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Are there any solids with hydronium ion in its crystal structure?

If so, what are they?

If not, why not? I understand there are hydroxide-containing solids and gels such as aluminium hydroxide but why not hydronium ions? Does this have to do with the positive charge of hydronium ion? I could see the positive charge of hydronium ion excluding the possibility of forming any salts/solids with the group 1 and 2 metals.

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2 Answers 2

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There is a section in this Wikipedia link entitled, "Solid hydronium salts." It says that any acid with an ionization constant of 10^9 or higher may form stable hydronium ion salts. Examples they give include hydronium perchlorate and carborane-based hydronium ion salts.

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  • $\begingroup$ Why 10^9? What's the reason for this Ka value? $\endgroup$
    – Dissenter
    Commented Jun 22, 2014 at 21:18
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    $\begingroup$ They note it as a general rule of thumb, based on experimental observation. They cite HCL with an ionization constant of 10^7 not forming a stable hydronium ion salt with water at any HCL:water ratio at RT. $\endgroup$
    – ron
    Commented Jun 22, 2014 at 21:23
  • $\begingroup$ What do you mean salt with water? $\endgroup$
    – Dissenter
    Commented Jun 22, 2014 at 21:24
  • $\begingroup$ I see, monohydrates. Why must hydronium ion form nonohydrate salts? Bc of the water bound up in hydronium? $\endgroup$
    – Dissenter
    Commented Jun 22, 2014 at 21:28
  • $\begingroup$ 1)not forming a stable hydronium ion salt when mixed with water, 2) monohydrates probably form do to space limitations in the crystal $\endgroup$
    – ron
    Commented Jun 22, 2014 at 21:36
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Are there any solids with hydronium ion in its crystal structure? If so, what are they?

There are crystal structures of acid monohydrates where the acid is deprotonated and the crystal water is protonated, i.e. the hydronium salt has formed. In addition to the hydronium perchlorate (DOI: 10.1107/s0108270103010461) mentioned in ron's answer, there are structures of hydronium tosylate (Acta Cryst B 27(7), 1293-1298, (1971)) and a crown ether complexed to hydronium with perchlorate as counter ion (http://scripts.iucr.org/cgi-bin/paper?S1600536810048622) shown below.

In the animated GIF, the hydronium ions are shown as space-filling while the crown ethers and the perchlorate ions are shown as ball-and-stick. One hydrogen atoms in each hydronium ion hydrogen bonds with perchlorate. If there were a proton transfer between the hydrogen bonding partners, the structure would be perchloric acid monohydrate, but the crystal structure shows that the hydrogen atom is closer to the oxygen of the hydronium than that of the perchlorate, so calling it a hydronium salt it appropriate.

enter image description here

Here is another view showing the hydrogen bond (1.7 Å distance vs 1.2 Å H-O bond length), with the hydronium ions as ball-and-stick and the remainder of the structure in all-bond representation.

enter image description here

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