Nitric Acid
HAZARDOUS MATERIALS
Chemical Data Notebook Series:
Pure nitric acid (non-fuming) is a colorless liquid that is a very strong, corrosive acid and a powerful oxidizer. In fact, its oxidizing power is what has lead the Department of Transportation (DOT) to regulate nitric acid as an oxidizer. This classification may cause first responders to mis-identify the material unless both the oxidizer and the corrosive placards are displayed. They may spot the oxidizer placard and be unaware of the product’s corrosive hazard.
Throughout this article, reference may be made to nitric acid, whose chemical formula is HNO3, or fuming nitric acid (which has the same chemical formula). The fuming acid may be white fuming nitric acid (WFNA) or red fuming nitric acid (RFNA). These fuming types are really “super” concentrated forms of nitric acid.
Although nitric acid is used in varying concentrations (from fairly dilute to highly concentrated) depending on the process employed by the user of the acid, it is usually shipped as one of the fuming types. This “super”concentration of the acid is produced to save money in shipping. The principle of shipping an acid in this manner is similar to shipping gases in a liquified form. Just as you can increase the amount of gas shipped in a container by liquifying it (packing more material into the same space), you can increase the amount of concentrated nitric acid in a shipping container by dissolving more of the acid-producing oxides of nitrogen in the already concentrated acid. This “packing in” of more acid-producing gases produces the fuming effect, and allows more non-fuming concentrated nitric acid to be produced at its destination by simply adding water (very carefully, of course).
The more gas dissolved in the acid, the more it fumes. The fuming liquid itself may be pale yellow to reddish-brown in color (as opposed to the colorless condition of “ordinary” concentrated nitric acid). When the acid vapors escape from the liquid and mix with the air, a white fume is created. When the nitrogen oxides themselves are liberated from the acid, the fumes are reddish-brown in color. Hence the designations white fuming nitric acid and red fuming nitric acid.
IDENTIFICATION
The UN/NA designation for nitric acid is 2032, and, as mentioned, it is placarded as an oxidizer. The corrosive placard may also be displayed, and, if it is, will be a great help for first responders. The Standard Transportation Commodity Code (STCC) identification number is 4918529, and the International Maritime Organization (IMO) designation is Corrosive, 8. The National Fire Protection Association (NFPA) 704 designation is 3-O-O-OXY.
Synonyms for nitric acid, white fuming nitric acid, and red fuming nitric acid include azotic acid, hydrogen nitrate (which is what one might call it by looking at the chemical formula), nitryl hydroxide, Nital, nitrous fumes, WFNA, RFNA, engraver’s acid, and aqua fortis (Latin for “strong water,” an understatement if there ever was one).
PROPERTIES
Nitric acid is soluble in water in all proportions, but may break down in water, liberating heat and the toxic nitrogen oxides.
Nitric acid has a specific gravity of about 1.5, depending upon its concentration. It boils near 197°F, again depending on its concentration, and it freezes near-54°F. The approximate boiling and freezing points depend on the amount of acid-producing gas in solution.
The molecular weight of nitric acid is 63 AMU (atomic mass unit).
SHIPPING CONCERNS/USES
Nitric acid will be found in any industrial setting where fertilizers, dyes, plastics, varnishes, and explosives may be manufactured. It is also used in the production of chemicals varying from pesticides to rocket fuels. Nitric acid is used in many manufacturing operations that work with metals needing to be cleaned, etched, electroplated, or photoengraved.
Nitric acid will be used wherever a strong oxidizer is required in a manufacturing process, and/or in any process that may require nitration. In a recent year, nitric acid was the tenth largest volume material shipped in the United States (with sulfuric acid being number one). Nitric acid is shipped in carboys, tank trucks, railcars, and barges. If the container in which nitric acid is shipped or stored is not glass, as it is in many carboys, it must be aluminum or stainless steel, since nitric acid will react with other metals.
This reaction with metals is hazardous in ways other than destroying the container in which it is stored or shipped. Nitric acid, in its chemical reaction (which is scientific language for “eating through”) with other metals, will release hydrogen gas (which is flammable and explosive) and the nitrogen oxides (NOx), some of which are oxidizing agents in themselves, and most of which are toxic. Therefore, the contact of nitric acid with many metals could result in an explosion, a fire, and/ or the release of toxic fumes.
HAZARDS
Reactivity
Nitric acid will not burn, but like other powerful oxidizing agents, it will make all ordinarily combustible materials nearly explosive. There is a tremendous amount of oxygen (77% by weight) in the nitrate ion that is present in the aqueous solution of the acid, and this oxygen will be released at the slightest urging.
The release of hydrogen during a reaction with a metal in itself is sufficient to cause an explosion. This generation of hydrogen gas is accelerated tremendously if the metal is powdered. Nitric acid will also react with other oxidizers like inorganic chlorates (and perchlorates), explosive compounds such as inorganic picrates, toxic compounds like inorganic cyanides, and water-reactive compounds like some of the carbides.
Personal
Nitric acid will attack paper, cloth, wood, and some rubber and plastic compounds. Needless to say, it is very harmful to living tissue, and will cause tremendous damage to human skin and other organs, destroying such tissue and organs when the concentration of the acid is high or if the acid is not immediately washed from the contacted area. A splash contacting a sizable portion of the human body will be fatal.
The vapors of nitric acid include the toxic nitrogen oxides (see the glossary in “Ammonium Nitrate,” FIRE ENGINEERING, September 1986). Most of these oxides, collectively designated as NO* (not a real chemical formula, but an accepted notation for the various binary gases known as the oxides of nitrogen), are very corrosive and are oxidizers. The release of these gases from the acid is the reason for an assigned TLV-TWA (threshold limit value-time weighted average) of 2 ppm (parts per million) or 5 mg/m3 (milligrams per cubic meter). The STEL (short-term exposure limit) is 4 ppm or 10 mg/ m3.
Nitrogen oxide poisoning can produce symptoms that may be delayed for as long as 36 hours. When the poisoning is severe, pulmonary edema and chemical pneumonitis may result in death. Anyone exposed to nitrogen oxides without respiratory protection may have inhaled enough toxic gases to kill him, but may not display any symptoms until it is too late. The symptoms, when they do appear, may resemble a heart attack. It should be mandatory that anyone exposed to the nitrogen oxides seek medical treatment immediately.
Corrosiveness
Again, nitric acid is a very powerful corrosive, and contact with the skin will produce tremendous damage and possibly death. Knowing this, remember that the oxidizing power of nitric acid is even more dangerous. And if that weren’t enough, the addition of water to fuming nitric acid can produce corrosive vapors that can be deadly.
All of these hazards must be considered simultaneously in making decisions to try to handle the incident with the safety of people in mind. Couple this with the environmental and property damage possible with this acid, and the incident commander has a real nightmare on his hands. Pre-planning and practice with the proper safety equipment will be invaluable.
STORAGE
Remember that nitric acid should be stored only in approved containers made from materials impervious to the acid’s corrosiveness. It should never be stored near a fuel (a strict definition of a fuel is anything that will burn), nor should it ever be stored where an accidental release might allow the liquid to flow into areas where combustible materials are stored.
Nitric acid may be found in all concentrations in industry and laboratories (educational, research, industrial, etc.). Pre-planning is essential since the concentration of the acid will dictate the degree of protection required by emergency responders in attempting to handle an incident involving nitric acid.
The strength of nitric acid dictates that it will be very active chemically, and it will, therefore, be dangerous to some degree in any concentration. The implication here is that explosive hydrogen gas may be liberated in any reaction with metals, even if the acid involved is not highly concentrated.
PERSONAL PROTECTION
Once the hazard of being a powerful oxidizer is overcome, and reactions liberating explosive hydrogen and the toxic nitrogen oxides have been handled, the emergency responder who must now actually get near the acid must, under all circumstances, protect himself from contact with the liquid and vapors. Normal turnout gear will not afford protection from the acid itself nor from high concentrations of vapors coming from the acid.
All exposed skin and other organs must be protected. Impervious boots, gloves, goggles, and faceshields must be utilized, and positive pressure self-contained breathing apparatus (SCBA) is mandatory.
Care must be taken in the selection of material from which the responder’s total encapsulating suits are made to be sure that total protection is afforded. Manufacturers of these total encapsulating suits provide lists of those chemicals that will not attack their suits, but some experimentation on the part of the user should be carried out. All manufacturers will provide sample swatches of the material from which their suit is constructed, and these should be tested in contact with various chemicals, such as nitric acid, to verify the chemical resistance.
Remember, protection is a relative term, and protection depends on several different factors, including, but not limited to:
- The type of material used in the construction of the suit;
- The thickness of the material;
- The integrity of the seams and other covered openings;
- The concentration of the acid (red fuming nitric acid is the most concentrated, followed by white fuming nitric acid, and then pure nitric acid, which is extremely corrosive in itself);
- The time spent exposed (including time before decontamination);
- Whether the exposure was with liquid or vapor (or both);
- Whether the suit was exposed to some material previously that might have lowered its overall integrity (or just in one spot, which could be dangerous also).
Some of the materials from which suits are manufactured that are claimed to protect wearers from nitric acid include polyvinyl chloride (PVC), polyethylene (PE), chlorinated polyethylene (CPE), natural rubber, nitrile rubber, nitrile-butadiene rubber (NBR), Neoprene, Saranex, and Viton. However, you must read the charts provided by the manufacturer carefully, since protection might be good against concentrated or dilute nitric acid, but not recommended for white or red fuming nitric acid. Again, it will be very helpful and informative if samples of the material used in the manufacture of the suit are tested prior to the first use of the suit to test resistance.
Tools
Because of the corrosiveness and oxidizing power of nitric acid, the proper tools must be used in handling the incident. Stainless steel will resist the corrosiveness of nitric acid, but the production of any sparks could set off a fire or explosion if any combustible or flammable material is involved in the incident.
The substitution of non-sparking tools may result in severe corrosion of the tools, and the possible generation of small amounts of hydrogen gas.
Aluminum will not spark and will resist the attack of the acid, but the availability of aluminum tools may cause a problem.
The incident commander may have to “make do” with what he has in the hope of controlling the situation without making it worse.
HANDLING PROCEDURES
Spills or other accidental releases of nitric acid are very dangerous. In any of the following procedures presented for mitigation of such an incident, one must be constantly aware of the vapors being generated by the liquid acid. Evacuation downwind may not be necessary in every case, but it should always be an early action for consideration by every incident commander.
Conditions to consider when determining whether or not to evacuate include current and impending weather conditions, topography of the affected land area, population patterns, endangered systems (such as highways, communications, drinking water, etc.), the manpower, tools, and other resources available, the volume of the spill, and, of course, the concentration of the spilled acid.
If large amounts of vapor are being generated and those vapors are presenting a danger, hose lines may be laid and a high-pressure spray or fog may be used in a sweeping motion to dissolve the vapors out of the air. This, of course, will cause runoff problems, but they are easier to control and less hazardous than a deadly cloud of corrosive fumes.
In the event of a spill, and after a decision is made concerning whether to evacuate or not, containment of the acid should be the next priority.
Containment can be effected by diking with sand, soil, or any other absorbent material that will not react with the acid. Priority must be given to actions that will prevent the acid from entering sewers or waterways. One consequence of containment is that the acid will seep into the ground in the containment pond or pit and contaminate the soil, which will eventually have to be removed and disposed of according to federal, state, and local regulations. Much of the acid contained may also become contaminated, and perhaps useless for the purpose for which it was originally intended. It too will have to be disposed of properly.
In addition to a containment pond, a pit may be dug for containment, and a trench or ditch dug to lead the spilled product to the pit. An acid-resistant cover of some type may be spread over the pit to decrease the evolution of acid vapors. Needless to say, from the beginning of the incident to its successful conclusion, a large safety zone around the spill must be secured to prevent injuries to nonemergency personnel.
Once the acid is contained, decisions must now be made as to its safe disposition. These decisions will depend upon the resources at hand and the other above-mentioned conditions.
Recovery
If another secure container (carboy, tank truck, or tank car) is available, the material may be pumped from the containment pond or pit into the secure container. Care must be taken, however, to use only pumps that will have nitric acid-resistant hoses, fittings, and other internal surfaces, or the pump will be destroyed.
If the ruptured container is leaking in an advantageous spot, that is, there is one leak and it may be caught in or directed to another container (resistant to the acid), the acid may be pumped back into the leaking container, thus controlling the acid until a secure container large enough to contain the entire load may be located and delivered to the incident scene. In any event, as long as any of the acid is exposed to the atmosphere, it may continue to release toxic and corrosive vapors.
If a decision is made not to attempt to recover the spilled acid, there are other techniques that may be used. Again, the procedure selected will depend on the amount of product spilled, its concentration, weather conditions, and the resources available.
Neutralization
Neutralization is a technique that depends upon the amount of neutralizing agent available, its effectiveness, whether or not it will accelerate fuming, and whether it is hazardous itself. Not all acids react in the same way to application of the same neutralizing agent. The manner in which the agent is added (as a solid or in solution) is important, especially if the acid is water-reactive, as nitric acid is.
The neutralization reaction is a specialized one in the chemistry laboratory, and it may not be the same reaction at the incident. In the lab, an acid and a base (the chemical opposite of an acid) such as sodium hydroxide or potassium hydroxide in a water solution of precisely known concentrations, will be added together. The products of this reaction are a salt and the water in which it will be dissolved. The acidity of the acid and the alkalinity of the base will be completely gone (neutralized) and the pH of the resulting solution will be 7.0. If sodium hydroxide, NaOH, is used, the products of the reaction with nitric acid will be sodium nitrate, NaNC>3, and water, HOH or H2O. If potassium hydroxide, KOH, is used, the products will be potassium nitrate, KNO3, and water. Both potassium nitrate (saltpeter) and sodium nitrate (Chile saltpeter) are salts that are very soluble in water (and are oxidizing agents).
Although the technique of adding a base to an acid to neutralize the acid (or vice versa, to neutralize the base with an acid) will work, it is almost never used in a spill situation.
The most effective bases, sodium hydroxide (caustic soda or lye) and potassium hydroxide (caustic potash or lye) are very hazardous materials themselves, and are relatively expensive. They are very corrosive and water reactive.
Ammonium hydroxide,NH4OH, and calcium hydroxide, Ca(OH)2 (slaked lime), are also hazardous and nearly as expensive as sodium hydroxide and potassium hydroxide. In addition, all the bases are soluble in water (calcium hydroxide is only slightly soluble), and nitric acid is water reactive.
By process of elimination, the next best and lower cost neutralizing agents are sodium carbonate, Na2CC>3; sodium bicarbonate, NaHCC>3; and calcium carbonate, CaCC>3. Of these, the most effective (and most expensive) neutralizer of inorganic acids is sodium bicarbonate (baking soda), followed by sodium carbonate (soda ash), and the least effective (it still does a satisfactory job) and least expensive is calcium carbonate (ground limestone).
The neutralization takes place as the metal ion of the bicarbonate (or carbonate) combines with the negative ion (nitrate) of nitric acid, forming sodium nitrate or calcium nitrate, water, and releasing carbon dioxide (CO2). The CO2 will bubble out of the solution, giving visual evidence of the neutralization taking place. However, when the bubbling stops, it doesn’t mean that the pH of the solution has reached 7.0 (neutral). It may just mean that the neutralizing agent has all been consumed, and the solution may still be acidic (although less so than the original solution). These three agents may be added as a powder or in a water solution, depending upon circumstances and the amount of acid involved. If added as a solution, one must remember the water-reactivity of nitric acid.
One enterprising fire department used an absolutely brilliant method of applying a powdered neutralizing agent to a very large acid spill by using a snow blower to throw the agent a great distance onto the surface of the spill, thus allowing application of a great deal of neutralizer without requiring emergency personnel to get close to the hazard.
As always, before applying the neutralizer to the acid, run an experiment by putting a small amount of the acid in a resistant container and adding the neutralizer to this sample. This will allow you to see close up, and yet safely, just what the results are. This might even give you some hint of the amount of neutralizer that will be required to do the job.
Neutralization will remove the acidity, but it will leave nitrates in solution, which could be a problem downstream or in the sewers if the solution is allowed into a waterway or the sewer system. The solution after neutralization must still be contained. All metallic nitrates are oxidizing agents, and there will be health aspects to also consider.
Dilution
Another technique to handle a nitric acid spill is dilution, even though nitric acid is water reactive. This reactivity is not as violent as that of sulfuric acid, but should still be respected because of the generation of toxic and corrosive vapors.
Dilution is effective only if the volume of water used:
- Is sufficient to rapidly overcome the water-reactive reaction;
- Is sufficient to reduce the acidity to a safe region;
- Will not produce a total volume of solution impossible to contain.
If water is added, it should be in flooding volumes, and added in a manner to minimize splashing and fuming. Again, experimentation may be necessary to determine the amount of deadly vapors that may be formed and allow positioning of hose lines to combat those vapors with fog patterns.
Absorption
A third technique, absorption, may be effective depending upon the amount of nitric acid spilled, the availability of absorbent material, and the ability to remove the contaminated sorbent. This technique may be forced on the incident commander if rain is imminent (which might produce tremendous clouds of toxic vapors), no neutralization is possible, and the acid cannot be diluted or pumped into a secure container.
Glossary
Base—The chemical opposite of an inorganic acid, made up of a metal (or the ammonium) ion and the hydroxide ion (OH-1). Many of the metallic hydroxides are very corrosive and are referred to as caustics.
Binary—A compound made up of two elements.
Fuming—The generation of vapors, usually of the gases dissolved in the acid or water. Fuming may also occur as the result of a chemical reaction.
inorganic—Any chemical compound not having a hydrocarbon or hydrocarbon derivative as its origin. Usually referred to as being of mineral origin.
Ion—An atom or group of atoms bound together chemically that have gained or lost one or more electrons, and is electrically charged according to how many electrons were gained or lost.
Nitration—A reaction in which a nitro group (-NO3) replaces a hydrogen attached to a carbon atom in a hydrocarbon compound.
pH—Refers to the acidity or alkalinity of a substance. A pH of 1 to 6.9 is acidic; a pH of 7.1 to 14 isalkaline. A pH of 7 is neutral. Technically, the pH is the logarithm of the reciprocal of the concentration of hydrogen ions in solution.
STEL—Short-term exposure limit. The amount of material to which a person may be exposed for a short period, usually 15 minutes, without harm.
TLV—The threshold limit value. The amount of a substance to which an average person in average health may be exposed in a 40-hour work week without harm. The values may be averaged over time, and the TLV may be referred to as the TWA, or time weighted average.
Soil, sand, and commercial sorbents may be used, as long as care is taken to keep ignition sources from the mixture if the soil contains a large amount of organic material. Once absorbed, the mixture must be disposed of in a safe and legal manner.
Sorbents for nitric acid, in addition to any commercial products that may be available, include soil, sand, clay, cement powder, and fly ash.
Experimentation with other materials should be part of a pre-plan, whether your district contains a manufacturing operation that uses nitric acid or not. Knowing what can be used, how much is available, where it is stored, and how to get it to the incident scene will make the job of the incident commander a lot easier and safer.
Water contamination
If the nitric acid does enter a waterway, any water treatment plants and/or downstream users of the water must be informed of the contamination immediately. If containment or diversion dams cannot be built at once, it will be impossible to keep the contamination from spreading.
If nitric acid enters a waterway or sewer, solutions of neutralizing agents added to the water at an appropriate place may be helpful, but the proper environmental authorities must be contacted before any chemical is deliberately added to a waterway.
If nitric acid enters a sewer system, a potential explosion hazard will exist. Sewers invariably contain organic materials (fuel), and nitric acid is a powerful oxidizer. Two sides of the fire triangle will now exist, and the mixture will wait for an ignition source.
To date, the chemicals covered in this series, which began with our June 1986 issue, include: sulfuric acid, chlorine, vinyl chloride, ammonium nitrate, and nitric acid.
