I was wondering how one could test the hypothesis that boric acid behaves not as a Brønsted acid but as a Lewis acid in that boric acid cleaves water molecules as to make hydrogen protons.

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rad Could one run an experiment involving boric acid in heavy water ($\ce{D_2O}$)?

Thus, if boric acid behaves as a Brønsted acid, the concentration of deuterium cation should stay the same, right? Boric acid might release hydrogen protons but those wouldn't affect the deuterium protons, right? Wouldn't heavy water still undergo auto-ionization and create a certain equilibrium concentration of the heavy equivalents of hydronium and hydroxide ions?

And thus, if boric acid behaves as a Lewis acid, couldn't we somehow precipitate the $\ce{B(OH)_3(DO)^-}$, and test whether the precipitate has D rather than H? I.e. introduce sodium chloride into the system and evaporate to dryness. Wouldn't that leave behind a precipitate consisting partly of sodium ion and partly of the borate? On the other hand what would happen to the excess chloride ion?

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    $\begingroup$ There will always be exchange of hydrogen and deuterium, even for highly covalent bonds like carbon hydrogen bonds, resulting in a wild mixture according to a shared Boltzmann statistics. $\endgroup$ – Martin - マーチン Aug 11 '14 at 16:21
  • $\begingroup$ Why does this exchange happen? Kinetics? $\endgroup$ – Dissenter Aug 11 '14 at 16:25
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    $\begingroup$ It would be helpful if your diagram better reflected your question. Also, deuterium is not radioactive. $\endgroup$ – ron Aug 11 '14 at 17:11

As for acidity of boric acid you can find on https://en.wikipedia.org/wiki/Boric_acid that it dissociates in both ways, but is very weak. However it's acidic enough that it's only a matter of time until it would become almost completely deuterised if dissolved in D2O. It happens because in mixture there would be much more deuterium then normal hydrogen. The acid is too weak to capture or detect its anions. Maybe kinetics of deuterisation would give us some info about properties of its anions.

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