Perchloric acid

Perchloric acid

}} Perchloric acid, HClO4, is an oxoacid of chlorine and is a colorless liquid soluble in water. It is a strong acid comparable in strength to sulfuric and nitric acids. It is useful for preparing perchlorate salts, but it is also dangerously corrosive and readily forms explosive mixtures.


Perchloric acid is produced by treatment of sodium perchlorate with hydrochloric acid and by the electrochemical oxidation of aqueous chlorine.


Perchloric acid is a superacid, and one of the strongest Brønsted-Lowry acids. Its pKa is −7.

Anhydrous perchloric acid is an oily liquid. It forms a series of at least five hydrates, several of which have been characterized crystallographically. The dihydrate is representative: the solid consists of the perchlorate anion linked to H2O and H3O+ centers via hydrogen bonds. Perchloric acid forms an azeotrope with water, consisting of about 72.5% perchloric acid. This form of the acid is stable indefinitely and is commercially available. Such solutions are hygroscopic, that is, if left unsealed, concentrated acid dilutes itself by absorbing water from the air.

A 0.100 molar solution in glacial acetic acid is used as an analytical reagent. Titration of weak bases is made easier if the usual medium, water, is replaced by glacial acetic acid. Glacial acetic acid is a much weaker base than water, so the base being titrated appears to be stronger. As a counterpart, the strength of acids is reduced. This shows the difference in strength among the strong acids.

Laboratory preparation

The diluted acid can be prepared by distillation of a solution of sodium perchlorate in concentrated sulfuric acid.
NaClO4 + H2SO4 → NaHSO4 + HClO4
In a related method, barium perchlorate reacts with dilute sulfuric acid to precipitate barium sulfate, leaving perchloric acid. It also can be made by mixing nitric acid with ammonium perchlorate. The reaction gives nitrous oxide and perchloric acid due to a concurrent reaction involving the ammonium ion.


Anhydrous and monohydrated perchloric acid are explosive, but the usual aqueous solutions are stable in the absence of organic compounds. It is very corrosive to skin and eyes. Upon contact with perchloric acid organic material such as cloth and wood inflame. Salts of perchloric acid are also powerful oxidizers that can be explosive. Perchlorate salts tend to be less unstable than their chlorate counterparts, which has led to their increased use in pyrotechnic compositions due to safety concerns.


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