Hydrogen Fluoride - Acidity

Acidity

The acidity of hydrofluoric acid solutions vary with concentration owing to hydrogen-bond interactions of the fluoride ion. Dilute solutions are weakly acidic with an acid ionization constant K_a = 6.6×10−4 (or pK_a = 3.18), in contrast to corresponding solutions of the other hydrogen halides which are strong acids. Concentrated solutions of hydrogen fluoride are much more strongly acid than implied by this value, as shown by measurements of the Hammett acidity function H₀ (or “effective pH”). For 100%, HF has an H₀, estimated to be between −10.2 and −11, which is comparable to the value −12 for sulfuric acid.

In thermodynamic terms, HF solutions are highly non-ideal, with the activity of HF increasing much more rapidly than its concentration. The weak acidity in dilute solution is sometimes attributed to the high H—F bond strength, which combines with the high dissolution enthalpy of HF to outweigh the more negative enthalpy of hydration of the fluoride ion. However, Giguère and Turrell have shown by infrared spectroscopy that the predominant solute species is the hydrogen-bonded ion-pair, which suggests that the ionization can be described as a double equilibrium:

H₂O + HF H₃O+ + F−

The first equilibrium to the right and the second to the left, meaning that HF is extensively dissociated, but that the tight ion pairs reduce the thermodynamic activity coefficient of H₃O+, so that the solution is effectively less acidic.

In concentrated solution, the additional HF causes the ion pair to dissociate with formation of the hydrogen-bonded hydrogen difluoride ion.

+ HF H₃O+ + HF₂−

The increase in free H₃O+ due to this reaction accounts for the rapid increase in acidity, while fluoride ions are stabilized (and become less basic) by strong hydrogen bonding to HF to form HF₂−. This interaction between the acid and its own conjugate base is an example of homoconjugation. At the limit of 100% liquid HF, there is autoionization

2 HF H₂F+ + F−

that forms an extremely acidic solution (H₀ = −11).

The acidity of anhydrous HF can be increased even further by the addition of Lewis acids such as SbF₅, which can reduce H₀ to −21.