Question

What is the colour of phenolphthalein?

Answer 1

Phenolphthalein is used as an indicator for acid-base titration. At a suitable pH the colour of phenolphthalein changes. In this way, we can identify that the titration is over.

Complete answer:
Phenolphthalein is an organic compound with the formula ${{C}_{20}}{{H}_{14}}{{O}_{4}}$. It is a dye and it belongs to the class of phthalein dyes. It is mainly dissolved in alcohols for organic experiments but it is also slightly soluble in water.
Phenolphthalein is an acid-base indicator and it has a different colour in acidic and basic solution. Let us see the colour of phenolphthalein in different conditions.
- In strongly acidic condition, the colour of phenolphthalein is orange since it exists in the protonated form $\left( {{\text{H}}_{3}}\text{I}{{\text{n}}^{+}} \right)$.
- In between strongly acidic and slightly basic condition, it is colourless and it exists in lactone form $\left( {{\text{H}}_{2}}\text{In} \right)$.
- When the pH further increases, the colour changes to pink and it exists in doubly deprotonated phenolate form $\left( \text{I}{{\text{n}}^{2-}} \right)$.
- In the strongly basic condition, the pink colour starts to fade and when the pH is greater than 13, it becomes colourless since it is converted into $\text{In}{{\left( \text{OH} \right)}^{3-}}$ form.
Hence, the colours of phenolphthalein in different pH are:
Strongly acidic – Orange
Slightly acidic or near neutral – Colourless
Slightly basic – Pink
Strongly basic – Colourless

Note:
Acid-base titration is used for volumetric analysis of a given acid or base. If we use phenolphthalein as an indicator in the analysis, the colour changes to pink in the basic medium. So we have to remember this for the analysis.