HCN Technical

Hydrogen Cyanide technical information

Chemical composition: HCN
Names: hydrocyanic acid, prussic acid, hydrogen cyanide
Boiling point: 25.7C/78.3F at 760 mm Hg
Specific gravity: 0.69 at 18C/64F
Vapor density: 0.947 (air = 1)
Melting point: -13.2C/8.2F
Vapor pressure: 750mm Hg at 25C/77F 1200mm Hg at 38C/100F; evaporates easily
Solubility in water: 100%
Appearance: clear
Color: colorless to slightly bluish depending upon concentration
Odor: bitter almond, very mild, non-irritating (odor is not considered a safe method of determining presence of the poison)

Hazards

Unstable with heat, alkaline materials and water.
Will explode if mixed with 20% sulfuric acid.
Polymerization (decomposition) will occur violently with heat, alkaline material or water. Once started, reaction is autocatalytic and uncontrollable. Will explode.
Flash point: -18C/0F
Autoignition temperature: 538C/1000F
Flammable limits in air (by volume): lower 6%, upper 41%

Source: Hydrogen Cyanide , Dupont Publication 7-83.

Commercial sources

The Dictionary of Chemical Compounds

"Derivation: (a) By catalytically reacting ammonia and air with methane or natural gas. (b) By recovery from coke oven gases. (c) From bituminus coal and ammonia at 1250 degrees".

Toxicity by inhalation

Concentration (mg/m3)	Effect
300	Immediately lethal
200	Lethal after 10 minutes
150	Lethal after 30 minutes
120-150	Highly dangerous (fatal) after 30-60 min.
50-60	Endurable for 20 min. - 1 h without effect
20-40	Light symptoms after several hours

Note these are rules of thumb and are not to be taken as absolutes. Death is caused by a total amount per kilogram of body weight that enters into the body. In light of that, simply examing the concentrations and times reveals there is no direct relationship between them.
Many factors could cause this as HCN interferes with the nervous system that of course controls breathing.

Means of toxicity

Cyanide binds cytochromes much in the same way that oxygen does, by conjugating at its open site. Unlike oxygen, cyanide cannot receive electrons from cytochrome a3.

           
   -:C=N: (note - actually a triple bond between C and N)
     |
---Fe(+2)--
     |
    His

With the ETS deprived of its electron "sink", the whole system backs up. Without the ETS, oxidative phosphorylation will dissipate the H+ gradient, ATP synthesis will stop, and the cell will die. Cyanide binds cytochromes more tightly than oxygen, and as a result is lethal at very low concentrations, at about 300 ppm. The effect also occurs at hemoglobin, as cyanide will bind to that too, preventing oxygen from reaching cells. In essence, this is how cyanide kills cells and whole organisms.

Hemoglobin
      Cyanide is most effective on warmblooded animals such as mammals, but is less effective on insects. While insect mitochondria and vertebrate mitochondria are not radically different, one thing is: Hemoglobin. Vertebrates carry oxygen in their blood via hemoglobin, while insects do not carry oxygen in their blood at all. Instead, insects have air tubules that carry oxygen directly to all cells in their body. Because cyanide poisons hemoglobin too, animals that use it are all the more susceptible. Also (while I am not sure of this) insects may be more tolerant of anaerobic metabolism than vertebrates.
      Since cyanide binds to hemoglobin much in the same fashion as it binds cytochrome a3, cyanide takes hemoglobin out of commission as well {9}. With their oxygen carrying molecules bound by cyanide, vertebrates die all the faster from asphyxiation. Mammals are also very dependent on oxygen- utilizing metabolism, and will die in minutes if it is shut off. Insects, lacking hemoglobin, die more slowly as their cells must be starved of ATP. Insects may also be able to survive longer on anaerobic (non-O2 utilizing) metabolism.
      Cyanide kills by binding to cytochrome a3 in the electron transport system. As this site is usually bound by oxygen, the passage of electrons from the ETS to oxygen is prevented, backing up the system. Unable to maintain a proton gradient without a properly functioning ETS, ATP synthesis stops and the cell dies. In vertebrate organisms, cyanide also binds to the porphyrin ring in hemoglobin, exacerbating cyanide's toxic effects.