thumb|old drawing of a chloralkali process plant (edgewood, maryland) the chloralkali process (also chlor-alkali and chlor alkali) is an industrial process for the electrolysis of sodium chloride. it is the technology used to produce chlorine and sodium hydroxide (caustic soda), which are commodity chemicals required by industry. to give some sense of scale, 35 million tons of chlorine were prepared by this process in 1987. industrial scale production began in 1892. usually the process is conducted on a brine (an aqueous solution of nacl), in which case naoh, hydrogen, and chlorine result. when using calcium chloride or potassium chloride, the products contain calcium or potassium instead of sodium. related processes are known that use molten nacl to give chlorine and sodium metal or condensed hydrogen chloride to give hydrogen and chlorine. the process has a high energy consumption, for example over 4 billion kwh per year in west germany in 1985. because the process gives equal (molar) amounts of chlorine and sodium hydroxide, it is necessary to find a use for these product in equal proportions. ==procedures== 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. 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): :2clâ â  + 2eâ at the cathode, positive hydrogen ions pulled from water molecules are reduced by the
Chemical_processes Electrolysis Industrial_gases