Background and Overview of Hazards

Perchloric acid is one of the strongest acids known. At room temperature, aqueous solutions up to 72% do not have significant oxidizing power, and the corrosive properties are very similar to other mineral acids. However, the oxidizing power increases with concentration and temperature. Concentrated perchloric acid (72%) heated above 150⁰C is a strong oxidizer, and reacts violently with organic material, which has resulted in devastating explosions in the past. 

The monohydrate of perchloric acid (85%) is a good oxidizer at room temperature.

Anhydrous perchloric acid is highly unstable, explodes upon contact with organic material, and explodes spontaneously at room temperature after storage for a few days. Its preparation should be avoided. 

Perchloric acid forms an azeotrope with water at a concentration of 72.5% perchloric acid. Therefore, aqueous solutions do not form anhydrous perchloric acid by evaporation. However, dangerous anhydrous perchloric acid can form when an aqueous solution is subjected to strong dehydrating conditions such as exposure to concentrated sulfuric acid, acetic anhydride, or phosphorous pentoxide.

At elevated temperatures, vapors from perchloric acid can condense on surfaces in the ductwork of the hood, where they form perchlorate salts that are often highly shock-sensitive and that pose a serious explosion hazard.

Perchloric acid reacts with alcohols and certain other organic compounds to form highly unstable and explosive perchlorate esters.

Safe Handling

• Wear appropriate Personal Protective Equipment (lab coat, safety glasses, and acid-resistant gloves) when handling perchloric acid.
• Do not handle perchloric acid on a wooden surface, and do not let it come into contact with oxidizable materials such as cloths, paper towels, or grease. Such materials can become highly flammable and may ignite spontaneously or even explode after absorbing perchloric acid liquid or vapor.
• Do not subject perchloric acid to strong dehydrating conditions.
• Dilute by adding perchloric acid to water, not by adding water to acid.
• If solutions containing perchloric acid are filtered through a paper filter, the filter (and precipitate) should be washed thoroughly with water to remove all perchlorate before being allowed to dry. 
• Do NOT mix concentrated perchloric acid (>72%) with organic chemicals if temperatures could rise above ambient levels. 
• Perchloric acid digestions and other uses at elevated temperatures require that the procedures be conducted in a specially designed fume hood with a water wash-down system. This system is required to prevent the buildup of explosive perchlorates in the ductwork.
• Do not heat perchloric acid in an oil bath. Use a sand bath, a heating mantle, or a hot plate.

If you know of or suspect that perchloric acid digestions have been performed in a fume hood not specifically designed for perchloric acid, inform Facilities and Services about the location of the hood.

Emergencies Procedures

Accidental Exposure

Skin Contact

Rinse off affected skin immediately with copious amounts of water for about 15 min; if necessary, use the safety shower. Remove contaminated clothing.

Eye Contact

Use the eye wash to rinse the eye thoroughly for at least 15 min, occasionally lifting upper and lower eyelids and rolling the eyeballs. 

Inhalation

Move into fresh air immediately.

Ingestion

Do not induce vomiting. Rinse mouth with water. 

If any symptoms persist after following first aid procedures, seek medical attention immediately. Provide the medical team with the Safety Data Sheet (SDS) for perchloric acid.

Spills

Spills of perchloric acid must be cleaned up thoroughly; dried acid residues can cause unexpected explosions in the future. The spill should be neutralized immediately with sodium bicarbonate or other inorganic acid neutralizer. Sweep up the neutralized spill with a non-flammable material and then clean the spill area thoroughly with water. 

Do NOT use rags, paper towels, or sawdust to soak up perchloric acid spills. Such materials may spontaneously ignite once dried out. Likewise, spills on wood may present a fire hazard after the liquid dries. 

In the event of a fire, the best extinguishing agent is water. 

Storage

Store perchloric acid with other inorganic acids and away from organic chemicals and reducers, especially alcohols, glycerol, and hypophosphites. The containers should be stored in secondary containment preferably made from glass, porcelain, ceramic, or other non-absorbing, non-combustible material. 

Limit stored quantities to what is needed for the next 6-12 months; do not store perchloric acid over extended periods of time. 

Dispose of bottles with discolored perchloric acid solutions immediately. 

Inform DRS if crystals have formed around the neck and cap of the bottle.

Disposal 

Collect all perchloric acid and contaminated waste and dispose of it through DRS. The UI codes are 587 for perchloric acid and 50043 for debris contaminated with perchloric acid.

References

Schilt, A. A. Perchloric acid and perchlorates, G. F. Smith Chemical Company: Columbus, OH, 1979.

Muse, L. A. Safe Handling of Perchloric acid in the Laboratory, J. Chem Educ. 1972, 49, A463-A464.

Furr, A. K., CRC Handbook of Laboratory Safety, 4th ed.; CRC Press LLC: Boca Raton, Florida, 1995.

NRC (National Research Council). Prudent Practices in the Laboratory. Handling and Management of Chemical Hazards. National Academy Press: Washington, DC, 2011.

Pitt, M. J. In Bretherick’s Handbook of Reactive Chemical Hazards, 6th ed.; Urben, P. G. Ed.; Butterworth-Heinemann Ltd: Oxford, 1999; Vol. 1, pp 1352-1364.