What Is the World's Strongest Superacid?

What you need to know about fluoroantimonic acid

This is the two-dimensional chemical structure of fluoroantimonic acid, the strongest superacid.
This is the two-dimensional chemical structure of fluoroantimonic acid, the strongest superacid. LAGUNA DESIGN / Getty Images

You may be thinking the acid in the Alien blood in the popular movie is pretty far-fetched, but the truth is, there's is an acid that is even more corrosive! Learn about the word's strongest superacid: fluoroantimonic acid. 

Strongest Superacid

The world's strongest superacid is fluoroantimonic acid, HSbF6. It is formed by mixing hydrogen fluoride (HF) and antimony pentafluoride (SbF5). Various mixtures produce the superacid, but mixing equal ratios of the two acids produces the strongest superacid known to man.

Properties of Fluoroantimonic Acid Superacid

  • Rapidly and explosively decomposes upon contact with water. Because of this property, fluoroantimonic acid cannot be used in aqueous solution. It is only used in a solution of hydrofluoric acid.
  • Evolves highly toxic vapors. As the temperature is increased, fluoroantimonic acid decomposes and generates hydrogen fluoride gas (hydrofluoric acid).
  • Fluoroantimonic acid is 2×1019 (20 quintillion) times stronger than 100% sulfuric acid. Fluoroantimonic acid has a H0 (Hammett acidity function) value of -31.3.
  • Dissolves glass and many other materials and protonates nearly all organic compounds (such as everything in your body). This acid is stores in PTFE (polytetrafluoroethylene) containers.

What Is It Used For?

If it's so toxic and dangerous, why would anyone want to have fluoroantimonic acid? The answer lies in its extreme properties. Fluoroantimonic acid is used in chemical engineering and organic chemistry to protonate organic compounds, regardless of their solvent.

For example, the acid can be used to remove H2 from isobutane and methane from neopentane. It is used as a catalyst for alkylations and acylations in petrochemistry. Superacids in general are used to synthesize and characterize carbocations.

Reaction Between Hydrofluoric Acid and Antimony Pentafluoride

The reaction between hydrogen fluoride and antimony pentrafluoride that forms fluoroantimonic acid is exothermic.

HF + SbF5 → H+ SbF6-

The hydrogen ion (proton) attaches to the fluorine via a very weak dipolar bond. The weak bond accounts for the extreme acidity of fluoroantimonic acid, allowing the proton to jump between anion clusters.

What Make Fluoroantimonic Acid a Superacid?

A superacid is any acid that is stronger than pure sulfuric acid, H2SO4. By stronger, it means a superacid donates more protons or hydrogen ions in water or has a Hammet acidity function H0 lower than -12. The Hammet acidity function for fluorantimonic acid is H0 = -28.

Other Superacids

Other superacids include the carborane superacids [e.g.,  H(CHB11Cl11)] and fluorosulfuric acid (HFSO3). The carborane superacids may be considered the world's strongest solo acid, as fluoroantimonic acid is actually a mixture of hydrofluoric acid and antimony pentafluoride. Carborane has a pH value of -18. Unlike fluorosulfuric acid and fluoroantimonic acid, the carborane acids are so noncorrosive that they may be handled with bare skin. Teflon, the non-stick coating often found on cookware, may contain carborante. The carborane acids are also relatively uncommon, so it's unlikely a chemistry student would encounter one of them.