Potassium cyanide

Template:Short description Template:Use dmy dates Template:Chembox Potassium cyanide is a compound with the formula KCN. It is a colorless salt, similar in appearance to sugar, that is highly soluble in water. Most KCN is used in gold mining, organic synthesis, and electroplating. Smaller applications include jewellery for chemical gilding and buffing.<ref name=Ullmann>Template:Cite book</ref> Potassium cyanide is highly toxic, and a dose of 200 to 300 milligrams will kill nearly any human.

The moist solid emits small amounts of hydrogen cyanide due to hydrolysis (reaction with water). Hydrogen cyanide is often described as having an odor resembling that of bitter almonds.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Not everyone, however, can smell cyanide; the ability to do so is a genetic trait.{{#ifeq:|none||{{#switch: | short = OMIM: | shortlink = OMIM: | plain = Online Mendelian Inheritance in Man: | full | #default = Online Mendelian Inheritance in Man (OMIM):}}}} {{#if: |{{{2}}} - }} 304300</ref>

The taste of potassium cyanide has been described as acrid and bitter, with a burning sensation similar to lye.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Potassium cyanide kills so rapidly its taste hasn't been reliably documented, in 2006 an Indian man named MP Prasad committed suicide using potassium cyanide. He was a goldsmith and was aware of the mystery behind its taste. In the suicide note Prasad left, the final words written were that potassium cyanide "burns the tongue and tastes acrid", for obvious reasons this description hasn't been independently confirmed.<ref name="Sydney Herald">Template:Cite news</ref><ref name="Labatut">Template:Cite book</ref>

Production

KCN is produced by treating hydrogen cyanide with an aqueous solution of potassium hydroxide, followed by evaporation of the solution in a vacuum:<ref name=Ullmann/> Template:Block indent

About 50,000 tons of potassium cyanide are produced yearly.<ref name=Ullmann/> For laboratory purpose it is easier to pass hydrogen cyanide through an alcoholic solution of potassium base because the crystals of potassium cyanide are not soluble in alcohol .

Historical production

Before 1900 and the invention of the Castner process, potassium cyanide was the most important source of alkali metal cyanides.<ref name=Ullmann /> In this historical process, potassium cyanide was produced by decomposing potassium ferrocyanide:<ref name=Wagner>Template:Cite book</ref>

Structure

In aqueous solution, KCN is dissociated into hydrated potassium (K⁺) ions and cyanide (CN⁻) ions. As a solid, KCN has structure resembling sodium chloride: with each potassium ion surrounded by six cyanide ions, and vice versa. Despite being diatomic, and thus less symmetric than chloride, the cyanide ions 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. <ref>Crystallography Open Database, Structure of KCN</ref><ref name="Stokes1993">Template:Cite journal.</ref>

Applications

KCN and 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.

KCN is used as a photographic fixer in the wet plate collodion process.<ref>J. Towler, MD. "The Silver Sunbeam (Facsimile 1864 edition, 1969)" p. 119</ref> The KCN dissolves silver where it has not been made insoluble by the developer. This reveals and stabilizes the image, making it no longer sensitive to light. Modern wet plate photographers may prefer less toxic fixers, often opting for sodium thiosulfate, but KCN is still used.

Template:Anchor In the 19th century, cyanogen soap, a preparation containing potassium cyanide, was used by photographers to remove silver stains from their hands.<ref>Template:Cite journal</ref>Template:Rp<ref>Reports of Trials for Murder by Poisoning; by Prussic Acid, Strychnia, Antimony, Arsenic, and Aconita. Including the trials of Tawell, W. Palmer, Dove, Madeline Smith, Dr. Pritchard, Smethurst, and Dr. Lamson, with chemical introduction and notes on the poisons used, G. Lathom Browne and C. G. Stewart, London: Stevens and Sons, 1883; redistributed by Project Gutenberg.</ref>Template:Rp<ref>"Cyanuret of potassium", Paper, Shadows and Light, Robert Douglas. Accessed 2024-01-20.</ref>

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 NaCN to produce sodium gold cyanide (NaAu(CN₂)).

Analytical chemistry

In analytical chemistry, potassium cyanide is used as complexing agent for chemical analysis of zinc in water and wastewater. The cyanide group complexes zinc and other heavy metals, which is later extracted and analyzed in a spectro-photometer.<ref>Template:Cite journal</ref>

Toxicity

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Potassium cyanide is a potent inhibitor of cellular respiration, acting on mitochondrial cytochrome c oxidase, hence blocking oxidative phosphorylation. 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 cyanide and sodium cyanide are identical, and symptoms of poisoning typically occur within a few minutes of ingesting the substance: the person loses consciousness, and brain death eventually follows. During this period the victim may suffer convulsions. Death is caused by histotoxic hypoxia/cerebral hypoxia. The expected LD100 dose (human) for potassium cyanide is 200–300 mg while the median lethal dose LD50 is estimated at 140 mg.<ref>John Harris Trestrail III. Criminal Poisoning –Investigational Guide for Law Enforcement, Toxicologists, Forensic Scientists, and Attorneys (2nd ed.) p. 119</ref>

Disposal

Due to toxicity considerations, the disposal of cyanide is subject to stringent regulations. Industrial cyanide effluent is typically destroyed by oxidation using peroxysulfuric acid, hydrogen peroxide, sulfur dioxide/copper salts ("Inco process") or all three ("Combiox Process"). Use of sodium hypochlorite, traditional for laboratory-scale wastes, is impractical on a commercial scale. Hydrolysis at higher temperatures is highly effective, but requires specialized equipment. Lastly, cyanide wastes can be acidified for recovery of hydrogen cyanide.<ref name=Ullmann>Template:Cite book</ref>

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
• 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. Template:In lang