Hydrogen cyanide

from Wikipedia, the free encyclopedia
Structural formula
Structure of hydrogen cyanide
Surname Hydrogen cyanide
other names
  • Formic acid nitrile
  • Prussic acid
  • Hydrocyanic acid
  • Formonitrile
  • Hydrogen cyanide
  • Acidum borussicum
  • Zyklon B
Molecular formula HCN
Brief description

colorless liquid with a smell of bitter almonds

External identifiers / databases
CAS number 74-90-8
EC number 200-821-6
ECHA InfoCard 100,000,747
PubChem 768
ChemSpider 748
Wikidata Q26075
Molar mass 27.03 g mol −1
Physical state



0.69 g cm −3

Melting point

−13 ° C

boiling point

26 ° C

Vapor pressure

817 h Pa (20 ° C)

pK s value



Fully miscible with water

Refractive index

1.2614 (20 ° C)

safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary
02 - Highly / extremely flammable 06 - Toxic or very toxic 05 - Corrosive 09 - Dangerous for the environment


H and P phrases H: 224-300 + 310 + 330-314-410
P: 210-241-260-273-280-303 + 361 + 353-304 + 340-309 + 310-403 + 233-405

DFG / Switzerland: 1.9 ml m −3 or 2.1 mg m −3

Toxicological data
Thermodynamic properties
ΔH f 0

108.9 kJ / mol

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . Refractive index: Na-D line , 20 ° C

Hydrogen cyanide ( Hydrocyanic acid ) is a colorless to slightly yellow, flammable, highly volatile and water-soluble liquid . The name hydrocyanic acid comes from the earlier extraction from iron hexacyanidoferrate ( Berlin blue ), a lightfast, deep blue pigment. Hydrocyanic acid can be regarded as the nitrile of formic acid (the nitrile carbon has the same oxidation state as the carboxyl carbon ), hence the common name formic acid nitrile .

Hydrocyanic acid is highly toxic. Its lethal effect has been used against people on various occasions in history, above all in the mass murders in the Auschwitz concentration camp during the Nazi era , and also found its way into literature (detective novels). Hydrocyanic acid is used industrially as a preliminary product and process material as well as for pest control.

It is widely believed that hydrocyanic acid emits a characteristic odor of bitter almonds . However, the actual smell of the substance is not described unanimously in the literature and perceived differently by some people, e.g. B. "dull" or "sharp". A significant part of the population does not even notice the smell of hydrogen cyanide (see also handling ).


Hydrocyanic acid in its highly pure form is a colorless, easily mobile liquid that can be mixed with water and alcohol in any proportion. The boiling point is 26 ° C. The substance evaporates so quickly at room temperature that part of it can solidify because of the cold from evaporation .

Hydrocyanic acid in its diluted form has a dull, dull smell reminiscent of bitter almonds, but which differs significantly from e.g. B. bitter almond aroma. In concentrated form, hydrocyanic acid smells unpleasant and indefinable, intensely pungent-sharp and scratchy, irritates the mucous membranes and the throat and leaves a bitter taste and brief burning sensation in the nose. However, even in very small amounts, the substance paralyzes the olfactory and taste buds after a short time.

Hydrocyanic acid is a very weak acid in water , which is driven by carbon dioxide from its salts, the cyanides , and only dissociates to a small extent :

Of the salts of hydrogen cyanide , those of the alkali and alkaline earth metals as well as mercury (II) cyanide are easily soluble in water, all others are sparingly soluble.

Depending on the source, their pKa value is given as 9.04 to 9.31. The dissociation constant is 4.0 · 10 −10 . Hydrocyanic acid is extremely flammable; mixtures with air are explosive in the range of 5.4–46.6% by volume. Since hydrocyanic acid can also be mixed with water in any proportion, there is a risk of groundwater contamination when a fire is extinguished . Therefore, if necessary, controlled burning is considered.

Hydrocyanic acid can spontaneously polymerize in an autocatalyzed reaction or break down into the elements. This reaction is highly exothermic and explosive. It is initiated by small amounts of bases and accelerated by additional bases that are formed in the process. Hydrocyanic acid is more unstable than completely anhydrous. A brown polymer is formed. For this reason, hydrocyanic acid is stabilized by adding small amounts of acids such as phosphoric or sulfuric acid . The acid neutralizes the bases and avoids a runaway reaction .


The name prussic acid goes back to the pigment Berlin blue , from which the substance was first made. In 1782 a publication by Carl Wilhelm Scheele appeared , which describes the production of hydrogen cyanide from yellow blood liquor salt and sulfuric acid as well as from Berlin blue and sulfuric acid. Scheele had already begun attempts to do this in 1768. The syllable cyan was introduced by Joseph Louis Gay-Lussac .


Poisonous effect

Hydrocyanic acid is extremely toxic, as little as 1–2 mg hydrogen cyanide per kg of body mass are fatal. In addition to direct ingestion, absorption can also take place via the respiratory tract and skin. The latter is favored by sweat , since hydrocyanic acid is highly water-soluble. In Germany, the substance was classified by the Federal Environment Agency in water hazard class 3 (highly hazardous to water).

Ribbon model of cytochrome c oxidase . Hydrogen cyanide blocks the binding site for oxygen in the active center .

The primary toxicity is the blockage of oxygen - binding site in the respiratory chain of body cells . In this case, the cyanide irreversibly binds to the central iron (III) ion of the heme a 3 - cofactor in the cytochrome c oxidase in the mitochondria . When the enzyme is inactivated , cell respiration comes to a standstill, the cell can no longer use the oxygen for energy production, and this leads to what is known as " internal suffocation ". The body reacts to the supposed lack of oxygen by increasing the breathing rate. Since the oxygen in the blood cannot be used and consequently also accumulates in the venous blood, the skin becomes light red in color. Eventually, cells die from a lack of ATP , which is normally produced by cell respiration. The binding of the cyanide to iron (II) ions is comparatively weak. The inactivation of hemoglobin therefore plays a minor role in poisoning.

Antidotes are 4-dimethylaminophenol (4-DMAP), sodium thiosulfate , hydroxycobalamin , and isoamyl nitrite .

The hydrogen cyanide contained in low concentrations in many foods is converted by the human enzyme rhodanase into the harmless substance rhodanide .

Natural occurrence

Apricot kernels

The kernels of some stone fruit ( almond , especially bitter almond , apricot , peach , cherry ) and other rose plants contain small amounts of hydrogen cyanide; this serves partly as a protection against eating of the seeds and also as a chemical germination inhibitor by inhibiting the breathing of the seeds. Only after the pericarp (endocarp) has rotted can the hydrogen cyanide escape and thus activate the germination process. In the tropics often as foods used tuber of cassava also contains a cyanogenic glycoside -bonded hydrogen cyanide by the processing before consumption of the plant is removed. Other important foods with toxicologically relevant hydrocyanic acid content are yams , sweet potatoes (certain types), sugar millet , bamboo , linseed and lima beans . Immature bamboo shoots , which are considered a delicacy in Eastern countries, contain high levels of hydrogen cyanide, and cases of poisoning are known. During preparation (intensive cooking) the hydrogen cyanide is split off from the glycosides and released into the air.

Cyanogenic poisonous plants are widespread among the higher plants and can release HCN from cyanogenic glycosides by means of the enzyme hydroxynitrile lyase if the plant tissue is damaged by herbivores . Some examples of cyanogenic plants are the tropical golden spotted fern ( Phlebodium aureum ), a member of the spotted fern family , or the Brazilian rubber tree ( Hevea brasiliensis ). White clover contains the prussic acid glycloside linamarine , which is particularly poisonous for small animals (e.g. snails ) when parts of plants are ingested orally , as hydrogen cyanide can split off from it. Amygdalin is one of the best-known substances that split off hydrogen cyanide and occur in the kernels of some stone fruit .

Hydrocyanic acid as a neuromodulator and endogenous formation of hydrocyanic acid in the human organism

Hydrocyanic acid is also formed endogenously in the human organism and apparently has the role of a neuromodulator . Furthermore, hydrogen cyanide z. B. also generated by the administration of opioids via the activation of µ- opioid receptors . The endogenous formation of hydrogen cyanide is also important in forensics . Thus, when corpses are stored at 4 ° C after about 2 weeks, putrefaction and autolysis form hydrogen cyanide, with the concentration reaching its maximum after about 6 weeks and then slowly falling slightly.


Industrial production

The following processes are important for the production of hydrogen cyanide:

In the laboratory

If small amounts of hydrogen cyanide are required in the laboratory and there is no corresponding gas cylinder available, it can easily be obtained from its salts by adding a stronger acid :


Due to this slight release of hydrogen cyanide, when working with its salts in the laboratory, care must always be taken that the pH value of the solution does not become acidic, otherwise an (unintended) release occurs.

Waste and by-product

Hydrocyanic acid is released from process steps in electroplating if handled incorrectly .

When burning nitrogen-containing polymers ( plastics ), hydrogen cyanide can be produced to a considerable extent.

When smoking tobacco and burning Esbit , small amounts of hydrogen cyanide are released.


Genetically determined impairment of perception

More than a quarter of the population cannot perceive the odor of hydrogen cyanide, which is often prevented by paralysis of the olfactory nerve cells. Special safety measures must therefore be taken when handling hydrogen cyanide. The Bavarian State Office for Health and Food Safety takes this into account when examining the suitability of applicants for fumigation and pest control .


Anhydrous hydrocyanic acid must be stored in a cool place and vessels with hydrocyanic acid may only be opened carefully when they are strongly cooled. Otherwise, due to the low boiling point, they are under high pressure, and if they are opened carelessly, considerable amounts of them can suddenly escape in gaseous form and, in the worst case, liquid substance can splash. The purest, water-free hydrocyanic acid is stable for a few months. However, it must not be stored without hesitation, as hydrogen cyanide can polymerize explosively after a certain time (formation of so-called azulmic acid, a brown, flaky solid). The polymerization can be accelerated by traces of alkalis (the glass surface is also important here!) Or heavy metal oxides - especially in combination with small amounts of water - and delayed by adding small amounts of mineral acids or oxalic acid. An incipient yellow or later brown color is an alarm sign that this spontaneous decomposition is to be expected.

Aqueous solutions of hydrocyanic acid can only be kept for a very limited time, as slow hydrolysis occurs with the formation of formic acid and ammonia:


In order to avoid the transport of this hazardous substance , hydrogen cyanide is usually processed immediately at the place of manufacture.


According to EINECS , the European register of chemical substances present before the REACH regulation came into force, hydrogen cyanide belongs to the list of existing substances and has the number 200-821-6. The English synonym prussic acid is a reference to its historical usage.

Executions / murders

As a biocide

Hydrocyanic acid is used to combat vermin. For this purpose, a carrier material, e.g. B. diatomaceous earth, soaked with hydrocyanic acid and added fragrances as a warning.


Hydrocyanic acid was first used as a poison gas by the French army on July 1, 1916. Due to its high volatility, however, the mission was ineffective. In 1918 it was also used by the USA and Italy.

Industrial use

Hydrocyanic acid is used in many processes in industry and mining, for example for the production of cyanogen chloride , cyanuric chloride , amino acids (especially methionine ), sodium cyanide and many other derivatives as well as for leaching gold :

The gold solution is then reduced with zinc. The cyanido complex can also be absorptively bound by added coconut shell activated charcoal . From the activated carbon loaded with the cyanido complex, the gold can be extracted as “ash” after the organic part has been burned. In more modern plants, the cyanido complex is obtained from the separated loaded activated carbon by eluting with hot sodium cyanide solution in concentrated form (because of the better handling, a sodium cyanide solution is not used here, not liquid hydrogen cyanide). This process, as well as the mercury amalgam process, which is only rarely used as an alternative , leads to the sometimes catastrophic water pollution in the gold mining areas of the Third World .

Hydrocyanic acid is used in large quantities to produce adiponitrile and acetone cyanohydrin , both intermediate products in plastics production. In the production of adiponitrile, hydrocyanic acid is added to 1,3-butadiene using a nickel catalyst ( hydrocyanation ). To produce acetone cyanohydrin, hydrocyanic acid is added catalytically to acetone. From prussic acid and the α- be on an industrial scale in multi-stage processes amino acid DL - methionine (use in the feed - supplementation ) and heterocycle cyanuric chloride produced. Of cyanuric chloride are pesticides and other derivatives synthesized.

Web links

Wiktionary: Hydrocyanic acid  - explanations of meanings, word origins, synonyms, translations
Commons : Hydrogen Cyanide  - Collection of Pictures, Videos and Audio Files

Individual evidence

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