Estimation of pH - Acid-Base Indicators

More Detail About Acid-Base Indicators

Acid-base indicators are weak acids or bases themselves. They are one color in the acid form and another in the base form. Phenolphthalein is a common indicator with a color change region of 8 to 10. While its structure is relatively complex, its reaction can be simply expressed:

PhH

Ph^- + H⁺

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.