Chevreul's salt

Chevreul's salt

Names
IUPAC name Copper (I, II) sulfite dihydrate
Other names Chevreul's salt
Identifiers
CAS Number	15293-86-4
3D model (JSmol)	Interactive image
ChemSpider	154599
PubChem CID	177582
UNII	4IVM1X62DM
CompTox Dashboard (EPA)	DTXSID90929993
InChI InChI=1S/3Cu.2H2O3S/c;;;2*1-4(2)3/h;;;2*(H2,1,2,3)/q2*+1;+2;;/p-4 Key: SJZXVCGNSKQUGN-UHFFFAOYSA-J
SMILES [O-]S(=O)[O-].[O-]S(=O)[O-].[Cu+].[Cu+].[Cu+2]
Properties
Chemical formula	Cu3H4O6S4
Molar mass	418.90 g·mol−1
Appearance	brick red powder
Density	3.57
Solubility	aqueous ammonia
Thermal conductivity	0.1 kWcm−1K−1
Structure
Crystal structure	monoclinic
Space group	P21/n
Lattice constant	a = 5.5671 Å, b = 7.7875 Å, c = c = 8.3635 Å α = 90°, β = 91.279o°, γ = 90°
Lattice volume (V)	362.5 Å3
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Chevreul’s salt (Copper (I,II) Sulfite Dihydrate, Cu₂SO₃ . CuSO₃ . 2 H₂O (Cu₃(SO₃)₂ . 2 H₂O)), is a copper salt which was prepared for the first time by a French chemist Michel Eugène Chevreul in 1812. Its unique property is that it contains copper in both oxidation states. It is insoluble in water. What was known as Rogojski's salt is actually a mixture of Chevreul's salt and metallic copper.

Preparation

Chevreul's salt is prepared very simply by dissolving copper sulfate in water and mixing it with a solution of potassium metabisulfite. The solution changes colour from blue to green immediately. The compound which causes the green colour is not known. Then the solution is heated until it reaches its boiling point, and the product starts to fall out of the solution as a brownish-red powder. The solution is set to cool to room temperature, decanted and the resulting product is filtered, washed with water and dried. The ollowing reaction describes the product formation:

3 CuSO₄ + 4 K₂S₂O₅ + x H₂O → Cu₃(SO₃)₂·2H₂O + 4 K₂SO₄ + 4 SO₂ + H₂SO₄

The reaction produces sulfur dioxide so it should be carried outside or in a fumehood.

When sodium ions are present in the solutions that form the salt, sodium can substitute for some of the copper(I), as it has a similar size.

Reactions

Chevreul's salt can be tested for actually containing both copper (I) and copper (II) by making it react with dilute hydrochloric acid. Copper (II) ions dissolve and copper (I) salts react with the acid and a white precipitate appears (Copper(I) chloride). If too much acid is added, the precipitate dissolves. 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.

On heating in an inert atmosphere it is stable to 200°C. It gives off water and sulfur dioxide to give CuSO₄•Cu₂O and CuSO₄•2CuO. At 850°C CuO is formed and from 900°C to 1100°C Cu₂O appears. Heating in air or oxygen yields CuSO₄, CuSO₃, and ultimately CuO (cupric oxide)

Properties

The infrared spectrum of Chevreul's salt contains strong bands with maxima at 473, 632 cm⁻¹, medium ones at 915, 980, and 1025 cm⁻¹, and a weak band at 860 cm⁻¹. 980 cm⁻¹ is due to symmetric stretch of the sulfite group, 632 cm⁻¹ due to symmetric bend, 915 due to asymmetric stretch, and 473 cm⁻¹ is due to asymmetric bend. The absence of splitting in these bands indicates that the sulfite group is not distorted by the other components in the compound.

The reflectance spectrum shows absortion around 425 nm witha shoulder to 500 nm. This is due to a cuprous sulfite chromophore. An absorption peaking at 785 nm with a shoulder to 1000 nm, is due to Jahn-Teller splitting in cupric ions. Maximum reflectance is around 650 nm in the red spectrum.

Chevreul's salt is a a representative member of an isomorphic series of double salts with formulae Cu₂SO₃•FeSO₃•2H₂O, Cu₂SO₃•MnSO₃•2H₂O, and Cu₂SO₃•CdSO₃•2H₂O.

The thermal conductivity is 0.1 kWcm⁻¹K⁻¹. Heat capacity is 0.62 Jcm⁻³K⁻¹, and thermal diffusivity is 0.154 cm²s⁻¹.

The specific susceptibility is 3.71×10⁻⁶emu/g.

Application

Chevreul’s salt is used in a hydrometallurgical process to extract copper from ore. Firstly the ore is oxidised, then extracted with an ammonium sulfate-ammonia solution. THis is then injected with sulfur dioxide resulting in the precipitation of Chevreul’s salt. pH must be between 2 and 4.5 for the precipitation to take place.

Chevreul’s salt is formed as a corrosion product on copper metal in the presence of humid air contaminated with sulfur dioxide. When first formed the salt has an unstable orthorhombic for with a = 5.591, b = 7.781 and c = 8.356 Å which changes to the normal monoclinic form over a month, or faster when heated.