Perchloric acid is a strong inorganic acid with formula HClO4. At high concentrations it is a dangerously strong oxidizer, but at low concentrations it has little oxidizing potential.



Perchloric acid is one of several strong acids, and is a potent oxidizer at high concentrations. It reacts with bases to form perchlorates.


Perchloric acid is an oily liquid at room temperature. Like many other concentrated acids, samples of perchloric acid that are highly concentrated are also hygroscopic, absorbing moisture from the air. Perchloric acid forms an azeotrope with water at approximately 72.5% concentration that boils at 203°C. It's density is 1.67 g/cm3.


Perchloric acid is available to concentrations up to ~70%, but its purchase requires paperwork and sometimes a business license, due to its strong explosive potential. Small amounts can be purchased from HMS Beagle.


Perchloric acid can be prepared by mixing a saturated solution of sodium perchlorate and hydrochloric acid. Sodium chloride will precipitate and perchloric acid will remain in solution. The acid can be concentrated up to its azeotrope by distillation.

Alternatively, a mixture of potassium perchlorate or calcium perchlorate, sulfuric acid, and water can be distilled. However, care must be taken to not let the concentration of perchloric acid in the distillate rise too high, especially since perchloric acid forms a negative azeotrope with water.




Perchloric acid is a strong acid, and when concentrated is a dangerous oxidizer capable of exploding violently in contact with organic compounds. Anhydrous perchloric acid is known to spontaneously detonate. Aqueous solutions up to 70% are safe to handle and store however. Additionally, perchlorate is toxic to the thyroid.

The biggest hazard in working with perchloric acid are its fumes. Normal fume hoods are insufficient in dealing with them, as the vapors will react with the construction materials and when they build up, there is a serious risk of explosion. Special fume hoods with washing down capabilities are mandatory when working with perchloric acid, to prevent accumulation of perchlorates in the ductwork.


Perchloric acid is safe to store at concentrations lower than 70%.


Perchloric acid and its compounds should not be released in the environment.


Relevant Sciencemadness threadsEdit

Community content is available under CC-BY-SA unless otherwise noted.