Fluoroantimonic Acid: The Strongest Acid Known to Humankind

Update Note, 30 January 2024: I used to have an image from the film Indiana Jones and the Raiders of the Lost Ark at the top of this article. It showed the face melting scene at the end of the film. However, Google constantly sent me violation reports because of this rather famous image, so I removed it. That said, enjoy what follows for the strongest known acid.

The strongest known acids on the planet are something called superacids. These are acids that are more acidic than a pure solution of sulfuric acid! That's most definitely "face melting acidity". Even the word "superacid" sounds like something you don't want to dance with on a Saturday night.

There are many superacids that chemists have formed, but by far the worst of the worst (the most hated and cursed? well, no, it's just the strongest) is the superacid known as fluoroantimonic acid. Fluoroantimonic acid can be estimated to be over 10 quadrillion times stronger than sulfuric acid! (see the discussion below on the Hammett acidity function to see how I got that number)

Fluoroantimonic acid has the chemical formula H₂FSbF₆ which shows you that it's composed of bonded atoms of fluorine (F) and antimony (Sb) with some hydrogen (H) as well. Let's chat a tad about the chemistry of fluoroantimonic acid and why you most definitely don't want it on your skin (or anywhere near you, for that matter, unless you're a chemist who's working with the stuff).

Dropping Acid

From left-to-right: strong acid, dilute acid, base (image from SeattlePI)

Acid (which comes from the Latin word(s) acidus/acēre, which means "sour") is something that people have known about for a long time. Acids are literally what you are tasting when you taste something sour (hence the name). Lemons and other citrous fruits have a sour taste due to citric acid. We use microbial lactic acid fermentation (producing lactic acid from glucose) to make sauerkraut, sour beers, and kimchi (and, incidentally, lactic acid fermentation ruined a pot of stew I had sitting out last week). You may have noticed that your vomit has a sour taste. That's because of the acid in your stomach that normally helps you to digest your food, though the burning sensation you feel in your throat from vomit has more to do with your stomach enzymes which cleave amino acid bonds to break down proteins (update: I had hydrochloric acid listed as the main stomach acid that causes the acrid taste of vomit, but a reader clarified that the real nasty smell and flavor comes from butyric acid). 

There's a lot more to acids than taste. Put simply, an acid is a substance which can donate a proton. In chemistry, we tend to think of these proton donations as shuffling of hydrogen nuclei (a hydrogen atom, which has one proton and one electron, is only a single proton when it's ionized). There are lots of molecules that can donate a proton to water (to form the hydronium ion) or to another molecule (something that accepts a proton is called a base). Some molecules are much better at doing this than others. The molecules that are the best at donating protons are called strong acids. Strong acids include things like hydrochloric acid (HCl), nitric acid (HNO₃), and sulfuric acid (H₂SO₄). In these cases, the chloride (Cl^-), nitrate (NO₃^-), and sulfate (SO₄^2-) ions are very stable as ions in solution, which is why they're so good at giving away those protons. For instance, in the picture below, the behavior of hydrochloric acid in water is compared to that of acetic acid (HC₂H₃O₂):

As the picture illustrates, hydrochloric acid donates all of its hydrogen (protons) to water, while acetic acid tends to mostly remain as acetic acid and only a small amount dissociates to form hydronium ions and acetate ions. Because of this, we consider hydrochloric acid to be a strong acid and acetic acid to be a weak acid.

Superacids 

When considering the acidity of a substance, many acids are compared in their acidity to that of sulfuric acid. The sulfate ion is very stable in its ionic form in solution and so it's not a happy camper when bound to a proton or two (which is a state called "protonated"). Like I mentioned earlier, a superacid is one that is considered to be more acidic than 100% sulfuric acid. When you have that strong of an acid, a measure of something like pH (the negative log of the concentration of protons in solution) is no longer adequate. Instead, chemists can turn to something called the Hammett acidity function.

I won't explain the Hammett acidity function here, but it can loosely be thought of as what the pH of a solution would be if it were possible to pack trillions of trillions of hydronium ions into a solution. A pure solution of sulfuric acid would have a Hammett acidity function value of -12 (so, kind of like having a pH of -12, if that were possible). 

This is Magic Acid, the second most acidic superacid. It's Hammett acidity function is -19.2!

What About Fluoroantimonic Acid?

Fluoroantimonic acid, the strongest known acid, has a Hammett acidity function value of -28! (though there are also sources out there stating an unconfirmed value of -31.3)

This is what the pH of a solution would be if it were possible to pack 10²⁸ moles of hydronium into each liter of solution. To give you an idea of how crazy that is, a solution with a pH of 1 (which is easily acidic enough to burn your skin) has 0.1 moles of hydronium per liter. 

Since the molar mass (the mass per mole) of hydronium is 19.02 g/mol, a quick calculation will show that fluoroantimonic acid is as acidic as a solution would be if it contained over 10²⁶ kg/L of hydronium. That's more than the known density of neutron stars! (Luckily, that's not really how these superacids work.)

As I mentioned above, it can be estimated that fluorantimonic acid is about 10 quadrillion times stronger than sulfuric acid. Since sulfuric acid has a Hammett function of -12 and fluorantimonic acid has a Hammett function of -28, the difference is 16, or about 10¹⁶ more moles of hydronium. That's 10 quadrillion times more! However, as I mentioned above, this isn't really how superacids work and the Hammett function can only loosely be idealized as the negative log of the acid concentration.

This is the structure of fluoroantimonic acid. White balls are hydrogen, green - fluorine, and purple - antimony.

Fluoroantimonic acid is a ridiculously strong acid. It will eat through glass and plastic. It will react explosively with water (so it is only mixed in solution of hydrofluoric acid). It can protonate almost any organic molecule (force a proton onto the molecule), and it will most definitely cause some massive trauma to any living organism (by massive trauma, I mean it will certainly destroy any and all flesh it comes into contact with). There's really only one way to store it. 

A bond between fluorine and carbon is the strongest chemical bond in organic chemistry. Polytetrafluoroethylene (PTFE), brand name known as Teflon, is composed of repeating units of carbon bonded to fluorine atoms. For a container to hold fluoroantimonic acid, it has to be made out of PTFE. Yup, you read that right: the strongest acid known to humanity can be stored in the same stuff that keeps your eggs from sticking to the pan when you make breakfast.

PTFE - Keeps your breakfast from sticking to the pan and kicks the snot out of the world's strongest acid

I bet you'll never look at your non-stick frying pans the same way again!

There are some uses of superacids like fluoroantimonic acid. These acids are great at creating substances known as carbocations (molecules with ionized carbon atoms) and providing environments for studying such substances. Carbocations are intermediates in some economically important reactions, so studying their behavior when isolated is pretty important. 

Outside of the need to protonate things that normally aren't protonated or to create these carbocations, it's pretty safe to say that there's absolutely no need to have superacids like fluoroantimonic acid around. Well, maybe not. Here's a TEDxGhent talk from a couple years ago by Lennart Joos where he suggests using a superacid known as phosphotungstic acid to combat smog:

So maybe there are some great uses for superacids. Still, I don't think I have any need to play with something like fluoroantimonic acid during this lifetime. My hats off to those chemists who deal with this stuff safely and securely (and my hopes that it remains that way in their future work). The science behind fluorantimonic acid is awesome, but when I think of the stuff all I can imagine is all of my skin melting off in the most painful of ways. Scary.