Potassium cyanide

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Potassium cyanide
IUPAC name

Potassium cyanide

Identifiers
CAS number 151-50-8 YesY
PubChem 9032
ChemSpider 8681 YesY
UNII MQD255M2ZO YesY
EC number 205-792-3
UN number 1680
RTECS number TS8750000
Jmol-3D images {{#if:[K+].[C-]#N|Image 1
Properties
Molecular formula KCN
Molar mass 65.12 g/mol
Appearance White crystalline solid
deliquescent
Odor faint, almond-like
Density 1.52 g/cm3
Melting point 634.5 °C; 1,174.1 °F; 907.6 K
Boiling point 1,625 °C; 2,957 °F; 1,898 K
Solubility in water 71.6 g/100 ml (25 °C)
100 g/100 mL (100 °C)
Solubility in methanol 4.91 g/100 mL (20 °C)
Solubility in glycerol soluble
Solubility in formamide 14.6 g/100 mL
Solubility in ethanol 0.57 g/100mL
Solubility in hydroxylamine 41 g/100 mL
Acidity (pKa) 11.0
Refractive index (nD) 1.410
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 T+ 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
 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

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 and buffing.[2]

KCN is highly toxic. The moist solid emits small amounts of hydrogen cyanide due to hydrolysis, which tastes like bitter almonds[citation needed]. Not everyone, however, can taste this: the ability to do so is a genetic trait.[3]

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:[4]

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.[2]

Structure

In aqueous solution, KCN is dissociated into hydrated potassium (K+) ions and cyanide (CN−) 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. [5][6]

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.[citation needed]

Toxicity

Main article: Cyanide poisoning

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 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.[7] 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.[8]

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 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

  1. ↑ Bernard Martel. Chemical Risk Analysis: A Practical Handbook. Kogan, 2004, page 361. ISBN 1-903996-65-1.
  2. ↑ 2.0 2.1 2.2 Andreas Rubo, Raf Kellens, Jay Reddy, Joshua Wooten, Wolfgang Hasenpusch "Alkali Metal Cyanides" in Ullmann's Encyclopedia of Industrial Chemistry 2006 Wiley-VCH, Weinheim, Germany. doi:10.1002/14356007.i01_i01
  3. ↑ Online 'Mendelian Inheritance in Man' (OMIM) 304300
  4. ↑ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
  5. ↑ Crystallography Open Database, Structure of KCN
  6. ↑ H. T. Stokes, D. L. Decker, H. M. Nelson, J. D. Jorgensen (1993). "Structure of potassium cyanide at low temperature and high pressure determined by neutron diffraction". Physical Review B 47 (17): 11082–11092. doi:10.1103/PhysRevB.47.11082. .
  7. ↑ John Harris Trestrail III. Criminal Poisoning - Investigational Guide for Law Enforcement, Toxicologists, Forensic Scientists, and Attorneys (2nd edition). pg 119
  8. ↑ John Emsley. The Elements of Murder: A History of Poison. pg 73

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