Potassium cyanide is a compound with the formula KCN. This colorless crystalline salt, which resembles sugar in appearance, is highly soluble in water. Most KCN is used in gold mining, organic synthesis, and electroplating. Smaller applications include jewelry for electroless gold plating and polishing.
Potassium cyanide is highly toxic. The moist solid releases small amounts of hydrocyanic acid through hydrolysis, which smells like bitter almonds.
In aqueous solution, KCN is dissociated into hydrated potassium ions (K+) and cyanide ions (CN−). The common form of solid KCN, stable at ambient pressure and temperature, has the same cubic crystal structure as sodium chloride, with each potassium ion surrounded by six cyanide ions and vice versa.
Although the cyanide ions are diatomic and thus less symmetric than chloride, they 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 layered cyanide ions.
KCN and sodium cyanide (NaCN) are used extensively in organic synthesis to produce nitriles and carboxylic acids, particularly in the von Richter reaction. It also finds use in 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 collodion wet plate process. The KCN dissolves silver where it was not insolubilized by the developer. This uncovers and stabilizes the image, making it no longer sensitive to light. Modern wet plate photographers may prefer less toxic fixatives and often opt for sodium thiosulfate, but KCN is still used. It was used extensively in the closing days of World War II by senior Nazi officials to commit suicide, such as Hermann Goering taking a pod the night before his execution.
Potassium cyanide is a potent inhibitor of cellular respiration that acts on mitochondrial cytochrome c oxidase, thereby blocking oxidative phosphorylation. Lactic acidosis then occurs as a result of anaerobic metabolism. Acute cyanide poisoning initially causes the victim to have a red or ruddy complexion because the tissues cannot utilize the oxygen in the blood.
The effects of potassium cyanide and sodium cyanide are identical, and symptoms of intoxication typically appear within minutes of ingesting the substance: the person loses consciousness, and eventually brain death ensues. During this time, the victim may experience convulsions. Death i
s caused by cerebral hypoxia. The expected LD100 dose (human) for potassium cyanide is 200–300 mg, while the median lethal dose LD50 is estimated at 140 mg