Let's look at another example. Thymol blue is another weak acid that is used as an indicator. The fully protonated species (abbreviated H₂Thy) is red, the monoprotonated species (HThy^-) is yellow, and the fully deprotonated species (Thy^2-) is blue. The ionization reactions for thymol blue are shown below:

At any pH, the color of a thymol blue solution can be predicted using the acid ionization constants. For example, at a pH of 4, the [H⁺] is 1 x 10^-4. If this substituted in both acid ionization expressions, two different ratios can be calculated:

This tells you that the concentration of HThy^- is 200 times greater than the concentration of H₂Thy and over 70,000 times greater than Thy^2- (the inverse of 1.3 x 10^-5 gives the ratio of HThy^- to Thy^2-). So HThy^- is definately the most prevalent thymol blue species in solution. Since HThy^- is yellow, the solution will be yellow.