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Chevreul’s salt is a double salt of copper(I) sulfite and copper(II) sulfite. It usually occurs as a dihydrate with the empirical formula Cu2SO3∙CuSO3∙2H2O. The color of its crystals and powder is often described in the literature as “brick red”. An anhydrous version of the salt1 also exists.
The salt was first prepared in 1812 by the prolific French chemist Michel Eugène Chevreul, whose work ranged from oils and fats to dyes to medicines. Although his eponymous salt is an inorganic compound, he is mainly known as a pioneer in organic chemistry; many biographical accounts of Chevreul, who lived to age 102, do not even mention the salt.
Chevreul’s salt was originally formed by heating an aqueous solution of copper(II) sulfate (CuSO4) and potassium metabisulfite (K2S2O5). The metabisulfite partially reduces Cu(II) to Cu(I) and displaces the sulfate with sulfite. The insoluble salt precipitates, leaving an acidic solution.
In 1999, Michiko B. Inoue and collaborators at the University of Sonora (Hermosillo, Mexico) and the University of Arizona (Tucson) described the preparation of Chevreul’s salt by heating aqueous CuSO4 with sodium hydrogen sulfite (NaHSO3) at 60–70 °C. They went on to measure the salt’s spectroscopic and magnetic properties. The following year, L. A. Silva, J. R. Matos, and J. B. de Andrade* at the Federal University of Bahia (Salvador, Brazil) and the University of São Paulo (Brazil) reported another synthesis in which an aqueous solution of CuSO4 is saturated with sulfur dioxide gas at ambient temperature. These authors prepared other Cu2SO3 double salts and determined their thermal properties.
Chevreul’s salt has few practical uses. A 1987 Polish patent described its use as a copper catalyst for making photopolymeric positive photoresists. In 2006, Turan Çalban*, Sabri Çolak, and Murat Yeşilyurt at Atatürk University (Erzurum, Turkey) demonstrated how dissolved copper extracted from the ore can be recovered by injecting sulfur dioxide and precipitating the salt.
1. CAS Reg. No. 15293-86-4.
Chevreul’s salt hazard information*
|Hazard class**||GHS code and hazard statement|
|Acute toxicity, oral, category 4||H302—Harmful if swallowed|
|Skin corrosion/irritation, category 2||H315—Causes skin irritation|
|Serious eye damage/eye irritation, category 2A||H319—Causes serious eye irritation|
|Short-term (acute) aquatic hazard, category 1||H400—Very toxic to aquatic life|
*Hazard information for Chevreul’s salt is unavailable. Data are given for copper(I) sulfate, CAS Reg. No. 17599-81-4.
**Globally Harmonized System (GHS) of Classification and Labeling of Chemicals. Explanation of pictograms
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Chevreul’s salt fast facts
|CAS Reg. No.||13814-81–8|
|SciFinder nomenclature||Sulfurous acid, copper(1+) copper(2+) salt (2:2:1), dihydrate|
|Molar mass||386.78 g/mol|
|Appearance||Dark red-orange crystals or powder|
|Melting point||>200 °C (dec.)|
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