CHEMICAL DATA NOTEBOOK SERIES #101: ACETIC ANHYDRIDE
BY FRANK L. FIRE
A corrosive, water-reactive, combustible, moderately toxic, irritating, clear, colorless liquid with a sour, pungent, vinegar-like odor, acetic anhydride is the waterless form of acetic acid (the word anhydride means “with no water present”). Acetic anhydride is used to manufacture cigarette filters, coatings, explosives, fibers, films, lacquers, perfumes, pharmaceuticals, plastics, tapes, and weed killers; it also is used as an intermediate in the manufacture of many other chemicals. Although a stable chemical, acetic anhydride will react with strong oxidizers and caustics and various other chemicals including acid gases, alcohols, amines, chromic acid, the aqueous solution of any chemical, and several other individual chemicals. It is corrosive to most metals and will attack some plastics and rubber. It reacts slowly with water. Acetic anhydride may be detected by smell at its threshold odor concentration of 0.14 to 0.36 ppm (parts per million of air).
PROPERTIES
Acetic anhydride has a flash point of 120°F, an ignition temperature of 600°F, and a flammable range of 2.7 percent to 12.4 percent. Its specific gravity is 1.08, molecular weight 102, and vapor density 3.52. It boils at 282°F and freezes at -101°F. It reacts with water; the reaction may be slow (with the evolution of some heat) or rapid and violent (with the evolution of enough heat to ignite the acetic anhydride)–the speed and violence of the reaction depend on the amount and speed with which the acetic anhydride is added to the water and the amount of water available. Its chemical formula is (CH3CO)2O.
Corrosivity is acetic anhydride`s major hazard; one reference claims that the corrosivity is equal to 117 percent concentrated acetic acid. An organic acid, acetic acid does not ionize appreciably in water. Although classified as a weak acid, it is a powerful corrosive. Acetic anhydride, therefore, is also a powerful corrosive and will destroy human tissue and many metals, including iron and nonstainless steels.
A water-reactive material, acetic anhydride will react with water instead of dissolve in it. If acetic anhydride enters a significant amount of water (such as in a fast-moving river or stream), the water will rapidly dissipate the heat evolved from the reaction. If, however, acetic anhydride is rapidly added to the water, the reaction may be so violent that it would produce heat energy faster than it can be dissipated, and the acetic anhydride will ignite. If the volume of water is fairly small, the reaction will be violent, and a fire or explosion (from the ignition of any acetic anhydride vapors generated by the heat) is very likely.
The acetic acid produced in the reaction of acetic anhydride with water contaminates the water and causes serious problems for any animal or vegetable life near the entry site. Human skin contacted by this water will blister, burn, itch, redden, and become inflamed; tissue damage could result. Blurred vision, conjunctivitis, pain, and redness; possible conjunctival and corneal destruction; and permanent injury, including blindness, could result from the water`s contacting the eyes.
Acetic anhydride is a combustible liquid (defined as having a flash point of 100°F or higher), and its flash point of 120°F is not much higher than the boundary between flammable and combustible liquids. It therefore is considerably more of a fire hazard than animal, vegetable, and other oils used in industry. Its relatively low ignition temperature of 600°F is easily reachable by all common ignition sources. Its wide explosive or flammable range of 9.7 percent (the “spread” between the lower explosive or flammable limit and the upper explosive or flammable limit) means that it will seldom be found in a “too-rich-to-explode” condition.
A very mobile liquid (one with very low viscosity and surface tension and which flows very rapidly), acetic anhydride will flow into crevices and tightly packed materials because its reduced surface tension allows it to act “wetter” than water.
Acetic anhydride is slightly to moderately toxic. Ingesting the liquid–in addition to burning the mouth, throat, and esophagus–will constrict the throat and cause abdominal spasms, nausea, and vomiting. Ingesting large quantities is likely to cause death. Acetic anhydride`s TLV-TWA (threshold limit value-time weighted average) is five ppm; it has no referenced STEL (short-term exposure limit).
Inhaling acetic anhydride vapors or mist will burn the nose, throat, and mucous membranes and cause difficulty in breathing; respiratory irritation; bronchitis; chemical pneumonitis; pulmonary edema; and, if the edema is unchecked, death.
NONFIRE RELEASE
A release of a substantial quantity of acetic anhydride will pose a serious hazard to any community. The hazard created will necessitate the need to activate the community`s emergency response plan. The incident commander must determine that all the community`s resources, including human resources in each area of expertise needed to protect the population and the environment, are ready for mitigating the incident. All except essential personnel should be evacuated immediately.
Acetic anhydride is combustible as well as corrosive. Emergency responders, therefore, must be aware at all times that a vapor explosion could occur and must protect themselves, the community, and the environment from the consequences of such an occurrence.
The threat of explosion with a release of acetic anhydride is not as severe as that with a release of a flammable liquid because of the difference in their flash points. The lowest listed flash point for acetic anhydride is 120°F, which means the liquid (only the surface of the liquid) must reach 120°F before it will generate vapors sufficient to form an ignitable mixture with the air near the surface of the liquid or the container it is in. Flammable liquids have flash points below 100°F and, therefore, need not be heated as high as acetic anhydride before vapors within the explosive range are generated.
If the temperature of the released acetic anhydride can be kept below 120°F (the lower, the better), no threat of explosion or fire will exist. Therefore, under normal, ambient temperatures in most parts of the country, acetic anhydride will not reach its flash point unless, of course, the heat of the sun (or some other source of energy) heats the liquid, whether inside its container or out in the open.
Keep acetic anhydride from reaching any waterway, including sewers or puddles on the open ground. Should water enter a waterway, immediately notify all downstream users of the water.
If the product should enter a sewer system and contact water, as noted above, a reaction producing the corrosive, combustible acetic acid and heat will occur. The generated heat might cause ignition. The less water involved in the reaction, the more concentrated (and therefore more corrosive) the generated acetic acid will be. In addition, less liquid will be available to absorb the generated heat so that the liquid will heat much faster, which could cause the accumulated acetic anhydride and acetic acid vapors to explode. The accumulated corrosive material traveling through the sewer system endangers downstream sewage-treatment facilities, and the corrosive and toxic vapors filling the sewer system threaten nonhuman life within the pipes and anywhere outside the system. Do not allow water fog or spray water used to disperse acetic anhydride vapors to contact the released product.
Personnel wearing the proper skin, eye, and respiratory protection and using equipment that is sparkproof and will not be affected by contact with the liquid should construct containment dikes by pushing soil, sand, and other compatible materials around the spill or dig containment pits to hold the released acetic anhydride in place. Environmental authorities present during the incident will determine the extent of the water`s (soil`s) contamination and when it`s safe for use.
Dilution or neutralization of the released acetic anhydride that cannot be salvaged immediately can be rendered less dangerous by dilution and/or neutralization. If dilution is selected (it may not be recommended because of the hazards presented by the material`s reaction with water), consider the potential dangers of its violent reaction with water and carefully weigh the dangers pres-ent vs. the problems that will be caused during mitigation. Use dilution only when the products of the reaction (heat, with the possibility of a fire or explosion, and acetic acid) pose no danger to emergency responders, other people, or the environment. Add flooding amounts of water to absorb as much of the heat as possible and to dilute the resulting acetic acid as quickly as possible.
If neutralization is considered, the water problem still must be considered. Recommended neutralizing agents include sodium bicarbonate (baking soda), sodium carbonate (baking powder or soda ash), and calcium oxide (lime), a hazardous material in itself. These neutralizing agents must be dissolved in water, presenting the problems of water reactivity and violent reactions. Perhaps the dilution and neutralization strategies could be used on relatively small releases of acetic anhydride; water would be added first and then the neutralizing solution. Remember that calcium oxide itself is a water-reactive material and that tremendous amounts of heat will be generated if it is added to water.
Contained acetic anhydride may be salvaged (always by professional salvage firms–not by emergency responders) by suctioning or pumping the material into secure containers, using equipment that will not be damaged by the product. Remaining acetic anhydride may be diluted and neutralized by the methods mentioned above. All contaminated soil must be removed and disposed of under the supervision of the environmental experts present.
FIRE RELEASE
Handle fires involving acetic anhydride as you would those involving any other combustible liquid. Approach from upwind and uphill. Any acetic anhydride released will pose a fire and/or explosion hazard if any of the material is heated to its flash point. Burning acetic anhydride may be extinguished with alcohol-type foam, carbon dioxide, dry chemical, and water spray. Foams, carbon dioxide, and dry chemical will be effective only if the weather conditions are “right,” the material is contained, the volume of product burning is relatively small, and plenty of the extinguishing agent is available and can be delivered effectively. Water spray should be used in large quantities to absorb as much heat as possible, including that generated by the water reaching the acetic anhydride.
Cool all exposed containers of acetic anhydride with large volumes of water delivered by unmanned appliances from as far away as possible. Never get between containers of liquid and an approaching fire.
Do not extinguish burning vapors escaping from a container of acetic anhydride unless the leaking vapors can be stopped immediately after the flame has been extinguished. Otherwise, a violent explosion will occur once the fire has ignited the vapors.
Should applying water make the situation too dangerous to control and using other fire extinguishing agents prove ineffective and there is no danger to life, the environment, property, or any infrastructure system, the best strategy might be to let the acetic anhydride burn itself out. During this mitigation technique, confine firefighting activities to securing the fireground and protecting exposures.
Once the fire has been extinguished, stressed (but nonleaking) containers should be handled very carefully and only by experienced professional salvage teams. The environmental authorities will determine the extent to which the incident ground has been contaminated and what steps will be needed before the land can be used safely.
PROTECTIVE CLOTHING AND EQUIPMENT
Wear equipment, including rubber boots, gloves, face shields, and splashproof safety goggles, that will provide reasonable protection against the material`s contacting skin or eyes. Fully encapsulating suits made from materials impervious to acetic anhydride and self-contained breathing apparatus (which should be worn with full facepiece or equivalent at all times) should be used to prevent contact with high vapor or fume concentrations in air. These materials may include butyl rubber, neoprene, and TeflonTM. Contact the manufacturers of total encapsulating suits and acetic anhydride for their recommendations.
FIRST AID
Inhalation. Move the victim to fresh air and keep him warm and relaxed. If breathing becomes difficult or has stopped, administer artificial respiration. (Caution: Administering mouth-to-mouth resuscitation may expose the first-aid provider to chemical in the victim`s lungs or vomit.) Immediately summon medical attention.
Skin contact. Flush the affected body areas with large amounts of water. Remove all contaminated clothing. Obvious burn conditions require medical attention.
Ingestion. If the victim is conscious, immediately administer large quantities of water. Some references suggest that the victim be made to drink at least two glasses of milk. Other references agree that no attempt should be made to have the victim vomit. Immediately summon medical attention. n
SYNONYMS
acetic acid
anhydride
acetic oxide
acetyl anhydride
acetic ether
acetyl ether
acetyl oxide
ethanoic acid anhydride
ethanoic anhydrate
hydroxyl biacetyl
IDENTIFICATION NUMBERS AND RATING
CAS
(Chemical Abstracts Service)
108-24-7
STCC
(Standard Transportation Commodity Code)
4931304
RTECS
(Registry of Toxic Effects of Chemical Substances)
AK1925000
UN/NA
(United Nations/North America)
1715
CHRIS
(Chemical Hazard Response Information System)
ACA
DOT
(U.S. Department of Transportation)
corrosive
NFPA 704 Rating
(National Fire Protection Association)
2-2-1- W
IMO
(International Maritime Organization)
8, corrosive
FRANK L. FIRE is the vice president of marketing for Americhem Inc. in Cuyahoga Falls, Ohio. He is 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.
