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Why must chlorine gas and hydroxide ions be kept separate during chlorine-alkali electrolysis?

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A labeled diagram of the chlorine element from the periodic table shows its atomic number 17, symbol Cl, electron configuration, and atomic weight range. The box is shaded green to indicate chlorine is a halogen, with icons for orthorhombic crystal structure, gas state at room temperature, and strong acidity. A key explains the icons and colors used. — chlorine Properties of chlorine.

During the electrolysis of brine in the chlor-alkali process, it is essential to keep chlorine gas and hydroxide ions separate because they react with one another if they come into contact. If allowed to mix, the chlorine would be consumed by this reaction, significantly reducing the efficiency and yield of the production process.12

To prevent this, industrial cells utilize specific designs to maintain separation:

Diaphragm cells: A porous wall or diaphragm is placed between the electrodes to physically separate the chlorine produced at the anode from the hydroxide ions formed at the cathode.12
Mercury cells: A liquid mercury cathode is used instead of an iron one. This prevents the formation of hydroxide ions within the cell by discharging sodium metal into the mercury to form an amalgam, which is then reacted with water in a separate vessel.12