superacid
How acidic can an acid possibly get? Far more than concentrated sulfuric acid, it turns out. A superacid is, by the standard definition (due to Conant and later Gillespie), any acid stronger than 100 percent (anhydrous) sulfuric acid. To measure acidity this extreme, the ordinary pH scale is useless — there is no water — so chemists use the Hammett acidity function H0; pure sulfuric acid sits at about H0 = minus 12, and superacids go much lower, the strongest reaching around minus 20 or beyond, meaning unimaginably proton-donating.
Superacids are usually made by combining a strong Bronsted acid with a strong Lewis acid that mops up the conjugate base. The classic 'magic acid' is fluorosulfonic acid plus antimony pentafluoride (HSO3F + SbF5): the SbF5, a powerful Lewis acid, grabs F- and SO3F- to form huge, extremely stable, non-nucleophilic anions like SbF6- and Sb2F11-, which refuse to hold a proton, so the proton is left desperately free and the acidity skyrockets. Fluoroantimonic acid (HF + SbF5) is even stronger and is often cited as the strongest known acid, roughly H0 of minus 21 to minus 28. These media can protonate things that are normally not basic at all — even hydrocarbons, even some noble gases under the right conditions.
Superacids are not a curiosity; they unlock real chemistry. They generate and stabilize carbocations long enough to study them directly (work that earned George Olah the 1994 Nobel Prize), they drive industrial isomerization and alkylation in petroleum refining, and they let chemists protonate and activate stubborn molecules. They also vividly illustrate the leveling effect from the other side: water could never reveal these strengths because anything stronger than H3O+ is leveled in water, so superacid chemistry must be done in non-aqueous, often anhydrous-fluoride media. A note of respect: HF-based superacids are ferociously corrosive and dangerous, eating through glass and skin.
Magic acid (HSO3F + SbF5): the Lewis acid SbF5 sequesters fluoride into the very stable, non-nucleophilic Sb2F11- anion, leaving a proton with nowhere to go and a Hammett H0 far below pure sulfuric acid's minus 12. It can even protonate alkanes, which water-based chemistry treats as completely non-basic.
A Lewis acid sponges up the conjugate base, freeing the proton — that is how an acid out-acids sulfuric acid.
Superacids are defined relative to anhydrous sulfuric acid, not water, and their strength is reported as Hammett H0, not pH. The acidity comes as much from the Lewis acid stabilizing a non-nucleophilic anion as from the Bronsted acid itself.