Chromium

Chromium: the essentials

 Brief description: chromium is steel-gray, lustrous, hard, metallic, and takes a high polish. Its compounds are toxic. It is found as chromite ore. Siberian red lead (crocoite, PrCrO₄) is a chromium ore prized as a red pigment for oil paints.

Emerald is a form of beryl (a beryllium aluminium silicate) which is green because of the inclusion of a little chromium into the beryl crytal lattice in place of some of the aluminium ions. Similarly, traces of chromium incorporated into the crystal lattice of corundum (crystalline aluminium oxide, Al₂O₃) as a replacement for some of the Al³⁺ ions results in another highly coloured gem stone, in this case the red ruby.

This sample is from The Elements Collection, an attractive and safely packaged collection of the 92 naturally occurring elements that is available for sale.

The result of burning ammonium dichromate, (NH₄)₂Cr₂O₇, containing pellets of mercuric thiocyanate (HgCNS). This is a large scale version of the indoor firework "Serpents in the grass" (only to be demonstrated by a professionally qualified chemist following a legally satisfactory hazard asessment). Improperly done, this reaction is dangerous! Llighting not great - sorry.

Cartoon by Nick D Kim (nearingzero.net, used by permission).

Isolation

Isolation: it is not normally necessary to make chromium in the laboratory as it is so readily available commercially. The most useful source of chromium commercially is the ore chromite, FeCr₂O₄. Oxidation of this ore by air in molten alkali gives sodium chromate, Na₂CrO₄ in which the chromium is in the +6 oxidation state. This is converted to the Cr(III) oxide Cr₂O₃ by extraction into water, precipitation, and reduction with carbon. The oxide is then further reduced with aluminium or silicon to form chromium metal.

Cr₂O₃ + 2Al → 2Cr + Al₂O₃

2Cr₂O₃ + 3Si → 4Cr + 3SiO₂

Another kind of isolation is by electroplating processes. This involves the dissolution of Cr₂O₃ in sulphuric acid to give an electrolyte used for chromium electroplating.

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