Monday, September 14, 2015

Fluoroantimonic Acid: The Strongest Acid Known to Humankind

Will fluoroantimonic acid make your face melt like Toht in Raiders of the Lost Ark?
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/ac─ôre, 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 (SO42-) 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.

This image from the music video Skinned is pretty much what superacids should bring to mind


29 comments:

  1. Thanks. I thought H2SO2 was the max, and I'm a medical technologist. You've tauaght this old gal something new and exciting, so yay!

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    1. You're very welcome. It's crazy to think about how powerful some of these superacids are. Of course, in nature here on Earth, we won't find anything lower than a pH of about -1, and even that's pushing it. It would be interesting though if there were some alien world out there with superacidic or superbasic environments.

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    2. I've never seen an element that "reacts explosively" with water. I am definitely no scientist, but I get curious from time to time. This article was quite interesting! Thank you!

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    3. Thanks for reading. Curiosity is one thing that we certainly have in common!

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    4. Give the alkeline metals a bit of your time. They're a blast in water. Also, fluoroantimonic acid wouldn't hurt if you had it on your skin. It disrupts your nerve cells as it ionizes the tissue it contacts. It would be painless and horrifying.

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  2. very interesting and amazing
    my thinking was H2SO4
    but today i am very happy to know about this superacid
    Fluoroantimonic Acid

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  3. Really I have only worked with gentler acids such as Aqua Regia, HF, and mixtures containing hexavalent chromium. These strong acids are super cool! I had never seen a bottle of Magic Acid. I once ordered a bottle of Magic Flux though...great for cleaning platinum crucibles.

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    1. HF is a weak acid. No one asked but just thought i'd say anyways. Lol.

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  4. i love Fluoroantimonic Acid

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  5. What would happen if an alkaline solutinon such as bleach or any other high pH compound were to come into contact with these superacids such as Magic Acid or Fluoroantimonic acid?

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    1. Great question! I actually don't know the answer. I'd imagine that the reaction would be abrupt and likely exothermic, but not sure more than that. Might be worth seeing if anyone who gets to work with this stuff has a good answer.

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    2. Boom would happen. Extremely violent reaction. Imagine, if you can, 2 freight trains heading towards the same square inch of track at 90mph. Invision the explosive collision. That'll give you some idea of the reaction.

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  6. Vomit is given its taste and smell from Butyric acid, not really HCl

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    1. Thanks for the clarification. I updated the writing.

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  7. If there were an accidental spill, would it continue to dissolve everthing in its path, unstoppable, on through the earth? How would it be contained or neutralized?

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    1. If I'm not mistaken, it would go until the acid can't protonate (donate protons) anymore.

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  8. Even better than HF *_*
    love it, thanks for the article

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  9. Fluoroantimonc acid is my favorite acid of all time!

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  10. Thank u ,I got a clarity abt scids

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  11. What about carborane acid?i read that is the strongest acid

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    1. From what I've read, carborane acids only have Hammett functions around -18, so they're still no where near fluorantimonic acid in relative strength.

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  12. My science teacher showed us this post and we were astonished

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    1. That's really cool. I hope you enjoyed it!

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  13. Is there a possibility of having a neutralization reaction if a superacid reacts with superbase?? Will it be exothermic??

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    1. It would be incredibly exothermic and likely extremely explosive. It would have to be carried out by mechanical means as anyone anywhere near the vicinity of this reaction would be in incredible danger. Fluorine compounds react extremely violently with just about anything and can start uncontrollable fires that can't be put out. The reaction would have to be carried out under argon or neon conditions.

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    2. You can buy it here. 1500 Dollars for 50g.

      http://www.sigmaaldrich.com/catalog/product/aldrich/175102?lang=en&region=SE

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  14. Very valuable piece of info.Thanks a lot

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  15. Can it be manufactured cheaply to dissolve plastic?

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    1. I don't know what the manufacturing costs are, but I imagine not given how caustic it is. Did a quick search and didn't find much for how much it costs to make a given amount.

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