More Detail About Acid-Base Indicators
In acidic or weakly basic solutions (pH < 8), the H+ is "forced" to stay on the phenolphthalein molecule (the equilibrium is shifted to the left). Since the protonated form is colorless, so is the solution. Gradually, between the pH values of 8 and 9.5, the equilibrium begins to shift to the right and the proton is lost. The unprotonated form is purple so the solution begins to turn purple. This is the color change region. Above a pH of 9.5, the equilibrium shifts far to the right, nearly all of the protons are lost, and the solution is very purple.