Potassium cyanide
Potassium cyanide is a compound with the formula KCN. It is a colorless salt, similar in appearance to sugar, that is highly soluble in water. Most KCN is used in gold mining, organic synthesis, and electroplating. Smaller applications include chemical gilding and buffing of jewelry. Potassium cyanide is highly toxic, and a dose of will kill nearly any human.
The moist solid emits small amounts of hydrogen cyanide due to hydrolysis. Hydrogen cyanide is often described as having an odor resembling that of bitter almonds.
The taste of potassium cyanide has been described as acrid and bitter, with a burning sensation similar to lye. However, potassium cyanide kills so rapidly its taste has not been reliably documented. In 2006, an Indian man killed himself using potassium cyanide. In the suicide note he left, the final words written were that potassium cyanide "burns the tongue and tastes acrid".
Production
KCN is produced by treating hydrogen cyanide with an aqueous solution of potassium hydroxide, followed by evaporation of the solution in a vacuum:About of potassium cyanide are produced yearly.
Historical production
Before 1900 and the invention of the Castner process, potassium cyanide was the most important source of alkali metal cyanides. In this historical process, potassium cyanide was produced by decomposing potassium ferrocyanide:Structure
In aqueous solution, KCN is dissociated into hydrated potassium ions and cyanide ions. As a solid, KCN has structure resembling sodium chloride: with each potassium ion surrounded by six cyanide ions, and vice versa. Despite being diatomic, and thus less symmetric than chloride, the cyanide ions rotate so rapidly that their time-averaged shape is spherical. At low temperature and high pressure, this free rotation is hindered, resulting in a less symmetric crystal structure with the cyanide ions arranged in sheets.Applications
KCN and sodium cyanide are widely used in organic synthesis for the preparation of nitriles and carboxylic acids, particularly in the von Richter reaction. It also finds use for the synthesis of hydantoins, which can be useful synthetic intermediates, when reacted with a carbonyl compound such as an aldehyde or ketone in the presence of ammonium carbonate.KCN is used as a photographic fixer in the wet plate collodion process. The KCN dissolves silver where it has not been made insoluble by the developer. This reveals and stabilizes the image, making it no longer sensitive to light. Modern wet plate photographers may prefer less toxic fixers, often opting for sodium thiosulfate, but KCN is still used.
In the 19th century, cyanogen soap, a preparation containing potassium cyanide, was used by photographers to remove silver stains from their hands.