Several other phases have been identified or claimed. The beta phase (β-phase) is cubic body-centered (space group Ia3),metastable, and at temperatures above 500 °C (930 °F) converts to alpha phase. It can be prepared by reduction of hematite by carbon,[clarification needed]pyrolysis ofiron(III) chloride solution, or thermal decomposition ofiron(III) sulfate.[13]
The epsilon (ε) phase is rhombic, and shows properties intermediate between alpha and gamma, and may have useful magnetic properties applicable for purposes such as high densityrecording media forbig data storage.[14] Preparation of the pure epsilon phase has proven very challenging. Material with a high proportion of epsilon phase can be prepared by thermal transformation of the gamma phase. The epsilon phase is also metastable, transforming to the alpha phase at between 500 and 750 °C (930 and 1,380 °F). It can also be prepared by oxidation of iron in anelectric arc or bysol-gel precipitation fromiron(III) nitrate.[citation needed] Research has revealed epsilon iron(III) oxide in ancient ChineseJian ceramic glazes, which may provide insight into ways to produce that form in the lab.[15][non-primary source needed]
Additionally, at high pressure anamorphous form is claimed.[5][non-primary source needed]
MoltenFe2O3 is expected to have a coordination number of close to 5 oxygen atoms about each iron atom, based on measurements of slightly oxygen deficient supercooled liquid iron oxide droplets, where supercooling circumvents the need for the high oxygen pressures required above the melting point to maintain stoichiometry.[16]
Several hydrates of Iron(III) oxide exist.When alkali is added to solutions of soluble Fe(III) salts, a red-brown gelatinous precipitate forms. This isnotFe(OH)3, butFe2O3·H2O (also written asFe(O)OH).
Several forms of the hydrated oxide of Fe(III) exist as well. The redlepidocrocite (γ-Fe(O)OH) occurs on the outside ofrusticles, and the orangegoethite (α-Fe(O)OH) occurs internally in rusticles.WhenFe2O3·H2O is heated, it loses its water of hydration. Further heating at1670 K convertsFe2O3 to blackFe3O4 (FeIIFeIII2O4), which is known as the mineralmagnetite.
Fe(O)OH is soluble in acids, giving[Fe(H2O)6]3+. In concentrated aqueous alkali,Fe2O3 gives[Fe(OH)6]3−.[12]
The most important reaction is itscarbothermal reduction, which gives iron used in steel-making:
Another redox reaction is the extremelyexothermicthermite reaction withaluminium.[17]
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 about400 °C produces magnetite, a black magnetic material that contains both Fe(III) and Fe(II):[18]
Iron(III) oxide is insoluble in water but dissolves readily in strong acid, e.g., hydrochloric andsulfuric acids. It also dissolves well in solutions of chelating agents such asEDTA andoxalic acid.
Heating iron(III) oxides with other metal oxides or carbonates yields materials known asferrates (ferrate (III)):[18]
Iron(III) oxide is a product of the oxidation of iron. It can be prepared in the laboratory by electrolyzing a solution ofsodium bicarbonate, an inert electrolyte, with an iron anode:
The resulting hydrated iron(III) oxide, written here asFeO(OH), dehydrates around200 °C.[18][19]
The overwhelming application of iron(III) oxide is as the feedstock of the steel and iron industries, e.g., theproduction of iron, steel, and many alloys.[19] Iron oxide (Fe2O3) has been used in stained glass since the medieval period, with evidence suggesting its use in stained glass production dating back to the early Middle Ages, where it was primarily used to create yellow, orange, and red colors in the glass, and is still being used for industrial purposes today.[20][21]
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 metallicjewelry andlenses, and historically as acosmetic. Rouge cuts more slowly than some modern polishes, such ascerium(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 awax orgrease 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 ofultrasonic cleaning. Products sold as "stropping compound" are often applied to aleather strop to assist in getting a razor edge on knives, straight razors, or any other edged tool.
Iron(III) oxide is also used as apigment, under names "Pigment Brown 6", "Pigment Brown 7", and "Pigment Red 101".[22] Some of them, e.g., Pigment Red 101 and Pigment Brown 6, are approved by the USFood and Drug Administration (FDA) for use in cosmetics.Iron oxides are used as pigments in dental composites alongside titanium oxides.[23]
Hematite is the characteristic component of the Swedish paint colorFalu red.
Iron(III) oxide was the most commonmagnetic particle used in all types ofmagnetic storage and recording media, including magnetic disks (for data storage) andmagnetic tape (used in audio and video recording as well as data storage). Its use in computer disks was superseded by cobalt alloy, enabling thinner magnetic films with higher storage density.[24]
α-Fe2O3 has been studied as aphotoanode for solar water oxidation.[25] However, its efficacy is limited by a short diffusion length (2–4 nm) of photo-excited charge carriers[26] and subsequent fastrecombination, requiring a largeoverpotential to drive the reaction.[27] Research has been focused on improving the water oxidation performance ofFe2O3 using nanostructuring,[25] surface functionalization,[28] or by employing alternate crystal phases such as β-Fe2O3.[29]
Calamine lotion, used to treat milditchiness, is chiefly composed of a combination ofzinc oxide, acting asastringent, and about 0.5% iron(III) oxide, the product's active ingredient, acting asantipruritic. The red color of iron(III) oxide is also mainly responsible for the lotion's pink color.
