Chlorine gas, Cl2, prepared industrially by the electrolysis of molten NaCl or by the electrolysis of aconcentrated aqueous NaCl solution (called brine). The latter process is called the chlor-alkali process (where chlor denoted chlorine and alkali denotes an alkali metal, such as sodium). Two of the common cell used in the chlor-alkali process are the mercury cell and the diaphragm cell. In both cell the overall reaction is