Perchloric acid

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Perchloric acid
IUPAC name
Perchloric acid
Systematic IUPAC name
Chloric(VII) acid
Other names
Hyperchloric acid
Hydrogen tetraoxochlorate(VII)
Molar mass 100.46 g/mol
Appearance Colorless liquid
Odor Odorless
Density 1.7703 g/cm3 (at 20 °C)
Melting point −112 °C (−170 °F; 161 K) (anhydrous)
−17 °C (1 °F; 256 K) (azeotrope)
Boiling point 203 °C (397 °F; 476 K) (azeotrope)
Solubility Reacts with amines
Readily soluble in trifluoroacetic acid
Insoluble in carbon tetrachloride
Vapor pressure 29.35 mmHg (at 20 °C)
Acidity (pKa) -15.2 (±2.0) ≈ −10
160-180 J·mol-1·K-1
-40.6 kJ/mol
Safety data sheet BioExpress
Flash point Non-flammable
Related compounds
Related compounds
Hypochlorous acid
Chlorous acid
Chloric acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

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. The addition of perchloric acid to a solution containing potassium ions will precipitate the poorly soluble potassium perchlorate out of the solution.


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. Its density is 1.67 g/cm3.

Perchloric acid, like many other mineral acids, comes in three typical concentrations: dilute (below 70%), concentrated (72.5%), and fuming (80-100%). Anhydrous (pure) perchloric acid is also used, although it is a very dangerous chemical. Each of these concentrations comes with its own set of hazards, although dilute is the safest.


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 and its salts are restricted in the EU countries.


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. Silver perchlorate, if available can also be used.

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.

If you cannot or won't want to perform distillation, you can get perchloric acid by using barium perchlorate instead. Filter the insoluble barium sulfate to obtain conc. perchloric acid.

Perchloric acid can also be made by boiling down a solution of chloric acid. Heat and dehydration decompose chloric acid, cause it to release toxic fumes of hydrochloric and perchloric acid, which condense on a cold surface.




Perchloric acid is a strong acid, and when concentrated is a dangerous oxidizer capable of exploding violently in contact with organic compounds. Anhydrous or fuming perchloric acid is known to spontaneously detonate. Aqueous solutions up to 70% are safe to handle and store however. The azeotropic (concentrated) acid is mostly safe under normal conditions. 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%. It must be kept away from any organic vapors and metals. Glass bottles with Teflon seal cap are suitable for storing this dangerous reagent.


Perchloric acid and its compounds should not be released in the environment. Perchloric acid is best precipitated to the insoluble potassium perchlorate, which can then be mixed with a combustible material and burned.

Perchlorate salts can also be neutralized by using iron powder and UV light, under anaerobic conditions.



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