Chromium(III) oxide

Last updated
Chromium(III) oxide
Chromium oxide cropped.jpg
Corundum struct.png
Names
Other names
Chromium sesquioxide
Chromia
Chrome green
Eskolaite
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.013.783 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 215-160-9
11116
PubChem CID
RTECS number
  • GB6475000
UNII
UN number 3077
  • InChI=1S/2Cr.3O Yes check.svgY
    Key: QDOXWKRWXJOMAK-UHFFFAOYSA-N Yes check.svgY
  • 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
Appearancelight 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
Std molar
entropy (S⦵298)
81 J·mol−1·K−1
−1128 kJ·mol−1
Hazards
GHS labelling:
GHS-pictogram-exclam.svg GHS-pictogram-silhouette.svg
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).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

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. [3]

Contents

Structure and properties

Cr
2O
3 has the corundum structure, consisting of a hexagonal close packed array of oxide anions with 2⁄3 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 is antiferromagnetic up to 307 K, the Néel temperature. [5] [6] It is not readily attacked by acids.

Occurrence

Eskolaite mineral Eskolaite.jpg
Eskolaite mineral

Cr
2O
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. [4]

Production

The Parisians Pannetier and Binet first prepared the transparent hydrated form of Cr
2O
3 in 1838 via a secret process, sold as a pigment. [7] It is derived from the mineral chromite, (Fe,Mg)Cr
2O
4. The conversion of chromite to chromia proceeds via Na
2Cr
2O
7, which is reduced with sulfur at high temperatures: [8]

Na
2Cr
2O
7 + S → Na
2SO
4 + Cr
2O
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)
2Cr
2O
7 → Cr
2O
3 + N
2 + 4 H
2O

The reaction has a low ignition temperature of less than 200 °C and is frequently used in “volcano” demonstrations. [9]

Applications

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: [8]

Cr
2O
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.

From 1962, it has been used as an inert marker in livestock intake research. However, its use in animal science research has been diminishing with the increased use of Titanium dioxide due to the latter being consider more food safe, allowing for the public sale of animals used in research trials that use an inert digestion marker. [10]

Reactions

Chromium(III) oxide is amphoteric. Although insoluble in water, it reacts with acid to produce salts of hydrated chromium ions such as [Cr(H
2O)
6]3+
. [11] 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
2O
3 + 2 Al → 2 Cr + Al
2O
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
2O
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
2O
3 + 4 MO + 3 O
2 → 4 MCrO
4

See also

Related Research Articles

<span class="mw-page-title-main">Chromium</span> Chemical element with atomic number 24 (Cr)

Chromium is a chemical element; it has symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.

<span class="mw-page-title-main">Iron(III) oxide</span> Chemical compound

Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It occurs in nature as the mineral hematite, which serves as the primary source of iron for the steel industry. It is also known as red iron oxide, especially when used in pigments.

Chromic acid is jargon for a solution formed by the addition of sulfuric acid to aqueous solutions of dichromate. It consists at least in part of chromium trioxide.

<span class="mw-page-title-main">Chromate and dichromate</span> Chromium(VI) anions

Chromate salts contain the chromate anion, CrO2−
4. Dichromate salts contain the dichromate anion, Cr
2O2−
7. They are oxyanions of chromium in the +6 oxidation state and are moderately strong oxidizing agents. In an aqueous solution, chromate and dichromate ions can be interconvertible.

<span class="mw-page-title-main">Chromite</span> Crystalline mineral

Chromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. It can be represented by the chemical formula of FeCr2O4. It is an oxide mineral belonging to the spinel group. The element magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite (MgCr2O4). Substitution of the element aluminium can also occur, leading to hercynite (FeAl2O4). Chromite today is mined particularly to make stainless steel through the production of ferrochrome (FeCr), which is an iron-chromium alloy.

<span class="mw-page-title-main">Copper(II) oxide</span> Chemical compound – an oxide of copper with formula CuO

Copper(II) oxide or cupric oxide is an inorganic compound with the formula CuO. A black solid, it is one of the two stable oxides of copper, the other being Cu2O or copper(I) oxide (cuprous oxide). As a mineral, it is known as tenorite, or sometimes black copper. It is a product of copper mining and the precursor to many other copper-containing products and chemical compounds.

<span class="mw-page-title-main">Potassium dichromate</span> Chemical compound

Potassium dichromate, K2Cr2O7, is a common inorganic chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is acutely and chronically harmful to health. It is a crystalline ionic solid with a very bright, red-orange color. The salt is popular in laboratories because it is not deliquescent, in contrast to the more industrially relevant salt sodium dichromate.

<span class="mw-page-title-main">Chromium(III) chloride</span> Chemical compound

Chromium(III) chloride (also called chromic chloride) is an inorganic chemical compound with the chemical formula CrCl3. It forms several hydrates with the formula CrCl3·nH2O, among which are hydrates where n can be 5 (chromium(III) chloride pentahydrate CrCl3·5H2O) or 6 (chromium(III) chloride hexahydrate CrCl3·6H2O). The anhydrous compound with the formula CrCl3 are violet crystals, while the most common form of the chromium(III) chloride are the dark green crystals of hexahydrate, CrCl3·6H2O. Chromium chlorides find use as catalysts and as precursors to dyes for wool.

<span class="mw-page-title-main">Chromium trioxide</span> Chemical compound

Chromium trioxide is an inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name. This compound is a dark-purple solid under anhydrous conditions and bright orange when wet. The substance dissolves in water accompanied by hydrolysis. Millions of kilograms are produced annually, mainly for electroplating. Chromium trioxide is a powerful oxidiser, a mutagen, and a carcinogen.

<span class="mw-page-title-main">Copper chromite</span> Chemical compound

Copper chromite often refers to inorganic compounds with the formula Cu2Cr2Ox. They are black solids. Cu2Cr2O4 is a well-defined material. The other copper chromite often is described as Cu2Cr2O5. It is used to catalyze reactions in organic chemistry.

<span class="mw-page-title-main">Sodium dichromate</span> Inorganic compound

Sodium dichromate is the inorganic compound with the formula Na2Cr2O7. However, the salt is usually handled as its dihydrate Na2Cr2O7·2H2O. Virtually all chromium ore is processed via conversion to sodium dichromate and virtually all compounds and materials based on chromium are prepared from this salt. In terms of reactivity and appearance, sodium dichromate and potassium dichromate are very similar. The sodium salt is, however, around twenty times more soluble in water than the potassium salt (49 g/L at 0 °C) and its equivalent weight is also lower, which is often desirable.

<span class="mw-page-title-main">Chromium compounds</span> Chemical compounds containing chromium

Chromium compounds are compounds containing the element chromium (Cr). Chromium is a member of group 6 of the transition metals. The +3 and +6 states occur most commonly within chromium compounds, followed by +2; charges of +1, +4 and +5 for chromium are rare, but do nevertheless occasionally exist.

<span class="mw-page-title-main">Ammonium dichromate</span> Chemical compound

Ammonium dichromate is an inorganic compound with the formula (NH4)2Cr2O7. In this compound, as in all chromates and dichromates, chromium is in a +6 oxidation state, commonly known as hexavalent chromium. It is a salt consisting of ammonium ions and dichromate ions.

<span class="mw-page-title-main">Chromium(III) sulfate</span> Chemical compound

Chromium(III) sulfate usually refers to the inorganic compounds with the formula Cr2(SO4)3.x(H2O), where x can range from 0 to 18. Additionally, ill-defined but commercially important "basic chromium sulfates" are known. These salts are usually either violet or green solids that are soluble in water. It is commonly used in tanning leather.

<span class="mw-page-title-main">Jones oxidation</span> Oxidation of alcohol

The Jones oxidation is an organic reaction for the oxidation of primary and secondary alcohols to carboxylic acids and ketones, respectively. It is named after its discoverer, Sir Ewart Jones. The reaction was an early method for the oxidation of alcohols. Its use has subsided because milder, more selective reagents have been developed, e.g. Collins reagent.

<span class="mw-page-title-main">Chromyl fluoride</span> Chemical compound

Chromyl fluoride is an inorganic compound with the formula CrO2F2. It is a violet-red colored crystalline solid that melts to an orange-red liquid.

Chromium(III) hydroxide is a gelatinous green inorganic compound with the chemical formula Cr(OH)3. It is a polymer with an undefined structure and low solubility. It is amphoteric, dissolving in both strong alkalis and strong acids.

<span class="mw-page-title-main">Chromium(III) bromide</span> Chemical compound

Chromium(III) bromide is an inorganic compound with the chemical formula CrBr3. It is a dark colored solid that appears green in transmitted light but red with reflected light. It is used as a precursor to catalysts for the oligomerization of ethylene.

<span class="mw-page-title-main">Guyanaite</span> Chromium oxide mineral

Guyanaite (CrOOH) is a chromium oxide mineral that forms as an intergrowth with other chromium oxide minerals known as bracewellite (CrOOH) and grimaldiite (CrOOH) as well as eskolaite (Cr2O3) which in early findings were nearly indistinguishable from one another. These oxides formed so closely as intergrowths with one another that they were initially, and erroneously, identified as a single definite mineral previously known as merumite. Because of its complex history and the previously undiscovered nature of these chromium oxide polymorphs, the relevance of any information found in many early experiments involving the mineral formerly known as merumite in regard to guyanaite is unknown and it is implied that in any further reference of merumite it will have been composed of a mineral assemblage including guyanaite. The rare occurrence and complexity from intergrowth of naturally occurring guyanaite hinders experimental work, leading to laboratory synthesized samples which help to better experiment with the minerals.

References

  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. 4 (2). AIP Publishing: 027121. Bibcode:2014AIPA....4b7121A. doi: 10.1063/1.4867012 . ISSN   2158-3226.
  2. 1 2 3 NIOSH Pocket Guide to Chemical Hazards. "#0141". National Institute for Occupational Safety and Health (NIOSH).
  3. ↑ "Pigments through the Ages - Overview - Cobalt green".
  4. 1 2 "Eskolaite". Webminerals. Retrieved 2009-06-06.
  5. ↑ J.E Greedan, (1994), Magnetic oxides in Encyclopedia of Inorganic chemistry R. Bruce King, Ed. John Wiley & Sons. ISBN   0-471-93620-0
  6. ↑ A. F. Holleman and E. Wiberg "Inorganic Chemistry" Academic Press, 2001, New York. ISBN   0-12-352651-5.
  7. ↑ Eastaugh, Nicholas; Chaplin, Tracey; Siddall, Ruth (2004). The pigment compendium: a dictionary of historical pigments . Butterworth-Heinemann. p.  391. ISBN   0-7506-5749-9.
  8. 1 2 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
  9. ↑ "Ammonium dichromate volcano". www.rsc.org. Retrieved 2019-02-26.
  10. ↑ Williams, C.H. (1962). "The determination of chromic oxide in feces samples by atomic absorption spectrophotometry". Journal of Agricultural Sciences. 59: 381–385.
  11. ↑ R. Scholder "Sodium Hexahydroxochromate(III)" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 2, 1688ff.
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