Information about electrolysis of sea water or brine

Very dilute sodium chloride solution (brine):

at positive electrode(anode): oxygen
at negative electrode(cathode): hydrogen
residue: sodium chloride

moderately concentrated brine:

at anode: chlorine
at cathode: hydrogen
residue: sodium hydroxide

very concentrated brine:

at anode: chlorine
at cathode: sodium
residue: water

2006-11-17 16:45:30 補充：
Electrolysis of sea water is NOT recommended because many impurities like magnesium, potassium, sulpates, etc. will also be extracted.

electrolysis of brine using a diaphragm cell produces chlorine gas, hydrogen gas, and caustic liqour[NaOH(aq) + NaCl(aq)].

diagram at http://electrochem.cwru.edu/ed/encycl/fig/b01/b01-f06.gif
other cells are also show at http://electrochem.cwru.edu/ed/encycl/art-b01-brine.htm
mercury cell is less prefered due to mercury being volatile and toxic.