In a diaphragm cell, hydrogen is discharged by the reduction of ?

The diaphragm cell is an electrolytic cell used mainly for the electrolysis of brine (aqueous sodium chloride solution) to produce chlorine gas, hydrogen gas, and sodium hydroxide (NaOH). It consists of two compartments separated by a porous diaphragm, which allows ions to pass but prevents the mixing of chlorine and sodium hydroxide solutions.

In the diaphragm cell, the main reactions occur at two electrodes — the anode and the cathode.

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  At the anode, chloride ions (Cl⁻) are oxidized to chlorine gas (Cl₂).
  2Cl−→Cl2+2e−2Cl⁻ → Cl₂ + 2e⁻2Cl−→Cl2+2e−

  
  


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  At the cathode, water molecules are reduced to hydrogen gas (H₂) and hydroxide ions (OH⁻).
  2H2O+2e−→H2+2OH−2H₂O + 2e⁻ → H₂ + 2OH⁻2H2O+2e−→H2+2OH−

  
  

Although sodium ions (Na⁺) are present in the solution, they are not reduced at the cathode because the reduction potential of Na⁺ is much lower than that of water. This means that water is reduced more easily to produce hydrogen gas. The generated hydroxide ions combine with sodium ions in solution to form sodium hydroxide (NaOH).

The porous diaphragm (often made of asbestos or polymeric material) helps to separate the chlorine gas formed at the anode from the sodium hydroxide formed near the cathode, preventing undesirable side reactions such as the formation of sodium hypochlorite (NaOCl).

Thus, in a diaphragm cell, hydrogen gas is discharged due to the reduction of water molecules, not from sodium or sodium chloride. This process is fundamental in the chlor-alkali industry, which supplies chlorine, hydrogen, and caustic soda for industrial use.