CHEMICAL DATA NOTEBOOK SERIES #98: HYDROFLUOROSILICIC ACID
HAZARDOUS MATERIALS
Hydrofluorosilicic acid is a corrosive, reactive, irritating, colorless to light yellow fuming liquid with a sour, acrid odor. It is used as a cement and ceramic hardener, a fluoridation agent for drinking water, a wood preservative, and a raw material for making paints. It is used also in the manufacture of aluminum fluoride, cryolite, hydrogen fluoride and for sterilizing bottling and brewing equipment.
PROPERTIES
Hydrofluorosilicic acid is nonflammable and has the following properties: specific gravity 1.3, molecular weight 144, vapor density 4.97, boiling point 212°F (at which temperature it decomposes), and freezing point -4 to -24°F. It is totally soluble in water. Its chemical formula is HiSiFf,.
HAZARDS
Hydrofluorosilicic acid is very corrosive. An inorganic acid, it is classified as a “strong” acid, as are all inorganic acids. (An acid’s strength is defined as the percentage of ionization the acid undergoes when dissolved in water: it has nothing to do with the concentration (the amount of acid dissolved in water) of the acid.] Inorganic acids are highly hazardous even at low concentrations; their degree of ionization is nearly 100 percent. Other inorganic acids include sulfuric acid, hydrochloric acid, hydrofluoric acid, chromic acid, nitric acid, and phosphoric acid (among numerous others).
Organic acids, on the other hand, are classified as weak acids since their degree of ionization in water is extremely low. often considerably less than one percent. Examples of organic acids include acetic acid (present in vinegar), formic acid (the painful substance injected by most stinging insects), lactic acid (present in sour milk), and acrylic acid, a monomer for many plastics. Again, these four organic acids are taken from a list of thousands.
The strength of an acid commonly is erroneously associated with its corrosiveness. Generally speaking, while it is true that strong acids are more corrosive than weak acids, concentrated weak acids can be very corrosive to human tissue and other materials. Do not try to classify the danger an acid poses to human tissue or other materials on the basis of its “strong” or “weak” designation. To be sure of the danger posed, you must know exactly what the acid is and its level of concentration.
If hydrofluorosilicic acid’s concentration is high enough, it will severely damage any human tissue it contacts. Pure hydrofluorosilicic acid and most concentrations of this acid in water are extremely corrosive: Even relatively low concentrations can severely damage human tissue. The damage can range from simple first-degree bums to very deep, tissue-destroying third-degree burns. Chemical burns are always many times more severe than thermal burns, so severe harm (to the point of causing death) can occur with relatively small areas of tissue damage. Eyes and skin will be severely damaged by contact with concentrated hydrofluorosilicic acid fumes or vapors; the hazards are more far-reaching than those posed by contact with the liquid. If very low concentrations of the acid contact the skin, the damage may be as light and simple as reversible irritation of the tissue involved.
Inhaling the vapors or fumes will severely damage the mouth, nose, and the entire respiratory system. Such damage could cause pulmonary edema, which could lead to death. At the very least, respiratory tissue could be permanently damaged, resulting in lifelong breathing impairment.
Ingesting hydrofluorosilicic acid will damage the lips, mouth, esophagus, and stomach. The degree of damage would depend on the concentration of the acid.
Hydrofluorosilicic acid is also corrosive to plant tissue, and most contacts of the acid with plant tissue usually will result in the death of that tissue and the plant to which it belongs. Because of this, hydrofluorosilicic acid is very’ dangerous to the environment, and its free movement in that environment must be controlled.
Most metals will be rapidly destroyed by the chemical corrosion caused by contact with hydrofluorosilicic acid. The metal will fail, which will be very dangerous if it is in any engineering component or used in the container’s construction. The rate of corrosion would depend on the type of metal, the duration of exposure, and the concentration of the acid; the rate of failure of the material would depend on the concentration of the acid and the thickness of the metal at the point of contact.
An additional hazard posed by hydrofluorosilicic acid and other acids is that the chemical action of the acid on metal usually releases hydrogen, a very explosive gas (see Fire Engineering, November 1993). The amount of hydrogen released and the rate of release would depend on the type of metal, the concentration of the acid, and the amount of metal exposed to the acid. In some cases, the energy generated by the chemical reaction of the acid and the metal could be sufficient to ignite the gas. The size and severity of the resulting explosion, naturally, would depend on the amount of hydrogen evolved and its concentration in the air surrounding the ignition source. Although hydrofluorosilicic acid is nonflammable, the action that produces hydrogen gas makes it very hazardous when near or involved in a fire.
An additional hazard of hydrofluorosilicic acid (and other acids containing fluorine) is the generation of hydrogen fluoride (see Fire Engineering, May 1988). This very corrosive halogen acid gas usually evolves during a fire involving materials containing fluorine in their chemical makeup, and not during normal corrosive reactions. However, hydrogen fluoride is extremely corrosive, and all emergency responders should take the steps necessary to protect themselves from it.
NONFIRE RELEASE
Hydrofluorosilicic acid is such a hazardous material that the release of any appreciable amount should activate the community’s Emergency Response Plan so that all the experts needed to deal with such a release will be mobilized. Medical and environmental experts who can properly advise the incident commander concerning the peripheral hazards that could arise during the running of the incident must supplement the normal response by a hazardous materials response team. Such mobilization also will facilitate evacuation of nonemergency personnel from the danger zone, including a downwind area.
Since hydrofluorosilicic acid is a liquid, consider all techniques usually used to contain liquids. Containment of the material released from its container should be accomplished by digging a containment pit or
. pushing dirt up around the release to form a containment pond. However, hydrofluorosilicic acid may react with some of the materials in the ground, particularly stones and rocks. Some of these materials will be attacked by the acid, but this is not dangerous in the sense that dangerous gases will be released. Usually, limestone will be part of the material pushed up by an earthmovertype machine; and the acid may attack those stones, liberating carbon dioxide. This also will reduce the amount of acid left in the unreacted condition.
If vapors or fumes are being released, you can use water fog or high-pressure spray to dissolve those vapors and fumes out of the air. Contain any runoff water.
Once the acid is contained, only properly protected, trained, and equipped salvage employees should carry out salvage operations. They will use equipment that will not be attacked by the acid and the proper procedures for placing the material in secure containers. Any residual liquid may be absorbed by using sand, earth, clay, or commercial sorbents. There may be some reaction of the acid with sand. Once the material has been safely removed, the environmental experts can determine how much soil has been contaminated and must be removed.
If salvage is not possible, the contained hydrofluorosilicic acid may be diluted by carefully adding water; do this only if the additional volume can be safely contained; this technique, of course, will produce a much larger volume of material that must be removed.
The acid also can be neutralized. If done properly, adding a neutralizing agent will increase the pH to 7.0 (neutral condition). Normal neutralizing agents for inorganic acids include sodium bicarbonate (baking soda), sodium carbonate (baking soda or soda ash), and calcium carbonate (ground limestone or oyster shell). One reference suggests using a dilute water solution of soda ash. Again, the containment volume of the pond must be sufficient to hold the increased volume of liquid. Any time dilution is used, a sample of the acid must be removed to a safe place and the intended neutralizing agent added carefully to the sample to determine the extent of any unexpected reaction. Only the environmental authorities can advise if dilution and neutralization can be carried out to the extent that the remaining liquid may be allowed into waterways or if it must be removed for further treatment.
Prevent released, untreated hydrofluorosilicic acid from entering waterways. Entry of the acid into the water will be hazardous to aquatic life nearby, but will quickly be diluted, especially if the water is moving rapidly or if the volume of water in the waterway is large. Environmental experts must monitor any waterway hydrofluorosilicic acid has entered, and they alone must determine the dangers (or safety) of such contaminated water. Stagnant contaminated water may be treated with neutralizing agents, but rapidly moving water will not lend itself to such treatment.
Diversion of contaminated water into a holding area for treatment is recommended, if possible. In any event, notify all downstream users of the water of its contamination at once, and establish monitoring stations near those users.
Prevent hydrofluorosilicic acid from entering sewers. Any inorganic material within the pipes, and the pipe itself, may be corroded. Notify all sewage treatment facilities at once.
FIRE SCENARIO
Since hydrofluorosilicic acid is a liquid, the radiated heat of an approaching fire or the energy of impinging flames will cause an internal pressure rise within its container. Such an increase could cause the container to fail catastrophically, spewing hot, concentrated acid over great distances. This could be fatal to anyone caught in the fallout of liquid and vapors. Cool all containers with water applied by unmanned monitors from as far away as possible.
Although hydrofluorosilicic acid is not flammable, its decomposition with heat during reactions with other materials could evolve hydrogen fluoride under some special conditions.
One of the biggest hazards might be burning hydrogen released when the acid contacts metal. This may produce a hotter fire than expected, possibly making more hazardous the approach to certain areas from which to fight the fire. Also, the possibility of the evolution of hydrogen fluoride makes the wearing of proper respiratory protection mandatory.
SYNONYMS
dihydrogen hexafluorosilicate
fluorosilicic acid
fluosilicic acid
HFS
hexafluorosilicic acid
hexafluosilicic acid
hydrofluosilicic acid
hydrogen hexafluorosilicate
hydrosilicofluoric acid
sand acid
silicofluoric acid
silicon hexafluoride dihydride
IDENTIFICATION NUMBERS AND RATING
CAS
(Chemical Abstract Services)
16961-83-4
STCC
(Standard Transportation Commodity Code)
493(X)26
RTECS
(Registry of Toxic Effects of Chemical Substances
VV8225(XX)
UN/NA
(United Nations/North American)
1778
CHRIS
(Chemical Hazard Response Information System)
FSL
DOT
(U.S. Department of Transportation)
Corrosive, Class 8
NFPA 704 Rating
(National Fire Protection Association)
4-0-0
IMO
(International Maritime Organization)
8, corrosive
RCRA
(Resource Conservation and Recovery Act)
No designation
Any material remaining in containers after a fire could be more dangerous than the original material. All containers (secured and breached) should be handled by the professional salvage teams mentioned earlier. They will determine which material can be saved and what to do with the rest. Environmental experts will determine which soil must be removed or treated after the acid had been released during the fire.
PROTECTIVE CLOTHING AND EQUIPMENT
Full protective clothing capable of protecting the emergency responder from all contact with hydrofluorosilicic acid is absolutely necessary. No materials are specifically recommended for contact with hydrofluorosilicic acid. One reference simply states that “acid-resistant slicker suits” are adequate. Many materials are capable of giving adequate protection; consult manufacturers of total encapsulating suits and hydrofluorosilicic acid for their recommendations. Of course, positive-pressure, self-contained breathing apparatus must be used in conjunction with the protective clothing.
FIRST AID
Inhalation of fumes. Remove the victim to fresh air and keep him warm and quiet. Administer mouth-to-mouth resuscitation if the victim has difficulty breathing or has stopped breathing, being aware that such action may expose the first-aid giver to the chemical in the victim’s vomit or lungs. Summon medical attention immediately.
Eye contact. Flush the eyes immediately for at least 20 minutes, lifting the eyelids occasionally. Immediate medical attention is required.
Skin contact. Remove contaminated clothing; wash the affected areas of the body with large amounts of water. Seek immediate medical attention. One reference suggests treating hydrofluorosilicic acid burns in the same manner as hydrofluoric acid bums; that is, placing iced benzalkonium soaks on the burned areas.
Ingestion. If the victim is conscious, have him drink large quantities of water immediately; but do not induce vomiting.
Never try to make an unconscious person drink anything. Seek immediate medical attention.
