Industrial electrolytic processes.

The chloralkali industry

The electrolysis of brine is carried out on a huge scale for the industrial production of chlorine and caustic soda (sodium hydroxide). Because the reduction potential of Na⁺ is much higher than that of water, the latter substance undergoes decomposition at the cathode, yielding hydrogen gas and OH^–.

anode reactions	2 Cl– → Cl2(g) + 2 e–	-1.36 v	i
	4 OH– → O2(g) + 2 H2O + 4 e–	-0.40 v	ii
cathode reactions	Na+ + e– → Na(s)	-2.7 v	iii
	H2O + 2 e– → H2(g) + 2 OH–	+.41 v	iv

A comparison of the E°s would lead us to predict that the reduction (ii) would be favored over that of (i). This is certainly the case from a purely energetic standpoint, but as was mentioned in the section on fuel cells, electrode reactions involving O₂ are notoriously slow (that is, they are kinetically hindered), so the anodic process here is under kinetic rather than thermodynamic control. The reduction of water (iv) is energetically favored over that of Na⁺ (iii), so the net result of the electrolysis of brine is the production of Cl₂ and NaOH ("caustic"), both of which are of immense industrial importance:

2 NaCl + 2 H₂O → 2 NaOH + Cl₂(g) + H₂(g)

     Since chlorine reacts with both OH^– and H₂, it is necessary to physically separate the anode and cathode compartments. In modern plants this is accomplished by means of an ion-selective polymer membrane, but prior to 1970 a more complicated cell was used that employed a pool of mercury as the cathode. A small amount of this mercury would normally find its way into the plant's waste stream, and this has resulted in serious pollution of many major river systems and estuaries and devastation of their fisheries.

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