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How was mercury used in the production of chlorine and sodium hydroxide?

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A periodic table element box for mercury shows its symbol Hg, atomic number 80, atomic weight 200.592, electron configuration, and name. Icons indicate mercury is a transition metal, liquid at room temperature, has a rhombohedral crystal structure, and is weakly basic. — mercury Properties of mercury.

In the chlor-alkali industry, mercury was utilized as a cathode in a specialized electrolytic cell to produce chlorine and sodium hydroxide.1

In this process, a shallow layer of mercury flowed across the bottom of the cell, serving as the negative electrode (cathode), while graphite rods acted as the anodes. When a powerful direct current was applied to brine, the mercury's high overpotential allowed sodium ions to be reduced to metallic sodium without simultaneously reducing the water.1

The resulting metallic sodium dissolved into the mercury to form a liquid amalgam. This amalgam was then directed into a separate vessel where it reacted with water to produce sodium hydroxide and hydrogen. This method was particularly valued because it allowed for the production of sodium hydroxide that was free of salt contamination. Due to the significant toxicity of mercury, these mercury-cell plants have been largely phased out in the early 21st century.12