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 acid or nitric acid. It is a superacid, but it is not the strongest Brønsted-Lowry acid (which is fluoroantimonic acid, HFSbF₅). Its pK_a is −10.^[1]

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Anhydrous perchloric acid is an oily liquid. It forms a series of 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.^[2] Perchloric acid forms an azeotrope with water, 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 only perchloric acid in solution. 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..

Safety

Anhydrous and monohydrated perchloric acid can explode, and it slowly decomposes at normal temperature. According to the CRC "Handbook of Laboratory Safety", perchloric acid is extremely hazardous. It is very corrosive to skin and eyes and should be handled with the utmost care. It can also ignite or explode upon contact with organic material such as cloth or wood. The salts of perchloric acid are powerful oxidizers that are often used in explosive compositions. Perchlorate salts tend to be less reactive and more stable than their chlorate counterparts, which has led to their increased use in pyrotechnic compositions due to safety concerns.

References

1. ^ dissociation constants
2. ^ Almlöf, Jan; Lundgren, Jan O.; Olovsson, Ivar "Hydrogen Bond Studies. XLV. Crystal structure of perchloric acid 2.5 hydrate" Acta Crystallographica, Section B: Structural Crystallography and Crystal Chemistry (1971), 27(Pt. 5), 898-904.