The ions present are: Ions from sodium chloride - Na+(aq) and Cl-(aq) Ions from water - H+(aq) and OH-(aq)
At the anode, OH- and Cl- ions are attracted to the carbon anode. Cl- ions are selectively discharged as chlorine gas because there is a higher concentration of Cl- ions as compared to OH- ions. Ionic half equation: 2 Cl-(aq) --> Cl2O(g) + 2 e- The OH- ions remain in solution.
At the cathode, H+ and Na+ ions are attracted to the platinum cathode. Sodium is higher in the reactivity series. Thus, Na+ ions are not discharged. H+ ions are preferentially discharged as hydrogen gas. Ionic half equation: 2 H+(aq) + 2 e- --> H2(g) The Na+ ions remain in solution.
The overall chemical equation: 2 NaCl(aq) + 2 H2O(l) --> 2 NaOH(aq) + H2(g) + Cl2(g) In the electrolysis of concentrated sodium chloride solution, The ratio of Cl2 to H2 collected in the electrolysis is 1:1. The solution gradually becomes alkaline during electrolysis because Na+ and OH- are left in the solution, which combine to form sodium hydroxide. The presence of alkaline solution can be detected by using Universal Indicator. It turns violet when added to the electrolysed solution.