CHEMICAL DATA NOTEBOOK SERIES #109: BORON TRICHLORIDE
BY FRANK L. FIRE
Boron trichloride is a corrosive, toxic, irritating, nonflammable, colorless gas usually found as a fuming liquid. Used in many chemical processes and in the manufacture of many chemicals, it is found in chemical facilities where aluminum, copper, magnesium, and zinc are refined to remove carbides, oxides, and nitrides from the molten metals. It may also be found wherever diborane, electrical resistors, soldering fluxes, or optical fibers are produced.
Since its boiling point is relatively high and much more material can be put into a container as a liquid than as a gas, boron trichloride usually is shipped as a liquid. Containers of boron trichloride, which range in size from cylinders to tank cars, are therefore pressurized; the surface that contacts the product is stainless steel.
PROPERTIES
Boron trichloride is a stable chemical that boils at 54.5°F; freezes at -161.1°F; and has a molecular weight of 117, which means its vapor density is 4.04. (The vapor density of any liquid or gas may be calculated by dividing a substance`s molecular weight by 29, the average molecular weight of air. The molecular weight of any chemical compound is calculated by adding the atomic weight of each atom as it appears in the molecular formula.) Boron trichloride`s specific gravity (liquefied gas) is 1.35. It reacts with water to form hydrogen chloride and boric acid. Hydrogen chloride will dissolve in moisture in the air and on any surface to form hydrochloric acid. Boric acid also is formed in this reaction with water. Even though references list boric acid as toxic and irritating, it is not as hazardous as hydrochloric acid. Therefore, I am listing some of the hazards of hydrochloric acid below and am not including those of boric acid.
HAZARDS
Boron trichloride is very corrosive to human tissue and to many metals and minerals–particularly when moisture is present. Although the pure gas (in gaseous or liquefied form) will attack tissue and metals, the activity of boron trichloride increases when it reacts in water, forming hydrochloric acid. Therefore, tissue damage and metal corrosion will take place faster (and, depending on the concentration, more severely) when the gas has reacted with water to form hydrogen chloride, an extremely hazardous material. Some of its properties follow: It boils at -121°F; freezes at -175°F; and has a molecular weight of 36.5, which means its vapor density is 1.26. The specific gravity of the liquefied gas is 1.19. It is very soluble in water as a gas or liquid. The gas will dissolve in moisture in the air or on any surface. The liquefied gas will sink in water while dissolving, liberating moderate amounts of heat.
The ceiling limit for hydrogen chloride is 7.5 ppm (parts per million parts of air), and its STEL (short-term exposure limit) is five ppm. Its detection threshold by odor is about one ppm.
Although hydrogen chloride and hydro-chloric acid are not flammable, the reaction between the acid (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 or fire hazard. Again, the reaction is usually with the acid, as the gas dissolves in moisture in the air or on the surface of the metal. Exposure of moist skin or metal to hydrogen chloride will produce a reaction.
In the case of hydrogen chloride, the reaction will occur anywhere on the body where moisture contacts hydrogen chloride, including the eyes, nose, mouth, mucous membrane surfaces, and any areas in which perspiration is present. Inhaling hydrogen chloride will irritate the upper respiratory tract (protection must be worn). Since it is so soluble in water, it will dissolve in the first moisture encountered–in the nose and mouth. As the acid forms, a sharp, pungent odor and taste are experienced. An individual will not voluntarily stay in an atmosphere of hydrogen chloride; therefore, exposure usually is limited to one or two “whiffs.” If, however, one is trapped in such an atmosphere without respiratory protection, burning of the nose, mouth, and upper respiratory tract will occur. Theoretically, lung damage can occur if enough hydrogen chloride is inhaled, but the vast majority of the material will dissolve in moisture before it reaches the lungs. The extent of the damage caused depends on the amounts of boron trichloride and water present and the presence or absence of a breeze to disperse the hydrogen chloride.
Although boron trichloride is a toxic gas, the American Conference of Governmental Industrial Hygienists has listed no TLV-TWA (threshold limit value-time weighted average). It has an LC50 of 20 ppm for rats. (LC50 is defined as the lethal concentration of gas, vapor, fumes, or dust in the air that kills 50 percent of the group of test animals exposed to it for a specified period during the exposure or post-observation periods.)
Ingesting boron trichloride will cause chemical burns of the tongue, mouth, esophagus, throat, and stomach, producing severe chest and abdominal pain. Other symptoms include diarrhea, vomiting, collapse, coma, and death.
Boron trichloride that contacts the skin will be absorbed and cause symptoms similar to those of ingestion; the skin will be severely burned, promoting ulcers and severe scarring.
Inhaling the vapors will burn the throat and cause choking, coughing, and severe irritation of the upper respiratory tract, possibly damaging the lungs and causing pulmonary edema.
If boron trichloride contacts the eyes, they immediately will become irritated and painful and tear significantly. The eye injury can range from slight irritation to redness to opacification of the cornea and blindness.
NONFIRE RELEASE
A release of any appreciable amount of boron trichloride is serious enough to activate the community`s emergency response plan. Always consider downwind evacuation when liquid has been released. A vapor density of 4.04 means the gas will sink to the ground and flow along low spots in the surrounding terrain. It will accumulate in low-lying areas (if there is no wind) and in basements and other low, enclosed areas. Although there is no threat of fire or explosion, anyone without respiratory protection entering an area where boron trichloride (or generated hydrogen chloride) has accumulated is in great danger.
If the air is particularly moist, much of the boron trichloride will react with the moisture and convert to hydrogen chloride. This irritant gas then will dissolve in any available moisture to form hydrochloric acid. The concentration of this acid depends on the amounts of moisture and boron trichloride present, the quantity of hydrogen chloride generated, the speed of the wind, and the temperature. In some instances, sufficient boron trichloride can react in moisture in the air to produce a concentration that will pit and corrode ferrous (iron-containing) metal present.
If vapors of boron trichloride are leaking, emergency responders may use a water fog or spray immediately downwind of the release to deliberately dissolve it and the newly formed hydrogen chloride out of the air. The immediate concern then shifts to containing the acidic runoff.
If the release is due to a leak in a pressurized container, mitigation of the incident might involve patching the leak or closing or repairing a valve. Standard techniques such as holding an air bag against the container with straps, driving a plug into the opening, or cementing a patch over the opening may be indicated. In any event, the material selected to stop a leak must be resistant to the gas and boron trichloride and any hydrochloric acid formed during the leak. In any situation, all personnel entering the immediate area of the release must don full body and respiratory protection.
If liquid boron trichloride is released, tremendous amounts of the gas will be generated, since the liquid is stored under pressure in a refrigerated liquid form. Its boiling point of 54.5°F 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-case scenario of a boron trichloride release.
Use water spray or fog to remove rapidly generated gas from the air; contain the runoff. In addition, prevent the water from contacting the liquid, as the water is considerably warmer than the liquid and will accelerate the liquid-to-gas conversion. A containment pond or large pit may be dug to handle a spill of this type. As stated, the runoff water from air-sweeping techniques must be contained.
If any liquid boron trichloride or resulting hydrochloric acid enters a sewer or waterway, immediately notify all downstream users. All boron trichloride entering the water eventually will convert to hydrochloric and boric acids. Water-treatment facilities might be totally disabled by the intake of hydrochloride acid. If the acid enters industrial operations through piping and machinery that might be attacked by the acid, the piping and machinery could fail catastrophically; highly explosive hydrogen gas also might be generated.
If liquid boron trichloride or hydrochloric acid enters a waterway to which emergency responders or others have access, such as a bridge over the waterway, the proper neutralizing agents (discussed below) may be added to the flowing water. Downstream users still must be notified.
If the liquid boron trichloride or hy-drochloric acid is successfully contained, several techniques can be used to safely mitigate the incident.
If the liquid boron trichloride is contained, act immediately to reduce the generation of hydrogen chloride gas. If possible, it would be best to pump it into the proper secure containers. Pumping it back into the leaking container in a semiclosed hoop will not be effective because of the hydrogen chloride generated under a small amount of pressure. If pumping is used, it is absolutely necessary to select equipment that is resistant to the liquid and acids that may form.
It may be necessary to neutralize the hydrochloric acid formed: Add a neutralization agent to the acid to raise the solution`s pH to a neutral level. Solutions of bases, such as sodium hydroxide or potassium hydroxide, may be used, but these materials are hazardous in themselves; dissolving them in water in itself will produce tremendous quantities of heat.
A safer and better action is to add sodium carbonate or sodium bicarbonate directly to the acidic solution. These materials are relatively expensive, though not as costly as the hydroxides mentioned above. Calcium carbonate, or ground limestone, probably is the best material to use. It may be added directly to the acidic solution; the bubbling produced (as with sodium carbonate and sodium bicarbonate) is the generation of carbon dioxide. In all neutralization attempts, remove a small sample of the solution to be neutralized to a safe place and add a sample of the neutralizing agent to determine a safe reaction rate. The amount of calcium carbonate needed to neutralize a spill depends on the amount and concentration of the acid present. Acid test paper must be used to determine the pH of the solution (a pH lower than 7.0 is acidic; a pH above 7.0 is basic, or alkaline; and a pH of 7.0 is neutral). Environmental officials at the scene must declare the solution safe.
These authorities also will determine the extent of contamination and the proper cleanup procedures to be used. Emergency responders and local fire departments should not be involved in the actual cleanup and should enlist the help of outside experts, if available, to dilute the spill.
FIRE SCENARIO
Although boron trichloride is nonflammable, it is a gas and under pressure in its container. All containers, therefore, must be protected from heat, which will cause a pressure rise. If the gas is released, it will not contribute to the fire, and any water used to fight a fire will cause hydrogen chloride gas to be generated. This gas is also nonflammable, and continued application of water spray also will remove this gas from the air. This technique is used to protect downwind exposure from the gas; the acidic runoff water must be contained.
If no water is being used, the hazards for exposure to boron trichloride 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 liquid boron trichloride.
Keep all containers of liquefied gas (all containers containing either a gas, a liquefied gas, or a liquid) cool with water applied by unmanned appliances from a safe distance–ranging from 250 feet for a single drum to 1,500 feet or even 2,500 feet for several drums or a large container. Consider evacuating all populated areas downwind. If fire approaches a secure container, the container could catastrophically fail as a result of the rise in internal pressure caused by the heat. Such a failure would spray vapor and liquid boron trichloride in all directions (that`s why all in the area must be in full protective gear). Never allow yourself to get between the approaching fire and containers of boron trichloride or any other gas, liquefied gas, or liquid of any type. Even properly functioning safety-relief devices may not be able to safely handle the fast rise in pressure caused by the heat of the fire.
PROTECTIVE CLOTHING AND EQUIPMENT
In all situations, wear total encapsulating suits and positive-pressure self-contained breathing apparatus to prevent boron trichloride and hydrogen chloride from contacting the skin or respiratory system. Manufacturers of total encapsulating suits claim protection against hydrogen chloride when suits are manufactured from materials that include butyl, natural, and nitrile rubbers; neoprene, polyethylene, polyethylene/ethylene vinyl alcohol; polyvinyl chloride (PVC); Saranex® and Viton®. Gloves, boots, and face shields also must be acid- resistant. Consult manufacturers of encapsulating suits and boron trichloride, hydrogen chloride, and hydrochloric acid for special recommendations.
FIRST AID
Inhalation. Move the victim to fresh air and administer artificial respiration if breathing is difficult or has stopped. (Warning: Mouth-to-mouth resuscitation may expose the responder to the chemical in the victim`s lungs or vomit.)
Ingestion. If the victim is conscious, administer large amounts of water. Do not attempt to make the victim vomit. Do not try to administer water to an unconscious victim.
Eye contact. Flush eyes immediately with water for at least 20 minutes, occasionally lifting the eyelids.
Skin contact. Remove contaminated clothing, making sure to protect against contact. Wash affected areas with large quantities of water.
In all forms of contact, immediate medical attention is absolutely necessary. n
SYNONYMS
borane, trichloro-
boron chloride
trichloroborane
IDENTIFICATION NUMBERS AND RATINGS
CAS
(Chemical Abstract Services)
10294-34-5
STCC
(Standard Transportation Commodity Code)
4932011
RTECS
(Registry of Toxic Effects of Chemical Substances)
ED1925000
UN/NA
(United Nations/North America)
1741
CHRIS
(Chemical Hazard Response Information System)
BRT
DOT
(U.S. Department of Transportation)
corrosive
NFPA 704 Rating
(National Fire Protection Association)
not listed
IMO
(International Maritime Organization)
2.3, corrosive gas
FRANK L. FIRE is the vice president of marketing for Americhem Inc. in Cuyahoga Falls, Ohio. He`s an instructor of hazardous-materials chemistry at the University of Akron as well as an adjunct instructor of haz mats at the National Fire Academy. Fire is the author of The Common Sense Approach to Hazardous Materials and an accompanying study guide, Combustibility of Plastics, and Chemical Data Notebook: A User`s Manual, published by Fire Engineering Books. He is an editorial advisory board member of Fire Engineering.