potassium cyanide. (KCN)

Potassium cyanide is a compound with the formula KCN. This colorless crystalline compound, 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 andbuffing.

KCN is highly toxic. The moist solid emits small amounts of hydrogen cyanide due to hydrolysis, which tastes like bitter almonds. Not everyone, however, can taste this: the ability to do so is a genetic trait

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.

Structure

In aqueous solution, KCN is dissociated into hydrated K⁺ ions and CN⁻. As a solid, the salt crystallizes such that the cations and anions organize like Na⁺ and Cl⁻ in NaCl. The cations and anions six-coordinate. Each K⁺ is linked to two pi-bonds of the CN⁻ as well as two links each to C and N each. Since CN⁻ is diatomic, the symmetry of the solid is lower than that in NaCl. The cyanide anions form sheets. The CN⁻ ions rapidly rotate in the solid at ambient temperature such that the time averaged shape of the CN⁻ ions is spherical.

Applications

KCN and its close relative sodium cyanide (NaCN) are widely used in organic synthesis for the preparation of nitriles and carboxylic acids, particularly in the von Richter reaction.

Potassium gold cyanide

In gold mining, KCN forms the water-soluble salt potassium gold cyanide (or gold potassium cyanide) and potassium hydroxide from gold metal in the presence of oxygen (usually from the surrounding air) and water:

4 Au + 8 KCN + O₂ + 2 H₂O → 4 K[Au(CN)₂] + 4 KOH

A similar process uses sodium cyanide (NaCN, a close relative of potassium cyanide) to produce sodium gold cyanide (NaAu(CN₂)).

Very few other methods exist for this extraction process.

Toxicity

KCN can be detoxified most efficiently with hydrogen peroxide:

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 not by cardiac arrest, but 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 (using purpose-made suicide pills), 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 entomologists as a killing agent in collecting jars, as most insects succumb within seconds, minimizing damage of even highly fragile specimens.