Superacid

A superacid is an acid stronger than pure sulfuric acid, often defined by a Hammett acidity function below −12. It can protonate substances that ordinary mineral acids cannot.

Hammett acidity function: An acidity scale for highly concentrated acids, based on protonation of indicator bases. It gives the common quantitative threshold used to classify superacids.
Sulfuric acid: A strong diprotic mineral acid with formula H₂SO₄. Pure sulfuric acid supplies the conventional reference point for superacid strength.
Carbocation: An organic ion in which a carbon atom carries a positive charge. Superacids stabilize carbocations long enough for their structures and reactions to be studied.
Strong acid: An acid that dissociates extensively in a specified solvent, commonly water. Superacids exceed sulfuric acid's strength, while strong-acid labels depend on solvent behavior.
Brønsted–Lowry acid–base theory: A theory defining acids as proton donors and bases as proton acceptors. Superacid strength reflects exceptional ability to donate protons to weak bases.
Fluorosulfuric acid: A strong acid with formula HSO₃F, also called fluorosulfonic acid. It is a powerful component of the classic fluorosulfuric-acid–antimony-pentafluoride mixture.
Magic acid: A superacid mixture of fluorosulfuric acid and antimony pentafluoride. It can protonate hydrocarbons and generate persistent carbocations.
Acidity: A measure of a substance's tendency to donate protons or increase hydrogen-ion activity. Superacidity is an extreme of acid strength, not simply high concentration or corrosiveness.
Protonation: The addition of a proton to an atom, molecule, or ion. Superacids drive protonation of molecules that resist ordinary acids.
Antimony pentafluoride: A strong Lewis acid with formula SbF₅. Adding it to fluorosulfuric acid greatly increases the mixture's acidity.

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