Phenolphthalein is an indicator used for titrations of a weak acid and strong base, and itself is a weak acid. The weak acid is colorless and its ion is bright pink. It distinguishes the pH range from 8 to 9.6. Adding extra hydrogen ions shifts the position of equilibrium to the left, and turns the indicator colorless. Adding hydroxide ions removes the hydrogen ions from the equilibrium which shifts to the right to replace them - turning the indicator pink.

Methyl orange, Methyl red are used for the titration of strong acid and strong base or strong acid and weak base while Bromothymol blue is a dye used as an indicator in determining pH. Bromothymol blue is a weak acid. It can be in acid or base form, depending on the pH of the solution.