Hydronium - Solid Hydronium Salts

Solid Hydronium Salts

For many strong acids, it is possible to form crystals of their hydronium salt that are relatively stable. Sometimes these salts are called acid monohydrates. As a rule, any acid with an ionization constant of 109 or higher may do this. Acids whose ionization constant is below 109 generally cannot form stable H₃O+ salts. For example, hydrochloric acid has an ionization constant of 107, and mixtures with water at all proportions are liquid at room temperature. However, perchloric acid has an ionization constant of 1010, and if liquid anhydrous perchloric acid and water are combined in a 1:1 molar ratio, solid hydronium perchlorate forms.

The hydronium ion also forms stable compounds with the carborane superacid H(CB₁₁H(CH₃)₅B₆). X-ray crystallography shows a C_3v symmetry for the hydronium ion with each proton interacting with a boron atom each from three carborane anions 320 pm apart on average. The
11 salt is also soluble in benzene. In crystals grown from a benzene solution the solvent co-crystallizes and a H₃O·(benzene)₃ cation is completely separated from the anion. In the cation three benzene molecules surround hydronium forming 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 examples of hydrated hydronium ions known, such as the H₅O+
2 ion in HCl·2H₂O, the H₇O+
3 and H₉O+
4 ions both found in HBr·4H₂O.