Potassium cyanide is a compound with the formula KCN. This colorless crystalline salt, similar
in appearance to sugar, is highly soluble in water. Most KCN is used in gold mining,
organic synthesis, and electroplating. Smaller applications include jewelry for chemical
gilding and buffing. KCN is highly toxic. The moist solid emits
small amounts of hydrogen cyanide due to hydrolysis, which smells like bitter almonds. Not everyone,
however, can smell this: the ability to do so is a genetic trait.
Production Modern Production
KCN is produced by treating hydrogen cyanide with a 50% aqueous solution of potassium hydroxide,
followed by evaporation of the solution in a vacuum:
HCN + KOH → KCN + H2O or by treating formamide with potassium hydroxide:
HCONH2 + KOH → KCN + 2H2O Approximately 50,000 tons of potassium cyanide
are produced yearly. Historical Production
Prior to 1900 AD, before 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:
K4[Fe(CN)6] → 4 KCN + FeC2 + N2 Structure
In aqueous solution, KCN is dissociated into hydrated potassium ions and cyanide ions.
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. Despite the cyanide ions being 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 the cyanide ions arranged in sheets.
Applications KCN and its close relative 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.
Potassium gold cyanide In gold mining, KCN forms the water-soluble
salt potassium gold cyanide and potassium hydroxide from gold metal in the presence
of oxygen and water: 4 Au + 8 KCN + O2 + 2 H2O → 4 K[Au(CN)2]
+ 4 KOH A similar process uses sodium cyanide to produce
sodium gold cyanide). Very few other methods exist for this extraction
process. Toxicity
KCN can be detoxified most efficiently with hydrogen peroxide or with a solution of sodium
hypochlorite. Such solutions should be kept basic whenever possible so as to eliminate
the possibility of generation of hydrogen cyanide:
KCN + H2O2 → KOCN + H2O Cyanide is a potent inhibitor of cellular
respiration, acting on mitochondrial cytochrome c oxidase and hence blocking oxidative phosphorylation.
This prevents the body from oxidizing 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
loses consciousness, and death eventually follows over a period of time. During this
period, convulsions may occur. Death occurs by hypoxia of neural tissue.
The lethal dose for potassium cyanide is 200–300 mg. The toxicity of potassium cyanide when ingested
depends on the acidity of the stomach, because it must react with an acid to become hydrogen
cyanide, the deadly form of cyanide. Grigori Rasputin may have survived a potassium cyanide
poisoning because his stomach acidity was unusually low.
A number of prominent persons were killed or committed suicide using potassium cyanide,
including members of the Young Bosnia and members of the Nazi Party, such as Hermann
Göring and Heinrich Himmler, World War II era British agents, computer scientist Alan
Turing, and various religious cult suicides such as by the Peoples Temple and Heaven's
Gate. Danish writer Gustav Wied and members of the LTTE involved in the assassination
of Indian prime minister Rajiv Gandhi also committed suicide using potassium cyanide.
It is used by professional entomologists as a killing agent in collecting jars, as insects
succumb within seconds to the HCN fumes it emits, thereby minimizing damage to even highly
fragile specimens. References
External links International Chemical Safety Card 0671
Hydrogen cyanide and cyanides National Pollutant Inventory - Cyanide compounds
fact sheet NIOSH Pocket Guide to Chemical Hazards
CSST NIST Standard Reference Database
Institut national de recherche et de sécurité. "Cyanure de sodium. Cyanure de potassium".
Fiche toxicologique n° 111, Paris:INRS, 6pp.