Question

Why electrolysis of an aqueous solution of Sodium sulfate $N{a}_{2}S{O}_4$ containing litmus develops a blue colour at the cathode and red colour at the anode?

Answer 1

During the electrolysis of Sodium sulfate $N{a}_{2}S{O}_4$ the following reactions takes place at cathode and anode.

$$\begin{gathered} Cathode:2H_{2}O\left(l\right)+2e^{-}\to H_2\left(g\right)+2O{H}^{-}\left(aq\right) \\ Anode:2H_{2}O\left(l\right)\to 4H^{+}\left(aq\right)+O_2\left(g\right)+4e^{-}\left(aq\right) \end{gathered}$$

Due to the formation of $O{H}^{-}$ ion at cathode which is base, it turns red litmus blue whereas due to the formation of $H^{+}$ ions at anode which is acid, it turns blue litmus red.