The blue history of cyanide

Cyanides are old poisons, with a uniquely long, dark history, probably because they grow so bountifully around us.

They flavor the leaves of the yew tree, flowers of the cherry laurel, the kernels of peach and apricot pits, the fat pale crunch of bitter almonds. They ooze in secretions of insects like millipedes, weave a toxic thread through cyanobacteria, mass in the floating blue-green algae along the edges of the murkier ponds and lakes, live in plants threaded through forests and fields.

But cyanide didn't really become a widely used poison until the 18th century, beginning with some experiments by a German painter, who, in 1704 was only trying to improve the colors on his palette. The artist in question, one Heinrich Diesbach, was a born experimenter. He spent hours at the laboratory of a Berlin alchemist, trying to create a new shade of red paint. He swirled together wilder and wilder mixtures, eventually mixing dried blood, potash (potassium carbonate) and green vitriol (iron sulfate) and then stewing them over an open flame. Instead of the bloody crimson he expected, the flask yielded a different brilliance, the deep violet-blue glow of a fading twilight. Diesbach called the vivid pigment Berlin Blue; English chemists would later rename it Prussian Blue.

Almost 80 years later, a Swedish chemist mixed Berlin Blue with an acid solution, heated the witchy, foaming result, and produced a colorless gas, undetectable but for a faint smell of bitter almonds. The gas easily condensed into a clear liquid that, even diluted with water, was an exceptionally potent acid.  That corrosive liquid became popularly known as Prussic Acid although scientists preferred to call it hydrocyanic acid (from the Greek words hydro for water and kyanos for blue).

In the history of poisons, the story of cyanide is always colored blue.

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Always good to see Prussian Blue get a mention - the first synthetic coordination complex, so it plays a key role in inorganic chemistry. I always demonstrate it in my inorganic reaction mechanisms course as an example of a colour caused by intervalence charge transfer. One minor quibble though - in what sense is prussic acid 'potent'? As a poison, yes, but as an acid? It's pretty weak as a proton donor (pKa about 9, I think).

By Simon Higgins (not verified) on 27 Feb 2010 #permalink

thanks - and excellent point. i meant potent in the poison sense, but i could have written that a little more clearly. thanks for the terrific comment - it adds a lot to the chemistry background here.