Like $\ce{NH4+}$ ,Is there any possibility of formation of $\ce{H4O^{2+}}$ (of tetrahedral structure)?

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My theory is: it can be formed by osmosis setup where heavy acids like $\ce{H3PO4}$ or $\ce{H2S2O7}$ are passed through a semipermeable membrane like gelatinous copper ferrocyanide SPM, which allow only small ions like H+ to pass.

This then could form $\ce{H3O+}$, then its lone pair could attack the excess $\ce{H+}$ in solution to form forming $\ce{H4O^{2+}}$.

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    $\begingroup$ Semipermeable membranes just don't work like that. You can't separate cations from anions this way, and that's a very, very huge "can't" (as in "a mosquito can't lift the Empire State Building"). The possible (?) existence of $\ce{H4O^2+}$ in extreme conditions is another story. $\endgroup$ Apr 9, 2016 at 6:55
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    $\begingroup$ see chemistry.stackexchange.com/questions/24342/… $\endgroup$
    – Mithoron
    Apr 9, 2016 at 11:14
  • $\begingroup$ Osmosis can reject anions as stated in rejection of anions by osmosis @IvanNeretin $\endgroup$
    – Swastik
    Apr 10, 2016 at 7:46
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    $\begingroup$ Sure, but it would also place another ions instead of them. $\endgroup$ Apr 10, 2016 at 8:48

1 Answer 1


The existence of $\ce{H4O^{2+}}$ has been inferred from hydrogen/deuterium isotopic exchange monitored through $\ce{^{17}O}$ NMR spectroscopy in the most extremely acidic condensed phase superacid we can make, fluoroantimonic acid ($\ce{HF:SbF5}$ or $\ce{HSbF6}$). It seems that even the slightly weaker but still very much superacidic magic acid $\ce{HSO3F:SbF5}$ is not strong enough to effect measurable protonation of $\ce{H3O+}$ into $\ce{H4O^{2+}}$. It likely also can exist in the gas phase by exposing water to bare protons, and might have some importance in astrochemistry. Relatedly, $\ce{H4S^{2+}}$ also has been inferred to exist through the same type of measurements, and is easier to make, existing in magic acid mixtures.

A nice source for your superacidic needs is books by George Olah, the father of superacidic chemistry. In particular, Onium Ions has a section on the protohydronium dication $\ce{H4O^{2+}}$ on page 435. There are many other curious cations analyzed such as $\ce{CH6^{2+}}$ and even $\ce{CH7^{3+}}$. When it comes to superacids, nothing is sacred!

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    $\begingroup$ Minor point. Fluoroantimonic acid is not $\ce{HSbF6}$. The fluoroantimonate anion is such a weak proton acceptor that it never gets protonated at all; therefore the acid forms only ionic species. See here for a description of these ionic species. $\endgroup$ Apr 4, 2019 at 23:32

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