Chromium(III) sulfate usually refers to the

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

s with the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...

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. *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 perchlo ...

, 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 is ...

. *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 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. image:EntryWithCollCode26550.png, 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 Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...

, although other methods exist. The reduction could formally be written: : 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 conven ...

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

jargon Jargon, or technical language, is the specialized terminology associated with a particular field or area of activity. Jargon is normally employed in a particular Context (language use), communicative context and may not be well understood outside ...

it is known as 33% reduced). Products with higher basicities, e.g. 42% or 50% may be obtained by the addition of

sodium carbonate Sodium carbonate (also known as washing soda, soda ash, sal soda, and soda crystals) is the inorganic compound with the formula and its various hydrates. All forms are white, odourless, water-soluble salts that yield alkaline solutions in water ...

, 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.

Natural occurrence

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

References

{{Sulfates Chromium(III) compounds Sulfates Tanning (leather)