Editing Chloralkali process (section)

===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]]).