text: Process systems Three production methods are in use. While the mercury cell method produces chlorine-free sodium hydroxide, the use of several tonnes of mercury leads to serious environmental problems. In a normal production cycle a few hundred pounds of mercury per year are emitted, which accumulate in the environment. Additionally, the chlorine and sodium hydroxide produced via the mercury-cell chloralkali process are themselves contaminated with trace amounts of mercury. The membrane and diaphragm method use no mercury, but the sodium hydroxide contains chlorine, which must be removed.
  Membrane cell
  The most common chloralkali process involves the electrolysis of aqueous sodium chloride (a brine) in a membrane cell. A membrane, such as one made from Nafion, Flemion or Aciplex, is used to prevent the reaction between the chlorine and hydroxide ions.
  Saturated brine is passed into the first chamber of the cell where the chloride ions are oxidised at the anode, losing electrons to become chlorine gas (A in figure):
  2Cl− →  + 2e−
  At the cathode, positive hydrogen ions pulled from water molecules are reduced by the electrons provided by the electrolytic current, to hydrogen gas, releasing hydroxide ions into the solution (C in figure):
  2 + 2e− → H2 + 2OH−
  The ion-permeable ion-exchange membrane at the center of the cell allows the sodium ions (Na+) to pass to the second chamber where they react with the hydroxide ions to produce caustic soda (NaOH) (B in figure). The overall reaction for the electrolysis of brine is thus:
  2NaCl + 2 →  +  + 2NaOH