Phenolphthalein's common use is as an indicator in acid-base titrations. It also serves as a component of universal indicator, together with methyl red, bromothymol blue, and thymol blue.
Phenolphthalein adopts different forms in aqueous solution depending on the pH of the solution.  Inconsistency exists in the literature about hydrated forms of the compounds and the color of sulfuric acid.  Wittke reported in 1983 that it exists in protonated form (HIn) under strongly acidic conditions, providing an orange coloration. However, a later paper suggested that this color is due to sulfonation to phenolsulfonphthalein.
The lactone form (HIn) is colorless between strongly acidic and slightly basic conditions. The doubly deprotonated (In) phenolate form (the anion form of phenol) gives the familiar pink color. In strongly basic solutions, phenolphthalein is converted to its In(OH) form, and its pink color undergoes a rather slow fading reaction and becomes completely colorless when pH is greater than 13.
The pK values of phenolphthalein were found to be 9.05, 9.50 and 12 while those of phenolsulfonphthalein are 1.2 and 7.70.