- Scheele's Green
Scheele's Green Identifiers CAS number PubChem ChemSpider Jmol-3D images Image 1 Properties Molecular formula AsCuHO3 Molar mass 187.474 (what is: /?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Scheele's Green, also called Schloss Green, is chemically a cupric hydrogen arsenite (also called copper arsenite or acidic copper arsenite), CuHAsO3. It is a compound similar to Paris Green. It is a green pigment, of yellowish hue and was used in the past in some paints but has since fallen out of use due to its toxicity.
The pigment was originally prepared by making a solution of sodium carbonate at a temperature of around 90 °C, then slowly adding in arsenious oxide, while constantly stirring until everything had dissolved. This sodium arsenite solution was then added to a copper sulphate solution. The sodium would displace the copper, which resulted in the formation of the desired product in the form of a green precipitate. This copper arsenite was then filtered off and warmed to around 43 °C to dry the crystals. To enhance the color, the salt was subsequently heated to 60-70 °C. The intensity of the color depends on the copper : arsenic ratio, which in turn was affected by ratio of the starting materials, as well as the temperature.
Another method used to produce Scheele's green was by directly adding dilute copper sulphate solution to an alkali solution of arsenic. This would then precipitate—the precipitate also being Scheele's green.
It has been found that Scheele's green was composed of a variety of different compounds, including copper metaarsenite (CuO·As2O3), copper arsenite salt (CuAsHO3 and Cu(AsO3)2·3H2O)), neutral copper orthoarsenite (3CuO·As2O3·2H2O), copper arsenate (CuAsO2 and Cu(AsO2)2), copper diarsenite (2CuO·As2O3·2H2O).
Scheele's Green was more brilliant and durable than the then-used copper carbonate pigments. However it tended to fade and blacken when subjected to hydrogen sulfide containing atmosphere. It also can not be mixed with pigments based on sulfides or containing sulfur.
Emerald green, also known as Paris Green, was developed later in attempt to improve Scheele's Green. It had the same tendency to blacken, but was even more durable. By the end of 19th century, both greens were made obsolete by zinc oxide and cobalt green, also known as zinc green.
Scheele's Green was used as a color for paper, e.g., for wallpapers and paper hangings, and in paints, even on some children's toys. It was also used to dye cotton and linen. The wallpapers containing Scheele's Green are implicated in the arsenic poisoning of Napoleon Bonaparte. Tiny particles of the pigment tend to flake off and become airborne, and then are absorbed by the lungs. Also, when the wallpaper becomes damp and moldy, the pigment hydrolyzes and releases poisonous arsine containing gas. Poisoning events due to this gas produced by certain microbes was assumed to be associated with the arsenic in paint in the late 19th century. In 1893 the Italian physician Bartolomeo Gosio published his results on "Gosio gas" that was subsequently shown to contain trimethylarsine. Under wet conditions, the mold Scopulariopsis brevicaulis produces significant amounts of methyl arsines via methylation of arsenic-containing inorganic pigments, especially Paris green and Scheele's Green, which were once used in indoor wallpapers. Newer studies show that trimethylarsine has a low toxicity and could therefore not account for the death and the severe health problems observed in the 19th century.
Today it is intuitive to most that a complex containing arsenic is most likely to be toxic, but in the 19th century this was not readily known. 19th century journals reported of children wasting away in bright green rooms, of ladies in green dresses swooning and newspaper printers being overcome by arsenic vapors.
Arsenic poisoning was usually caused by the inhalation of arsine gas (AsH3), which can be released from compounds containing arsenic following certain chemical processes, such as heating, or its metabolism by an organism. Fungi such as Scopulariopsis or the Paecilomyces species release arsine gas, when they are growing on a substance containing arsenic. In these compounds, the Arsenic is either pentavalent or trivalent (arsenic is in group 15), depending on the compound. In humans, arsenic of these valences is readily absorbed by the gastrointestinal tract, which accounts for its high toxicity. Pentavalent arsenic tends to be reduced to trivalent arsenic and trivalent arsenic tends to proceed via oxidative methylation in which the trivalent arsenic is made into a mono, di and trimethylated products by the S adenosyl methionine (an enzyme) and its cofactor glutatione. Arsenic is not only toxic, but it also has carcinogenic effects.
Despite its high toxicity, Scheele's Green was also used as a food dye for sweets such as green blancmange, a fondness of traders in 19th century Greenock, leading to a long-standing Scottish prejudice against green sweets. The presence of Scheele's Green as a colorant in absinthe is now thought to be the source of the problems with the liquor that led to its banishment in most European countries and the US by 1915. Another popular poisonous colorant was chrome yellow, used for sweets, snuff and custard powder. There is one example of an acute poisoning of children attending a Christmas party where such candles were burned.
Scheele's Green and Napoleon
During Napoleon's exile in St. Helena, he resided in a very luxurious bright green room. His cause of death is generally believed to be stomach cancer, however, analysis of his hair samples revealed significant amounts of arsenic. As St. Helena has a rather damp climate, it is more than possible that fungus grew on the wall. So as to remove the toxic arsenic, the fungus would release it in the form of arsine gas, which when inhaled causes arsenic poisoning. It has also been suggested that the presence of such abnormally high levels of arsenic might be due to attempts at preserving his body.
- Not to be confused with copper arsenate.
- ^ StudioMara - History of Pigments
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- ^ Fungal Glossary
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- ^ a b Was Napoleon Murdered?
- ^ Timbrell, John (2005). "Butter Yellow and Scheele's Green". The Poison Paradox: Chemicals as Friends and Foes. Oxford University Press. ISBN 9780192804952. http://books.google.com/?id=qYYOtQU37jcC.
- ^ Acute Poisoning
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