4.2.2 Electrolysis of concentrated aqueous solution

4.2.2 Electrolysis of concentrated aqueous solution

Electrolysis of a concentrated aqueous solution of sodium chloride called brine is used for the industrial production of sodium hydroxide. Electrolysis is carried out by using inert electrodes such as platinum, or carbon (graphite). The electrolytic cell is shown in Figure 4.3. The brine contains sodium ions and chloride ions from NaCl and hydrogen ions and hydroxide ions from water.

During electrolysis, chloride ions move towards the anode. At anode chloride ions are oxidized to produce chlorine gas. Sodium ions move towards the cathode. However, sodium ions are not reduced to sodium metals in this process. Instead, hydrogen ions from water are discharged to produce hydrogen gas. More and more water molecules keep ionizing to replace $\mathbf{H}^{+}$ ions as soon as they are consumed. The electrolyte in the solution becomes sodium hydroxide (NaOH). Following reactions occur in the electrolytic cell.

At anode:

The solution contains ${\mathrm{N}\mathrm{a}}^{+}$$\mathrm{Na}^{+}$ and ${\mathrm{O}\mathrm{H}}^{-}$$\mathrm{OH}^{- }$ ions. Evaporation of water from this solution produces pure solid sodium hydroxide.

4.2.3 Electrolysis of dilute aqueous solution

a. Electrolysis of dilute aqueous Sodium chloride using platinum electrodes

Unlike electrolysis of concentrated aqueous sodium chloride, chloride ions are not oxidized at the anode. Instead, hydroxide ions are oxidized and oxygen gas is produced at the anode. Reaction at the cathode remains the same. And hydrogen is produced at the cathode. The following reactions take place during electrolysis.

At cathode:

At anode:

b. Electrolysis of dilute sulphuric acid using platinum electrodes

In an aqueous solution, sulphuric acid ionizes to form hydrogen ions and sulphate ions. Autoinization of water gives hydrogen ions and hydroxide ions.