Sodium chlorite is a compound used for water disinfection and purification. It is produced in large quantities as flakes or a solution from chlorine dioxide and sodium hydroxide. Its use as a bleach for textiles was first discovered during the 1920s. Today, sodium chlorite is an important specialty chemical with sales over $18 million annually.
In its dried state, sodium chlorite (NaClO 2 ) is a white or light yellow-green solid. The greenish tint comes from trace amounts of CdO2 or iron, which are production residuals. Sodium chlorite has a molecular weight of 90.44 and decomposes at about 392°F (200°C). It is generally soluble in water, but its solubility increases as the temperature of the water rises. Sodium chlorite is a powerful oxidizer that will not explode on percussion. The anhydrous salt does not absorb water and is stable for up to ten years.
Sodium chlorite is used for a variety of applications. It is used as a disinfectant and purification chemical for water. It is also employed as a textile-bleaching and water anti-fouling agent. Additionally, it is used in the paper and electronics manufacturing industries as a bleaching agent.
When put in an acid solution, sodium chlorite breaks down into chlorine dioxide. When added to a municipal water supply, chlorine dioxide helps control unwanted tastes and odors. It also aids in the removal of ions like iron and manganese. One added benefit is that it helps eliminate trihalomethanes in drinking water.
As a textile-bleaching agent, sodium chlorite is effective with various fibers. It can be used on cotton, bast fibers, and man-made fibers like nylon, Perlon, Dralon, and Rhovyl. It has an oxidizing effect on many of the natural waxes and pectins found in cellulose fibers. It helps solubilize them and makes the fiber more even and workable. It has the added benefit of destroying natural color matter without attacking the fibers themselves. This makes it useful for making permanent white fabrics without compromising tensile strength.
Sodium chlorite is also used for various industrial applications. It controls microbial contamination in industrial cooling systems and towers. It is used in place of chlorine in industrial ammonia plants because it does not react negatively with ammonia. Since it is an oxidizer, it is often a part of flue gas scrubber systems. Food-processing companies use it for washing fruits and vegetables because it is a fungicide. Meat and poultry are also washed with a solution, as is food processing equipment. Finally, it is an anti-mildew agent in detergent compositions and has been used in toothpaste and contact lens solutions.
The development of sodium chlorite as an industrial chemical began in 1921 when E. Schmidt found that cellulosic fibers could be purified with chlorine dioxide without being appreciably damaged. Unfortunately, chlorine dioxide gas is extremely explosive at high concentrations. These discoveries prompted researchers to look for safe and economical ways to deliver chlorine dioxide for bleaching purposes. The first company to introduce sodium chlorite for this purpose was the Mathieson Chemical Corporation.
In 1960, sodium chlorite became the standard material for continuous bleaching operations in the United States, replacing hydrogen. In subsequent years, other uses for sodium chlorite were discovered.
The primary raw materials used in the production of sodium chlorite are chlorine dioxide, sodium hydroxide, and hydrogen peroxide. Chlorine dioxide is a gas at room temperature. Its color is intensely greenish-yellow. Chlorine dioxide provides the source of chlorine that is converted to sodium chlorite. In production, it is stored as a liquid solution in glass-lined steel containers.
Sodium hydroxide is a fused solid with a crystalline structure. Also known as caustic soda, it is corrosive to skin and vegetable tissue, causing severe burns. It is typically produced through the electrolysis of sodium chloride solutions. Hydrogen peroxide is a colorless liquid that is caustic and bitter to taste. Pure H2 02 is a thick, syrupy liquid that rapidly decomposes into oxygen and water. In nature, it occurs only in trace amounts in snow or rain. It is naturally generated during lightening storms. It is typically used in dilute solutions during the manufacture of sodium chlorite.
Other materials are typically added to sodium chlorite powders or solutions before they are sold. Commercial sodium chlorite bleaching solutions contain special ingredients including anticorrosive agents, buffering agents, chlorine dioxide fume controllers, and surfactants. Anticorrosive agents are used to prevent the corrosion of stainless steel bleaching equipment. Buffer salts help liberate the chlorine dioxide that is produced during the bleaching process. Surfactants help stabilize solutions and allow for cleaning and penetration effects. Stabilized sodium chlorite solution can be stored for long periods without loss of activity. When the sodium chlorite is sold as a solid, sodium chloride is often included to make it safer to handle and store.
While a variety of chlorites are available, sodium chlorite is the only one produced commercially. It is sold in solution or as a solid. The technical grade is made up of about 80% sodium chlorite and the rest is sodium chloride. Large scale production is based on a reaction of chlorine dioxide in a sodium hydroxide solution. Hydrogen peroxide is also present as the reducing agent. Sodium chlorite is manufactured in three phases, chlorine dioxide production, sodium chlorite generation, and recovery.
To ensure the quality of the sodium chlorite that is produced, the production process is monitored at each stage. The starting raw materials and the final product are all subjected to a variety of chemical and physical tests to determine that they meet the required specifications. Some of the commonly tested characteristics include appearance, odor, pH, density, specific gravity, and melting point. If the final product is a solution, its chemical activity is tested to make sure it has the correct concentration. For solid granules, particle size is determined and modified if necessary.
Manufacturing sodium chlorite produces some undesirable byproducts, such as chlorine dioxide, that cannot be released into the immediate environment. Concentrated fumes of chlorine dioxide are toxic, and cause sickness, appetite loss, and nausea in line operators. In the production plant, circulation of fresh air is essential. The chlorine dioxide gas is also highly corrosive. For this reason, sodium chlorite solutions must be stored in specially coated containers. Materials such as glass, porcelain, some plastics, or earthenware are typically used. Titanium is the most resistant metal used today. In the textile industry, molybdenum alloy stainless steels are used to store the sodium chlorite bleach solutions.
With increased applications for chlorine dioxide, improvements in sodium chlorite production are currently being studied. Sodium chlorite research is focused on reducing the environmental impact of bleaching systems and finding quicker, less expensive production methods. New bleaching formulations are constantly being developed by formulating chemists.
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Busch, Gretchen. "Vulcan to Expand Treatment Business with Chlorite Buy." Chemical Marketing Reporter (June 15, 1992).
Rittmann, Douglas, and Joel Tenney. "Generating chlorine dioxide gas: Chlorate vs. chlorite." Water Engineering & Management (Sept. 1998).
— Perry Romanowski