Chrome alum or Chromium(III) potassium sulfate is the potassium double sulfate of chromium. Its chemical formula is KCr(SO4)2 and it is commonly found in its hydrate form as KCr(SO4)2·12(H2O). It is used in leather tanning.
Production and properties
Chromium alum is produced from chromate salts or from ferrochromium alloys. Concentrated aqueous solutions of potassium dichromate can be reduced, usually with
sulfur dioxide but also with alcohols or
formaldehyde, in the presence of sulfuric acid at temperatures <40 °C. Alternatively and less commonly, ferrochromium alloys can be dissolved in sulfuric acid and, after precipitation of the ferrous sulfate, the chrome alum crystallizes upon addition of potassium sulfate. Chromium alum crystallizes in regular
octahedra with flattened corners and is very soluble in water. The solution reddens
litmus and is an
astringent. Aqueous solutions are dark violet and turns green when it is heated above 50 °C.
In addition to the dodecahydrate, the hexahydrate KCr(SO
4)
2·6H
2O, dihydrate KCr(SO
4)
2·2H
2O, and the monohydrate KCr(SO
4)
2·H
2O are known.
[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. ]
Uses
Chromium alum is used in the tanning of
leather as chromium(III) stabilizes the leather by cross linking the
collagen fibers within the leather.
However, this application is obsolete because the simpler chromium(III) sulfate is preferred.
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It was also used in gelatine emulsions in
