The chlor-alkali process is a widely used electrolytic process that yields chlorine and caustic soda. It has been in use since the 19th century and is a primary industry in the United States, Western Europe, and Japan. There are three primary electrolytic processes for producing chlorine and caustic soda: the diaphragm cell process, the mercury cell process, and the membrane cell process. The first two have been used for over 100 years; the latter was developed in the past 60. The two oldest methods are used the most throughout the world and have been proven to be the most environmentally unfriendly through their use of asbestos and mercury, respectively. The membrane cell process is a superior method in its energy efficiency and lack of harmful chemicals. This article reviews the history of each method, addresses changes in its modern technology and use, and discusses the emissions and outputs of the processes.

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Mercury (Hg) has a long history of both medicinal uses and toxic effects. Hg chlorides were used as medicines; however, 'corrosive sublimate' (HgCl 2 ) was also used as a violent poison in the Middle Ages. In this work, certain thermodynamic principles of reactivity were used to show that ingested aqueous calomel (Hg 2 Cl 2 ) in the human stomach is almost entirely converted to HgCl 2 . This work opens the way to a new series of studies on poisoning cases in history.