Chromium(III) sulfate usually refers to the inorganic compounds with the formula Cr₂(SO₄)₃^.x(H₂O), 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(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 In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorat ...

, r(H₂O)₆sup>3+. The formula of this compound can be written more descriptively as r(H₂O)₆sub>2(SO₄)₃·6H₂O. Six of the eighteen water molecules in this formula unit are

water of crystallization In chemistry, water(s) of crystallization or water(s) of hydration are water molecules that are present inside crystals. Water is often incorporated in the formation of crystals from aqueous solutions. In some contexts, water of crystallization i ...

. *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 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 from chromate salts by reduction with sulfur dioxide, although other methods exist. The reduction could formally be written: :Na₂Cr₂O₇ + 3 SO₂ + H₂O → Cr₂(SO₄)₃ + 2 NaOH Since 33% of the

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

charges are due to hydroxy ions the basicity is 33% (but in tanning

jargon Jargon is the specialized terminology associated with a particular field or area of activity. Jargon is normally employed in a particular communicative context and may not be well understood outside that context. The context is usually a partic ...

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 Sodium formate, HCOONa, is the sodium salt of formic acid, HCOOH. It usually appears as a white deliquescent powder. Preparation For commercial use, sodium formate is produced by absorbing carbon monoxide under pressure in solid sodium hydroxide ...

. The sodium sulfate is often left in the technical product since it is inert with respect to the tanning process. It is important to fully reduce the

hexavalent chromium Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is chromium in any chemical compound that contains the element in the +6 oxidation state (thus hexavalent). Virtually all chromium ore is processed via hexavalent chromium, specifically the ...

to trivalent since the hexavalent is more likely to cause health problems for tanners and leather consumers.

Natural 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:

bentorite Bentorite is a mineral with the chemical formula Ca6(Cr,Al)2(SO4)3(OH)12·26H2O. It is colored violet to light violet. Its crystals are hexagonal to dihexagonal dipyramidal. It is transparent and has vitreous luster. It has perfect cleavage. It ...

, reddingtonite, and ptnisite.

References

{{Sulfates Chromium(III) compounds Sulfates