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

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Mercury thermometer — mercury processing Mercury in the bulb of a thermometer.

In the chlor-alkali industry, the mercury cell process utilizes a shallow, flowing layer of mercury as a cathode to produce chlorine and sodium hydroxide from brine.1

During electrolysis, a direct electric current is passed between graphite anodes and the mercury cathode. Because mercury has a high overpotential for the reduction of water, the voltage can be raised sufficiently to reduce sodium ions into metallic sodium without simultaneously reducing the water.1

The resulting metallic sodium dissolves into the mercury to form an amalgam. This amalgam flows out of the electrolytic cell into a separate vessel, where it reacts with water to produce sodium hydroxide and hydrogen gas. This method is advantageous because it allows for the production of sodium hydroxide that is completely free of salt, making it highly desirable for specific applications like rayon manufacturing. However, due to environmental concerns regarding mercury losses in sludge and wastewater, this process has largely been replaced by alternative technologies.12