Can pure hydronium exist?

Question

Can pure hydronium exist? If not, why not? It seems to me (and I am no chemistry expert) like pure hydronium should have the theoretical maximum acidity or minimum pH that a substance can reach.

Can't, for example, we obtain pure hydronium if we could somehow produce a container with a certain amount of pure ionized hydrogen (H⁺), and mixed it with an equal number of moles of water (H₂O) to produce pure hydronium (H₃O⁺)? Or if that example is too theoretical, couldn't we also simply mix an equal number of moles of hydrochloric acid (HCl) and water (H₂O) to produce something close enough, hydronium (H₃O⁺) with chloride ions (Cl⁻)?

Answer 1

$\ce{H3O+}$ is a ion – a charged particle.

As an isolated ion, "naked" in vacuum or gas, or hydrated in water, it is a common thing. But with macroscopic quantities, it is more complicated.

If we hypothetically collected about $\ce{20 g}$ of hydronium, it would contain electrostatic charge of about 1000 CG+ lightnings 10 km long. It would cause a gigantic Coulombic explosion.

OTOH, hydronium balanced with counter anion of a strong acid, dissolved in water, is a common molecular entity chemists works with. There can be easily $\pu{200 g/L}$ of hydronium ions.

Note that crystalline monohydrate of some very strong acids is de facto a hydronium salt $\ce{[H3O+][A-]}$. It happens with some exceptions for acidity constants $\mathrm{p}K_\mathrm{a} \le –9$.

Answer 2

Absolutely, and it's not even difficult. Sure, in the dilute gas phase naked $\ce{H3O+}$ is commonly found simply due to the relative ease of formation and abundance of $\ce{H+}$ and $\ce{H2O}$, in space and in your friendly neighbourhood mass spectrometry facility. But you don't need anything nearly as exotic.

As long as you have a sufficiently strong acid, its ionization in the presence of water is so favourable that in the right conditions it will force just about every single water molecule it touches to accept a proton and become a hydronium ion. Clear evidence of this is found in solid hydronium salts, where you can even directly determine the presence of hydronium cations through x-ray crystallography. These salts will often form with an exact 1:1 stoichiometric mixture of water and acid molecules.

Now, you might think it takes some crazily strong acid to do it. However, a nice example is para-toluenesulfonic acid ($\ce{TsOH}$), one of the most common organic acids in a chemistry laboratory, which is almost always encountered as the monohydrate, $\ce{TsOH.H2O}$. What not many people seem aware of is that "p-toluenesulfonic acid monohydrate" is perhaps more precisely called hydronium p-toluenesulfonate, $\ce{H3O+TsO^-}$. It's an easy-to-handle, very well-behaved solid with a stable composition in ambient conditions. It is indeed about as strong of an acid as you can get (in water), though this is not a particularly great achievement, as solvent levelling means a considerable number of acids, even those not containing hydronium ions, will form solutions of almost equal acidity when mixed in the same molar amount.

Lastly, as it turns out, hydrogen chloride ($\ce{HCl}$) is a fairly strong acid, but it cannot form hydronium chloride $\ce{H3O+Cl^-}$ in ambient conditions. This ultimately comes down to the fact that $\ce{HCl}$ is actually a gas in ambient conditions, with a very low boiling point of -85 °C, so before you can even get a 1:1 molar mixture of $\ce{H2O}$ and $\ce{HCl}$ the acid becomes too volatile and just boils away. However, if you lower the temperature, you can generate hydronium chloride $\ce{H3O+Cl^-}$ - it's a solid which happens to decompose at temperatures above -15°C, as shown by the rightmost point in this graph:

Source: Wikipedia

Answer 3

Neutral hydronium may also exist. Huang et al. [1] calculate that $\ce{H3O}$ is stable as a liquid metal phase under high pressure and temperature conditions existing in the interiors of Uranus and Neptune. They hypothesize that this metallic fluid is a source of the peculiar magnetic fields seen on these planets.

Reference

Peihao Huang, Hanyu Liu, Jian Lv, Quan Li, Chunhong Long, Yanchao Wang, Changfeng Chen, Russell J. Hemley  and Yanming Ma (March 3, 2020). "Stability of H3O at extreme conditions and implications for the magnetic fields of Uranus and Neptune". Proc. Nat. Acad. Sci. 117 (11), 5638-5643. https://doi.org/10.1073/pnas.1921811117.