What Is the Strongest Acid?

What is the world's strongest acid? It's probably not one you'd guess.

None of the strong acids traditionally listed in a chemistry text holds the title of World's Strongest Acid. The record-holder used to be fluorosulfuric acid (HFSO₃), but the carborane superacids are hundreds of times stronger than fluorosulfuric acid and over a million times stronger than concentrated sulfuric acid. The superacids readily release protons, which is a slightly different criterion for acid strength than the ability to dissociate to release a H⁺ ion (a proton). The strongest carborane superacid has the chemical structure H(CHB₁₁Cl₁₁). 

Strong Is Different from Corrosive

The carborane acids are incredible proton donors, yet they are not highly corrosive. Corrosiveness is related to the negatively-charged part of the acid. Hydrofluoric acid (HF), for example, is so corrosive it dissolves glass. The fluoride ion attacks the silicon atom in silica glass while the proton is interacting with oxygen. Even though it is highly corrosive, hydrofluoric acid is not considered to be a strong acid because it does not completely dissociate in water.

The carborane acid, on the other hand, is highly stable. When it donates a hydrogen atom, a negatively charged anion left behind is sufficiently stable that it doesn't react any further. The anion is the carborane portion of the molecule. It consists of one carbon and a cluster of 11 boron atoms arranged into an icosahedron.