How does the mercury cathode process produce sodium hydroxide?

In the mercury cathode process, sodium hydroxide is produced through the electrolytic decomposition of brine (sodium chloride).1

The process functions as follows:

• Electrolysis: A direct current is passed through the brine between graphite anodes and a flowing layer of mercury that serves as the cathode.12
• Amalgam Formation: Due to the high overpotential of mercury, sodium ions are reduced to metallic sodium at the cathode without reducing the water molecules. This metallic sodium dissolves into the mercury, forming a solution called an amalgam.12
• Reaction with Water: The amalgam flows out of the cell into a separate vessel where it is brought into contact with water. The sodium reacts with the water to form sodium hydroxide and hydrogen gas.12

This method is advantageous because it generates sodium hydroxide in the absence of salt, resulting in a high-purity product. However, due to the toxicity of mercury, its use has declined in favor of other processes.132