Indicators are the substances which indicate by change in color, the completion of the chemical reaction. We know that litmus solution is red in the acidic region and blue in the basic region. A number of weak organic acids can act as indicators if the unionized acid, HI ion and its anion have different colors.
For example:
In volumetric analysis, during the titration of sodium hydroxide and hydrochloric acid which are taken in the burette, phenolphthalein turns pink to colorless when the whole of sodium hydroxide has been neutralized by hydrochloric acid. All indicators show change in color over some pH range which varies considerably from one indicator to another.
