Dip a piece of litmus paper into the acid solution. Submerge it for only a few seconds, then allow it to dry on a nonreactive surface.
Compare the color of the litmus paper against the standards provided with its kit. To find the pH, match the developed color to the most similar color in the comparative charts.
Raise the negative of the pH to the base 10. For example, if the pH is 3.2, raising it to the base 10 will give 10^-3.2. This yields the hydrogen ion concentration in the acid solution.
Square the hydrogen ion concentration and divide it by the molarity of the acid. This operation yields the rate constant, known as the Ka, for the solution.
Take the negative logarithm of the Ka value. This yields the pKa of the unknown acid.