pH indicator Phenolphthalein (pH indicator) below pH 8.3 above pH 10.0 8.3 ⇌ 10.0 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.[3] Phenolphthalein adopts different forms in aqueous solution depending on the pH of the solution.[4][2][5][6] 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 (H3In+) under strongly acidic conditions, providing an orange coloration. However, a later paper suggested that this color is due to sulfonation to phenolsulfonphthalein.[2] The lactone form (H2In) is colorless between strongly acidic and slightly basic conditions. The doubly deprotonated (In2-) phenolate form (the anion form of phenol) gives the familiar pink color. In strongly basic solutions, phenolphthalein is converted to its In(OH)3− form, and its pink color undergoes a rather slow fading reaction[6] and becomes completely colorless when pH is greater than 13.