Sodium hydroxide production. This is the mercury cell method for the chlor-alkali electrolytic process to produce sodium hydroxide. Salt solution (sodium chloride) is piped in at upper left (26%) and out at lower left (24%). The electrolysis uses a carbon anode (positive) and a liquid mercury cathode (negative). Chlorine gas is produced at the anode, and a mixture (amalgam) of sodium and mercury (blue) is formed at the cathode. The amalgam is separated in the area at right (using graphite decomposers and water). The mercury is recycled, and sodium hydroxide and hydrogen are the products at far right. All 3 products are used in the chemical industry.