Acidity
Hydrofluoric acid is classified as a weak acid because of its lower dissociation constant compared to the strong acids. It ionizes in aqueous solution in a similar fashion to other common acids:
- HF + H2O H3O+ + F−
It is the only hydrohalic acid that is not considered a strong acid, i.e. it does not fully ionize in dilute aqueous solutions.
When the concentration of HF approaches 100%, the acidity increases dramatically because of the following equilibrium:
- 2 HF H+ + FHF−
The bifluoride (FHF−) anion is stabilized by the very strong hydrogen–fluorine hydrogen bond.
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