chrome green

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

Structure and properties

has the corundum structure, consisting of a hexagonal close packed array of oxide anions with two thirds of the octahedral holes occupied by chromium. Similar to corundum, is a hard, brittle material (Mohs hardness 8 to 8.5). It is antiferromagnetic up to , the Néel temperature. It is not readily attacked by acids.

Occurrence

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.

Production

The Parisians Pannetier and Binet first prepared the transparent hydrated form of in 1838 via a secret process, sold as a pigment. It is derived from the mineral chromite, . The conversion of chromite to chromia proceeds via , which is reduced with sulfur at high temperatures:
: + S → +

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

: → + + 4

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:
: + 3 → 5 +

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.

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 consider more food-safe. Titanium dioxide as a digestion marker allows the public sale of animals used in research trials.

Reactions

Chromium(III) oxide is amphoteric. Although insoluble in water, it reacts with acid to produce salts of hydrated chromium ions such as . It is also attacked by concentrated alkali to yield salts of .

When heated with finely divided carbon or aluminium, it is reduced to chromium metal:
: + 2 Al → 2 Cr +
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:
: + 3 + 3 C → 2 + 3 CO

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:
:
Several million kilograms are processed in this way as a means of winning chromium from its ore. The ore is fused with a base, typically sodium 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.

See also

* Chromium(II) oxide
* Chromium(IV) oxide
* Chromium trioxide
* Chromic acid
* Green pigments
* List of inorganic pigments
* Refractory

References

{{DEFAULTSORT:Chromium(Iii) Oxide
Chromium(III) compounds
Transition metal oxides
Inorganic pigments
Alchemical substances
Abrasives
Sesquioxides
Glass dyes