Question

What is the colour of phenolphthalein?

Answer 1

1. Phenolphthalein (${\mathrm{C}}_{20}{\mathrm{H}}_{14}{\mathrm{O}}_4$) is a white colour organic compound which belongs to the Phthalein family with a molar mass of $318.328{\mathrm{gmol}}^{-1}$.
2. It is a weak acid with low solubility in water or aqueous solution but more soluble in ethanol.
3. The short notation of Phenolphthalein is HPh, phph, Ph and is used for the identification of acid and base.
4. It is synthesized by the reaction of Phenol (${\mathrm{C}}_6{\mathrm{H}}_6\mathrm{O}$) and Phthalic acid (${\mathrm{C}}_8{\mathrm{H}}_6{\mathrm{O}}_4$) in the presence of concentrated Sulphuric acid by the heating process.

The Phenolphthalein role for the different colours is described below:

1. Acidic condition: It is a Benzenoid structure with a pH below than the 7 value. The concentration of ${\mathrm{H}}^{+}$ is high and ${\mathrm{Ph}}^{-}$ is low. There is a undissociation of compounds so It becomes colourless.
2. Basic condition: It is a quinonoid structure with a pH range of 8.3 to 10. The concentration of ${\mathrm{H}}^{+}$ is low and ${\mathrm{Ph}}^{-}$ is high. There is a dissociation of the carboxylic compound and it turns into pink colour. $$\begin{gathered} \mathrm{HPh}\left(\mathrm{aq}\right)\rightleftharpoons {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{Ph}}^{-}\left(\mathrm{aq}\right) \\ \mathrm{Unionized}\mathrm{colorless}\mathrm{Ionized}\mathrm{pink} \end{gathered}$$