Phenolphthalein is an indicator that turns from clear to pink in a basic solution, according to Reference.com. In a strongly acidic solution, the indicator is orange, but in the pH range of 0 to 8.2 it is clear. The indicator is pink in solutions with a pH of 8.2 to 12, but turns colorless again in strong bases above this pH range.
Reference.com indicates that phenolphthalein is a weak acid itself that in molecular form is colorless. The addition of a base to a solution of phenolphthalein removes hydrogen ions from the solution, creating more phenolphthalein ions, which causes the color to change.
Chemists use titrations to determine the concentration of an unknown acid or base. They add the opposite reagent of known concentration and carefully measure the amount needed to cause the color change. About.com says, "For a properly performed titration, the volume difference between the end point and the equivalence point is small." At the equivalence point, V1N1 = V2N2, where V is volume and N is the concentration of the acid or base.
Reference.com says that universal indicators are a mix of phenolphthalein, and the indicators methyl red, bromothymol blue, and thymol blue. By mixing several indicators, chemists create a single solution that is useful in determining pH over a wide range.