Hydrocyanic Acid

Hydrocyanic Acid


Chemical Data Notebook Series

Hydrocyanic acid (hydrogen cyanide, when found as a gas) is a deadly poison. Readily absorbed through the skin, the substance is extremely toxic whatever the route of entry into the body. It’s used in a multitude of industrial settings, including electroplating, plastics, dyes, cyanide salts, fumigants, insecticides, pesticides, pharmaceuticals, mining, and metallurgy. It’s also the gas that’s used in gas chambers.

The substance—chemical formula HCN—is flammable, unstable, and colorless in additon to being toxic. It has the odor and taste of bitter almonds, and some references indicate the odor can be detected at 0.2 to 5 parts per million of air. But testing for odor or taste clearly isn’t a safe way to identify the chemical.

HCN has an ignition temperature of 1,000° F, a boiling point of 78° F, and a freezing point of 8° F. Its molecular weight is 27, its flammable range 5.6% to 41%.

In its natural state, HCN is a gas which has a vapor density of .931. The substance can also exist as a solution of gas in water and as a liquefied gas. In these two forms, HCN’s almost always called hydrocyanic acid. The liquid has a specific gravity of 0.688 and a flash point of 0° F. HCN is usually shipped and stored in that form because its low boiling point makes it easy to liquefy.

The liquid state is the most dangerous, because it represents much more material present in a container than if the contents were in the gaseous state. In addition, some very hazardous reactions become more likely when HCN is a liquid.

The U.S. Department of Transportation considers both gaseous and liquefied HCN to be a Poison A, which it defines as “an extremely dangerous poison: poisonous gases or liquids of such a nature that a very small amount of the gas, or vapor of the liquid, mixed with air, is dangerous to life.” Some water solutions may carry the Poison B classification, which means a short, one-time exposure probably wouldn’t be fatal. HCN in both classes must be appropriately placarded or labeled.

When shipped in rail cars, HCN is identifiable at great distances because of the tank car’s distinctive coloration: white with one broad, horizontal stripe and two vertical stripes, all in red. This “candy striper” design isn’t designated by any regulatory agency, but some manufacturers have agreed on it.

The safety relief devices—or lack of them—on a container can also help identify HCN. There may be no such devices on containers carrying Poison A materials, because they could be the source of a lethalgas leak that would do more harm than the violent rupture the devices are meant to prevent.

The exact concentrations of HCN that cause death aren’t known, but good estimates can be made. The time-weighted average/ threshold limit value (TWA/TLV) is 10 ppm. The TLV is the amount of material to which a person may be exposed during a normal, 40hour work week without harm; the TWA indicates that actual amounts during that period may at times exceed the TLV. There’s usually a safety factor figured in, so that, depending on the sensitivity of the individual exposed, noticeable symptoms may not actually occur until prolonged exposure levels reach 20 ppm (the short-term inhalation limit).

Some references indicate that HCN becomes immediately dangerous to life and health at 50 ppm. Symptoms that become pronounced when the concentration is between 40 and 60 ppm are difficult breathing, palpitations, reddening of the eyes, salivation, confusion, headache, weakness, nausea, vomiting, and convulsions.

Levels of 100 to 200 ppm may be fatal in 30 to 60 minutes, and levels of 250 ppm will kill even sooner. Simple—or even elaborate—respiratory protection is no protection at all, because HCN is absorbed fairly fast through the skin. Selfcontained breathing apparatus with external air will delay the effects only a few minutes. And the liquid is absorbed even more quickly than the gas; contact with the material in any form must be avoided—particularly when a responder has a break in the skin.

Ingesting the liquid is more deadly still. Some references report that as little as 50 milligrams (there are more than 28,000 mg to the ounce) may be fatal.

At the same time hydrogen cyanide gas is reaching its toxic levels, it’s also very quickly building toward its lower flammable limit of 5.6%, and any common ignition source will provide the energy required to reach its ignition temperature. Always remember, though, that the first reaction of a flammable gas (or any flammable vapor) when ignited is an explosion, usually followed by a fire.

As a monomer, HCN is also subject to polymerization, a rather special chemical reaction that, if allowed to proceed uncontrolled, can produce a violent reaction that might resemble a boiling-liquid, expanding-vapor explosion (BLEVE).


Whenever there’s an accidental release of HCN, evacuation should be an automatic consideration, especially downwind. If any attempt is made to stop the leak, it should be done only after personnel are properly protected for entry into such a lethal atmosphere. Even if the spill threatens many lives, heroics such as rushing into the spill area without total encapsulating suits and proper respiratory protection will only cost more lives— those of the responders.

Rubber boots, gloves, other impervious clothing, and positivepressure, self-contained breathing apparatus may be sufficient protection, but total encapsulating suits are much safer. Protection is relative, and it depends on the thickness of the suit material, the integrity of the seams, the concentration of the hazardous material, and the duration and type of exposure.

Consult the manufacturer of your suits to confirm protection against HCN. Some manufacturers claim protection is afforded by total encapsulating suits constructed of Neoprene, Butyl rubber, polyvinyl chloride, or Viton. Always conduct tests (in laboratory hoods) to be sure that the suit won’t allow the hazardous material to pass through it.

When a spill occurs, contact the manufacturer, consignee, and transportation people involved immediately so they can begin getting their personnel into position to handle the situation. Put mitigation into the hands of the experts as soon as possible. Have your own people approach from upwind and eliminate ignition sources if no fire has occurred yet.

Coincidental with calling for help from experts, begin containment. Dikes and ponds can be constructed using soil, clay, sand, and other materials. Remember that the larger the surface area of the liquid, the faster the generation of deadly gases will be—so the preferred strategy might be to dig a containment pit, rather than a pond, as fast as possible. The deeper the pit, the more material it can hold with a smaller surface, which is also easier to cover with plastics, tarps, or foam.


Acid liquid HCN

Aero liquid HCN

Anhydrous hydrocyanic acid

Carbon hydride nitride

Cyclon (a trademark)

Cyclone B (a trademark)

Fluohydric acid gas

Formic anammonide


Hydrocyanic acid (aqueous solution)


Hydrocyanic acid (liquid)

Hydrocyanic acid (solution)

Hydrocyanic acid (stabilized or unstabilized)

Hydrogen cyanide anhydrous (stabilized)

Prussic acid Prussic acid (solution)

Prussic acid (stabilized or unstabilized)

Resource Conservation and Recovery Act Waste Number P063


Identification Numbers and Ratings

United Nations/North America

UN/NA 1051 for the liquefied acid

UN NA 1613 for solutions

UN NA 1614 for acid absorbed in solids

National Fire Protection Association rating



(Chemical Abstract Service)



(Registry of Toxic Effects of Chemical Substances) National Institute for Occupational Safety and Health

MW 6825000 for the pure material (gas or liquefied)

MW 6850000 for the solution

MW 6860000 for the unstabilized, liquefied material

MW 6880000 for a mixture of hydrocyanic acid and cyanogen chloride


(Standard Transportation Commodity Code)

Association of American Railroads, Bureau of Explosives

4920125 for the liquefied material

4921030 for a solution that’s 5% or more HCN

4921417 for solutions containing less than 5% HCN

The actual containment procedure should be decided upon after consultation with the manufacturer and with environmental agencies. All areas of the containment pond or pit will be contaminated by the product, and soil may have to be removed.

Alcohol-type foam will slow the release of gas from a pool of the liquefied gas. Or you can take advantage of HCN’s water-solubility by directing a water spray or fog over the spill or near the leak to dissolve the gas out of the air. The contaminated water from this action must be contained, or else it will spread the toxic and fire hazards.

Under no circumstances should the spilled product be allowed to enter sewers or waterways. If that does happen, notify all downstream users and everyone in the path of the sewer flow, and begin evacuation immediately. Contaminated water that enters a manufacturing process can produce lethal and explosive vapors in confined areas. If there’s any chance of damming, diking, or diverting this contaminated water to a holding or containment area, do it.

If a fire confronts you when you respond to an HCN incident, don’t extinguish it until you’re ready to control the flow of gas. The combustion products of HCN, in addition to water and carbon dioxide, are carbon monoxide and nitrogen oxides. Most firefighters realize the carbon monoxide can kill, but aren’t aware that nitrogen oxides are also very hazardous. There may also be some unburned HCN.

The combustion products, though dangerous, are less toxic than the HCN itself, and you should consider allowing the fire to totally consume the chemical. In fact, think early on during the incident about the tactic of a deliberate burn if you can avoid an explosion on ignition and the resulting fire won’t cause undue damage.

Check for containers exposed to flame impingement or radiant heat. If they don’t have safety relief devices, the odds of a violent rupture are high. If they do have such devices, a release of lethal gas may be imminent.

Depending on the size of the containers, which might range from a cylinder up to a rail tank car, evacuation distances should be 2,500 feet or more in all directions; downwind, it might be as far as two miles.

Addressing the effects

For anyone stricken by cyanide poisoning, proper and immediate medical attention is absolutely necessary; fortunately, those who recover are usually totally unharmed by the experience. If someone has inhaled the HCN and is still conscious, move the victim to fresh air and keep the person quiet and warm. If breathing has stopped or become labored, apply artificial respiration, but not moutb-to- mouth if it can be avoided. If mouth-to-mouth must be administered, beware of hydrogen cyanide in the victim’s lungs.


Monomer—A small molecule with the unusual ability to react with other molecules of the same kind to form a polymer.

Polymer—A long chain molecule made up of repeating units of certain small molecules called monomers.

Polymerization—A special type of chemical reaction in which monomers react with themselves to form polymers, liberating tremendous amounts of heat. This is usually a controlled reaction, under specific amounts of heat and pressure and carried out in a reactor which can be cooled.

Runaway polymerization—Polymerization in an uncontrolled environment, often occurring so fast that an explosion takes place.

Sparging—A process by which air or another gas is bubbled through a material.

If breathing hasn’t stopped, break an ampule of amyl nitrite in a handkerchief and hold it in front of the victim’s mouth for 15 seconds. Follow this with a rest for 15 seconds so the amyl nitrite doesn’t prevent oxygenation. Reapply it for 15 seconds, then rest again for 15 seconds. The ampule will be useless after five cycles, and you’ll have to break a new one. Emergency medical personnel should be well-versed in this process, and may follow it with the injection of sodium nitrite and sodium thiosulfate as directed by their training.

For anyone who might have come in contact with the HCN, decontamination is absolutely necessary; otherwise, injuries, and even death, could occur at some time after the incident.

Once the incident has been mitigated, consult the manufacturer about disposing of runoff, since the problem material is still present. Groundwater contamination is always possible and should be checked at this point. Properly trained and equipped personnel can remove the HCN from contained, contaminated water slowly and safely by agitation or sparging.

Liquefied gas can be salvaged from containment ponds by pumping it into secure containers, always with the proper personal protective equipment and with explosion-proof pumps and electrical connections. (Ignition sources must be eliminated at this stage of the incident, not only during the initial size-up and approach.)

Liquid can also be absorbed by inert materials, such as cement powder, fly ash, clay, sand, or soil. If effective neutralizers are available, the HCN manufacturer will suggest them. The contaminated neutralizers or sorbents then must be confined in secure containers and disposed of properly. But j again, these are jobs for experts trained in this type of work; emergency responders should avoid salvage and overhaul of dangerous spills whenever possible.

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