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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⁻)?

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$\ce{H3O+}$ 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 $\mathrm{p}K_\mathrm{a} \le –9$.

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  • $\begingroup$ It's theoretically possible to create a diffuse plasma with H3O+ and e- being the dominating particles. Not sure if there is a range of conditions where it would be thermodynamics stable. $\endgroup$
    – permeakra
    Oct 18, 2023 at 12:18
  • $\begingroup$ It is possible. But I bet OP is interested in more standard conditions. $\endgroup$
    – Poutnik
    Oct 18, 2023 at 12:40
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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

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  • $\begingroup$ Acids reportedly need to have pKa <=-9 to be able to form solid hydronium salts ( CH SE answer ) $\endgroup$
    – Poutnik
    Oct 18, 2023 at 6:59
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    $\begingroup$ @Poutnik That's a rule of thumb which applies in ambient conditions, but details such as the energetics of specific hydronium crystal lattices play a role to soften that boundary. $\endgroup$ Oct 18, 2023 at 7:02
  • $\begingroup$ Good to know. Interesting fact. $\endgroup$
    – Poutnik
    Oct 18, 2023 at 7:06
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    $\begingroup$ As a curiosity, p-toluenesulfonic acid monohydrate melts at 105-107 °C (whereupon it presumably loses its stoichiometric hydronium nature and becomes an equilibrium including some free water molecules), while sulfuric acid forms a stable monohydrate (i.e. hydronium bisulfate, or amusingly hydronium hydrogen sulfate) only below around 10 °C, even though it is a moderately stronger acid in principle. $\endgroup$ Oct 18, 2023 at 7:10
  • $\begingroup$ @NicolauSakerNeto hydronium bisulfate melts under ambient conditions but retains its ionic structure, which leads to a maximal viscosity and minimal ionic conductivity of sulfuric acid solutions at this composition. See here. $\endgroup$ Oct 18, 2023 at 12:20
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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.

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  • $\begingroup$ Although the recent papers on Ice-XIX may also impact Uranus and Neptune in a similar fashion. $\endgroup$
    – Jon Custer
    Oct 18, 2023 at 12:27
  • $\begingroup$ True, hence I (and the authors) say a "hypothesis". We would expect Ice XIX, with its mobile protons, to contain some mixture of species having the form $\ce{H_nO^{n-2}}$ with $n=1,2,3$ possibly also $n=0,4$. $\endgroup$ Oct 18, 2023 at 12:42
  • $\begingroup$ Not that it was not obvious before, but water really is the weirdest stuff in the universe. $\endgroup$
    – Jon Custer
    Oct 18, 2023 at 12:44
  • $\begingroup$ Other weird stuff: helium (needs pressure to form a solid, two liquud phases), silicon (beats water in percent contraction upon melting), many humans. $\endgroup$ Oct 18, 2023 at 12:46
  • $\begingroup$ Plutonium's 6 allotropes (3 unique to Pu) at 1 atmosphere (and an additional one at moderate pressures) is cool, but 19 solid ice phases across the P-T diagram is hard to beat, and their diversity is broad as well. $\endgroup$
    – Jon Custer
    Oct 18, 2023 at 12:49

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