Editing Chloralkali process (section)

==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 [[Sodium hydroxide#Production|membrane cell]]. A membrane, such as [[Nafion#Chlor-alkali production cell membrane|Nafion]], Flemion or Aciplex, is used to prevent the reaction between the chlorine and hydroxide ions.

[[Image:Chloralkali_membrane.svg|thumb|center|upright=3|Basic membrane cell used in the [[electrolysis]] of brine. At the anode ('''A'''), chloride (Cl<sup>−</sup>) is oxidized to chlorine. The ion-selective membrane ('''B''') allows the counterion Na+ to freely flow across, but prevents anions such as hydroxide (OH<sup>−</sup>) and chloride from diffusing across. At the cathode ('''C'''), water is reduced to hydroxide and hydrogen gas. The net process is the electrolysis of an aqueous solution of NaCl into industrially useful products sodium hydroxide (NaOH) and chlorine gas.]]

Saturated brine is passed into the first chamber of the cell. Due to the higher concentration of chloride ions in the brine, the [[chloride]] ions are [[redox|oxidised]] at the [[anode]], losing electrons to become [[chlorine]] gas ('''A''' in figure):
:2Cl<sup>−</sup> → {{chem|Cl|2}} + 2[[electron|e<sup>−</sup>]]

At the [[cathode]], positive [[hydrogen ion]]s pulled from water molecules are [[redox|reduced]] by the electrons provided by the electrolytic current, to hydrogen gas, releasing [[hydroxide]] ions into the solution ('''C''' in figure):
:2{{chem|H|2|O}} + 2e<sup>−</sup> → H<sub>2</sub> + 2OH<sup>−</sup>

The ion-permeable [[ion-exchange membrane]] at the center of the cell allows only the [[sodium]] ions (Na<sup>+</sup>) to pass to the second chamber where they react with the hydroxide ions to produce [[caustic soda]] (NaOH) ('''B''' in figure):<ref name = Du/> 

Na<sup>+</sup> + OH<sup>−</sup> → NaOH

The overall reaction for the electrolysis of brine is thus:
:2NaCl + 2{{chem|H|2|O}} → {{chem|Cl|2}} + {{chem|H|2}} + 2NaOH

===Diaphragm cell===
In the diaphragm cell process, there are two compartments separated by a permeable diaphragm, often made of [[asbestos|asbestos fibers]]. Brine is introduced into the anode compartment and flows into the cathode compartment. Similarly to the membrane cell, chloride ions are oxidized at the anode to produce chlorine, and at the cathode, water is split into caustic soda and hydrogen. The diaphragm prevents the reaction of the caustic soda with the chlorine. A diluted caustic brine leaves the cell. The caustic soda must usually be concentrated to 50% and the salt removed. This is done using an evaporative process with about three tonnes of steam per tonne of caustic soda. The salt separated from the caustic brine can be used to saturate diluted brine. The chlorine contains oxygen and must often be purified by liquefaction and evaporation.

===Mercury cell===
{{main|Castner–Kellner process}}
[[File:HgNaOHElectrolysis.png|thumb|upright=1.2|Diagram of the mercury-cell process, showing an "inner" cell sandwiched between two "outer" cells, with a layer of mercury common to all three.]]

In the mercury-cell process, also known as the [[Castner–Kellner process]], the "outer" electrolytic cells each contain an anode immersed in brine, which floats on a layer of mercury. The "inner" cells each contain a cathode in a sodium hydroxide solution, floating on the same mercury layer. The walls dividing the cells have gaps below the surface of the mercury layer. This allows mercury to flow between cells, while preventing the aqueous solutions from doing so. 

In the "outer" cell, chloride ions are oxidized at the anode, producing chlorine gas which bubbles out of the cell. The mercury layer acts as the cathode, here sodium ions in the brine are reduced and form [[Sodium amalgam|an amalgam]] with the mercury. Once in the amalgam, sodium atoms are free to move to the "inner" cell.

In the "inner" cell, the mercury layer now acts as the anode. Sodium atoms in the amalgam are oxidized and enter aqueous solution. Meanwhile at the cathode,  water is split into hydrogen gas and hydroxide ions.

Mercury cells are being phased out due to concerns about the high toxicity of mercury and [[mercury poisoning]] from mercury cell pollution such as occurred in Canada (see [[Ontario Minamata disease]]) and Japan (see [[Minamata disease]]).