Question

What is phenolphthalein?

Answer 1

Titration is the technique used for the quantitative analysis of any solution. This process determines the concentration of any substance in a solution. At the equivalence point or the end point of titration, where the reactants are fully utilized, then there is a color change that is indicated by an indicator. Phenolphthalein is an indicator.

Complete answer:
Quantitative analysis is used to determine the concentration of a substance in a solution. This is achieved by titration technique that involves the reaction of the known and unknown concentration of the standard solution with a titrant. The reaction involves an equivalence end point where there is a change in pH. This change in ph is detected by an indicator that creates a color change at this point.
Phenolphthalein is a weak acid – base indicator that is used in a basic standard solution to detect the change in pH. It gives colorless pink color change. Phenolphthalein is dissociated to give hydronium ions at a pH from 8.3 to 10 therefore used in basic solutions to detect the end point. It dissociates as:
$H{{I}_{n}}+{{H}_{2}}O\rightleftharpoons {{I}_{n}}^{-}+{{H}_{3}}{{O}^{+}}$
Hence, phenolphthalein is a weak acid that is used as an indicator in basic solutions that
changes from colorless to pink at the end point.

Note:
Phenolphthalein has a molecular formula as${{C}_{20}}{{H}_{14}}{{O}_{4}}$ which is an organic molecule. Apart from phenolphthalein, methyl orange is used as an acid – base indicator and changes its color from light yellow to orange at the end point, it is used in acidic solutions as methyl orange has a basic nature and dissociates to give hydroxide ions.