Editing Hydronium (section)

==Solid hydronium salts==
{{See also|Hydronium perchlorate}}
For many [[strong acids]], it is possible to form crystals of their hydronium salt that are relatively stable. These salts are sometimes called ''acid monohydrates''. As a rule, any acid with an [[ionization constant]] of {{10^|9}} or higher may do this. Acids whose ionization constants are below {{10^|9}} generally cannot form stable {{H3O+}} salts. For example, [[nitric acid]] has an ionization constant of {{10^|1.4}}, and mixtures with water at all proportions are liquid at room temperature. However, [[perchloric acid]] has an ionization constant of {{10^|10}}, and if liquid anhydrous perchloric acid and water are combined in a 1:1 molar ratio, they react to form solid [[hydronium perchlorate]] ({{chem2|H3O+*ClO4(-)}}).{{citation needed|date=November 2021}}

The hydronium ion also forms stable compounds with the [[carborane superacid]] {{chem2|H(CB11H(CH3)5Br6)}}.<ref>
{{Cite journal|doi=10.1021/ja0551335 | pmid=16464096| url = https://www.researchgate.net/publication/7310527| title = The Nature of the H<sub>3</sub>O<sup>+</sup> Hydronium Ion in Benzene and Chlorinated Hydrocarbon Solvents. Conditions of Existence and Reinterpretation of Infrared Data| journal = [[Journal of the American Chemical Society]]| volume = 128| issue = 6| pages = 1948–58| year = 2006| last1 = Stoyanov | first1 = Evgenii S. | last2 = Kim | first2 = Kee-Chan | last3 = Reed | first3 = Christopher A. | s2cid=33834275}}</ref> [[X-ray crystallography]] shows a {{chem2|C_{3v}|}} [[point group|symmetry]] for the hydronium ion with each proton interacting with a bromine atom each from three carborane anions 320 [[picometer|pm]] apart on average. The {{chem2|[H3O] [H(CB11HCl11)]}} salt is also soluble in [[benzene]]. In crystals grown from a benzene solution the solvent co-crystallizes and a {{chem2|H3O*(C6H6)3}} cation is completely separated from the anion. In the cation three benzene molecules surround hydronium forming [[Cation–π interaction|pi-cation interactions]] with the hydrogen atoms. The closest (non-bonding) approach of the anion at chlorine to the cation at oxygen is 348 pm.

There are also many known examples of salts containing hydrated hydronium ions, such as the {{chem2|H5O2(+)}} ion in {{chem2|HCl*2H2O}}, the {{chem2|H7O3(+)}} and {{chem2|H9O4(+)}} ions both found in {{chem2|HBr*4H2O}}.<ref>{{Greenwood&Earnshaw}}</ref>

[[Sulfuric acid]] is also known to form a hydronium salt {{chem2|H3O(+)HSO4(-)}} at temperatures below {{convert|8.49|C|F}}.<ref>I. Taesler and I. Olavsson (1968). "Hydrogen bond studies. XXI. The crystal structure of sulfuric acid monohydrate." Acta Crystallogr. B24, 299-304. https://doi.org/10.1107/S056774086800227X</ref>