The titration curve of weak acid vs. strong base with phenolphthalein as indicator is shown below. The ${\mathrm{K}}_{\text{phenolphthalein }}=4\times {10}^{-10}$

Given: $\log 2 = 0.3$

The number of following statement/s which is/are correct about phenolphthalein is _______

A. It can be used as an indicator for the titration of weak acid with weak base.

B. It begins to change colour at $\mathrm{pH} = 8.4$

C. It is a weak organic base

D. It is colourless in acidic medium

 

Phenolphthalein is an organic acid and can be represented as $\mathrm{HPh}$.

$\underset{\text{ (colourless) }}{\mathrm{HPh}}\rightleftharpoons {\mathrm{H}}^{+}+\underset{\text{ (Pink) }}{{\mathrm{Ph}}^{-}}$

Using Henderson equation for phenolphthalein

$\mathrm{pH}={\mathrm{pK}}_{\mathrm{In}}+\log \left[\frac{\left[{\mathrm{Ph}}^{-}\right]}{\left[\mathrm{HPh}\right]}\right]$

At equivalence point  $\left[{\mathrm{Ph}}^{-}\right]=\left[\mathrm{HPh}\right]$

$\therefore {\mathrm{pH}}_2={\mathrm{pK}}_{\mathrm{In}}=-\log \left[4\right]+10=-0.6+10=9.4$

Hence at $(9.4\pm 1)\mathrm{PH}$ , phenolphthalein starts changing colour. Phenolphthalein is colourless in acidic medium and pink in basic medium. Phenolphthalein indicator distinguish the pH change between $8$ to $10$. Therefore, it is used for strong acid and strong base titration or weak acid and strong base titration. Hence, A and C are incorrect statements.