Question

What type of indicator is phenolphthalein?
A. Acidic
B. Basic
C. Equivalent (titration)
D. Neutral

Answer 1

Hint: Try to recall that an indicator is something which shows that a change has happened and is used to check whether a substance is acid or base. Phenolphthalein is a chemical compound with chemical formula $C_{20}{H}_{14}{O}_4$ and is used as an indicator in acid-base titration. Now, by using this you can easily find the correct option from the given options.

Complete step by step answer:
- It is known to you that phenolphthalein is an acid base indicator used to test the $pH$ of the solution.
- Phenolphthalein is an organic compound with chemical formula $C_{20}{H}_{14}{O}_4$ and is a synthetic indicator.
- It is usually prepared by reacting phthalic anhydride and two molecules of phenol in an acidic medium.
- Phenolphthalein ($HIn$) is weakly acidic in nature and in aqueous solution, it dissociates into $H^{+}$ and $I{n}^{-}$ ions. Here, the $H$ denotes the hydrogen protons and the $In$ indicates the indicator part of the ion.
- The pink colour of the solution is due to the concentration of $I{n}^{-}$ ions in the solution.
- Under acidic conditions, the concentration of $I{n}^{-}$ in the solution is very low and concentration of $H^{+}$is high, hence it is colourless.
- Similarly, under basic conditions, the concentration of $$H^{+}$$ ions is very low and concentration of $I{n}^{-}$ is high, hence the solution is pink coloured.
- The structure of phenolphthalein is as follows:

So, the correct answer is “Option A”.

Note: - It should be remembered to you that the phenolphthalein indicator is a weak colourless acid that forms pink anions when it is dissolved in water. Although the anions are pink, the solution remains colourless in the presence of an acid.
- If the $pH$ of the solution is 8.2 or above, then the number of anions increases, causing the solution to pink, the anions of the phenolphthalein molecule actually act as an indicator.