What is the strongest acid?

According to Wikipedia, $\ce{HeH+}$ and fluoroantimonic acid are the strongest.


According to [a News article in Nature](http://www.nature.com/news/2004/041115/full/news041115-5.html), Carborane acid is the strongest, but Wikipedia says fluoroantimonic acid is stronger.


**Links:**


* [$\ce{HeH+}$ / Wikipedia](https://en.wikipedia.org/wiki/Helium_hydride_ion) - stating HeH+ the second most acidic
* [$\ce{HeH+}$ / Wikipedia 2nd source](https://en.wikipedia.org/wiki/Compounds_of_fluorine) - stating $\ce{HeH+}$ the most acidic (see in 'Notes' section)
* [Fluoroantimonic Acid / Wikipedia](https://en.wikipedia.org/wiki/Fluoroantimonic_acid) - stating it is the strongest acid (super\_acid)
* [Carborane Acid / nature.com](http://www.nature.com/news/2004/041115/full/news041115-5.html) - According to *Nature* and even Google itself, it is the strongest acid created but according to wikipedia Flouroantimonic acid is stronger.


**So, which is the strongest ?**




The problem with this question is that the exact answer is "**it depends...**"


First off, it depends on your definition of acidity and how you measure it. Everyone seems to be using Brønstead acids (i.e. $\ce{H+}$ donors). I see two different measures in other answers:


* **[Proton affinity](http://en.wikipedia.org/wiki/Proton_affinity)**: This is a gas-phase measurement of $\ce{A^{−} + H^{+} -> HA}$
* **[Hammett acidity ($H\_0$)](http://en.wikipedia.org/wiki/Hammett_acidity_function)**: This is a solution measurement, given by $\mathrm pK\_{\ce{BH^+}} - \log\left(\frac{[\ce{BH^+}]}{[\ce{B}]}\right)$


Secondly, it depends on medium as mentioned by LDC3.


* **Leveling effect**: The [solvent leveling effect](http://en.wikipedia.org/wiki/Leveling_effect) reflects the lowest possible $\mathrm pK\_\mathrm a$ in a particular solvent, based on the basicity of the conjugate base. So you need $\ce{HF}$ or fluorosulfuric acid to reach low $\mathrm pK\_\mathrm a$.


So the problem in my mind is that carboranes and fluoroantimonic acid are solution measurements, but $\ce{HeH+}$ is a gas-phase measurement. It does have the highest gas-phase proton affinity. But I'd put my money on things I can use in lab.


Incidentally, the Reed group prepared the fluorinated carborane acid this year: [*Angew. Chem. Int. Ed.* **2014,** *53* (4), 1131–1134](https://doi.org/10.1002/anie.201308586). So that compound, $\ce{H(CHB11F11)}$ wins the crown for strongest solution-phase Brønstead acid (for the moment, at least).


There's also a nice review article "Myths about the Proton. The Nature of H+ in Condensed Media" by the same group in [*Acc. Chem. Res.* **2013,** *46* (11), 2567–2575](https://doi.org/10.1021/ar400064q).


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From [Wikipedia](http://en.wikipedia.org/wiki/Superacid)  

It depends on the medium.


For plasma: [helium hydride ion](http://en.wikipedia.org/wiki/Helium_hydride_ion)  

For liquids: [fluoroantimonic acid](http://en.wikipedia.org/wiki/Fluoroantimonic_acid)


From [Wikipedia](http://en.wikipedia.org/wiki/Fluoroantimonic_acid)  

Fluoroantimonic acid - $H\_0$ Value = −31.3  

Carborane superacid - $H\_0$ Value = −18.0  

100% Sulfuric Acid - $H\_0$ Value = -12.0  

I didn't find any information about $\ce{HeH^+}$, I guess it's because it only exist in a plasma and difficult to study.


 15

Carborane Acid is the strongest acid.


The reason this is technically the world’s strongest acid is because it’s the strongest acid as a **single molecule**. Much stronger acids- the acids that melt through tables- can be created by mixing certain antimony compounds with **hydrofluoric acid ($\ce{HF}$)**. For example when you mix $\ce{SbF5}$ with $\ce{HF}$, you get an acid with a pH of **-31**.




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Reference: [Chemistrytwig](http://chemistrytwig.com/2011/05/09/what-is-the-strongest-acid/)


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