Hydronium perchlorate

Hydronium perchlorate is an inorganic chemical compound with the chemical formula [H3O]ClO4. It is an unusual salt due to it being a solid and stable hydronium salt. It consists of hydronium cations [H3O]+ and perchlorate anions ClO4−.

Production
Hydronium perchlorate is produced by the reaction of anhydrous perchloric acid and water in a 1:1 molar ratio:
 * HClO4 + H2O → [H3O]+ClO4−

A more analytically reliable method was reported using the macrocyclic Schiff base of sodium 2,6-diformyl-4-methylphenolate and 2,6-diformyl-4-methylphenol as a chelating agent to sequester Cu(II): transmetallation of the macrocycle with copper(II) perchlorate yielded colorless crystals of hydronium perchlorate.

It can also be produced by the reaction of anhydrous nitric acid and perchloric acid.