HF or hydrogen fluoride is a weak acid with a pKa value of 3.2. Compared with the other hydrogen halides, HF is the weakest in terms of acidity. The dissociation of HF in the solution of water can be expressed as:
`HF + H_2O <=> H_3O^(+) + F^(-)`
*double-headed arrow indicates that the reaction is reversible.
There are many methods in determining the acidity of HF in the solution. A physical method of using litmus paper can be done. it the blue litmus paper turns to rest the solution is confirmed to be an acid.
Another is the use of chemical indicators such as methyl red and phenolphthalein. You can also work out in calculation using the pKa value of HF at given concentration, temperature and pressure.
Lastly, we can titrate the unknown solution with a known standard reactant. With the use of an indicator, we can determine the amount of the standard reactant used. With mole ratio equivalents, we can be able to get the amount of HF in the solution and eventually solving for the pH of the solution, again using its pKa value.