copper hydrogen arsenite
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Scheele's Green, also called Schloss Green, is chemically a cupric hydrogen arsenite (also called copper arsenite or acidic copper arsenite), CuHAsO3. It is chemically related to Paris Green. It is a yellowish-green pigment and in the past it was used in some paints, but has since fallen out of use because of its toxicity and the instability of its colour in the presence of sulphides and various chemical pollutants.
The pigment was originally prepared by making a solution of sodium carbonate at a temperature of around 90 °C, then slowly adding arsenious oxide, while constantly stirring until everything had dissolved. This produced a sodium arsenite solution. Added to a copper sulfate solution, it produced a green precipitate of effectively insoluble copper arsenite. After filtration the product was dried at about 43°C. 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.
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 used as a color for paper, e.g., for wallpapers and paper hangings, and in paints, wax candles, and even on some children's toys. It was also used to dye cotton and linen. Scheele's Green is more brilliant and durable than the then-used copper carbonate pigments. However, because of its copper content it tends to fade and blacken when exposed to sulfides, whether in the form of atmospheric hydrogen sulfide or in pigment mixtures based on 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 more durable. By the end of 19th century, both greens were made obsolete by cobalt green, also known as zinc green, which is far less toxic.
Scheele's Green was used as an insecticide in the 1930s, together with Paris Green.
Despite evidence of 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.
In the 19th century the toxicity of arsenic compounds 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. There is one example of an acute poisoning of children attending a Christmas party where dyed candles were burned.
Two main theories on the cause of wallpaper poisoning events have been proposed: dust particles caused by pigment and paper flaking, and toxic gas production. Tiny particles of the pigment can flake off and become airborne, and then are absorbed by the lungs. Alternatively, toxic gas can be released from compounds containing arsenic following certain chemical processes, such as heating, or metabolism by an organism. When the wallpaper becomes damp and moldy, the pigment may be metabolised, causing the release of poisonous arsine gas (AsH3). Fungi genera such as Scopulariopsis or Paecilomyces release arsine gas, when they are growing on a substance containing arsenic. 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 produced significant amounts of methyl arsines via methylation of arsenic-containing inorganic pigments, especially Paris green and Scheele's Green.
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 mono, di and trimethylated products by methyltransferases and an S-adenosyl-methionine methyl donating cofactor. However, newer studies indicate that trimethylarsine has a low toxicity and could therefore not account for the death and the severe health problems observed in the 19th century.
Arsenic is not only toxic, but it also has carcinogenic effects.
Scheele's Green and Napoleon
During Napoleon's exile in St. Helena, he resided in a very luxurious room painted bright green, his favorite color. His cause of death is generally believed to be stomach cancer, and arsenic exposure has been linked to an increased risk of gastric carcinoma. Analysis of his hair samples revealed significant amounts of arsenic. As St. Helena has a rather damp climate, it is not unlikely that fungus grew on the walls. 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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