Chemical Data Notebook Series #24: Hydrogen Fluoride
FEATURES
HAZARDOUS MATERIALS
Both etched glass and frosted glass can be quite decorative, a welcome addition to a person’s china cabinet. But the material used to do the decorative work, hydrogen fluoride, is very unwelcome in a spill situation. Exposure to the material, in either gas or liquid form, could lead to serious illness or injury, and in some cases, death.
Hydrogen fluoride is used in many industries, particularly where its powerful corrosive action is desirable. In addition to dissolving glass, it can dissolve many metal ores and can clean steel (including stainless), copper, and brass. The petroleum industry uses it to acidize oil wells and in the manufacture of fluorinated chemicals.
The substance is a clear, colorless gas or fuming liquid (its boiling point of 67.1° F is so close to room temperature that references may differ on which it is) that’s usually shipped and stored as a liquid. It’s very corrosive, toxic, and water-reactive, and has a pungent, irritating odor. It dissolves in water to form hydrofluoric acid.
Although hydrogen fluoride, in itself, isn’t flammable, its corrosive action with many metals will generate hydrogen gas, which is highly flammable and explosive. Hydrogen fluoride will also react with many chemicals. This has serious consequences in an accidental spill or release, where the material may contact other chemicals. Hydrogen fluoride must be stored in a manner that will minimize such contact if there’s a release.
The liquid has a specific gravity of 0.992, a vapor density of 0.69, and a molecular weight of 20. It freezes at —134° F. When it dissolves in water, substantial amounts of heat are generated as particles are liberated.
Because it forms hydrofluoric acid when it dissolves in water, hydrogen fluoride is classified as an acid gas. The same property is possessed by hydrogen chloride (hydrochloric acid or HC1), hydrogen bromide (hydrobromic acid or HBr), and hydrogen iodide (hydriotic acid or HI). These four acids are collectively known as the halogen acids, and the gases are known as halogen acid gases.
The chemical formula for both hydrogen fluoride and hydrofluoric acid is HF.
Hydrogen fluoride is about as toxic as it is corrosive. Its threshold-limit value/time-weighted average (TLV/TWA) is 3 parts per million. Its immediately dangerous to life and health (IDLH) limit is 20 ppm. The American Conference of Governmental Hygienists, in its 1987-88 TLV publication, has eliminated hydrogen fluoride’s short-term exposure limit (STEL); however, it’s difficult to understand why such an action would be taken with such a hazardous material.
The odor of hydrogen fluoride will be noticed at concentrations of 0.03 milligrams per cubic meter.
The material in any of its forms—liquid hydrogen fluoride itself, high vapor concentrations in the air, or hydrofluoric acid—is extremely dangerous by all routes of entry into the body (meaning ingestion, inhalation, skin contact, and entry through a wound or other break in the skin). Absorption through the skin may not occur as rapidly because the skin will char on contact, cauterizing the capillaries that would otherwise carry the material away into the bloodstream. The chemical burns will be as severe as with any concentrated mineral acid, such as sulfuric or hydrochloric acid.
Inhalation of hydrogen fluoride may produce severe damage to the nose, throat, and upper respiratory tract. Hydrogen fluoride will dissolve in the first moisture presented to it to form hydrofluoric acid, causing severe irritation and possible burns. High concentrations of hydrogen fluoride in air may cause rapid reactions in the lungs, including inflammation and congestion of the lungs, which in turn may produce pulmonary edema and may result in death. In some cases, these reactions may be delayed for one to eight hours, and may not be recognized as stemming from exposure to hydrogen fluoride. Concentrations of 50 to 250 ppm are dangerous even in brief exposures.
Contact of hydrogen fluoride with the eyes is extremely dangerous, since the substance dissolves in the water in the eyes to form hydrofluoric acid. Severe irritation and deep burns resulting in permanent injury or blindness are possible reactions to a high concentration.
Ingestion of hydrogen fluoride or the acid will cause severe damage to the mouth, throat, esophagus, and stomach.
Entry of the hydrogen fluoride or acid through a wound or other break in the skin will cause destruction of all tissue exposed, including decalcification of bone.
Response
If a container of liquid hydrogen fluoride has been breached and the liquid has been released, it will produce great quantities of hydrogen fluoride vapor. With a vapor density of 0.69, the normal tendency of the gas will be to rise and disperse rapidly. If this occurs, there will still be a major hazard of exposure to the vapors near the spill and immediately downwind. However, if the liquid is very cold, the density of the gas might approach that of air, and the danger may spread farther downwind.
As with similar situations, approach the spill with extreme caution from upwind. The area of the release must be restricted to emergency response personnel. Among the personnel, only those properly protected can be allowed near the product.
Hydrogen fluoride is very soluble in water, and as vapors are released from a liquid pool, they must be swept from the air by use of a water spray or fog. But the water must not be allowed to contact the liquid hydrogen fluoride, because the reaction is highly exothermic, and the liberated heat will increase the liquid’s volatility, thus increasing the vapor production. Care must also be exercised to control the runoff caused by sweeping the air with water spray, because the runoff is a solution of hydrofluoric acid.
Synonyms
AHF
Anhydrous hydrofluoric acid
Fluorohydric acid gas
HF
HF-A
Hydrofluoric acid, anhydrous
Hydrofluoride
Resource Conservation and Recovery Act Waste Number U134
Identification Numbers and Ratings
UN/NA
(United Nations/North America)
1052
CAS
(Chemical Abstract Service)
7664-39-3
RTECS
(Registry of Toxic Effects of Chemical Substances)
MW7875000
STCC
(Standard Transportation Commodity Code)
Association of American Railroads,
Bureau of Explosives
4930024
CHRIS
(Chemical Hazard Response Information System)
U.S. Coast Guard
HFX
IMO
(International Maritime Organization)
2.3, poison gas, corrosive
National Fire Protection Association 704 rating
4-0-0
Of course, if the spill is in an area where no people or the environment are endangered, and the hydrogen fluoride is evaporating rapidly, the careful addition of the water will cause the hydrogen fluoride to dissolve in the water to form hydrofluoric acid, which will not release vapors as readily as the liquid hydrogen fluoride. This will allow it to then be salvaged by pumping it into a secure container. But this procedure may be troublesome: It will liberate heat and may spread the contamination.
Under all circumstances, liquid hydrogen fluoride or hydrofluoric acid must be prevented from entering sewers and waterways. If the liquid has reached a sewer, all downstream users must be notified immediately. Sewage treatment plant officials should be provided with such information as location of entry into the system, time of entry, type of material, and approximate quantity of material entering the system. Industrial users must also be warned, since bringing acidic material into their processes can damage equipment, and, if heat is applied, the hydrogen fluoride gas may be released and endanger the health of anyone exposed.
If it’s possible to divert the contaminated water to a holding area, this should be done, and then the acid solution should be removed, diluted, or neutralized.
Liquid hydrogen fluoride or hydrofluoric acid can be contained by building dikes to create a containment pond or by digging a containment pit. Soil or clay may be used, but sand could be dissolved by the hydrogen fluoride.
If a containment pond is small, or the area of a containment pit is small, the surface may be covered by plastic sheeting to lessen the evolution of vapors. And the smaller the surface area, the less vapor will be generated, even if the pond is uncovered.
Once the material is contained, there are several ways to mitigate the incident. One is to salvage the spilled product by pumping it into secure containers. This always requires equipment and containers that are impervious to hydrogen fluoride’s corrosive action. None of the metal parts of the pump or its connections and hoses should be exposed to the chemical.
The equipment and containers usually belong to the manufacturer of the hydrogen fluoride, who should be contacted immediately in any accidental release of its product. Fire departments shouldn’t attempt salvage procedures of this type, except in extreme situations, and then only if the responders are fully equipped and trained to handle the job.
Dilution should be attempted only where the resulting acceleration of vapor evolution won’t cause harm. Additionally, the containment pond or pit must be large enough to handle the increased volume of liquid.
Neutralization should be used only if the proper neutralizing agents are available, and these may be determined only by the manufacturer or qualified experts. However, there’s some agreement that sodium hydroxide (caustic soda) shouldn’t be used.
Effective neutralizing agents might include sodium bicarbonate (baking soda), sodium carbonate (baking powder or soda ash), calcium carbonate (ground limestone), and calcium hydroxide (slaked lime). A sample of the liquid hydrogen fluoride or hydrofluoric acid should be obtained in a plastic bucket and taken to a safe area, where the neutralizing agent should be added carefully and the reaction noted. Remember that the liquid will constantly release toxic and corrosive hydrogen fluoride vapors, and no liquid or vapor must be touched or inhaled.
Once hydrogen fluoride has been removed, diluted, or neutralized, all contaminated material must also be removed, including the diking material and all soil contacted by the hydrogen fluoride.
The amount of contaminated soil to be removed (indeed, the degree to which the environment has been contaminated) must be decided by the proper environmental authorities, who must subsequently supervise the decontamination of the spill site. Removal and disposal of contaminated material must be done by qualified personnel, usually employed by a commercial enterprise specializing in such work.
If the container of hydrogen fluoride has been breached above the liquid—that is, in the vapor space—there will be a release of vapors under some pressure, depending on the temperature of the liquid. The vapors may be colorless or white. They should rise and disperse rather quickly, but emergency responders may lessen the danger downwind by sweeping the vapors from the air with a high-pressure spray.
The leak, if small enough, may be patched or plugged using any available patching kit, if those doing the work are properly protected. Care must be exercised so that absolutely no part of the body is exposed to the fumes.
If a secure container is approached by fire, the heat may well raise the internal pressure enough to cause catastrophic failure. Such a failure would spray vapor and liquid hydrogen fluoride in all directions, which isn’t a pleasant thought for anyone not fully protected. Therefore, all containers threatened by radiant heat or flame impingement must be kept cool by water applied from as far away as possible, preferably with unmanned appliances. Safe distances depend on the size of the container, and may range from 250 feet for a single drum to 1,500 or even 2,500 feet for several.
Should liquid be released when there’s a fire, use of water may splash the hydrogen fluoride about, and will certainly cause the liberation of more heat by its contact. If water must be used on a fire near or around liquid hydrogen fluoride, be aware of the generation of toxic and corrosive vapors that will be present in the smoke and thermal column. The runoff will most likely be hydrofluoric acid, and must be contained.
Personal protective clothing should prevent any contact of the skin by the liquid or vapor. Rubber boots and gloves, splash-proof goggles, and face shields may provide protection. Positive-pressure, self-contained breathing apparatus must be used whenever vapors may be encountered. Total encapsulating suits should be worn by anyone required to work in or near the liquid or vapor. Manufacturers of such suits claim protection against hydrogen fluoride and hydrofluoric acid is afforded by neoprene, nitrile rubber, nitrile butadiene rubber, and Viton. However, swatches of these materials obtained from the manufacturers should be tested in liquid hydrogen fluoride and in concentrated hydrofluoric acid solutions.
Glossary
Corrosive—Any material that will attack and destroy, by chemical action, any living tissue with which it comes in contact.
Dilution—The act of adding water to a water-soluble material to lower its concentration, thereby weakening it.
Exothermic reaction—A reaction which liberates heat.
Fiuorination—A chemical reaction in which fluorine becomes attached to some other type of molecule.
Fuming liquid—A liquid that gives off visible vapors under normal conditions.
Molecular weight—The total weight of a molecule, calculated by adding up the atomic weights of all the atoms in the molecule.
Neutralization—The chemical reaction in which a base reacts with an acid, producing a salt and water; the pH of such a solution will be near 7.0. Also, the chemical reaction of an acid or base with another material, with the resulting pH near 7.0.
Volatility—The tendency of a liquid to evaporate. A liquid which is said to be volatile, or have high volatility, will evaporate rapidly.
Protection against such a powerful corrosive as hydrogen fluoride or hydrofluoric acid is relative and depends on many things, including the material used in the suit, its thickness, the integrity of the material (no punctures, slits, or tears), the tightness and integrity of the seams, the type of exposure (vapors, splash, spray, or immersion into the liquid), the concentration of the liquid, and the length of time of the exposure.
Inhalation victims should be moved to fresh air, and artificial respiration should be administered if breathing is difficult or has stopped. However, mouth-tomouth resuscitation may expose the responder to the chemical in the victim’s lungs or vomit. In any event, medical attention should be summoned immediately.
If hydrogen fluoride or hydrofluoric acid has been ingested and the victim is conscious, administer large amounts of water. Don’t attempt to make the victim vomit, but do attempt to administer water to an unconscious victim. Again, you should summon medical attention immediately.
If hydrogen fluoride liquid or vapors contact the eyes, they should be flushed immediately with water for at least 15 minutes, occasionally lifting the eyelids. It’s important to get qualified medical attention right away, because severe damage, including permanent blindness, can occur.
If there’s contact with the skin, remove all contaminated clothing, being sure to protect against contact. Wash all affected areas with water. Get medical attention immediately.
