In a diaphragm cell, why is the level of brine in the anode compartment kept slightly higher?

A diaphragm cell is an electrolytic cell used in the chlor-alkali process to produce chlorine, hydrogen, and sodium hydroxide from brine (NaCl solution). The diaphragm separates the anode compartment from the cathode compartment, allowing selective ion movement and preventing undesired reactions.

In the anode compartment, chloride ions (Cl⁻) are oxidized to chlorine gas (Cl₂). The hydroxide ions (OH⁻), formed in the cathode compartment, must not reach the anode because they would be oxidized to oxygen gas (O₂), reducing efficiency and contaminating the products.

To prevent OH⁻ ions from migrating to the anode, the level of brine in the anode compartment is kept slightly higher than in the cathode compartment. This creates a hydraulic pressure that counteracts the diffusion of OH⁻ ions across the diaphragm, ensuring that chlorine gas is produced at the anode without contamination.

Chlorine gas is collected separately, and hydrogen is evolved at the cathode. Sodium hydroxide remains in solution in the cathode compartment. The careful control of brine levels ensures high purity of products and prevents side reactions.

This design principle is important in industrial inorganic chemistry, particularly in the mass production of chlorine and caustic soda. It highlights how cell design, ion migration, and compartmentalization are crucial for efficient electrolysis. By maintaining a higher brine level at the anode, OH⁻ ions are prevented from reaching the anode, protecting the chlorine yield and maintaining product purity.

The key concept is the hydraulic control of ion movement in a diaphragm cell. This prevents unwanted oxidation of hydroxide ions and ensures that the electrolysis process remains efficient, safe, and industrially viable.