CHEMICAL DATA NOTEBOOK SERIES #72: ARGON
HAZARDOUS MATERIALS
Argon is a colorless, odorless, tasteless, nonreactive, nontoxic gas. An elemental gas, it is a member of the group or family of elements known as the “inert” or “noble” gases, which occupy Group VIIIA on the Periodic Table of the Elements. The numbering on the Tables may differ, but the noble gases occupy the last vertical column on all versions of this document.
Because of its resistance to react (to form any chemical compounds), argon is used wherever oxidation or some unwanted chemical reaction might occur, including the welding process using the electric arc method and the surface finishing of metals. It also is used as the inert gas in incandescent light bulbs, fluorescent light tubes, and glow tubes, as well as in the process of “growing” germanium and silicon crystals.
Argon exists naturally in the atmosphere at an average of 0.93 percent by volume (at sea level), making it the third most abundant gas (behind nitrogen and oxygen) making up the mixture we know as air. It is shipped as a compressed gas in cylinders or as a cryogenic liquid in special containers (described below).
PROPERTIES
As an inert gas, argon does not burn or support combustion. It has the following properties: a vapor density of 1.25 at room temperature and 1.78 at 32°F, an atomic weight of 40, and a freezing point of — 308.6°F (only 6.3°F below its boiling point!). Argon is almost totally insoluble in water, and the liquefied (cryogenic) gas has a specific gravity of 1.4 at its boiling point of — 302.3°F.
Argon hits no synonyms. However, when shipped, its physical state (compressed gas or refrigerated liquid) is placed after the word argon. Its chemical symbol is Ar.
HAZARDS
Argon’s hazards are relatively few, but they vary according to the form of the gas—compressed or liquefied. The hazards are the same whether the gas originates from a container of compressed or liquefied gas, but the extremely low temperature and the amounts of the gas generated from liquefied gas make it more hazardous.
Compressed gas. Although argon is a nontoxic gas, it can lead to asphyxiation whenever it is allowed to accumulate. Asphyxiation, of course, occurs when the air is replaced or diluted to the point where the oxygen content of the resulting argon-air mixture drops to below 15.7 percent by volume, requiring about 33 percent of argon in the air At this level, rapid respiration takes place, as the body begins working hard to acquire oxygen. Dizziness and nausea occur, as mental alertness drops and muscular coordination decreases. All sensations may be depressed, errors in judgment occur, fatigue comes on rapidly, and emotional instability becomes prominent Vomiting may occur, followed by prostration, unconsciousness, and finally —as the concentration of the asphyxiant increases—death.
There is no hazard from any chemical reaction involving argon, since argon will not enter into any chemical reactions—it is not possible for argon to burn, to support combustion, or to form any dangerous chemicals. This inertness comes from a basic law of nature: that the chemistry (and therefore the chemical activity) of every element is due to the number and configuration of the electrons in orbit around the nucleus of each of its atoms. This law’ states that an element will take part in chemical reactions to form compounds with which it either will gain, lose, or share electrons with other atoms of elements until it completely fills its outermost ring of electrons. All elements—except the noble or inert gases of Group VIIIA of the Periodic Table of the Elements (nitrogen, although considered inert, really isn’t inert, and it is not a member of this “family” of elements) —behave this way, and therefore are chemically active elements. Since these gases (in addition to argon, they are helium, neon, krypton, xenon, and radon) do not react with their own atoms or the atoms of other elements, they already must have a “full” or completed outer ring of electrons, and they do. Therefore, their chemical reactivity, which is zero, differs from other elements.
As gases, however, noble gases obey the same laws of physics as any other gas. As a compressed gas, argon presents the same hazards as any other compressed gas, minus the danger of possible chemical reactivity; that is, as a compressed gas in a container, argon’s major hazard would be any condition that might cause a catastrophic failure of the container. Such failure can be caused by an input of energy in any form but particularly from the heat radiating from a fire or any other heat source or from direct contact with flames or a heating element.
The Combined Gas Law states that as the temperature of a confined gas increases, its pressure increases (the pressure also increases if the volume is reduced or if more gas is added to the same container without increasing the volume of that container). If the internal pressure is allowed to rise above the design strength of the container, the container will fail—often as a pressure-relief explosion, propelling deadly shrapnel in all directions. If the failure occurs at one end, the container may behave like an unguided missile. All safety relief devices on the containers (as on the containers of any compressed gas), therefore, must be in proper working order. The exception is Class A poison containers, which have no safety relief device.
Liquefied form. The major hazards of argon probably are related to the liquefied form. Liquid argon is classified as a cry ogenic liquid because of its extremely low boiling point of — 302.3°F, which indicates that it is not an ordinary liquefied gas such as ammonia (boiling point — 28.3°F), propane (boiling point — 44.5°F), or carbon dioxide (boiling point — 34.6°F). Cryogenic means that a material is produced, stored, or used at a temperature of — 150°F or below (some references use — 200°F as the boundary, but — 150°F is more conservative and should be used).
Liquefied argon is just as nontoxic as the compressed gas form of argon, but there is at least one set of obvious hazards and one not-so-obvious hazard. The obvious hazards of liquefied argon have to do with its extreme coldness. The maximum temperature of the liquid is — 302.3°F; at this temperature, all ordinary materials, including human tissue, are destroyed. The obvious danger of frostbite may be so severe that the tissue damage will be irreversible. Avoid all contact with the liquid and the gas being liberated by the liquid, which also will be very cold.
Any material contacting the liquid will become very brittle and shatter on the slightest impact. This includes all safety clothing, equipment, tools, and parts of the human body. It is very difficult for the human mind to comprehend a temperature of — 302.3°F, which is 334.3°F below water’s freezing point. It is even more difficult to understand that this is the liquid’s boiling point, which is 514.3°F below water’s boiling point. We have been conditioned to think that boiling point means some high temperature instead of extremely low temperature. Remember that all liquefied gases are cold and present frostbite hazards, but cryogenic liquids are much colder and are proportionally more dangerous to anything that can be damaged by such extremely low temperatures.
The less obvious hazard of cryogenic gases is the extremely high vaporto-liquid ratio (the amount, in volume, of gas that evaporates from the liquid compared with the volume of the liquid). For every cubic foot of liquid argon. 840 cubic feet of gas will evolve. To put it another way, one gallon of liquid argon will generate more than 1 1 2 cubic feet of gas.
The rate at which gaseous argon will evolve from a liquid argon release depends on the type and temperature of the medium the liquid contacts. For example, if liquid argon is released onto the ground, a large amount of gas initially may be generated as the very cold liquid argon contacts the relatively warm earth. The evolution of gas, however, will slow because energy is required to convert the liquid argon to gaseous argon (the exact amount is known as its latent heat of vaporization), and that energy will be extracted from the medium the liquid argon contacts. In this case, when energy is removed from the earth to allow the argon to evaporate, the ground rapidly freezes and less energy is available for evaporation. After the initial large amount of gas evolves, the evolution is relatively slow.
This description of how the liquid argon boils rapidly at first and slows down considerably a few moments after the release demonstrates the “self-insulating” properties of all cryogenic liquids. When confined in a specially insulated container called a Dewar flask (a container within a container, similar to a thermos bottle), there is very little gas above the liquid — and thus very little pressure. This is so because there is little energy present in the system (the very cold liquid is inside an insulated container at — 302.3°F) to generate gas by evaporation. For this reason, vaporpressure values are given for specific conditions, such as 0°F. 6()°F, or “room temperature.” The vapor density for liquid argon is very low.
The source of argon for liquefaction is the atmosphere. Air is drawn into a compression unit through apparatus that removes water vapor and particulates. The compressed air (again according to the Gas haws), which has been heated by the compression, is cooled by refrigeration coils on the outside of the compressor. More air is brought into the unit and is cooled again. This process is repeated until the compressed air is at 2,000 psi (pounds per square inch) and cooled to — 28.3°F (the temperature of the liquid ammonia that is circulated in the cooling tubes on the outside of the tank holding the compressed air).
The cold compressed air then is allowed to expand rapidly into another, larger container. The Gas Laws state that when the pressure of a gas drops (for example, by expanding the volume of the gas, which is being accomplished by allowing the gas to escape from a small into a large container), its temperature drops.
In this case, by controlling the size of the container into which the gas is allowed to escape, the temperature of the gas may be driven down to — 320°F, at which point nearly all of it condenses into a liquid. Nitrogen (boiling point — 320°F), oxygen (boiling point -297°F), and argon (boiling point – 302.3°F) are collected by letting the liquid air boil and collecting the different gases at their boiling points (distillation).
NONFIRE RELEASE
Although argon gas is nonflammablc, noncorrosivc, nontoxic, nonreactive, stable, and not an oxidizer, it nonetheless is a hazardous material. Any appreciable release of argon, particularly liquid argon, should trigger the community’s emergency response plan. Although the release of argon, even liquefied argon, does not represent a threat to the environment, the release might initiate a chain reaction of events that could be threatening. A release on a public highway or rail crossing might trigger an accident that involves other hazardous materials that might require the community’s entire resources. In-plant accidents involving released liquid argon might cause even more serious accidents. It is better to overmobilize than to be understaffed with a spreading incident. Environmental authorities and other agencies with valuable input should be alerted and mobilized as soon as possible.
Evacuation may not be necessary unless the potential release is verylarge or there is danger that the container might explode from overpressurization. The vapor density of the gas released by a spill of liquid argon will be higher than 1.78 (the vapor density at 0°F), and the generated gas will seek low areas in the terrain and flow at some distance. Aside from the frostbite threat near the release site, the major hazard of the flowing gas will be the collection of asphyxiating argon gas in low-lying or confined areas. Any’ appreciable breeze will disperse the argon. If more rapid dispersion is desired, a high-pressure water spray or fog maybe used. The runoff water need not be confined, since argon is not soluble in water and would not cause a major problem if it were.
Any liquid argon released should be confined to as small an area as possible with a containment pond created by pushing up soil, sand, or other | materials to form a surrounding dike.
A containment pit also may be dug to ! hold the released liquid. The liquid j may be held there until salvage opera! tions by specialized personnel and . equipment are completed. Under no ; circumstances should emergency re! sponders untrained and unequipped | for this ty pe of salvage or cleanup be I involved.
The boiling and conversion to gas | will be as described above as the earth under the liquid freezes. Avoid all I contact with the liquid or newly evapI orated gas. If, under special conditions, it is desirable to speed up the evaporation, applying water will accomplish it.
Should liquid argon reach a body of ! water, it will evaporate at a much higher rate than it does on the ground — especially if the body of water is large and particularly if it is a rapidly moving stream or river. Water has such a high specific heat that it contains considerably more heat enI ergy than a corresponding weight of earth at the same temperature. It ; therefore has more energy to give to the liquid argon to speed up evaporation. Some localized freezing of the water in immediate contact with the liquid argon may occur, but it quickly will be melted by the surrounding water, which will continue to provide energy until the liquid argon totally has boiled away. The air within the river’s banks may contain 75 percent or more of argon (which is deadly), but a strong breeze will disperse the argon. Aside from the aquatic life or waterfowl contacted by the liquid, no damage to the environment or wildlife should occur.
Prevent liquid argon from becoming entrapped in a closed system. The gas generated may be sufficient to cause the system to explode from the rise in internal pressure.
FIRE SCENARIO
Argon will not burn or support combustion. However, flames or the radiated heat of a fire threatens any container holding compressed gaseous argon. Activate pressure safety relief devices and keep internal pressures below design strength. Keep the containers cool by applying water from a distance far enough away to be safe from the flying shrapnel of any tank that might explode from overpressurization. Do not get caught between a fire and any type of container.
Do not use water to cool containers of liquid argon unless it can be kept from the vent area. Any water near the vent area will be frozen immediately by the escaping gas and quickly form a huge block of ice on the safety relief device, which will keep it from working. The tank, which is not as strong as pressurized tanks, is liable to burst. Since the container is really one container inside another, water applied to the outer tank will not cool the inner tank. It may, however, cool the outer tank and keep the inner tank a little cooler. Consult engineers from the company providing the liquid argon (that company usually also owns the tank) for proper procedures concerning the tank.
Any gas leaking from a container or evaporating from a pool of liquid argon reduces the oxygen content of the surrounding air and makes it difficult for a nearby fire to grow. The cold gas will hug the ground and may even behave like a fire extinguisher.
IDENTIFICATION NUMBERS AND RATINGS
CAS
(Chemical Abstract Services)
7440-37-1
STCC
(Standard Transportation Commodity Code)
4904502
RTECS
(Registry of Toxic Effects of Chemical Substances) CF2300000
UN/NA
(United Nations/North America)
1006
DOT
(U.S. Department of Transportation)
nonflammable gas
IMO
(International Maritime Organization)
2.2, nonflammable gas
PROTECTIVE CLOTHING AND EQUIPMENT
Select clothing that protects against the extreme cold of liquid argon. The clothing—including total encapsulating suits made from any material — will freeze, become brittle, and break at the slightest touch if it contacts the liquid. All known total encapsulating suits will become brittle and break at — 302.3°F, as will all rubber gloves and boots.
Liquid argon seeping through the clothing and contacting the skin may cause instant, irreversible damage to the skin and any part of the body contacted. Wear full face shields and chemical splashproof goggles to protect the eyes from a splash. Use positive-pressure, self-contained breathing apparatus because of the possibility of reduced oxygen levels.
FIRST AID
Respiration. Remove to fresh air any person exposed to a reduced level of oxygen because of the potential asphyxiation hazards. If the victim is not breathing, start artificial respiration at once and provide immediate medical attention
Ingestion. Ingesting the cryogenic liquid is unlikely; doing so will severely and irreparably damage the mouth, esophagus, and stomach. Immediate medical attention is needed.
Skin. If the cryogenic liquid contacts the skin, it will cause severe frostbite and tissue damage. Remove frozen clothing with caution to avoid causing additional tissue damage. Wash the affected areas with large quantities of water. Do not use warm or hot water, and do not rub frostbitten areas. Provide immediate emergency medical attention.
Eyes. If the cryogenic liquid contacts the eyes, it will severely damage them, due to its extremely low temperature. Wash the eyes with large amounts of cool w ater for at least 1 5 minutes, occasionally lifting the lids. Provide immediate emergency medical attention.
