Chevreul's salt (copper(I,II) sulfite dihydrate, Cu₂SO₃•CuSO₃•2H₂O or Cu₃(SO₃)₂•2H₂O), is a copper

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...

which was prepared for the first time by a French chemist Michel Eugène Chevreul in 1812. Its unusual property is that it contains

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

in both of its common oxidation states. It is insoluble in

water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

and stable in air. What was known as Rogojski's salt is a mixture of Chevreul's salt and metallic copper.

Preparation

Chevreul's salt is prepared by treating aqueous

copper sulfate Copper sulfate may refer to: * Copper(II) sulfate, CuSO4, a common compound used as a fungicide and herbicide * Copper(I) sulfate Copper(I) sulfate, also known as cuprous sulfate, is an inorganic compound with the chemical formula Cu2 SO4. It ...

with a solution of

potassium metabisulfite Potassium metabisulfite, K2S2O5, also known as potassium pyrosulfite, is a white crystalline powder with a pungent odour. It is mainly used as an antioxidant or chemical sterilant. As a disulfite, it is chemically very similar to sodium metabi ...

. The

solution Solution may refer to: * Solution (chemistry), a mixture where one substance is dissolved in another * Solution (equation), in mathematics ** Numerical solution, in numerical analysis, approximate solutions within specified error bounds * Soluti ...

changes colour from blue to green immediately. The identity of the green species is unknown. 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.

Reactions

Chevreul's salt exhibits properties of both copper(I) and copper(II). Hydrochloric acid produces a white solid of

copper(I) chloride Copper(I) chloride, commonly called cuprous chloride, is the lower chloride of copper, with the formula CuCl. The substance is a white solid sparingly soluble in water, but very soluble in concentrated hydrochloric acid. Impure samples appear gre ...

. If too much acid is added, the precipitate dissolves. If an

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...

solution is added to the product, it is dissolved and a deep blue color appears - the presence of u(NH₃)₄sup>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 Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

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 optical reflectance spectrum shows absorption around 425 nm with a shoulder to 500 nm. This is due to a cuprous sulfite chromophore. An absorption peaking at 785 nm with a shoulder to 1000 nm, in the

near infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from arou ...

, is due to Jahn-Teller splitting in cupric ions. Maximum reflectance is around 650 nm in the red part of the spectrum. In the infrared range the

band gap In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference ( ...

is 0.85 eV. Chevreul's salt is a representative member of an isomorphic series of double salts with formulae Cu₂SO₃•FeSO₃•2H₂O, Cu₂SO₃•MnSO₃•2H₂O, and Cu₂SO₃•CdSO₃•2H₂O. The properties of these salts show the effect of ionic radius and ion hardness. Another analogue, Cu₂SO₃•NiSO₃•2H₂O, is brick-red in colour. It is made by bubbling sulfur dioxide through a nickel sulfate, copper sulfate mixed solution, heating to 80°C and changing pH to 3.5 to precipitate the salt. The

thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

of Chevreul’s salt is 0.1 kWcm⁻¹K⁻¹.

Heat capacity Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to an object to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K). Heat capacity ...

is 0.62 Jcm⁻³K⁻¹, and

thermal diffusivity In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It measures the rate of transfer of heat of a material from the hot end to the cold end. It has the SI ...

is 0.154 cm²s⁻¹. The specific susceptibility is 3.71×10⁻⁶ emu/g. In Chevreul's salt crystals there are two environments for copper. 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. The X-ray photoelectron spectrum of Chevreul's salt shows peaks at 955.6, 935.8, 953.3 and 943.9 eV that correspond to Cu(II) 2''p''_1/2, 2''p''_3/2, Cu(I) 2''p''_1/2, 2''p''_3/2. There are also secondary peaks for copper at 963.7, and 943.9 eV. Sulfur 2''p'' causes a peak at 166.7 eV and oxygen 1s causes a spike at 531.8.

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 Ammonium sulfate (American English and international scientific usage; ammonium sulphate in British English); (NH4)2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen a ...

-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 form 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.

References

{{DEFAULTSORT:Chevreul's salt Copper compounds Sulfites