Chemical Data Notebook Series #21: Hydrogen Chloride
Features
HAZARDOUS MATERIALS
A person will not, voluntarily, stay in an atmosphere of hydrogen chloride, due to its sharp, pungent odor and taste. That makes its detection—at as little as 1 part per million parts of air—relatively simple.
But anyone trapped in an atmosphere of the gas can suffer damage to the eyes, skin, and respiratory system. And, in very high concentrations, the chemical will be toxic to persons without respiratory protection.
Hydrogen chloride is a corrosive, irritating, nonflammable, and colorless gas. When it combines with water, it becomes hydrochloric acid. The gas can be removed from the air easily by a water spray or fog, and the resulting acid can be neutralized quickly. However, the gas poses a serious explosion and fire hazard when it contacts metal, liberating extremely flammable hydrogen gas.
The material, as a gas or in its more common liquefied form, is used in many chemical processes and in the manufacture of many chemicals. It will be found in the pharmaceutical and rubber industries, metal-cleaning facilities, the manufacture of food starch, and ore refining. It will also be found in transportation or industrial accidents when chlorine-containing products are involved and certain reactions take place.
In addition to being manufactured, hydrogen chloride is found in the human stomach. And it can be generated when certain materials containing hydrogen and chloride in their molecules burn, or when certain chemical reactions occur between water and water-reactive materials that also contain chloride in the molecules.
Hydrogen chloride is usually shipped as a liquid, since more material can be put into a container as a liquid rather than as a gas. Therefore, any containers will be pressurized. These pressurized containers range in size from cylinders to tank cars, and the surface contacted by the hydrogen chloride must be stainless steel.
Hydrogen chloride is a stable chemical that boils at — 121° F and freezes at -175° F. Its molecular weight is 36.5, which means its vapor density is 1.26. (Vapor density can be calculated by dividing a substance’s molecular weight by 29, the average molecular weight of air.) The specific gravity of the liquefied gas is 1.19.
The chemical is very soluble as a gas or liquid. The gas will dissolve in moisture in the air or on any surface. The liquid will sink in water while dissolving in it, liberating moderate amounts of heat. The resulting solution of either gaseous or liquid hydrogen chloride in water is hydrochloric acid. Highly concentrated hydrochloric acid is about a 37 percent solution of hydrogen chloride in water.
The molecular formula for both hydrogen chloride and hydrochloric acid is HC1, and the structure of the gas molecule is a simple one: H-Cl. Because it produces an acid when in water, hydrogen chloride is called an acid gas or, more specifically, a halogen acid gas. A molecule of a halogen acid gas contains one atom of hydrogen and one atom of a halogen (fluorine, chlorine, bromine, or iodine).
Hydrogen chloride’s hazards are many. For one, it’s very corrosive, not merely to human tissue, but to many metals and minerals, particularly in the presence of moisture. Although in either gaseous or liquefied form, hydrogen chloride will attack tissue and metals without moisture present, its reactivity increases tremendously when it dissolves in water and forms hydrochloric acid. Therefore, tissue damage and metal corrosion will take place faster and, depending on the concentration, more severely when the gas has dissolved in water to form the acid.
Although hydrogen chloride and hydrochloric acid aren’t flammable, the reaction between the add (and sometimes the gas) and many metals will liberate very flammable hydrogen gas. In some cases, the heat evolved in the reaction will be enough to ignite the hydrogen, producing an explosion and fire hazard.
Again, the reaction is usually with the acid, since moisture in the air or on the surface of the metal turns it into acid. Any exposure of moist skin or metal in the presence of hydrogen chloride will produce a reaction. Body moisture areas include the eyes, nose, mouth, mucous membrane surfaces, and anywhere perspiration is present.
The first body moisture encountered is likely to be in the nose and mouth, so hydrogen chloride can become an upper respiratory irritant. A sharp, pungent odor and taste occur as the acid is formed.
Most people exposed to hydrogen chloride get about one or two “whiffs” before moving away from the atmosphere. But someone trapped without respiratory protection will experience burning of the nose, mouth, and upper respiratory tract. Theoretically, lung damage can also occur, but nearly all of the substance will dissolve in moisture before it reaches the lungs.
The eyes are particularly vulnerable, due to their sensitivity and the presence of moisture. Eye protection is absolutely required. Keeping the eyes closed can protect them, but one would have to keep them closed during the entire exposure. Severe damage and blindness can occur without proper optical protection.
Glossary
Concentration—The percentage of an acid that’s dissolved in water.
Halogen—An atom, or group of atoms bound together chemically, that has gained or lost one or more electrons and is electrically charged: fluorine, chlorine, bromine, and iodine.
pH—A measure of the acidity or alkalinity of a substance. A pH from 1 to below 7 is acidic, and a pH of above 7 to 14 is alkaline. A pH of 7 is neutral.
Reactivity—The ability to enter into chemical reactions.
The threshold-limit value/timeweighted average (TLV/TWA) for hydrogen chloride is 5 ppm. Its short-term exposure limit (STEL) is 5 ppm for 5 minutes. Its immediately dangerous to life and health (IDLH) figure is 100 ppm.
Like any other irritant gas, the higher the concentration, the greater the hazard. Death can occur if enough hydrogen chloride is breathed. As more and more gas is inhaled, severe coughing and choking may occur. Concentrations of 1,000 to 2,000 ppm may be dangerous for even short periods. But hydrogen chloride is more readily avoided than some other irritant gases, or deadly, odorless carbon monoxide, because of its 1ppm detection threshold.
Incident mitigation
Although hydrogen chloride is nonflammable, it is a gas and under pressure in its container, so all containers must be protected from heat that will cause a pressure rise. If the gas is released, it won’t contribute to the fire, and any water used to fight a fire will dissolve the gas from the air. Indeed, this is a technique used to protect downwind exposures from the gas, but the runoff must be contained.
If no water is being used, the hazards for exposure to hydrogen chloride mentioned above will be present. If excessive pressure is allowed to build, catastrophic failure of the container may occur, with resulting shrapnel and massive discharge of hydrogen chloride: Thus, containers must be kept cool from a safe distance, using unmanned appliances, and all populated areas downwind must be considered for evacuation.
Whenever hydrogen chloride has been released, downwind evacuation must be considered. Its vapor density of 1.26 means the gas will sink to the ground and flow along low spots in the surrounding terrain. The tendency will be to accumulate in low-lying areas (if there’s no wind), and in basements and other low, enclosed areas. There will be considerable danger to anyone who, without wearing proper respiratory protection, enters an area where hydrogen chloride has accumulated.
If the air is particularly moist, much of the hydrogen chloride will dissolve in that moisture and convert to hydrochloric acid. The concentration of this acid depends on how much moisture is present, how much hydrogen chloride is present, how fast the wind is blowing, and what the temperature is. There have been occasions where enough hydrogen chloride has dissolved in moisture or air to produce enough acid of sufficient concentration to pit or corrode any machinery present.
This solubility in water can be used to protect downwind populations and other exposures. Emergency responders may use a water fog or spray immediately downwind of the release to dissolve the hydrogen chloride out of the air. The immediate concern then shifts to the acidic runoff.
The solubility can also be used to effect an emergency escape by holding a wet towel or cloth over the mouth and nose. As the air is pulled through the damp medium, the hydrogen chloride will dissolve in the water. This technique will work for only a short time, since the cloth which is being held against the face will eventually be saturated with hydrochloric acid. The possible minor burns suffered in this manner are preferable to the coughing and choking which occur when the gas is inhaled, causing more rapid breathing and damage to the respiratory system. Frequent rinsing of the cloth will prolong its usefulness for this.
If the release is due to a leak in a pressurized container, mitigation might involve patching the leak or closing or repairing a valve. Standard techniques may be indicated, such as holding an air bag against the container with straps, or driving a plug into the opening, or cementing a patch over the opening. Any material selected to stop a leak must be resistant to the gas and hydrochloric acid. And entry into the immediate area of the release must be done with full body and respiratory protection.
If it’s liquid hydrogen chloride that’s released, tremendous amounts of the gas will be generated right away, since the liquid is stored under pressure in a refrigerated liquid form. Its -121° F boiling point means that at any ambient temperature, the liquid will be boiling and converting large amounts of liquid to gas very rapidly. This is probably the worst type of a hydrogen chloride release.
Water spray or fog may be used to remove gas from the air. The runoff must be contained. The water itself must be prevented from contacting the liquid, because it’s considerably warmer than the liquid and will accelerate the liquidto-gas conversion.
A containment pond or large pit is almost always a necessity in using the dilution technique on a spill of this type. Although it’s not recommended that water be allowed to contact liquid hydrogen chloride, circumstances may require the rapid dilution of the spill. In this case, flooding amounts of water are rapidly added to the spill. Then it will be imperative to contain the solution, which will probably be a fairly high concentration of hydrochloric acid.
Should any liquid hydrogen chloride or resulting hydrochloric acid enter a sewer or waterway, all downstream users must be notified immediately. Intake of hydrochloric acid into water treatment facilities might totally disable them, and intake into industrial operations through piping and machinery that might be attacked by the acid could produce catastrophic failure of the piping and machinery. In addition to these dangers, highly explosive hydrogen gas might be generated.
If liquid hydrogen chloride or hydrochloric acid enters a waterway to which emergency responders or others have access, such as a bridged waterway, the proper neutralizing agents (discussed below) may be added to the flowing water. Downstream users must still be notified.
Should the liquefied gas or acid be contained, there are several techniques that can be used to safely mitigate the incident; any one of them must be combined with immediate action to reduce generation of the gas. The best approach is to pump it into the proper secure containers. Pumping it back into the leaking container would just agitate the liquid inside and cause it to generate more gas, more pressure, and a faster leak. However, if the original container has been effectively sealed, this technique can be used. If pumping is used, it’s absolutely necessary to select equipment that will be resistant to the liquid.
Synonyms
Anhydrous hydrochloric acid Basilin Chlorohydric acid
Hydrochloric acid, anhydrous Hydrochloric acid gas Hydrochloride
Identification Numbers and Ratings
UN/NA United Nations/North America
1050 for the gas 2186 for the liquid
CAS (Chemical Abstract Service)
7647-01-0
RTECS
(Registry of Toxic Effects of Chemical Substances)
MW9610000
STCC (Standard Transportation Commodity Code) Association of American Railroads, Bureau of Explosives
4904270 for the gas
4904271 for the refrigerated liquid
CHRIS (Chemical Hazards Response Information System)
U.S. Coast Guard HDC
IMO (International Maritime Association)
2.2, nonflammable gas, corrosive
National Fire Protection Association 704 rating
3-0-0
U.S. Department of Transportation
Nonflammable gas
If the dilution method is selected, the resulting hydrochloric acid solution may be pumped into secure containers. In this case, it may require 10 to 20 times the capacity of the original container, depending on the amount of water used.
Once diluted and contained, neutralization may be a course of action. A neutralization agent is added to the acid to raise the pH of the solution to a safe level. Solutions of bases such as sodium hydroxide or potassium hydroxide may be used, but these materials are hazardous in themselves, and dissolving them will produce tremendous quantities of heat.
Sodium carbonate and sodium bicarbonate may be safely added directly to the acidic solution. But these materials are relatively expensive—though not as costly as the hydroxides just described. The best material to use is probably calcium carbonate, or ground limestone. It may be added directly to the acidic solution. The bubbling produced (as with sodium carbon-ate and sodium bicarbonate) is the generation of carbon dioxide.
In all neutralization attempts, samples of the solution to be neutralized and of the neutralizing agent to be added should be taken to determine a safe reaction rate. (The mixing has to be done without splashing.) The amount of calcium carbonate required to neutralize a spill depends on the amount and concentration of the acid present. Acid test paper (litmus paper) must be used to determine the pH of the solution. Local environmental officials must be the authority present to declare the solution safe.
These officials will also determine the extent of contamination and the proper clean-up procedures to be employed. Local fire departments shouldn’t be involved in the clean-up, and they should enlist the help of outside authorities to perform the dilution, as well. Outside authorities must be notified as early in the incident as possible, and should be constantly consulted for their advice in handling the matter. Careful notes should be taken concerning their instructions, so that if actions taken are later questioned, accurate records can be produced showing the origin of such advice.
Protection and first aid
In all situations, protection must be maintained to prevent any and all contact with the skin or respiratory system. The best protection will come from total encapsulating suits and positive-pressure, selfcontained breathing apparatus. Manufacturers of total encapsulating suits claim protection against hydrogen chloride when the suits are manufactured from materials which include Butyl rubber, natural or nitrile rubber, neoprene, polyethylene, polyvinyl chloride (PVC), Saranex, and Viton. Gloves, boots, and face shields must also be acid-resistant.
Victims who have inhaled the gas must be moved to fresh air. Artificial respiration must be administered if breathing is difficult or has stopped, but mouth-tomouth resuscitation may expose the responder to the chemical in the victim’s lungs or vomit.
In ingestion cases, if the victim is conscious, administer large amounts of water. Don’t attempt to make the victim vomit, and don’t administer water to an unconscious victim.
For eye contact, flush the eyes immediately with water for at least 15 minutes, occasionally lifting the eyelids.
When there’s skin contact, remove contaminated clothing, being sure to protect against contact. Wash affected areas with large amounts of water.
In all contacts, immediate medical attention is necessary.
