Chromium(III) sulfate

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. It is isostructural with anhydrous aluminum sulfate.
• 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, [Cr(H2O)6]3+. The formula of this compound can be written more descriptively as [Cr(H2O)6]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(III) sulfates are known, but also contain hydroxide or oxide ligands. Most important commercially is basic chromium sulfate, which is thought to be [Cr2(H2O)6(OH)4]SO4 (CAS#39380-78-4). It results from the partial neutralization of the hexahydrates. Other chromium(III) hydroxides have been reported. 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(III) 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(III) oxide byproduct is generated, which is readily extracted into sulfuric acid. Evaporation of these acidic solutions affords salts if hydrate chromium(III) sulfate. 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(III) 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% (but in tanning jargon it is known as 33% reduced). 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(III) sulfates: bentorite, reddingtonite, and putnisite.

References

References

1. {{PGCH. 0141
2. (1993). "Beiträge zum thermischen Verhalten von Sulfaten IX. Einkristallstrukturverfeinerung der Metall(III)-sulfate Cr₂(SO₄)₃ und Al₂(SO₄)₃". R. Oldenbourg Verlag.
3. Covington, A. D.; Lampard, G. S.; Menderes, O.; Chadwick, A. V.; Rafeletos, G.; O'Brien, P. "Extended X-ray absorption fine structure studies of the role of chromium in leather tanning" Polyhedron 2001, volume 20, 461–466. {{doi. 10.1016/S0277-5387(00)00611-2
4. (1982). "Structure de l'Hydroxysulfate de Chrome monohydrate". Acta Crystallographica B.
5. "Chromium Compounds".
6. "Chromium sulfate tanning powder". Farmers Fertiliser Ltd - a subsidiary of Fernz Corporation Ltd.
7. "Bentorite".
8. "Redingtonite".
9. "Putnisite".
10. (21 March 2011). "List of Minerals".