CHEMICAL DATA NOTEBOOK SERIES #87: PHOSPHORIC ACID
HAZARDOUS MATERIALS
Phosphoric acid is a corrosive, toxic, irritating, nonflammable, colorless, odorless liquid or crystalline solid. In a recent year, it ranked seventh in production volume among chemicals produced in the United States. It is used in the manufacture of animal feed, ceramic binders, detergents, fertilizers, gasoline additives, gelatins, inorganic phosphates, metal coatings, pharmaceuticals, polishes, rust inhibitors, soaps, soil stabilizers, yeasts, and waxes. It also is used for such diverse applications as preparation of activated carbon, catalyst production, dye manufacturing, food and carbonated beverage preparation, metal pickling, sugar refining, and water treatment.
PROPERTIES
Phosphoric acid in its 100 percent pure state is a solid material in a crystalline form. The solid material melts at 108°F and has a specific gravity of 1.83. In an 85-percent solution in water, it is a thick, syrupy, colorless solution; at concentrations below 75 percent, the solution is clear and mobile. But since phosphoric acid usually is transported, stored, and used at concentrations above 75 percent, most of the discussion of the material will involve that state of the product.
Since phosphoric acid is nonflammable, it has no flash point, ignition temperature, or flammable range. However, the corrosive action phosphoric acid has on many metals evolves the highly explosive gas hydrogen. The specific gravity of phosphoric acid as a liquid is 1.89, and its molecular weight is 98. Its freezing point depends on the concentration of the acid, and it is soluble in water in all proportions. Adding the pure acid in solid form to water will cause a mild reaction, generating heat as it dissolves. The chemical formula for phosphoric acid is H3PO4.
HAZARDS
As a strong acid, phosphoric acid’s major hazard is its corrosivity. Since more of the solid product can be loaded into a given container if it is dissolved in water, phosphoric acid normally is shipped in solution form. Of course, the more highly concentrated the solution, the more economical it is to ship the material, so most shipments are at an 85-percent concentration, although concentrations of 80, 75, 72, and from 52 to 54 percent may be found, depending on the end use for which the phosphoric acid is destined. Phosphoric acid is very corrosive at all of these levels, the most dangerous being the 85percent concentration.
A strong warning is necessary here. Since pure (anhydrous) phosphoric acid contains no w’ater, you might draw the conclusion that, when released from its transportation or storage container, the dry material is nonhazardous. ‘Ibis is not the case. To be kept in its anhydrous state, phosphoric acid must be isolated from the normal atmosphere, which usually contains a small amount of moisture. This small amount of moisture will react with the anhydrous phosphoric acid, producing a very concentrated solution of the acid on the surface of the crystals, and those crystals will continue to dissolve in whatever moisture is available. Phosphoric acid is extremely soluble in water and will readily dissolve in any water that is available. Though not very likely, it is possible that the reaction may generate enough heat to ignite nearby combustibles that have very low ignition temperatures.
Even though phosphoric acid is used as a flavor enhancer in certain soft drinks, do not get the impression that it can be ingested safely. The amount of phosphoric acid present in a soft drink is very low (that is, it is present in a low concentration). In that form, it is harmless when ingested. If ingested in higher concentrations, severe burns to the mouth, tongue, throat, esophagus, and stomach will occur. If the acid is high enough in concentration, death will occur following its ingestion.
If a mist of phosphoric acid is present, eye, nose, and throat irritation will occur. The TLV-TWA (threshold limit value-time weighted average) for phosphoric acid is one mg/m’. If concentrated phosphoric acids (any commercial concentration and many that are weaker) come in contact with the eyes, permanent damage and blindness will occur.
Phosphoric acid is considered a stable chemical, but it will react violently with such classes of chemicals as acetylides, bases, carbides, carbonates, cyanides, fluorides, metals, phosphides, silicides, strong oxidizing agents, strong reducing agents, and sulfides.
NONFIRE RELEASE
A phosphoric acid release of any appreciable amount should trigger the community’s emergency response plan as mandated by SARA Title III. All community resources must be mobilized, including local environmental authorities, who will be able to determine how far any contamination has spread, how to dispose of contaminated product and/or contaminated soil, and whether ground water or other water sources have been contaminated. These authorities also will be able to offer advice on which mitigation techniques will cause the least amount of damage to the environment.
A phosphoric acid solution release must be treated as a release of any other corrosive liquid. The first active intervention effort should be to try to contain the spread of the liquid. If firefighters are to actively intervene in a hazardous-materials incident, they must be trained, educated, and equipped to handle the class of material released. An incident commander who is familiar (or who quickly can become familiar) with the material released must be present. All the resources necessary for the safe mitigation of the problem also must be present.
The major hazards presented by a release of phosphoric acid are the corrosive action the acid has on human tissue and metals and the contamination of the environment. Any contact with a commercial grade of phosphoric acid will cause severe burns to human tissue and the possibility of death. Other living tissue, such as that of animals and plants, also may be destroyed by the acid. In addition, phosphoric acid will weaken ferrous (iron-containing) metals.
The possible generation of explosive hydrogen gas is another hazard presented by a phosphoric acid release. This hazard will be greatest right next to the point of contact with ferrous metals or in enclosed spaces. Hydrogen is very light (its vapor density is 0.07) and will rise immediately after its evolution from the chemical reaction. Because of the possible generation of hydrogen, use only sparkproof tools when working at or around a phosphoric acid release.
Any condition that may create a mist of the acid will pose a significant hazard. A strong wind or heat that will cause the phosphoric acid to boil will throw phosphoric acid mist into the air, causing problems for humans, plants, and structures that are downwind.
Containment methods for hazardous liquids include building a containment dike by pushing up soil around the spill or digging a containment pit and allowing the liquid to flow into it. Once the liquid has been contained, a professional salvage team or the product’s shipper, manufacturer, or buyer can begin salvage operations. The I phosphoric acid can be suctioned into secure containers, and the remaining | material may be neutralized by the | addition of a suitable neutralizing i agent. Consult the manufacturer of the phosphoric acid for its recommendations, but some MSDSs (material safety data sheets) call for the use of lime (calcium oxide). Calcium oxide is itself a hazardous material (see Fire Engineering, August 1993) and should be handled with care and only in conjunction with the proper personal protective equipment.
If no appropriate neutralizing agent is available, the acid may be diluted by the addition of water. Of course, this will add to the volume of contaminated material to be removed, but the diluted acid may be so dilute that it no longer is dangerous and it, too, may be suctioned into secure containers. It may be disposed of in a manner different from that used for other contaminated materials, if the environmental experts so decree.
If dilution is not possible because of the volume created by the addition of water, the remaining phosphoric acid can be absorbed by adding cement powder, clay, soil, or a commercial sorbent recommended by the manufacturer.
Once the acid has been salvaged and the remaining material neutralized, it is up to the environmental authorities to determine how much soil has been contaminated and must be removed. The contaminated soil, as well as any absorbent used, will contain phosphoric acid and thus must be disposed of in accordance with all federal, state, and local regulations.
Prevent the acid from entering waterways and sewer systems by using dams at all manholes, catch basins, and waterway entrances. If phosphoric acid does enter a sewer, immediately notify all sewage treatment facilities on the system. Likewise, if the acid enters a river or stream, immediately notify all downstream users and/or processors of the water.
If the acid enters a moving stream or river, it will dissolve very rapidly in the water, quickly becoming diluted. If the volume of w ater is large or the water itself is moving rapidly, the acid may be rapidly diluted to the point at which it ceases to be a hazard. The area at the point of entry will be dangerous to waterfowl and aquatic life. Once again, environmental representatives will be able to determine when the water is safe again.
A phosphoric acid release into a lake, pond, or other stationary body of water also will present danger to waterfowl and aquatic life near the point of entry. Since the water is not moving, the hazard will be greater and will last longer. If the volume of water in the lake or pond is large enough or the amount of acid entering the water is small enough, a slow dilution of the acid will occur, and the danger eventually will pass.
If a large volume of phosphoric acid enters a moving body of water, some relatively difficult mitigation techniques can be attempted to prevent the contamination from moving downstream. Dams may be quickly constructed to hold back the contaminated water until it can be treated, and diversion dams can be built to direct the contaminated water into low-lying areas, where the water can be treated. Once contained, the water can be removed or treated and released back into the stream. This second technique might be the easier one, but it threatens contamination over a wider area. In either case, the techniques must be applied very soon after the material enters the water.
Once water has been removed from a containment area or treated and released back into the stream, the area that held it must be decontaminated, which might be difficult and costly. In all cases, environmental authorities can offer advice as to which techniques are the least threatening to the environment and regarding the purity of the water, once treated. Treatment can consist of neutralization or the addition of chemicals to remove phosphates caused by chemical reactions.
FIRE SCENARIO
Phosphoric acid will not burn, so in the event of a fire, the concern is something other than flammability or combustibility. Since phosphoric acid usually is stored as a liquid, all con* tainers holding the acid must be protected from the radiated heat of the fire or the heat of flame impingement. Cool exposed containers with master streams of water applied by unmanned appliances placed as far away as possible. The possibility always exists that the acid will become heated and produce vapors that can raise the pressure inside the container to a point above the container’s design strength, causing catastrophic decomposition of the container accompanied by deadly shrapnel and equally deadly hot acid. Firefighters must never allow themselves to be caught between the fire and containers of any liquid or gas.
Fire approaching pools of released phosphoric acid could cause the acid to boil, releasing hot acid vapors that are damaging to the eyes, the skin, and the lungs, Firefighters approaching the released material always must wear the proper protective clothing.
If phosphoric acid comes in contact with ferrous metals, hydrogen will be released from the acid, contributing fuel to the fire. The higher the temperature of the phosphoric acid, the more reactive the acid will be with metals or any other material it contacts.
PROTECTIVE CLOTHING AND EQUIPMENT
Protective clothing and equipment should provide maximum respiratory, eye, and skin protection against phosphoric acid. Regular turnout gear may provide temporary protection against dilute phosphoric acid, but wear acidresistant clothing when encountering phosphoric acid. Emergency responders who might be contacted by the acid should wear total encapsulating suits constructed of butyl rubber, natural rubber, neoprene, nitrile-butadiene rubber, nitrile rubber, polyethylene, or polyvinyl chloride (PVC). One reference makes no recommendation regarding butyl rubber but adds SaranexTM to the above list. Contact the manufacturers of phosphoric acid and the total encapsulating suits to determine the best protective material. Emergency responders always should use self-contained breathing apparatus in phosphoric acid releases.
IDENTIFICATION NUMBERS AND RATINGS
CAS
(Chemical Abstract Services)
7664 38-2
STCC
(Standard Transportation Commodity Code)
4930248
RTECS
(Registry of Toxic Effects of Chemical Substances) TB6300000
CHRIS
(Chemical Hazard Response Information System) PAC
DOT
(U.S. Department of Transportation)
corrosive
NFPA 704 Rating
(National Fire Protection Association)
1805
LMO
(International Maritime Organization)
8, corrosive
SYNONYMS
o-phosphoric acid orthophosphoric acid phos acid
white phosphoric acid
Protection is relative. It depends on such factors as the thickness of the suit material: integrity of the seams, zippers, and other closures; concentration of the acid; elapsed time of contact; and the total encapsulating suit’s material and overall condition.
FIRST AID
Inhalation. Move the victim to fresh air and keep him/her calm and warm. If the victim’s breathing has stopped or becomes labored, administer artificial respiration, being aware that such action might expose the first-aid giver to the material in the victim’s lungs and/or vomit. Seek immediate medical attention.
Eye contact. Immediately flush the eyes for at least 20 minutes, lifting the eyelids occasionally. Be careful not to wash the product into the clean eye. Seek immediate medical attention.
Skin contact. Wash the affected body areas with large amounts of water. Medical attention is necessary for all phosphoric acid burns.
Ingestion. If the victim is conscious, he or she immediately should drink large quantities of milk, milk of magnesia, or water. There is disagreement as to whether to induce vomiting. Vomiting usually is not induced when a corrosive has been ingested. Never try to force an unconscious person to drink anything or vomit. Immediate medical attention is mandatory; follow the medical authority’s directions as to whether to induce vomiting.
