Chromium(III) oxide

Summary

Chromium(III) oxide (or chromia) is an inorganic compound with the formula Cr
2
O
3
. It is one of the principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.

Chromium(III) oxide
Cr2o3 gruener farbstoff.jpg
Corundum struct.png
Names
Other names
Chromium sesquioxide
Chromia
Chrome green
Eskolaite
Identifiers
  • 1308-38-9 checkY
3D model (JSmol)
  • Interactive image
ChEBI
  • CHEBI:48242 checkY
ChemSpider
  • 451305 checkY
ECHA InfoCard 100.013.783 Edit this at Wikidata
EC Number
  • 215-160-9
11116
  • 517277
RTECS number
  • GB6475000
UNII
  • X5Z09SU859 checkY
UN number 3077
  • DTXSID4043721 Edit this at Wikidata
  • InChI=1S/2Cr.3O checkY
    Key: QDOXWKRWXJOMAK-UHFFFAOYSA-N checkY
  • InChI=1/2Cr.3O/rCr2O3/c3-1-5-2-4
    Key: QDOXWKRWXJOMAK-LUXALHLMAR
  • O=[Cr]O[Cr]=O
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 in alcohol insoluble in alcohol, acetone, acids
+1960.0×10−6 cm3/mol
2.551
Structure[1]
Corundum
R3c (No. 167)
a = 495 pm, c = 1358 pm
Thermochemistry
81 J·mol−1·K−1
−1128 kJ·mol−1
Hazards
GHS labelling:
GHS07: Exclamation markGHS08: Health hazard
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]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Structure and propertiesEdit

Cr
2
O
3
has the corundum structure, consisting of a hexagonal close packed array of oxide anions with 23 of the octahedral holes occupied by chromium. Similar to corundum, Cr
2
O
3
is a hard, brittle material (Mohs hardness 8 to 8.5).[3] It is antiferromagnetic up to 307 K, the Néel temperature.[4][5] It is not readily attacked by acids.

OccurrenceEdit

 
Eskolaite mineral

Cr
2
O
3
occurs naturally as the mineral eskolaite, which is found in chromium-rich tremolite skarns, metaquartzites, and chlorite veins. Eskolaite is also a rare component of chondrite meteorites. The mineral is named after Finnish geologist Pentti Eskola.[3]

ProductionEdit

The Parisians Pannetier and Binet first prepared the transparent hydrated form of Cr
2
O
3
in 1838 via a secret process, sold as a pigment.[6] It is derived from the mineral chromite, (Fe,Mg)Cr
2
O
4
. The conversion of chromite to chromia proceeds via Na
2
Cr
2
O
7
, which is reduced with sulfur at high temperatures:[7]

Na
2
Cr
2
O
7
+ S → Na
2
SO
4
+ Cr
2
O
3

The oxide is also formed by the decomposition of chromium salts such as chromium nitrate, or by the exothermic decomposition of ammonium dichromate.

(NH
4
)
2
Cr
2
O
7
Cr
2
O
3
+ N
2
+ 4 H
2
O

The reaction has a low ignition temperature of less than 200 °C and is frequently used in “volcano” demonstrations.[8]

ApplicationsEdit

Because of its considerable stability, chromia is a commonly used pigment. It was originally called viridian. 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 pigment chromium dioxide, by the following reaction:[7]

Cr
2
O
3
+ 3 CrO
3
→ 5 CrO
2
+ O
2

Along with many other oxides, it is used as a compound when polishing (also called stropping) 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 of refractories due to its high melting point.

ReactionsEdit

Chromium(III) oxide is amphoteric. Although insoluble in water, it reacts with acid to produce salts of hydrated chromium ions such as [Cr(H
2
O)
6
]3+
.[9] It is also attacked by concentrated alkali to yield salts of [Cr(OH)
6
]3−
.

When heated with finely divided carbon or aluminium, it is reduced to chromium metal:

Cr
2
O
3
+ 2 Al → 2 Cr + Al
2
O
3

Unlike the classic thermite 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 with chlorine and carbon yields chromium(III) chloride and carbon monoxide:

Cr
2
O
3
+ 3 Cl
2
+ 3 C → 2 CrCl
3
+ 3 CO

Chromates can be formed by the oxidation of chromium(III) oxide and another oxide in a basic environment:

2 Cr
2
O
3
+ 4 MO + 3 O
2
→ 4 MCrO
4

See alsoEdit

ReferencesEdit

  1. ^ Abdullah, M. M.; Rajab, Fahd M.; Al-Abbas, Saleh M. (2014). "Structural and optical characterization of Cr2O3 nanostructures: Evaluation of its dielectric properties". AIP Advances. AIP Publishing. 4 (2): 027121. doi:10.1063/1.4867012. ISSN 2158-3226.
  2. ^ a b c NIOSH Pocket Guide to Chemical Hazards. "#0141". National Institute for Occupational Safety and Health (NIOSH).
  3. ^ a b "Eskolaite". Webminerals. Retrieved 2009-06-06.
  4. ^ J.E Greedan, (1994), Magnetic oxides in Encyclopedia of Inorganic chemistry R. Bruce King, Ed. John Wiley & Sons. ISBN 0-471-93620-0
  5. ^ A. F. Holleman and E. Wiberg "Inorganic Chemistry" Academic Press, 2001, New York. ISBN 0-12-352651-5.
  6. ^ Eastaugh, Nicholas; Chaplin, Tracey; Siddall, Ruth (2004). The pigment compendium: a dictionary of historical pigments. Butterworth-Heinemann. p. 391. ISBN 0-7506-5749-9.
  7. ^ a b Gerd Anger, Jost Halstenberg, Klaus Hochgeschwender, Christoph Scherhag, Ulrich Korallus, Herbert Knopf, Peter Schmidt, Manfred Ohlinger, "Chromium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. doi:10.1002/14356007.a07_067
  8. ^ "Ammonium dichromate volcano". www.rsc.org. Retrieved 2019-02-26.
  9. ^ R. Scholder "Sodium Hexahydroxochromate(III)" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 2, 1688ff.