 | |
 | |
| Names | |
|---|---|
| Other names | |
| Identifiers | |
3D model (JSmol) | |
| ChEBI | |
| ChemSpider |
|
| ECHA InfoCard | 100.013.783 |
| EC Number |
|
| 11116 | |
| RTECS number |
|
| UNII | |
| UN number | 3077 |
| |
| |
| Properties | |
| Cr2O3 | |
| Molar mass | 151.9904 g/mol |
| Appearance | light to dark green, fine crystals |
| Density | 5.22 g/cm3 |
| Melting point | 2,435 °C (4,415 °F; 2,708 K) |
| Boiling point | 4,000 °C (7,230 °F; 4,270 K) |
| insoluble | |
| Solubility inalcohol | insoluble inalcohol,acetone, acids |
| +1960.0×10−6 cm3/mol | |
Refractive index (nD) | 2.551 |
| Structure[1] | |
| Corundum | |
| R3c (No. 167) | |
a = 495 pm,c = 1358 pm | |
| Thermochemistry | |
Std molar entropy(S⦵298) | 81 J·mol−1·K−1 |
Std enthalpy of formation(ΔfH⦵298) | −1128 kJ·mol−1 |
| Hazards | |
| GHS labelling: | |
  | |
| Danger | |
| H302,H317,H319,H360 | |
| P201,P202,P261,P264,P270,P272,P280,P281,P301+P312,P302+P352,P305+P351+P338,P308+P313,P321,P330,P333+P313,P337+P313,P363,P405,P501 | |
| NIOSH (US health exposure limits): | |
PEL (Permissible) | TWA 1 mg/m3[2] |
REL (Recommended) | TWA 0.5 mg/m3[2] |
IDLH (Immediate danger) | 250 mg/m3[2] |
| Related compounds | |
Related compounds | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Chromium(III) oxide (orchromia) is aninorganic compound with theformulaCr
2O
3. It is one of the principal oxides ofchromium and is used as a pigment. In nature, it occurs as a rare mineral calledeskolaite.[3]
Cr
2O
3 has thecorundum structure, consisting of ahexagonal close packed array of oxide anions with two thirds of the octahedral holes occupied by chromium. Similar to corundum,Cr
2O
3 is a hard, brittle material (Mohs hardness 8 to 8.5).[4] It isantiferromagnetic up to 307 K (34 °C), theNéel temperature.[5][6] It is not readily attacked by acids.
Cr
2O
3 occurs naturally as the mineraleskolaite, which is found in chromium-richtremolite skarns, metaquartzites, andchlorite veins. Eskolaite is also a rare component of chondrite meteorites. The mineral is named after Finnish geologistPentti Eskola.[4]
The Parisians Pannetier and Binet first prepared the transparent hydrated form ofCr
2O
3 in 1838 via a secret process, sold as a pigment.[7] It is derived from the mineralchromite,(Fe,Mg)Cr
2O
4. The conversion of chromite to chromia proceeds viaNa
2Cr
2O
7, which is reduced with sulfur at high temperatures:[8]
The oxide is also formed by the decomposition of chromium salts such as chromium nitrate, or by the exothermic decomposition ofammonium dichromate:[9]
Because of its considerable stability, chromia is a commonly used pigment. It was originally calledviridian. It is used in paints, inks, and glasses. It is the colorant in "chrome green" and "institutional green". Chromium(III) oxide is a precursor to the magnetic pigmentchromium dioxide by the following reaction:[8]
Along with many other oxides, it is used as a compound when polishing (also calledstropping) the edges of knives, razors, surfaces of optical devices etc. on a piece of leather, balsa, cloth or other material. It is available in powder or wax form, and in this context it is known as "green compound".
It is used as a component ofrefractories due to its high melting point.
Once used as an inert marker in livestock intake research, its use in animal science research has been diminishing with the increased use of titanium dioxide due to the latter being considered more food-safe. Titanium dioxide as a digestion marker allows the public sale of animals used in research trials.[10]
Chromium(III) oxide isamphoteric. Although insoluble in water, it reacts with acid to produce salts of hydrated chromium ions such as[Cr(H
2O)
6]3+
.[citation needed] It is also attacked by concentrated alkali to yield salts of[Cr(OH)
6]3−
.[11]
When heated with finely divided carbon or aluminium, it is reduced to chromium metal:[12]
Unlike the classicthermite reaction involving iron oxides, the chromium oxide thermite creates few or no sparks, smoke or sound, but glows brightly. Because of the very high melting point of chromium, chromium thermite casting is impractical.
Heating withchlorine and carbon yieldschromium(III) chloride andcarbon monoxide:
Chromates salts form by the oxidation of chromium(III) oxide in the presence an alkali metal oxide (M2O):
A similar oxidation proceeds with alkaline earth oxides:[11]
Several million kilograms are processed in this way as a means of winning chromium from its ore.[8] The ore is fused with a base, typicallysodium carbonate, at around 1000 °C in the presence of air (source of oxygen):
This step solubilizes the chromium and allows it to be extracted into hot water, separating the chromium from aluminium- and iron-containing compounds.
