Alkali and Chlorine Products, Chlorine and Sodium Hydroxide
Authors: Curlin L. Calvert , Bommaraju Tilak V. , Hansson Constance B. 
Published: August 2004

Alkali and chlorine products are a group of commodity chemicals which include chlorine, Cl2; sodium hydroxide (caustic soda), NaOH; sodium carbonate (soda ash), Na2CO3; potassium hydroxide (caustic potash), KOH; and hydrochloric acid (muriatic acid or anhydrous), HCl. Chlorine and caustic soda are the two most important products in this group, ranking among the top ten chemicals in the United States. The applications for chlorine and the alkalies are so varied that there is hardly a consumer product which is not dependent on one or both of them at some manufacturing stage. Chlorine and caustic soda are coproducts of the electrolysis of aqueous solutions of sodium chloride, NaCl (commonly called brine). Conversion of aqueous NaCl to Cl2and NaOH is achieved in three types of electrolytic cells: the diaphragm cell, the membrane cell, and the mercury cell. The distinguishing feature of these cells is the manner by which the electrolysis products are prevented from mixing with each other, thus ensuring proper purity. Solution mining of salt and the availability of asbestos resulted in the dominance of the diaphragm process in North America, whereas solid salt and mercury availability led to the dominance of the mercury process in Europe. Japan imported its salt in solid form and, until the development of the membrane process, also favored the mercury cell for production. Chlorine, a member of the halogen family, is a greenish‐yellow gas having a pungent odor at ambient temperatures and pressures and a density 2.5 times that of air. Chlorine is soluble in water and in salt solutions. Chlorine generally exhibits a valence of\documentclass{article}\pagestyle{empty}\begin{document}${{\rm{-}}1}$\end{document}in compounds. Molecular chlorine is a strong oxidizer and a chlorinating agent. Dry chlorine can be handled safely below 120°C in equipment made from iron, steel, stainless steels, Monel metal, nickel, copper, brass, bronze, and lead. Liquid chlorine is generally stored in vessels made from nonalloyed carbon steel or cast steel. Chlorine is classified by the U.S. Department of Transportation (DOT) as a nonflammable compressed gas requiring a green label. No attempt should be made to handle chlorine for any purpose without a thorough understanding of its properties and the hazards involved. This information is available from the Chlorine Institute and from various handbooks. Chlorine gas is a respiratory irritant and at high concentrations and in extreme situations, increased difficulty in breathing can result in death through suffocation. Sodium hydroxide, NaOH, mol wt 39.998, is a brittle, white, translucent crystalline solid. Because of its corrosive action on all human body tissue, it is also known as caustic soda. Aqueous solutions of caustic soda are highly alkaline. Hence caustic soda is primarily used in neutralization reactions to form sodium salts. Reactions of NaOH with natural products are complex. They include solubilization of cotton in rubber reclaiming, cotton scouring, refining of vegetable oils, and removal of lignin and hemicellulose in the Kraft pulping process. The only caustic soda production process besides electrolysis is the soda–lime process, practiced mainly in isolated areas in some process operations. It is not as efficient a route as electrolytic production. Three forms of caustic soda are produced to meet customer needs: purified diaphragm caustic (50% Rayon grade), 73% caustic, and anhydrous caustic. Regular 50% caustic from the diaphragm cell process is suitable for most applications and accounts for about 85% of the NaOH consumed in the United States. However, it cannot be used in certain operations such as the manufacture of rayon because of the presence of salt, sodium chlorate, and heavy metals. Caustic soda is classified as a corrosive material by the DOT, and it has a marked corrosive action on all body tissue. Inhalation of the dust or mist can cause damage to the upper respiratory tract. During handling, all persons should wear proper protective clothing, safety goggles (sometimes a full face shield), rubber gloves, boots, and a caustic‐resistant apron or suit. Disposal of waste or spilled caustic soda must be carried out by properly trained personnel.