potassium cyanide

Potassium cyanide
IUPAC name	Potassium cyanide
Identifiers
CAS number	151-50-8 Y
EC number	205-792-3
UN number	1680
RTECS number	TS8750000
Properties
Molecular formula	KCN
Molar mass	65.12 g/mol
Appearance	White crystalline solid deliquescent
Density	1.52 g/cm3
Melting point	634.5 °C
Boiling point	1625 ºC
Solubility in water	71.6 g/100 ml (25 °C) 100 g/100 mL (100 °C)
Solubility in methanol	4.9 g/100 mL (20 °C)
Solubility in glycerol	soluble
Thermochemistry
Std enthalpy of formation ΔfHo298	−131.5 kJ/mol
Standard molar entropy So298	127.8 J K−1 mol−1
Hazards
MSDS	ICSC 0671
EU Index	006-007-00-5
EU classification	Very toxic (T+) Dangerous for the environment (N)
R-phrases	R26/27/28, R32, R50/53
S-phrases	(S1/2), S7, S28, S29, S45, S60, S61
NFPA 704	0 4 0
Flash point	Non-flammable
LD50	5–10 mg/kg (oral in rats, mice, rabbits)[1]
Related compounds
Other anions	Potassium cyanate Potassium thiocyanate
Other cations	Sodium cyanide
Related compounds	Hydrogen cyanide
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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.^[2]

KCN is highly toxic. Due to hydrolysis, the moist solid emits small amounts of hydrogen cyanide, which smells like bitter almonds. However, not everyone can smell this odor: the ability to do so is a genetic trait^[3]. 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.

Production

KCN is produced by treating hydrogen cyanide with potassium hydroxide,

HCN + KOH → KCN + H₂O

or by treating formamide with potassium hydroxide:

HCONH₂ + KOH → KCN + 2H₂O

Approximately 50,000 tons are produced yearly.^[2]

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.^[4]

Applications

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 in the von Richter reaction.

Toxicity

KCN can be detoxified most efficiently with hydrogen peroxide:^[2]: 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 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 Black Hand Gang (unsuccessfully) and members of the Nazi Party, such as Hermann Göring and Heinrich Himmler, WWII era British agents (using purpose-made suicide pills) and various religious cults such as in Jonestown. Potassium cyanide (and other forms of cyanide) is a popular method of murder in fiction, especially in the books written by Agatha Christie.

References

External links

• International Chemical Safety Card 0671
• Hydrogen cyanide and cyanides (CICAD 61)
• National Pollutant Inventory - Cyanide compounds fact sheet
• NIOSH Pocket Guide to Chemical Hazards
• European Chemicals Bureau
• CSST (Canada)
• NIST Standard Reference Database
• Institut national de recherche et de sécurité (1997). "Cyanure de sodium. Cyanure de potassium". Fiche toxicologique n° 111, Paris:INRS, 6pp. (PDF file, in French)