Sodium Hypochlorite

Resources

Sodium hypochlorite (NaClO) is a chemical compound consisting of sodium (Na), oxygen (O), and chlorine (Cl) that has been used for centuries for bleaching and disinfecting. Today, sodium hypochlorite (commonly called chorine bleach or simply bleach) is mass produced by the chlorination of soda ash and is employed in many household products, including laundry bleaches, hard surface cleaners, mold and mildew removers, and drain cleaners.

Sodium hypochlorite is the salt formed by a negatively charged hypochlorite ion (ClO^-) and a positively charged sodium ion (Na⁺). Pure hypochlorite is highly reactive and unstable; therefore, it is usually supplied as a dilute aqueous solution. In solution, hypochlorite eventually decomposes to yield a variety of byproducts including oxygen, chlorine gas, and salt. One of these byproducts, hypochlorous acid, is a powerful oxidizing agent (meaning it can accept electrons from other materials) that lends hypochlorite excellent bleaching and disinfecting abilities. The term “available chlorine’ is often used to describe the concentration of hypochlorous acid in solution (which provides a measure of the solution’s oxidative ability).

Due to its reactive nature, hypochlorite is particularly sensitive to the presence of trace metals such as copper, nickel, iron, chromium, cobalt and manganese that catalyze its decomposition. In fact, it is so reactive that it will aggressively attack many materials, including rubber, most types of fabrics, and certain plastics. Therefore, care must be taken in handling and storing hypochlorite solutions; all vessels should be glass, PVC (polyvinyl chloride) plastic, porcelain, or glazed earthenware.

Hypochlorite was first produced in 1789 in Javelle, France, by passing chlorine gas through a solution of sodium carbonate. The resulting liquid, known as Eau de Javelle or Javelle water was a weak solution of sodium hypochlorite. However, this process was not very efficient and alternate production methods were sought. One such method involved the extraction of chlorinated lime (known as bleaching powder) with sodium carbonate to yield low levels of available chlorine. This method was commonly used to produce hypochlorite solutions for use as a hospital antiseptic, which was sold under the trade names Eusol and Dakin’s solution. Near the end of the nineteenth century, E. S. Smith patented a method of hypochlorite production involving hydrolysis of brine to produce caustic soda and chlorine gas, which then mix to form hypochlorite. Both electric power and brine solution were in cheap supply at this time and various enterprising marketers took advantage of this situation to satisfy the market’s demand for hypochlorite. Bottled solutions of hypochlorite were sold under numerous trade names; one such early brand produced by this method was called Parozone. Today, an improved version of this method, known as the Hooker process, is the only large scale industrial method of sodium hypochlorite production.

Over the last few hundred years, one of the primary uses for sodium hypochlorite has been for the bleaching of fabrics, particularly cotton. Virgin cotton fibers are not pure white and must be processed to remove their natural coloration. Cotton bleaching has been practiced since the time of the ancient Egyptians who exposed fabric to sunlight to cause whitening. Even as late as the end of the eighteenth century, the British textile industry would bleach linen fabric by soaking it in sour milk for at least 48 hours, then exposing it to sunlight by laying out miles of treated fabrics on specially designated grasslands. In the 1800s, Claude Louis Berthellot attempted to take advantage of chlorine’s bleaching ability, but, because it is a gas in its natural state, the chlorine was difficult to control. Subsequently, a process was developed to deliver chlorine as a dry powder by treating calcium carbonate with chlorine gas. However, this method of bleaching was far from ideal since it resulted in damage to the fabric wherever the concentrated hypochlorite powder came into contact with the fibers. Industrial fabric bleaching was vastly improved with the development of commercial bottled solutions of hypochlorite (also called chlorine bleach). Sodium hypochlorite gained widespread use not only as for industrial fabric treatment but also as a home laundry bleach. It is still sold today as a 5% solution in water.

Another important use for hypochlorite is as a sanitizer or disinfectant. Both of these uses rely on the hypochlorite’s ability to destroy microorganisms. The same oxidative mechanism responsible for hypochlorite’s bleaching ability also makes it an effective germicide. Although this mechanism was not understood at the time, hypochlorite (in the form of bleaching powder) was used as early as 1800 to counteract bad odors associated with disease. In fact, it has been said that no single element has played so important a role in combating disease over the nineteenth century as chlorine in its various forms. It should also be noted that hypochlorite is corrosive at high concentrations and was only used on the skin at very dilute levels. Its disinfectant properties have also been utilized for the sanitization of food processing equipment, particularly milking utensils used in the dairy industry. One marked advantage of hypochlorite for these applications is the fact

KEY TERMS

Available chlorine —A measure of the oxidative potential of a chlorine containing solution.

Bleaching powder —A dry bleach made by treating calcium carbonate with chlorine gas.

Chlorine —A chemical element whose strong oxidizing abilities make it useful as a disinfectant and deodorizer.

Dakin’s solution —An aqueous solution of hypochlorite (approximately 0.5%) in water used as a hospital antiseptic.

Javelle water —The first known production of hypochlorite that was made by passing chlorine gas through a solution of sodium carbonate.

Sodium hypochlorite —A chemical compound (NaClO) consisting of sodium, oxygen, and chlorine that has been used for centuries for its bleaching and disinfectant properties.

that it, in addition to working quickly, rapidly breaks down to innocuous compounds. For this reason, it is also useful in chlorination of sewage effluents and swimming pool water. Today, its primary uses are in lavatory bowl deodorizers and sanitizers.

New and improved ways to use hypochlorite are still being developed. In recent years, a number of improved bleach-containing products have been brought to market as chemists have learned to combine sodium hypochlorite with cleaning agents, thickeners, and fragrance compounds to create efficacious products with improved aesthetic properties. For example, hypochlorite-based hard surface cleaners for kitchen counter tops, mold and mildew removers for showers and baths, and drain cleaners for kitchen and bathroom sinks are now commercially available.

See also Antisepsis.

Resources

BOOKS

Block, Seymour S., ed. Disinfection, Sterilization, and Preservation. Philadelphia, PA: Lippincott Williams & Wilkins, 2001.

Chalmers, Louis. Household and Industrial Chemical Specialties. Vol. 1. Chemical Publishing Co. Inc., 1978.

Schwarcz, Leonard. Sanitary Chemicals. New York: Mac Nair-Dorland Co., 1953.

Randy Schueller

Sodium Hypochlorite

OVERVIEW

Sodium hypochlorite (SO-dee-um hye-po-KLOR-ite) is the active ingredient in liquid chlorine bleaches, used in the home and many industries to whiten fabric and other materials and to disinfect surfaces and water. The anhydrous compound is very unstable and explodes readily. The pentahydrate is a pale-green crystalline solid that is relatively stable. The compound is usually made available as an aqueous solution that contains anywhere from 3 to 6 percent sodium hypochlorite (for household use) to as high as 30 percent (for industrial applications). In solution form, sodium hypochlorite is quite stable and can be stored for long periods of time out of sunlight.

Sodium hypochlorite decomposes by two mechanisms. In one case, it breaks down to form sodium chloride and sodium chlorate:

3NaOCl → 2NaCl + NaClO₃

KEY FACTS

OTHER NAMES:

Sodium oxychloride; hypochlorite; bleach; chlorine bleach

FORMULA:

NaClO

ELEMENTS:

Sodium, chlorine, oxygen

COMPOUND TYPE:

Oxy salt (inorganic)

STATE:

Solid or aqueous solution; See Overview

MOLECULAR WEIGHT:

74.44 g/mol

MELTING POINT:

Solid NaClO explodes on heating. The pentahydrate (NaClO·5H₂O) is more stable; its melting point is 18° C (64°F)

BOILING POINT:

Not applicable; decomposes

SOLUBILITY:

Soluble in water

In the second case, it breaks down to form sodium chloride and nascent (free single atoms) oxygen:

NaOCl → NaCl + (O)

Nascent oxygen is a very active form of oxygen that is responsible for the bleaching and disinfectant properties of sodium hypochlorite.

Humans have long made efforts to bleach fabrics. Neither cotton nor linen, two very popular fabrics, are naturally very white, so efforts were made to find ways to convert them to white materials. Those efforts were not very successful until the discovery of chlorine by the Swedish chemist Karl Wilhelm Scheele (1742–1786) in the 1770s. The powerful oxidizing powers of chlorine made it a likely candidate for use as a bleach (although chemists at the time did not understand how bleaching occurred). The first person to take advantage of chlorine's bleaching powers was the French chemist Claude Louis Berthollet (1748–1822), who lived in Javelle, France. Berthollet produced a weak solution of sodium hypochlorite by passing chlorine gas through sodium carbonate. The product had excellent bleaching powers, and it became known by the name of eau de Javelle or eau de Berthollet. Berthollet's invention came at just the right time. The Industrial Revolution was just getting under way, and the invention of machines like the spinning jenny and the power loom mechanized the commercial production of cotton and linen cloth and rapidly increased the demand for bleaching agents.

HOW IT IS MADE

Sodium hypochlorite is made commercially by passing chlorine gas (Cl₂) through a cold aqueous solution of sodium hydroxide (NaOH):

Cl₂ + 2NaOH → NaClO + NaCl + H₂O

The pentahydrate then can be extracted by crystallization.

COMMON USES AND POTENTIAL HAZARDS

Sodium hypochlorite is used almost exclusively for one of two purposes: bleaching or purification. The compound is available commercially for household use under a number of trade names, including Antiformin®, B-K Liquid®, Clorox®, Dakin's Solution®, Dazzle®, Hychlorite®, Javelle water, Piochlor®, Purex®, and Saniton Toothbrush Sanitizer®. About two-thirds of the sodium hypochlorite made in the United States is used as a laundry bleach and sanitizer, in restaurants and institutional kitchens for bleaching and sanitation, and for water purification in residential pools and spas. Some examples of the ways in which sodium hypochlorite is used include:

• For the sterilization of milking equipment and containers at dairy farms;
• For the cleaning and sterilization of work surfaces by amateur and professional beer and wine makers;
• As an ingredient in home cleaning agents, such as toilet bowl sanitizers, mold removers, and drain cleaners; and
• As a disinfectant in private water wells to prevent the growth of microorganisms.

About half of all the sodium hypochlorite used for industrial purposes is consumed in municipal and water treatment systems. About a third of the compound production goes to the sterilization of municipal and commercial swimming pools. The remaining sodium hypochlorite is used in commercial, municipal, and industrial cleaning and bleaching operations.

Sodium hypochlorite is a fire and health hazard. It reacts strongly with metals and organic materials. The rate of reaction increases with the concentration of sodium hypochlorite in solution, so industrial and municipal formulations present a greater environmental threat than do household products. One combination of special concern to consumers is the reaction between sodium hypochlorite and compounds that contain ammonia. For example, the combination of household bleach and household ammonia can produce an explosive or flam-mable mixture. The fumes from this combination can also be harmful, even deadly. Similarly, household bleach should not be use to clean spills that contain urine since urine itself contains ammonia. Sodium hypochlorite is also incompatible with hydrogen peroxide and acidic products.

Interesting Facts

• Until the discovery of chlorine, cloth was usually bleached by soaking it in sour milk or buttermilk and letting it sit in the sun. The process often took up to eight weeks and required large "bleaching fields" on which the cloth could be laid out.
• The first attempt to apply chemical principles to the practice of bleaching was documented in a book on the subject by the Scottish physician Francis Home, published in 1756. Home suggested using a weak solution of sulfuric acid for bleaching, a practice that reduced bleaching time to about 12 hours.

Words to Know

ANHYDROUS

Lacking water of hydration.

AQUEOUS SOLUTION

A solution that consists of some material dissolved in water.

OXIDATION

A chemical reaction in which oxygen reacts with some other substance or, alternatively, in which some substance loses electrons to another substance, the oxidizing agent.

PENTAHYDRATE

A form of a crystalline compound that occurs with five molecules of water.

Sodium hypochlorite is an irritant to the skin, eyes, and respiratory system. It can produce inflammation, burning, and blistering of the skin; burning of the eyes, with subsequent damage to one's vision; and irritation of the gastrointestinal system that can result in stomach pain, nausea, vomiting, coughing, and ulceration of the digestive tract.

FOR FURTHER INFORMATION

Chalmers, Louis. Household and Industrial Chemical Specialties. Vol. 1. New York: Chemical Publishing Co., Inc., 1978.

Fletcher, John, and Don Ciancone. 'Why Life's a Bleach (The Sodium Hypochlorite Story).' Environmental Science & Engineering. May 1996. Also available online at http://www.esemag.com/0596/bleach.html (accessed on November 8, 2005).

"Medical Management Guidelines (MMGs) for Calcium Hypochlorite (CaCl₂O₂) Sodium Hypochlorite (NaOCl)." Agency for Toxic Substances and Disease Registry. http://www.atsdr.cdc.gov/MHMI/mmg184.html (accessed on January 12, 2006).

"Sodium Hypochlorite." Hill Brothers Chemical Co. http://hillbrothers.com/msds/pdf/sodium-hypochlorite.pdf (accessed on November 8, 2005).

"Sodium Hypochlorite." Medline Plus. http://www.nlm.nih.gov/medlineplus/ency/article/002488.htm (accessed on November 8, 2005).

Sodium hypochlorite

Sodium hypochlorite (NaOCl) is a chemical compound consisting of sodium, oxygen , and chlorine that has been used for centuries for bleaching and disinfecting. Today, sodium hypochlorite (commonly called chorine bleach ) is mass produced by the chlorination of soda ash and is employed in many household products, including laundry bleaches, hard surface cleaners, mold and mildew removers, and drain cleaners.

Sodium hypochlorite is the salt formed by a negatively charged hypochlorite ion (OCl-) and a positively charged sodium ion (Na+). Pure hypochlorite is highly reactive and unstable; therefore, it is usually supplied as a dilute aqueous solution . In solution, hypochlorite eventually decomposes to yield a variety of byproducts including oxygen, chlorine gas, and salt. One of these byproducts, hypochlorous acid, is a powerful oxidizing agent (meaning it can accept electrons from other materials) that lends hypochlorite excellent bleaching and disinfecting abilities. The term "available chlorine" is often used to describe the concentration of hypochlorous acid in solution (which provides a measure of the solution's oxidative ability).

Due to its reactive nature, hypochlorite is particularly sensitive to the presence of trace metals such as copper , nickel, iron , chromium, cobalt and manganese that catalyze its decomposition . In fact, it is so reactive that it will aggressively attack many materials, including rubber, most types of fabrics, and certain plastics . Therefore, care must be taken in handling and storing hypochlorite solutions; all vessels should be glass , PVC plastic, porcelain, or glazed earthenware.

Hypochlorite was first produced in 1789 in Javelle, France, by passing chlorine gas through a solution of sodium carbonate . The resulting liquid, known as "Eau de Javelle" or "Javelle water" was a weak solution of sodium hypochlorite. However, this process was not very efficient and alternate production methods were sought. One such method involved the extraction of chlorinated lime (known as bleaching powder) with sodium carbonate to yield low levels of available chlorine. This method was commonly used to produce hypochlorite solutions for use as a hospital antiseptic which was sold under the trade names "Eusol" and "Dakin's solution." Near the end of the nineteenth century, E. S. Smith patented a method of hypochlorite production involving hydrolysis of brine to produce caustic soda and chlorine gas which then mix to form hypochlorite. Both electric power and brine solution were in cheap supply at this time and various enterprising marketers took advantage of this situation to satisfy the market's demand for hypochlorite. Bottled solutions of hypochlorite were sold under numerous trade names; one such early brand produced by this method was called Parozone. Today, an improved version of this method, known as the Hooker process, is the only large scale industrial method of sodium hypochlorite production.

Over the last few hundred years, one of the primary uses for sodium hypochlorite has been for the bleaching of fabrics, particularly cotton . Virgin cotton fibers are not pure white and must be processed to remove their natural coloration. Cotton bleaching has been practiced since the time of ancient the Egyptians who exposed fabric to sunlight to cause whitening. Even as late as the end of the eighteenth century, the British textile industry would bleach linen fabric by soaking it in sour milk for at least 48 hours, then exposing it to sunlight by laying out miles of treated fabrics on specially designated grasslands . In the 1800s, C. Berthellot attempted to take advantage of chlorine's bleaching ability, but, because it is a gas in its natural state, the chlorine was difficult to control. Subsequently, a process was developed to deliver chlorine as a dry powder by treating calcium carbonate with chlorine gas. However, this method of bleaching was far from ideal since it resulted in damage to the fabric wherever the concentrated hypochlorite powder came into contact with the fibers. Industrial fabric bleaching was vastly improved with the development of commercial bottled solutions of hypochlorite (also called chlorine bleach). Sodium hypochlorite gained widespread use not only as for industrial fabric treatment but also as a home laundry bleach. It is still sold today as a 5% solution in water .

Another important use for hypochlorite is as a sanitizer or disinfectant. Both of these uses rely on the hypochlorite's ability to destroy microorganisms . The same oxidative mechanism responsible for hypochlorite's bleaching ability also makes it an effective germicide. Although this mechanism was not understood at the time, hypochlorite (in the form of bleaching powder) was used as early as 1800 to counteract bad odors associated with disease . In fact, it has been said that no single element has played so important a role in combating disease over the last century as chlorine in its various forms. It should also be noted that hypochlorite is corrosive at high concentrations and was only used on the skin at very dilute levels. Its disinfectant properties have also been utilized for the sanitization of food processing equipment, particularly milking utensils used in the dairy industry. One marked advantage of hypochlorite for these applications is the fact that it, in addition to working quickly, rapidly breaks down to innocuous compounds. For this reason it is also useful in chlorination of sewage effluents and swimming pool water. Today, its primary uses are in lavatory bowl deodorizers and sanitizers.

New and improved ways to use hypochlorite are still being developed. In recent years, a number of improved bleach-containing products have been brought to market as chemists have learned to combine sodium hypochlorite with cleaning agents, thickeners and fragrance compounds to create efficacious products with improved aesthetic properties. For example, hypochlorite-based hard surface cleaners for kitchen counter tops, mold and mildew removers for showers and baths, and drain cleaners for kitchen and bathroom sinks are now commercially available.

See also Antisepsis.

Resources

books

Chalmers, Louis. Household and Industrial Chemical Specialties. Vol. 1. Chemical Publishing Co. Inc., 1978.

Schwarcz, Leonard. Sanitary Chemicals. New York: Mac Nair-Dorland Co., 1953.

Randy Schueller

KEY TERMS

Available chlorine

—A measure of the oxidative potential of a chlorine containing solution.

Bleaching powder

—A dry bleach made by treating calcium carbonate with chlorine gas.

Chlorine

—A chemical element whose strong oxidizing abilities make it useful as a disinfectant and deodorizer.

Dakin's solution

—An aqueous solution of hypochlorite (approximately 0.5%) in water used as a hospital antiseptic.

Javelle water

—The first known production of hypochlorite which was made by passing chlorine gas through a solution of sodium carbonate.

Sodium hypochlorite

—A chemical compound consisting of sodium, oxygen and chlorine (NaOCL) which has been used for centuries for its bleaching and disinfectant properties.