Chromium(III) sulfates
Three chromium(III) sulfates are well characterized: *Anhydrous chromium(III) sulfate, Cr2(SO4)3, (CAS #10101-53-8) is a violet solid that dissolves in water upon addition of a reducing agent, which generates chromium(II) sulfates. *Hydrated chromium(III) sulfate, Cr2(SO4)3·18H2O, (CAS #13520-66-6) is a violet solid that readily dissolves in water to give the metal aquo complex, 2O)6">r(H2O)6sup>3+. The formula of this compound can be written more descriptively as 2O)6">r(H2O)6sub>2(SO4)3·6H2O. Six of the eighteen water molecules in this formula unit are water of crystallization. *Hydrated chromium(III) sulfate, Cr2(SO4)3·15(H2O), (CAS #10031-37-5) 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(VI) sulfates are known, but also contain hydroxide or oxide ligands. Most important commercially is basic chromium sulfate, which is thought to be 2(H2O)6(OH)4">r2(H2O)6(OH)4O4 (CAS#39380-78-4). It results from the partial neutralization of the hexahydrates. Other chromium(III) hydroxides have been reported. 244px, left, Structure of Cr(SO4)(H2O)(OH), showing the CrO6 coordination sphere typical of many chromium(III) compounds.Production
The most useful source of chromium(III) sulfate are the Cr(III) wastes from the chromate oxidation of various organic compounds. Anthroquinone and quinone are produced on large scale by treatment of anthracene and phenol with chromic acid. A chromium(III) oxide coproduct is generated which is readily extracted into sulfuric acid. Evaporation of these acidic solutions affords the hydrate salt described above. The hydrated salts of chromium sulfate can also be produced, albeit impure, by extraction of various other chromium compounds, but these routes are not economically viable.. Extraction of chromite ore with sulfuric acid in the presence of some chromate gives solutions of chromium(III) 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 fromNatural occurrence
Pure chromium(III) sulfate, both in anhydrous and hydrous form, is as yet (2020) unknown among mineral species. There are 3 complex minerals being, in part, Cr(III) sulfates:References
{{Sulfates Chromium(III) compounds Sulfates