What is HCN

Hydrocyanic acid, hydrogen cyanide HCN

Teflon closure



Colorless liquid, from which hydrogen cyanide evaporates even at room temperature, this has a bitter almond odor.
Bitter almond kernels

molar mass 27.026 g / mol

AGW 0.9 ml / m3 (TRGS 900)
pKsValue (HCN)  +9,21
density 0.6876 g / cm3   
Melting point −13.28 ° C
boiling point + 25.63 ° C
Water solubility mixable in any ratio
Explosion size6 to 40% by volume (air)
Flash point −18 ° C
Ignition point +538 ° C

GHS 02
GHS 05
GHS 06
GHS 09
Hazard classes + category 

Flammable liquids 1
Acute toxicity oral / inhalative / dermal 1
Skin corrosion 1A
Hazardous to the aquatic environment. acute / chronic 1
HP rates (see note) 
H 224, 300, 310, 314, 330, 400, 410
P 210, 241, 260, 273, 280.1-4,6,7, 303 + 361 + 353, 304 + 340, 309 + 310, 403 + 233, 405

special instructions

German name 
English name 
CAS 74-90-8Hydrocyanic acid 
Hydrogen cyanide
Hydrocyanic acid 
Hydrogen Cyanide

Note for schools: Hydrocyanic acid and hydrogen cyanide may not be stored in the school laboratory in Germany due to their very high toxic potential. Storage or production in schools is strongly discouraged. There is an acute danger to life if the vapors are inhaled or if the skin comes into contact with the hydrocyanic acid.
Effect on the human body 
Since the liquid evaporates at room temperature, the vapors are easily inhaled. The liquid can be easily absorbed through the skin. Hydrocyanic acid blocks cell respiration in the cells. In the event of poisoning, the cells can no longer use the vital oxygen. The skin turns bright red. The course of the poisoning is the same as with potassium cyanide. Some people cannot perceive the typical smell of hydrogen cyanide. The lowest lethal oral dose (LDLo oral) is 0.57 milligrams per kilogram of body weight (source: Internet> ChemIDplus). Hydrocyanic acid is more toxic than potassium cyanide. The intravenous administration of sodium nitrite and sodium thiosulphate is recommended as countermeasures in the event of poisoning. The nitrite forms methemoglobin, which reacts with the cyanide in the tissue and makes it harmless. The sodium thiosulfate provides sulfur, which is required for the natural breakdown of cyanide in the body by enzymes.

In its pure state, hydrocyanic acid is a colorless liquid that smells like bitter almonds or marzipan from which hydrogen cyanide evaporates at room temperature. The name goes back to the original extraction from the blue pigment Berlin blue. Hydrocyanic acid occurs naturally in bitter almonds or in some plants such as Columbine in small quantities. Therefore the columbine is avoided by grazing cattle.

In the leaves of the Common Columbine (Aquilegia vulgaris)
hydrocyanic acid occurs in small quantities.
Hydrogen cyanide gas is highly flammable; with air, explosive mixtures in the range of 5.4 to 46.6 percent by volume are formed. Hydrocyanic acid can be mixed with water and ethanol as required. It is a very weak acid. The pKsValue is +9.21. The property as an acid is caused by the hydrogen atom, which a proton can give off:
Hydrocyanic acid + water Cyanide ion + H3O+-Ion
HCN + H2O CN−  + H3O+    
This creates the salts of hydrogen cyanide, the cyanides, which include, for example, the well-known potassium cyanide. Hydrocyanic acid represents a borderline case between inorganic and organic chemistry. If it is understood as the nitrile of formic acid, then it would be an organic substance. The nitrile group as a functional group contains a carbon atom that forms a triple bond with a nitrogen atom. If the nitrile group is connected to an organic radical (R), the substance group of organic nitriles is obtained.


Structural formula of the nitriles
The Swedish chemist Carl Wilhelm Scheele (1742-1786) discovered hydrogen cyanide in 1782 when potassium hexacyanoferrate (II) was reacted with dilute sulfuric acid. In the laboratory, hydrogen cyanide can be produced by dripping 50% sulfuric acid onto potassium cyanide. The escaping HCN gas is then liquefied with the aid of dry ice cooling. Such a production is, however, strongly discouraged if suitable protective measures are not in place. Smoking tobacco and burning plastics also release small amounts of hydrogen cyanide. In industrial technology, hydrogen cyanide is mainly produced using the Andrussow process. A mixture of methane and ammonia is partially oxidized with atmospheric oxygen with the aid of platinum-rhodium catalysts:
2 CH4 + 2 NH3 + 3O2  2 HCN + 6 H2O ΔHR. = +480 kJ / mol
In the past, hydrogen cyanide was often used in ships, mills and storage tanks as a fumigant against vermin. There have been numerous accidents with hydrogen cyanide in the past. In 1947, a Los Angeles house filled with hydrogen cyanide while fighting termites exploded. In 1995 an attempt was made to rid a church of woodworms in the holiday resort of Lovran in Croatia. As the building was not properly sealed, toxic fumes were released and the entire place had to be evacuated.
The deadly material probably wrote its worst chapter in the National Socialist extermination camps. During the Holocaust, millions of Jews were poisoned by the Zyklon B gas. In some states of the United States, hydrogen cyanide was used in the gas chambers to carry out the death penalty. The deadly gas was generated by the reaction of a cyanide salt with an acid.
In the chemical industry today, hydrogen cyanide is an important intermediate product for the production of other substances, for example in the dye or fertilizer industry. In mining, gold can be leached out of the gangue with the help of cyanide leaching:
4 Au + 8 HCN + O2 + 4 OH  4 [Au (CN)2] + 6 H.2O
The resulting gold solution can be reduced with zinc. Since using hydrogen cyanide would be too dangerous, cyanides such as potassium cyanide are used in this process. Due to the environmentally hazardous effects of hydrogen cyanide and cyanides, this type of gold mining often leads to catastrophic water pollution in third world countries.