The presence of water in a solution of sodium chloride must be examined in respect to its reduction and oxidation in both electrodes. Usually, water is electrolysed as mentioned above in electrolysis of water yielding gaseous oxygen in the anode and gaseous hydrogen in the cathode. On the other hand, sodium chloride in water dissociates in Na and Cl ions. The cation, which is the positive ion, will be attracted to the cathode (−), thus reducing the sodium ion. The chloride anion will then be attracted to the anode (+), where it is oxidized to chlorine gas.
The following half reactions should be considered in the process mentioned:
# Cathode: Na + e → NaE° = –2.71 V
# Anode: 2 Cl → Cl + 2 eE° = +1.36 V
# Cathode: 2 HO + 2 e → H + 2 OHE° = –0.83 V
# Anode: 2 HO → O + 4 H + 4 eE° = +1.23 V
Reaction 1 is discarded as it has the most negative value on standard reduction potential thus making it less thermodynamically favorable in the process.
When comparing the reduction potentials in reactions 2 and 4, the oxidation of chloride ion is favored over oxidation of water, thus chlorine gas is produced at the anode and not oxygen gas.
Although the initial analysis is correct, there is another effect, known as the overvoltage effect. Additional voltage is sometimes required, beyond the voltage predicted by the E°. This may be due to kinetic rather than thermodynamic considerations. In fact, it has been proven that the activation energy for the chloride ion is very low, hence favorable in kinetic terms. In other words, although the voltage applied is thermodynamically sufficient to drive electrolysis, the rate is so slow that to make the process proceed in a reasonable time frame, the voltage of the external source has to be increased (hence, overvoltage).
The overall reaction for the process according to the analysis is the following:
:Anode (oxidation): 2 Cl → Cl + 2 e
:Cathode (reduction): 2 HO + 2 e → H + 2 OH
:Overall reaction: 2 HO + 2 Cl → H + Cl + 2 OH
As the overall reaction indicates, the concentration of chloride ions is reduced in comparison to OH ions (whose concentration increases). The reaction also shows the production of gaseous hydrogen, chlorine and aqueous sodium hydroxide.