CHEMICAL DATA NOTEBOOK SERIES #61: NITROGEN
HAZARDOUS MATERIALS
Nitrogen is a nonflammable, nontoxic, colorless, ordorless gas that makes up about 78 percent of our atmosphere. It is generally inert or unreactive, and its uses reflect that property. It is used to fill light bulbs and thermometers, to provide an inert atmosphere for some industrial operations, and to synthesize various chemicals. As a cryogenic liquid it is used to freeze plastic so it can be ground to a powder, to treat skin lesions by freezing (cryotherapy), to freeze foods rapidly, and to provide an inert atmosphere for food packaging (a drop of liquid nitrogen is inserted into a food package just before it is sealed to keep the product fresh). Nitrogen may be contained as a gas under high pressure or as a cryogenic liquid at temperatures below its boiling point of – 320°F.
Nitrogen is an element, and its chemical symbol is N. However, the nitrogen in our atmosphere—and the subject of this article—is molecular nitrogen, whose chemical formula is N2.
PROPERTIES
Nitrogen is considered a stable, inert, unreactive gas—and it is, under normal conditions. Lightning strikes and other inputs of very high energy, however, can cause the nitrogen to oxidize, as will be explained later. It has a vapor density of 0.97, a molecular weight of 28, and a specific gravity of 0.81 for the liquid. It freezes at – 346°F, boils at 320°F, and is almost nonsoluble in water.
HAZARDS
Nitrogen essentially is nontoxic, but it can replace oxygen to a level below that required to support human life and, therefore, is classified as a simple asphyxiant —that is, it brings about hypoxia when it replaces enough oxygen.
Nitrogen is considered nonflammable and could be used as a fire extinguishing agent, but since carbon dioxide is so effective an extinguishing agent, nitrogen is used only as a propellant for some dry chemical extinguishers.
As unreactive as nitrogen is, it oxidizes under extreme conditions — most specifically, when a tremendous amount of energy is encountered. The most famous example of this is its conversion to nitrogen dioxide in the atmosphere by the energy released by a lightning bolt—described as the origin of the material needed to form nitrates in the soil, providing fertilizer for growing plants.
More dangerous to emergency responders is nitrogen’s capacity to oxidize into nitrogen oxides at large fires. This group of toxic gases includes nitrous oxide (N2O), which is not particularly toxic; nitrogen oxide (NO); nitrogen trioxide, also known as nitrogen sesquioxide (N2O3); nitrogen peroxide, also known as dinitrogen tetroxide (NO); nitrogen dioxide (NO); dinitrogen pentoxide (NO); and trinitrogen tetroxide (N3O4).
Nitrogen oxides can form in very large and/or very hot fires. As the fire progresses past the smoldering or incipient stage into the growth stage, flames appear and a thermal column is established. As the heat of the fire warms the air and causes it to move upward, more air is pulled into the base of the fire. As this air moves through the flames, the oxygen is used to support the combustion of the fuel, and the nitrogen, unreacted oxygen, carbon, carbon monoxide, carbon dioxide, water, and intermediate products of combustion move upward and into the thermal column, causing it to grow.
As the fire grows hotter and hotter, the covalent bonds holding molecular nitrogen together fracture and release the free radical, elemental nitrogen (it is a free radical because of the unpaired electrons that were created when the covalent bonds broke). These free radicals are very active and seek to react with other available free radicals. Many of them react with other nitrogen free radicals, thus reforming molecular nitrogen. Other free radicals reacting with oxygen free radicals (the covalent bonds holding O2 together also break) form the various nitrogen oxides.
Many of these combustion products are carried away by the thermal column and do not pose danger. However, anyone close to the fire and without respiratory protection is exposed to them and may inhale enough of the nitrogen oxides to cause lung damage. The insidious aspect of nitrogen oxide poisoning is that its effects may be delayed for six to 48 hours. Sy mptoms of nitrogen oxide poisoning include shortness of breath, perspiration, and chest pain. In a short time, the chest pain becomes intense and spreads down the left arm. Breathing becomes impossible, and the victim collapses and dies. When this occurs to an overweight, out-ofshape emergency responder, the diagnosis is heart attack, and usually no autopsy is performed.
Firefighters who have not worn their masks on the fireground during a large and/or hot fire should seek immediate medical attention to determine whether inhaled nitrogen oxides have converted (or are converting) to nitrous or nitric acid in their lungs and are damaging the alveoli or destroying their lungs’ ability to remove oxygen from the air and oxygenate their blood. Exposed responders should inform the emergency room physician that they have been exposed to nitrogen oxides.
The compressed form of nitrogen presents the same hazard as any other compressed gas: It expands with heat and could cause catastrophic destruction of its container if the pressure rises faster than the safety-relief devices can control it. If compressed nitrogen escapes from a leak or valve, the gas becomes very cold; its pressure drops from approximately 2,000 psi inside the container to approximately 14.7 psi outside the container. Contact with this gas can cause frostbite.
The gas in its liquefied form has a maximum temperature of — 320°F and is stored in an insulated container called a Dewar’s flask—a container within a container, separated from one another by an empty space. The pressure in the air space in this container is very low, usually lower than 1.0 psi. This is due to the fact that the liquid requires energy to convert to gas to form the vapor pressure. There is very little energy in the extremely cold liquid, and the insulated container allows no—or very little—energy provided by the ambient temperature of the surroundings to reach the liquid.
The liquid is extremely cold and instantly destroys any tissue —especially human tissue—it contacts. The damage is instantaneous and irreversible at – 320°F.
Released liquid nitrogen generates 694 cubic feet of gas for every cubic foot of liquid. This expansion ratio produces enough nitrogen (which is ground-hugging because it is so cold) to displace oxygen and produces an asphyxiation hazard for anyone in the area.
NONFIRE RELEASE
A container of compressed nitrogen that has developed a leak presents no great danger to the population at large. If the leaking container is indoors or in an enclosed space, however, it could cause asphyxiation. Emergency responders need not act unless the leaking container is in enclosed or restricted places, in which case exposed persons should be evacuated. The leaking container may be plugged or patched if it is large and there is an interest in salvaging the remaining nitrogen. This would be possible only if the pressure is not too high and the proper plugging and/or patching materials and tools are available. Watch personnel working on the leaking container closely to avoid their being asphyxiated. In most instances, however, emergency responders simply can monitor the situation and allow the leaking container to empty.
The case of a leaking cryogenic liquid Dewar’s flask may be different. Again, if gas is leaking, the situation is not necessarily hazardous. Request the company that provided the liquid nitrogen to handle the leak. In many instances, the producers of the cryogenic liquid own the container on their customer’s property.
Liquid nitrogen presents a greater exposure danger. Once the liquid touches the ground or any other environment outside its container, it boils vigorously at first and then releases very large quantities of gas. If this occurs, secure the area. Do not let anyone come in contact with the liquid. Consider evacuating downwind if the nitrogen does not disperse rapidly and has the capability of diluting the oxygen in the air.
If the liquid enters a sewer, the water within the sewer will freeze immediately, and the liquid nitrogen will flow down through the system. Chances are that all the liquid will boil vigorously and evaporate to gas before it covers a great distance. There is no danger of fire or explosion from the gas; nonetheless, notify the sewage treatment plant at once. The oxygen level in the sewer will be severely lowered.
Liquid nitrogen reaching a waterway boils rapidly on contact with the water. It converts rather rapidly to the gas, because the water has a large capacity to warm the liquid nitrogen. It should cause no harm to fish or other wildlife unless they contact the liquid nitrogen. The release should not permanently harm the environment.
CAS
IDENTIFICATION NUMBERS AND RATINGS
(Chemical Abstract Services)
7727-37-9
STCC
(Standard Transportation Commodity Code)
4904565, compressed gas 4904566, cryogenic liquid
UN/NA
(United Nations/North America)
1066, compressed gas 1977, cryogenic liquid
CHRIS
(Chemical Hazard Response Information System)
NXX
DOT
(U.S. Department of Transportation)
nonflammable gas
NFPA 704 Rating
3-0-0
IMO
(International Maritime Organization)
2, nonflammable gas
The extreme cold of the liquid could damage pavement or metal. Rubber tires on vehicles (including fire apparatus) will be destroyed by contact.
FIRE SCENARIO
Do not allow any container holding compressed nitrogen or any other compressed gas to be heated excessively. Gas containers, even though they possess a pressure-relief device, can heat so rapidly that the pressure may rise faster than the device can relieve it, creating the possibility of catastrophic disintegration of the container and the production of deadly shrapnel.
If a fire is approaching a container of nitrogen, or if the container is being heated from the radiated heat of a fire, cool the container with water applied from as far away as possible; do this, however, only if it can be done safely. If the pressure-relief device is functioning, chances are that the container will not blow up, but it is safer to stay out of the range of flying metal.
If a fire is approaching a container of liquid nitrogen, do not apply water on the container unless the water can be precisely controlled. Since the container is insulated, chances are that the heat of an approaching fire will not overheat the cryogenic liquid inside. If the inner container does receive heat energy, the spring-loaded valve will operate and reduce any pressure inside. Any water directed on the top of the container will instantly freeze and form a huge block of ice that will seal off and defeat the purpose of the safety-relief device. Therefore, if water cannot be precisely controlled, do not apply it to a cryogenic container—which may not need cooling anyway.
If the fire is uncontrollable and attacks the cryogenic container, the inner tank may fail if the heat input is great enough. The release of liquid nitrogen is hazardous to any person it contacts; its release onto the fire scene, however, might help extinguish the fire, as it cools the fuel and reduces the oxygen level of any air near the release.
PROTECTIVE CLOTHING AND EQUIPMENT
Select protective clothing that protects against the extreme cold of the liquid nitrogen. The clothing will freeze, become brittle, and break at the slightest touch if it comes in contact with liquid nitrogen. Liquid nitrogen seeping through the clothing causes instant, irreversible damage to the skin and any part of the body it contacts. All known total-encapsulating suits and rubber gloves and boots will become brittle and break at — 320°F. Wear full face shields and chemical splashproof goggles to protect the eyes. Because of the possibility of reduced oxygen levels, wear positive-pressure, self-contained breathing apparatus.
FIRST AID
Any person exposed to a reduced level of oxygen because of the presence of a larger amount of nitrogen than normal must be removed to fresh air immediately. If the victim is not breathing, artificial respiration must be started at once. Immediate medical attention is necessary.
Ingestion of the cryogenic liquid is unlikely, but it causes severe and irreparable damage to the mouth, esophagus, and stomach. Seek medical attention immediately.
Contact of the cryogenic liquid with the skin causes severe frostbite and tissue damage. Remove frozen clothing with caution to avoid further tissue damage. Wash the affected area with large quantities of water. Do not use warm or hot water, and do not rub frostbitten areas. Seek immediate medical attention.
Contact of the cryogenic liquid with the eyes causes severe damage due to the liquid’s extremely low temperature. Wash the eyes with large amounts of cool water for at least 15 minutes, occasionally lifting the lids. Seek immediate medical attention.
