Its unusual property is that it contains copper in both of its common oxidation states, making it a mixed-valence complex.

[5] Chevreul's salt is prepared by treating aqueous copper(II) sulfate with a solution of potassium metabisulfite.

Heating this solution produces a reddish solid precipitate: 3 CuSO4 + 4 K2S2O5 + 3 H2O → Cu3(SO3)2•2H2O + 4 K2SO4 + 4 SO2 + H2SO4 When sodium ions are present in the solutions that form the salt, sodium can substitute for some of the copper (I), as the ions have the same charge and similar sizes.

If an ammonia solution is added to the product, it is dissolved and a deep blue color appears - the presence of [Cu(NH3)4]2+ complex.

The properties of these salts show the effect of ionic radius and ion hardness.

The +1 oxidation state copper is in a distorted tetrahedral space surrounded by three oxygens and a sulfur atom.

The +2 oxidation state copper (or other metal in the isomorphic series) is in a distorted octahedral coordination surrounded by four oxygen atoms and two water molecules.

[8] Chevreul's salt is formed as a corrosion product on copper metal in the presence of humid air contaminated with sulfur dioxide.