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What’s the first image that comes to mind if I say “the world’s strongest acid”? Is it that scene from a certain chemistry-informed show where hydrofluoric acid is used to dissolve a corpse, and then the bathtub the corpse was inside of, and then the bathroom floor? Sounds pretty strong!

But in the grand scheme of things, hydrofluoric acid is not the strongest acid. And if you’ve seen our previous episode on “The Strongest Acids in the World”, you might be thinking, “Well of course not. It’s fluoroantimonic acid!”. But truthfully, that’s more of an acid cocktail. Kind of a, two-acids-in-a-trench coat thing. So by some measures, that makes it what scientists call “a big ol’ cheater”. But by disqualifying that particular acid, we find a surprising winner. The strongest singular acid is one that can’t burn through a corpse, or a tub, or a floor. In fact, it’s so gentle that one hypothesized application is to help make your daily vitamin supplement. 

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If you’re desperately struggling to remember anything from high school chemistry, never fear, we’ve got you. There’s a lot of ground we need to cover, so we’ll start with a review on acids and bases before we place our measuring stick against a league of very strong acids called "superacids." Then, we can crown one superacid the strongest, and finally explore why it can’t even burn through a bathtub. And no, you don’t have to take notes unless you want to. There won't be a pop quiz.

Acids can be defined as substances that donate protons, otherwise known as positively charged hydrogen ions, to other atoms. And when you drop an acid into water, one or more protons will separate from the original molecule to mingle and bond with all the free-floating H2Os. And it’s how readily that separation happens that determines an acid’s strength. That’s right. It’s got nothing to do with how easily a given acid can dissolve your bones. Weak acids are compounds which only sometimes let go of their protons. And a classic example is formic acid, which many ant species will squirt out of their abdomens as a defense mechanism. Meanwhile, strong acids will very willingly give their protons up. Like hydrochloric acid, which is so willing that it's like, “Here take my fruitcake, no, I insist.”

But the labels “strong” and “weak” are super vague, so whether or not chemists are looking for the strongest acid in the world, they need something else that’s way more specific. And they get that specificity by calculating a value called the "acid dissociation constant," or Ka. It’s a simple division problem that compares the number of acid molecules that have dissociated, or have broken up, against the number of acid molecules that are left intact.

In a strong acid, most of the acid molecules have donated their protons. So the numerator is larger and the denominator is smaller than it will be for a weak acid. And that means its Ka will be a much larger value. But you can see how we might run into a problem when trying to crown the strongest known acid. For stronger and stronger acids, the denominator gets closer and closer to zero. And the answer to our little division problem rapidly approaches infinity. So trying to get legitimate values for a bunch of similarly strong acids and compare them to one another gets a little bit impractical.

But what about the pH scale we all learned about in school? If it can help us compare stomach acid and vinegar, can it help us find the strongest acid? Well unlike Ka, the pH scale doesn’t bother with how many intact acid molecules are floating around. It only cares about those protons. Or more specifically, it cares about the fleeting particles that those protons create by temporarily bonding with water molecules, called "hydronium ions."

To calculate a pH, you take the negative log of how much hydronium you’ve got per liter of water. Pure water has a pH of 7, and anything with a lower number is classified as an acid. And the stronger an acid is, the more protons it can donate to form more hydronium ions, so it’ll have a lower pH.

For example, sulfuric acid, which is found in lead-acid batteries, often has a pH in the 1 to 3 range. But that sulfuric acid has been diluted by some amount of water. You can’t measure the pH of pure sulfuric acid, because there are no hydronium ions without water. And even if you added just a few drops of H2O to get that hydronium forming, the pH you calculated wouldn’t be reliable. According to the math, the pH would come out to be a negative number. But you wouldn’t be able to confirm that with actual experimental evidence, because the equipment chemists use to measure pH only works if the pH is at or above zero.

So not only can the pH scale not accurately describe the true strength of something as common as sulfuric acid, it can’t be used for anything stronger than sulfuric acid, either. These acids are collectively known as superacids, and it’s here where we need to look to find the strongest known acid. When measuring the strength of superacids, chemists often use something called the Hammett acidity function. It was designed to be a sort of continuation to the pH scale, with its H0 value roughly translating to a pH. But the way that it’s calculated means an acid doesn’t need to be diluted in water.

So far, our measurement methods have focused on the hydrogen ion side of an acid - the proton that the acid molecule spits out. But the Hammett acidity function looks at the rest of the molecule, the slightly negative leftover bit, called the "conjugate base." As we discussed, strong acids really want to give away their protons. And because of this, their conjugate bases also really don’t want to get that proton back. This makes them weak bases since they don’t want to act like regular bases, which readily accept protons. And this is a fundamental part of acids and bases: the stronger an acid, the weaker its conjugate base. So what the Hammett acidity function does is look at an acid’s conjugate base, determines how weak that is, then figures out how strong the original acid was based on that.

It’s kinda like figuring out how angry someone is based on how quiet they’ve gotten, and the Hammett activity function is no less daunting. But once you get past the scary looking equation, it can be used to figure out the acidity of very strong acids, like pure sulfuric acid, which has an H0 of -12. And -12 is only our starting point, because remember, superacids are all stronger than sulfuric acid. The trench coat acid we mentioned at the beginning, fluoroantimonic acid, has an H0 of a whopping -31.3.

But even the Hammett acidity scale has a problem: it only works if your acid is currently a liquid. And some superacids aren’t found in liquid form, including the superacid that many chemists crown the strongest known acid in the world - the super strong, yet super gentle, carborane acids.

Yes, you heard that right. Acids. Plural.

There’s technically more than one kind, but we’re going to focus on chlorinated carborane acid, which has this chemical formula. Typically, it’s a solid, and while that didn’t stop chemists from calculating an approximate H0 value of at least -18, they knew they’d need a completely different method to figure out exactly how strong it is. It might sound a bit weird, but they basically took their solid acid, mixed the ions of another compound in, shot a laser beam at the whole thing, and then measured the vibes between the two.

See, we can think of the bonds between molecules or ions like springs. and springs, rather famously, will vibrate when you “boing” them. And okay, “boing” is not a technical term, but in this case, scientists were measuring how one proverbial spring inside that ion - a bond between a nitrogen and hydrogen atom - would act in the presence of an acid’s conjugate base. Because it turns out, the weaker that conjugate base, the less it will mess with that bond, and the faster it will vibrate. And remember, the weaker the conjugate base, the stronger the acid. So to find the strongest acid, you just need to find the spring with the quickest vibes. And back in 2006, one team of researchers published their measurements for a selection of known superacids, and chlorinated carborane acid came out on top. But that was just this acid in its typical solid form. Even more research has proven its strength while it’s a gas by taking a measurement called "proton affinity."

Remember that weak conjugate bases don’t like protons. So by extension, strong acids treat protons like they’re playing a game of hot potato. Once they toss that bad boy to you, they really don’t want it back. The stronger the acid, the better they are at their hot potato game, and the lower their proton affinity. And according to a study published in 2009, researchers conducted their version of a hot potato tournament to find their superacid champion. Okay, I’m the one calling it a hot potato tournament, not the chemists, but go with me here.

First, they took chlorinated carborane acid’s conjugate base, along with some other carborane acid’s conjugate bases, and exposed them to several superacid molecules with known proton affinities, including the then-record holder for the lowest known proton affinity as a gas. And the team wanted to see if any of those other superacids were strong enough to hand their potato - or their proton - over to a carborane’s conjugate base. They took the conjugate base of the strongest of those other superacids and checked whether a chlorinated carborane acid molecule could offload its proton onto that one. And surprise, surprise - it could!
Chlorinated carborane acid became the team’s, quote, “strongest measured acid in the gas phase” and my hot potato champion. So we’ve got a carborane acid topping the charts when it’s a solid and when it’s a gas. And that just leaves liquid, right?

Well in 2004, scientists were able to measure the strength of various liquid superacids - including multiple carborane acids - by diluting them with sulfur dioxide, which is a liquid if you’ve got it at a high enough pressure. The strength was still based around how readily the acid molecule donates a proton to something else. It just wasn’t happening inside a solution swimming with water molecules. And not only did carborane acids outperform previously established strong acids like sulfuric and fluorosulfuric acid, chlorinated carborane acid came out on top again! In other words, carborane acids have been shown to be the strongest singular acid across solid, liquid, and gas phases. Which is more than you can say for the only other real contender for strongest acid, the helium hydride ion. Which you can only really find in interstellar space, and the only experiments we’ve done on it have been while it’s a gas.

And now that we’ve established that, we can move on to how carborane acids can also be gentle. A lot of superacids, including the ones that pack multiple acids into proverbial trench coats, leave behind very reactive conjugate bases after donating their protons. And because they’re so reactive, they just kind of go around destroying everything in their path - like corpses and bathtubs and bathroom floors.

Carborane acids, however, leave behind fairly non-reactive conjugate bases, and a lot of that’s to do with their shape. The conjugate base will look a little different depending on exactly which carborane acid you’re dealing with, but generally, they’ve got a central cage structure that’s mostly made of a bunch of boron atoms. And at least according to one scientist, this probably makes it "the most stable cluster of atoms in all of chemistry,” which is a pretty big claim, if I do say so myself.

But chemists have observed the gentle nature of carborane acids using one particular test that no other known superacid can pass. It involves molecules nicknamed "buckyballs," which are 60 carbon atoms arranged in a soccer ball pattern. Expose buckyballs to your standard superacid, and they’ll get ripped apart . But expose them to a carborane acid, and you can safely modify them. Carborane acids can gently plop individual protons onto the buckyball’s carbon cage without inflicting any damage. And if they don’t damage buckyballs, they won’t damage other molecules that might have more wide-reaching applications.

Vitamin supplements, for example, often use acidified molecules, which are molecules that have lovingly, or perhaps begrudgingly, accepted a proton. So hypothetically, carborane acids could be used to make those acidified molecules accept protons a bit less begrudgingly. But unfortunately, there doesn’t appear to be any published research that suggests scientists or supplement companies are actually looking into this. It may not be a cost-effective solution, which -to be honest - a little bit disappointing, because I’d kill to take a vitamin made with the world’s strongest acid. Well, not kill. And I definitely wouldn’t dissolve a body in a bathtub, either. But I would definitely tell my high school chemistry teacher all about it.

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