The World’s Strongest Acid – The Superacids

The world’s strongest acid is fluoroantimonic acid, one of the superacids. Superacids are so powerful they aren’t even measured using the regular pH or pK_A scales. Here’s a look at fluoroantimonic acid and other superacids and how they work.

What Are Superacids?

A superacid is a strong acid with an acidity greater than that of pure sulfuric acid. Chemists describe superacid strength using the Hammett acidity function (H₀) or other special acidity functions because the pH scale only applies to dilute aqueous solutions.

How Superacids Work

Many superacids form by mixing a Brønsted acid and a Lewis acid. The Lewis acid binds and stabilizes the anion formed by dissociation of the Brønsted acid. This removes a proton acceptor, making the acid a better proton donor.

You may hear superacids have “naked” or “unbound” protons, but this isn’t true. The acid donates protons to substances that normally don’t accept them, but initially the protons are bound to molecules in the acid and not floating free. However, these protons rapidly move between one proton acceptor and the next. What happens is that the superacid is an extremely poor proton acceptor. So, it’s easier for a proton to attach to the other substance than to return to the acid.

World’s Strongest Acid

The world’s strongest acid is the superacid called fluoroantimonic acid (HSbF₆). It is over a billion times stronger than pure sulfuric acid. In other words, fluoroantimonic acid donates protons around a billion times better than sulfuric acid.

Mixing equal amounts of hydrogen fluoride (HF) and antimony pentafluoride (HSbF₆) makes the most potent fluoroantimonic acid, but other mixtures also yield a superacid.

HF + SbF₅ → H⁺ SbF₆^–

Fluoroantimonic acid is nasty stuff. It’s highly corrosive and releases toxic vapors. It explosively decomposes in water, so it only finds use in hydrofluoric acid solutions. Fluoroantimonic acid decomposes with heat to release hydrogen fluoride gas. The acid protonates glass, most plastics, and human tissue.

The Carborane Acids

Fluoroantimonic acid results from a mixture of acids, but the carborane acids [e.g., H(CHB₁₁Cl₁₁)] are solo acids. The H₀ of the carborane acids is at least -18, but the nature of the acid molecule makes it difficult to calculate its strength. Carborane acids may be even stronger than fluoroantimonic acid. They are the only acids able to protonate C₆₀ and carbon dioxide. Despite their strength, the carborane acids are not corrosive. They don’t burn skin and can be stored in ordinary containers.

List of Superacids

Superacids have acidity greater than sulfuric acid, which has a Hammett activity of -11.9 (H₀ = -11.9). So, superacids have H₀ < -12. The pH of 12M sulfuric acid is negative using the Henderson-Hasselbalch equation. While the equation uses assumptions that don’t apply to superacids, you could say the superacids all have negative pH values.

How Are Superacids Stored?

There is no one-size-fits-all container material for the superacids. It’s safe to store carborane acids in glass. Fluorosulfuric acid and fluoroantimonic acid eat through glass and normal plastic. They require polytetrafluorethylene (Teflon) containers. The combination of carbon with fluorine protects against acid attack.

Uses of the Strongest Acids

Why would anyone use such a strong acid, much less one as toxic and corrosive as fluoroantimonic acid? These acids aren’t used in daily life or even a normal chemistry lab. Rather, they find use in organic chemistry and chemical engineering to protonate compounds that don’t normally accept protons. Also, they are useful because they work in solvents besides water.

Superacids are catalysts in petrochemistry. Solid forms of acids alkylate benzene with propene and ethene and acylate chlorobenzene. Reactions like this help produce high-octane gasoline and synthesize plastics. Superacids are used to manufacture explosives, make ethers and olefins, etch glass, isomerize hydrocarbons, and stabilize carbocations.

References

• Ghosh, Abhik; Berg, Steffen (2014). Arrow Pushing in Inorganic Chemistry: A Logical Approach to the Chemistry of the Main-Group Elements. Wiley.
• Hall, N.F.; Conant, J.B. (1927). “A Study of Superacid Solutions”. Journal of the American Chemical Society. 49 (12): 3047-3061. doi:10.1021/ja01411a010
• Hammett, L. P. (1940). Physical Organic Chemistry. New York: McGraw-Hill.
• Herlem, Michel (1977). “Are reactions in superacid media due to protons or to powerful oxidising species such as SO₃ or SbF₅?”. Pure and Applied Chemistry. 49: 107–113. doi:10.1351/pac197749010107