The negative logarithm of the hydrogen ion concentration, the pH, is expressed as follows:
pH = -log10 [H+]
The pH scale is a measure of hydrogen ion concentration that eliminates dealing with large powers of 10 and compresses a large range of concentrations onto a more convenient scale, between 1 and 14 as show in the figure below:
Strong acids are considered to be completely dissociated into ions in dilute solutions. However, weak acids (or bases) are only partially dissociated in solution, and thus an equilibrium is established between the ions and the undissociated molecules.
This equilibrium can be represented by the equation:
This equation can be rearranged to give a Ka value, which is a measure of how strong an acid is.
Stronger acids will dissociate more and will have a higher Ka value.
Use of Ka values is not very useful, as the differences in dissociation are massive between strong and weak acids, and the values calculated can vary by many orders of magnitude. As is done with the pH scale, we take the negative log of the Ka value to give the pKa value.