What's the World's Strongest Superacid?

Fluoroantimonic acid — Here we see the chemical structure of fluoroantimonic acid, the strongest superacid known to man. Wikimedia/(CC BY-SA 3.0)

Some acids are safe enough that even a homeowner can use them, like muriatic acid. It's designed to be used as a household cleaner, as long as you follow the directions and all safety precautions.

Then there are other acids that are simply too caustic and corrosive to be handled by anybody. So what happens when you mix two of those? You get something altogether stronger. For instance, when equal amounts of the acids hydrogen fluoride (HF) and antimony pentafluoride (SbF₅) are combined, the results are too remarkable to ignore. You don't get just any other acid. You end up with the strongest superacid known to humankind — fluoroantimonic acid, aka HSbF₆.

What Is Flouroantimonic Acid?

Fluoroantimonic acid is a colorless liquid with toxic vapor. It's fatal if it's swallowed or inhaled, and it can cause severe burns if it comes in contact with skin. Its corrosiveness is part of why it's top dog in a group of acids called superacids.

Superacids are any category of strong acids with an acidity greater than sulfuric acid. Superacids are so strong they can't be measured using the regular pH or pKA scales. Instead they are measured using the Hammett acidity function — written as H₀ — or other special functions in place of the standard pH scales. Fluoroantimonic acid's H₀ is -21. In comparison, sulfuric acid's H₀ is −12.

To put that in perspective, fluoroantimonic acid is 20×10¹⁹ (20 quintillions) times stronger than pure sulfuric acid. That's like if Captain Marvel started a galactic battle with a newborn kitten, though perhaps even worse. In this scenario, however, fluoroantimonic acid sounds more like a villain than a superhero.

Fluoroantimonic Acid's Kryptonite: Teflon

Unfortunately for the Captain Marvel of superacids, fluoroantimonic acid can't wear a cape unless it's made of polytetrafluoroethylene (PTFE), aka Teflon. That means the only container suitable to store the stuff is PTFE. It also can be stored in a hydrofluoric acid solution, the one solution in which HSbF₆ doesn't explosively decompose.

Fluoroantimonic acid otherwise will dissolve glass, most plastics, every organic compound (including the human body) and will explode in water. Though it might sound like a bit of hazardous fun to drop fluoroantimonic acid in a flask of water to see what happens, it's incredibly toxic and dangerous and not for your junior high science class. It's best left to the pros in chemical engineering and organic chemistry.

Protonation: Fluoroantimonic Acid's Superpower

Fluoroantimonic acid's true superpower is protonation, or giving protons to organic compounds. Pronation changes a substance's chemical properties like mass, ability to dissolve in water (solubility), and reaction when mixed in water (hydrophilicity). That means fluoroantimonic acid is helpful for chemists in making chemical reactions. It's also used to etch glass, enhance the quality of gasoline by raising octane levels, make plastics and engineer explosives like grenades.

Fluoroantimonic acid is the strongest acid and one of the most corrosive, but experts say that in some ways, hydrofluoric acid is still the most dangerous. Why? Because you're a lot more likely to encounter some pharmaceuticals, cleaning products and herbicides that contain hydrofluoric acid. It's a lot harder for the average person to get a hold of fluoroantimonic acid accidentally.

If you do take on any superacid, especially fluoroantimonic acid, be prepared to suit up in some serious chemical personal protective equipment (PPE), including a respirator and protective eyewear. That's the super-suit of modern chemists and anyone who doesn't want to watch their own flesh and bones dissolve into a puddle of poor decisions.