Hydrofluoric acid ionization constant

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 Ka = 6.6 x 10 " (or pKa = 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 Ho (or effective pH ). For 100%, HF has an Ho, estimated to be between -10.2 and -11, which is comparable to the value -12 for sulfuric acid. 

In dilute aqueous solutions, the three halides, HC1, HBr, and HI, are essentially fully ionized, but in the poorer ionizing solvent, methanol, ionization is partial. In the latter solvent, HC1 is the most weakly ionized of the three whereas HI is the most strongly ionized this order of acidities has already been discussed (p. 90 see also Exercise 14). The ionization constant for hydrofluoric acid in dilute solutions is 7.2 X 10 4, but in very concentrated solutions the degree of ionization appears to rise sharply, a behavior opposite to that of almost all other weak electrolytes. This increase in acid strength is almost certainly due to the increase in concen-... 

Most acids are weak acids, which ionize only to a limited extent in water. At equilibrium, aqueous solutions of weak acids contain a mixture of nonionized acid molecules, ions, and the conjugate base. Examples of weak acids are hydrofluoric acid (HF), acetic acid (CH3COOH), and the ammonium ion (NH4). The limited ionization of weak acids is related to the equilibrium constant for ionization, which we will study in the next section. 

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