Perchloric acid has the formula HClO4 and is a colourless liquid soluble in water. It is a strong acid like sulphuric or nitric acid.
This acid is very dangerous and can explode when concentrated. Its color turns yellow before it explodes. That is why perchloric acid is often diluted with water.
An M/10 soloution 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 up the difference in strengths 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 sulphuric acid. It also can be made by mixing nitric acid with ammonium perchlorate. The reaction gives nitrous oxide and perchloric acid.
The salts of perchloric acid are powerful oxidizers that are often used in explosive compositions.
It forms an azeotrope with water that boils at 203°C.