Perchloric acid has the formula HClO₄ and is a colorless liquid soluble in water. It is a strong acid like sulfuric or nitric acid. M.P. -17°C, B.P. 181°C, r.d.1.664. It is a superacid, but it is not the strongest Brønsted-Lowry acid (which is fluorosulfuric acid). Its pKa is -7^*.

Perchloric acid forms an azeotrope with water, about 72.5% perchloric acid. This form of the acid is stable indefinitely. Commercially available concentrated acid is around this concentration. If left unsealed, concentrated acid dilutes itself by pulling water from the air.

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 when combined with common organic material such as cloth or wood.

Pure anhydrous perchloric acid is an oily liquid; it can explode, and it slowly decomposes at normal temperature. Perchloric acid monohydrate, a crystalline substance, is more stable, but can also explode. Neither of these is commonly used or prepared; neither can be made by boiling the usual concentrated acid, since this at best forms the azeotrope.

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. Perchloric acid, being the strongest of all readily available acids, is therefore the acid of choice.

The diluted acid can be made by distillation of a solution of sodium perchlorate mixed with sulfuric acid. It also can be made by mixing nitric acid with ammonium perchlorate. The reaction gives nitrous oxide and perchloric acid. An alternative way is to react barium perchlorate with dilute sulfuric acid, which precipitates the barium sulfate, leaving only perchloric acid in solution.

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.

External links

• International Chemical Safety Card 1006

Perchlorates | Acids

Perchlorsäure | Acide perchlorique | Acido perclorico | חומצה על-כלורית | 過塩素酸 | Kwas chlorowy(VII) | Хлорная кислота | Перхлорна киселина | Perkloorihappo