Potassium cyanide

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Potassium cyanide
Potassium-cyanide-phase-I-unit-cell-3D-SF.png
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 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 molar
entropy
So298
127.8 J K−1 mol−1
Std enthalpy of
formation
ΔfHo298
−131.5 kJ/mol
Hazards
MSDS ICSC 0671
EU Index 006-007-00-5
EU classification Very Toxic T+ Dangerous for the Environment (Nature) N
R-phrases R26/27/28, R32, R50/53
S-phrases (S1/2), S7, S28, S29, S45, S60, S61
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 4: Very short exposure could cause death or major residual injury. E.g., VX gas Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point Non-flammable
LD50 5 mg/kg (oral, rat)[1]
Related compounds
Other anions Potassium cyanate
Potassium thiocyanate
Other cations Sodium cyanide
Related compounds Hydrogen cyanide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

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.[2]

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

Production[edit]

Modern Production[edit]

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]

Historical Production[edit]

Prior to 1900AD, before the invention of the Castner process, potassium cyanide was the most important source of alkali metal cyanides.[2] In this historical process potassium cyanide was produced by decomposing potassium ferrocyanide:[5]

K4[Fe(CN)6] → 4 KCN + FeC2 + N2

Structure[edit]

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. [6][7]

Applications[edit]

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. 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[edit]

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[edit]

Main article: Cyanide poisoning

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:[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 by hypoxia of neural tissue.

The lethal dose for potassium cyanide is 200–300 mg.[8] 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.[9]

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[edit]

  1. ^ http://chem.sis.nlm.nih.gov/chemidplus/rn/151-50-8
  2. ^ a b c d 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. ^ Von Wagner, Rudolf (1897). Manual of chemical technology. New York: D. Appleton & Co. p. 474 & 477. 
  6. ^ Crystallography Open Database, Structure of KCN
  7. ^ 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. .
  8. ^ John Harris Trestrail III. Criminal Poisoning - Investigational Guide for Law Enforcement, Toxicologists, Forensic Scientists, and Attorneys (2nd edition). pg 119
  9. ^ John Emsley. The Elements of Murder: A History of Poison. pg 73

External links[edit]