Superacid

According to the classical definition superacid is an acid with an acidity greater than that of 100% pure sulfuric acid, which has a Hammett acidity function (H₀) of −12. According to the modern definition, superacid is a medium, in which the chemical potential of the proton is higher than in pure sulfuric acid. Commercially available superacids include trifluoromethanesulfonic acid (CF₃SO₃H), also known as triflic acid, and fluorosulfuric acid (FSO₃H), both of which are about a thousand times stronger (i.e. have more negative H₀ values) than sulfuric acid. The strongest superacids are prepared by the combination of two components, a strong Lewis acid and a strong Brønsted acid. The strongest known superacid is fluoroantimonic acid.

Unstable gas-phase heteroelement-hydride ions can be extremely acidic, e.g. the helium hydride ion. However, they are not considered as such because they exist only in extremely hot plasma not in the liquid state.