Chromium(III) sulfate


Chromium sulfate usually refers to the inorganic compounds with the formula Cr23.x, 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.

Chromium(III) sulfates

Three chromium sulfates are well characterized:
  • Anhydrous chromium sulfate, Cr23, is a violet solid that dissolves in water upon addition of a reducing agent. It is isostructural with anhydrous aluminum sulfate.
  • Hydrated chromium sulfate, Cr23·18H2O, is a violet solid that readily dissolves in water to give the metal aquo complex, 3+. The formula of this compound can be written more descriptively as 23·6H2O. Six of the eighteen water molecules in this formula unit are water of crystallization.
  • Hydrated chromium sulfate, Cr23·15, is a green solid that also readily dissolves in water. It is obtained by heating the 18-hydrate material above 70 °C. Further heating yields the anhydrous sulfate.
A variety of other chromium sulfates are known, but also contain hydroxide or oxide ligands. Most important commercially is basic chromium sulfate, which is thought to be SO4. It results from the partial neutralization of the hexahydrates. Other chromium hydroxides have been reported.
[image:EntryWithCollCode26550.png|thumb|244px|left|Structure of solid Cr(SO4)(H2O)(OH), showing the CrO6 coordination sphere typical of many chromium(III) compounds. Color code: red = O, yellow = S, dark blue = Cr.]

Production

Chromium sulfate are commonly obtained from the wastes of chromate oxidations of various organic compounds. Anthraquinone and quinone are produced on large scale by the x treatment of respectively anthracene and phenol with chromic acid. A chromium oxide byproduct is generated, which is readily extracted into sulfuric acid. Evaporation of these acidic solutions affords salts if hydrate chromium sulfate. Extraction of chromite ore with sulfuric acid in the presence of some chromate gives solutions of chromium sulfate contaminated with other metal ions. Similarly, dissolution of chrome alloys gives chromium sulfate together with ferrous sulfate.

Basic chromium(III) sulfate

Basic chromium sulfate is produced from chromate salts by reduction with sulfur dioxide, although other methods exist. The reduction could formally be written:
Since 33% of the anion charges are due to hydroxide ions, the basicity is 33%. Products with higher basicities, e.g. 42% or 50% may be obtained by the addition of sodium carbonate, these are often used in combination with sodium formate. The sodium sulfate is often left in the technical product since it is inert with respect to the tanning process.

Natural occurrence

Three complex minerals that are in part Cr sulfates: bentorite, reddingtonite, and putnisite.