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scheele green

Scheele's Green

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.

Scheele's Green was invented in 1775 by Carl Wilhelm Scheele. By the end of the 19th century, it virtually replaced the older green pigments based on copper carbonate.

Preparation

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 colour, the salt was subsequently heated to 60-70°C. The intensity of the colour 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.

Uses

Scheele's Green was used as an insecticide in 1930's, together with Paris Green. Scheele's Green can be used also to color wax candles.

Scheele's Green was used as a color for paper, eg. for wallpapers and paper hangings, and in paints, even on some children 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 gas.

Toxicity

Today it is intuitive to most that a complex containing arsenic is most likely to be toxic, however 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 vapours.

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 oxidised 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, drink: the green in absinthe is now thought to be the source of the problems with the liquor instead of the ingredient thujone (leading to its banishment in most European countries and the US by 1915) eg. green blancmange and others, a fondness of traders in 19th century Greenock, leading to a long-standing Scottish prejudice against green sweets. 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.

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