Potassium cyanide

Potassium cyanide

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Potassium cyanide is an inorganic compound with the formula KCN. This colorless crystalline compound, similar in appearance to sugar, is highly soluble in water. The vast majority of KCN is used in gold mining followed by use in organic synthesis, and electroplating. Smaller applications include jewelry for chemical gilding and buffing.

Highly toxic, KCN is odorless but due to hydrolysis, the moist solid emits small amounts of hydrogen cyanide, which smells like bitter almonds (not everyone can smell it—the ability thereof is due to a genetic trait). It is also used by entomologists as a killing agent in collecting jars, as most insects succumb within seconds, minimizing damage of even highly fragile specimens.


KCN is produced by treating hydrogen cyanide with potassium hydroxide. Approximately 50,000 tons are produced yearly (the production of sodium cyanide is 10x that amount). It is detoxified most efficiently with hydrogen peroxide:
KCN + H2O2 → KOCN + H2O


In gold mining, KCN and NaCN form water-soluble salts from gold metal in the presence of air:
4 Au + 8 KCN + O2 + 2 H2O → 4 K[Au(CN)2] + 4 KOH
Very few alternative methods exist for this extraction process.

KCN and the related NaCN are widely used in organic synthesis for the preparation of nitriles and carboxylic acids. Illustrative is the Von Richter reaction.


Cyanide is a potent inhibitor of cellular respiration, acting on mitochondrial cytochrome c oxidase and hence blocking oxidative phosphorylation. This prevents the body from oxidising food to produce useful energy. Lactic acidosis then occurs as a consequence of anaerobic metabolism. Initially, acute cyanide poisoning causes a red or ruddy complexion in the victim because the tissues are not able to use the oxygen in the blood. The effects of potassium and sodium cyanide are identical. The person may die within two hours if not treated medically. During this period, convulsions may occur. Death occurs mainly by cardiac arrest.

A number of prominent persons were killed or committed suicide using potassium cyanide, including members of the Nazi and various religious cults. Potassium cyanide (and other forms of cyanide) are a popular method of murder in fiction.


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