Hydronium ion is the strongest acid

Oxonium (also Oxidanium) is the name for protonated water (H3O+) and belongs to the hydrogen ions according to IUPAC. The names Hydroxonium or Hydronium are out of date and should no longer be used according to the recommendations of the chemical nomenclature, but are still widely used in the literature.
The pKa value of the oxonium ion is -1.74.

H+ · H2O ≡ H3O+
Oxonium ion

Oxonium ions are formed by autoprotolysis of the water, whereby a proton (H.+) passes from one water molecule to another. In equilibrium in neutral water, at a temperature of 25 ° C, there is a molar concentration of the oxonium ions (as well as the hydroxide ions) of 10−7 mol / l, which defines the neutral pH value of 7.

As a result of the autoprotolysis of the water, oxonium and hydroxide ions are constantly produced, even in neutral water, which react back to form water molecules.

The addition of acids increases this equilibrium concentration through the transfer of protons from the acid to water molecules, and the pH value is lowered. In alkaline solutions, the pH increases as the concentration of oxonium ions decreases.

The lifespan of the oxonium ion is very short (around 10−13Seconds), since the attached proton is very easily passed on to another water molecule:

Transfer of a proton from one water molecule to another.

In solution there is a continuous transition between differently hydrated protons. During the transfer, a proton is always passed on from one oxygen atom to the next. The positions of the individual atoms change only minimally. This defect migration, which is also known as the Grotthuss mechanism, is the reason for the high ion equivalent conductivity of protons of 315 S · cm compared to other ions3· Mol-1.

The Zundel ion and the Self-ion are the limiting species in this process. The Zundel ion can formally be viewed as a proton that is hydrated by two water molecules:

H+ • 2 H.2O ≡ H5O2+
Zundel Ion (+1)

In contrast, the Eigen ion (formally [H9O4]+ or Tetraoxidanium) formally an oxonium hydrated by three water molecules.

H3O+ · 3 H2O ≡ H9O4+
Self-ion (+1)

Since this hydration in aqueous solution does not play a role in calculations, the notation H is usually used3O+ (the actual oxonium ion) or even just H+ (Hydron) used. Free protons are practically non-existent in aqueous solutions.

Oxonium salts

Stable oxonium salts are only used by the strongest acids, e.g. B. perchloric acid:

Water is protonated by perchloric acid. This creates an oxonium ion and the perchlorate anion. Both together form the oxonium salt.

See also

Categories: Chemical Compound | Acid-base reaction