Sodium hypochlorite
Overview
 
Sodium hypochlorite is a chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 NaClO. Sodium hypochlorite solution, commonly known as bleach
Bleach
Bleach refers to a number of chemicals that remove color, whiten, or disinfect, often via oxidation. Common chemical bleaches include household chlorine bleach , lye, oxygen bleach , and bleaching powder...

 or chlorox, is frequently used as a disinfectant or a bleaching agent.
Sodium hypochlorite was first produced in 1789 by Claude Louis Berthollet
Claude Louis Berthollet
Claude Louis Berthollet was a Savoyard-French chemist who became vice president of the French Senate in 1804.-Biography:...

 in his laboratory on the quay Javel
Javel - André Citroën (Paris Metro)
Javel - André Citroën is a station of the Paris Métro, serving Line 10 and offering transfer to the RER C via Javel RER station in the 15th arrondissement....

 in Paris
Paris
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...

, France, by passing chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 gas through a solution of sodium carbonate. The resulting liquid, known as "Eau de Javel" ("Javel water"), was a weak solution of sodium hypochlorite.
Encyclopedia
Sodium hypochlorite is a chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 NaClO. Sodium hypochlorite solution, commonly known as bleach
Bleach
Bleach refers to a number of chemicals that remove color, whiten, or disinfect, often via oxidation. Common chemical bleaches include household chlorine bleach , lye, oxygen bleach , and bleaching powder...

 or chlorox, is frequently used as a disinfectant or a bleaching agent.

Production

Sodium hypochlorite was first produced in 1789 by Claude Louis Berthollet
Claude Louis Berthollet
Claude Louis Berthollet was a Savoyard-French chemist who became vice president of the French Senate in 1804.-Biography:...

 in his laboratory on the quay Javel
Javel - André Citroën (Paris Metro)
Javel - André Citroën is a station of the Paris Métro, serving Line 10 and offering transfer to the RER C via Javel RER station in the 15th arrondissement....

 in Paris
Paris
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...

, France, by passing chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 gas through a solution of sodium carbonate. The resulting liquid, known as "Eau de Javel" ("Javel water"), was a weak solution of sodium hypochlorite. However, this process was not very efficient, and alternative 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 that 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 sodium hypochlorite production involving electrolysis of brine
Brine
Brine is water, saturated or nearly saturated with salt .Brine is used to preserve vegetables, fruit, fish, and meat, in a process known as brining . Brine is also commonly used to age Halloumi and Feta cheeses, or for pickling foodstuffs, as a means of preserving them...

 to produce sodium hydroxide and chlorine gas, which then mixed to form sodium hypochlorite. This is known as the chloralkali process
Chloralkali process
The chloralkali process is an industrial process for the electrolysis of sodium chloride solution . Depending on the method several products beside hydrogen can be produced. If the products are separated, chlorine and sodium hydroxide are the products; by mixing, sodium hypochlorite or sodium...

. Both electric power and brine solution were in cheap supply at the time, and various enterprising marketers took advantage of the situation to satisfy the market's demand for sodium hypochlorite. Bottled solutions of sodium hypochlorite were sold under numerous trade names.

Today, an improved version of this method, known as the Hooker process, is the only large scale industrial method of sodium hypochlorite production. In the process, sodium hypochlorite (NaClO) and sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...

 (NaCl) are formed when chlorine is passed into cold and dilute sodium hydroxide solution. It is prepared industrially by electrolysis
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...

 with minimal separation between the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

 and the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

. The solution must be kept below 40 °C (by cooling coils) to prevent the undesired formation of sodium chlorate
Sodium chlorate
Sodium chlorate is a chemical compound with the chemical formula . When pure, it is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 250 °C to release oxygen and leave sodium chloride...

.
Cl2 + 2 NaOH → NaCl + NaOCl + H2O


Hence, chlorine is simultaneously reduced and oxidized; this process is known as disproportionation
Disproportionation
Disproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....

.

The commercial solutions always contain significant amounts of sodium chloride (common salt) as the main by-product
By-product
A by-product is a secondary product derived from a manufacturing process or chemical reaction. It is not the primary product or service being produced.A by-product can be useful and marketable or it can be considered waste....

, as seen in the equation above.

Packaging and sale

Household bleach sold for use in laundering clothes is a 3-6% solution
Percentage solution
"Percentage solution" is an ambiguous term which is used to describe a solution with the unit "%". It may refer to:* Mass fraction if mass/mass is meant...

 of sodium hypochlorite at the time of manufacture. Strength varies from one formulation to another and gradually decreases with long storage.

A 12% solution is widely used in waterworks for the chlorination
Chlorination
Chlorination is the process of adding the element chlorine to water as a method of water purification to make it fit for human consumption as drinking water...

 of water, and a 15% solution is more commonly used for disinfection of waste water in treatment plants. Sodium hypochlorite can also be used for point-of-use disinfection of drinking water.

It can also be found on store shelves in Daily Sanitizing Sprays, as the sole active ingredient at 0.0095%.

Reactions

Sodium hypochlorite reacts with metals gradually, such as zinc, to produce the metal oxide or hydroxide:
NaClO + Zn → ZnO + NaCl


It reacts with hydrochloric acid to release chlorine gas:
NaClO + 2 HCl → Cl2 + H2O + NaCl


It reacts with other acids, such as acetic acid, to release hypochlorous acid
Hypochlorous acid
Hypochlorous acid is a weak acid with the chemical formula HClO. It forms when chlorine dissolves in water. It cannot be isolated in pure form due to rapid equilibration with its precursor...

:
NaClO + CH3COOH → HClO + CH3COONa


It decomposes when heated or evaporated to form sodium chlorate and sodium chloride:
3 NaClO → NaClO3 + 2 NaCl


In reaction with hydrogen peroxide it gives off molecular oxygen:
NaClO + H2O2 → H2O + NaCl + O2

Bleaching

Household bleach is, in general, a solution containing 4-6% sodium hypochlorite and 0.01-0.05% sodium hydroxide; the sodium hydroxide is used to delay the breakdown of sodium hypochlorite into sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...

 and sodium chlorate
Sodium chlorate
Sodium chlorate is a chemical compound with the chemical formula . When pure, it is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 250 °C to release oxygen and leave sodium chloride...

.

In household form, sodium hypochlorite is used for removal of stains from laundry. It is particularly effective on cotton fiber, which stains easily but bleaches well. Usually 50 to 250 mL of bleach per load is recommended for a standard-size washer. The properties of household bleach that make it effective for removing stains also result in cumulative damage to organic fibers, such as cotton, and the useful lifespan of these materials will be shortened with regular bleaching. The sodium hydroxide (NaOH) that is also found in household bleach (as noted later) causes fiber degradation as well. It is not volatile, and residual amounts of NaOH not rinsed out will continue slowly degrading organic fibers in the presence of humidity. For these reasons, if stains are localized, spot treatments should be considered whenever possible. With safety precautions, post-treatment with vinegar
Vinegar
Vinegar is a liquid substance consisting mainly of acetic acid and water, the acetic acid being produced through the fermentation of ethanol by acetic acid bacteria. Commercial vinegar is produced either by fast or slow fermentation processes. Slow methods generally are used with traditional...

 (or another weak acid) will neutralize the NaOH, and volatilize the chlorine from residual hypochlorite. Old
T-shirts and cotton sheets that rip easily demonstrate the costs of laundering with household bleach. Hot water increases the effectiveness of the bleach, owing to the increased reactivity of the molecules.

Disinfection

A weak solution of 2% household bleach in warm water is used to sanitize smooth surfaces prior to brewing of beer or wine. Surfaces must be rinsed to avoid imparting flavors to the brew; these chlorinated byproducts of sanitizing surfaces are also harmful.

US Government regulations (21 CFR Part 178) allow food processing equipment and food contact surfaces to be sanitized with solutions containing bleach, provided that the solution is allowed to drain adequately before contact with food, and that the solutions do not exceed 200 parts per million (ppm) available chlorine (for example, one tablespoon of typical household bleach containing 5.25% sodium hypochlorite, per gallon of water). If higher concentrations are used, the surface must be rinsed with potable water after sanitizing.

A 1-in-5 dilution of household bleach with water (1 part bleach to 4 parts water) is effective against many bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 and some virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

es, and is often the disinfectant of choice in cleaning surfaces in hospitals (primarily in the United States). The solution is corrosive
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

, and needs to be thoroughly removed afterwards, so the bleach disinfection is sometimes followed by an ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

 disinfection. Even "scientific-grade", commercially produced disinfection solutions such as Virocidin-X usually have sodium hypochlorite as their sole active ingredient, though they also contain surfactants (to prevent beading) and fragrances (to conceal the bleach smell).

Water treatment

For shock chlorination
Shock chlorination
Shock chlorination is a process used in many swimming pools, water wells, springs, and other water sources to reduce the bacterial and algal residue in the water. Shock chlorination is performed by mixing a large amount of sodium hypochlorite, which can be in the form of a powder or a liquid such...

 of wells or water systems, a 3% solution of household bleach is used. For larger systems, sodium hypochlorite is more practical because lower rates can be used. The alkalinity of the sodium hypochlorite solution also causes the precipitation of minerals such as calcium carbonate, so that the shock chlorination is often accompanied by a clogging effect. The precipitate also preserves bacteria, making this practice somewhat less effective.

Sodium hypochlorite has been used for the disinfection of drinking water. A concentration equivalent to about 1 liter of household bleach per 4000 liters of water is used. The exact amount required depends on the water chemistry, temperature, contact time, and presence or absence of sediment. In large-scale applications, residual chlorine is measured to titrate the proper dosing rate. For emergency disinfection, the United States Environmental Protection Agency
United States Environmental Protection Agency
The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...

 recommends the use of 2 drops of 5%ac household bleach per litre of water. If the treated water does not smell of bleach, 2 more drops are to be added.

The use of chlorine-based disinfectants in domestic water, although widespread, has led to some controversy due to the formation of small quantities of harmful byproducts such as chloroform
Chloroform
Chloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous...

.

An alkaline solution (pH 11.0) of sodium hypochlorite is used to treat dilute (< 1 g/L) cyanide wastewater, e.g., rinsewater from an electroplating shop. In batch treatment operations, sodium hypochlorite has been used to treat more concentrated cyanide wastes, such as silver cyanide plating solutions. A well-mixed solution is fully treated when an excess of chlorine is detected.

Endodontics

Sodium hypochlorite is now used in endodontics
Endodontics
Endodontics is one of the dental specialties recognized by the American Dental Association, Royal College of Dentists of Canada, and Royal Australasian College of Dental Surgeons, and deals with the tooth pulp and the tissues surrounding the root of a tooth...

 during root canal treatments. It is the medicament of choice due to its efficacy against pathogenic organisms and pulp digestion. In previous times, Henry Drysdale Dakin
Henry Drysdale Dakin
Henry Drysdale Dakin FRS was an English chemist.He was born in London as the youngest of 8 children to a family of steel merchants from Leeds. As a school boy he did water analysis with the Leeds City Analyst. He studied chemistry at the University of Leeds with Julius B...

's solution (0.5%) had been used. Its concentration for use in endodontics today varies from 0.5% to 5.25%. At low concentrations it will dissolve mainly necrotic tissue; whereas at higher concentrations tissue dissolution is better but it also dissolves vital tissue, a generally undesirable effect. It has been shown that clinical effectiveness does not increase conclusively for concentrations higher than 1%.

Oxidation

Household bleach, with a phase-transfer catalyst, has been reported to oxidize alcohols to the corresponding carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....

 compound.

Nerve Agent Neutralization

At the various nerve agent destruction facilities throughout the United States, 50% sodium hypochlorite is used as a means of removing all traces of nerve agent or blister agent from PPE (Personal Protection Equipment) after an entry is made by personnel into toxic areas. 50% sodium hypochlorite is also used to neutralize any accidental releases of nerve agent in the toxic areas. Lesser concentrations of sodium hypochlorite are used in similar fashion in the PAS (Pollution Abatement System) to ensure that no nerve agent is released in furnace flue gas.

Safety

Sodium hypochlorite is a strong oxidizer. Oxidation reactions are corrosive, solutions burn skin and cause eye damage, in particular, when used in concentrated forms. However, as recognized by the NFPA, only solutions containing more than 40% sodium hypochlorite by weight are considered hazardous oxidizers. Solutions less than 40% are classified as a moderate oxidizing hazard (NFPA 430, 2000).

Chlorination of drinking water can oxidize organic contaminants, producing trihalomethane
Trihalomethane
Trihalomethanes are chemical compounds in which three of the four hydrogen atoms of methane are replaced by halogen atoms. Many trihalomethanes find uses in industry as solvents or refrigerants. THMs are also environmental pollutants, and many are considered carcinogenic...

s (also called haloforms), which are carcinogenic.

Household bleach and pool chlorinator solutions are typically stabilized by a significant concentration of lye (caustic soda, NaOH) as part of the manufacturing reaction. Skin contact will produce caustic irritation or burns due to defatting
Defatting (medical)
Defatting is a medical term that describes the chemical dissolving of dermal lipids, from the skin, on contact with defatting agents. This can result in water loss from the affected area and cause the whitening and drying of the skin which may result in cracking, secondary infection and chemical...

 and saponification
Saponification
Saponification is a process that produces soap, usually from fats and lye. In technical terms, saponification involves base hydrolysis of triglycerides, which are esters of fatty acids, to form the sodium salt of a carboxylate. In addition to soap, such traditional saponification processes...

 of skin oils and destruction of tissue. The slippery feel of bleach on skin is due to this process.

Sodium thiosulfate
Sodium thiosulfate
Sodium thiosulfate , also spelled sodium thiosulphate, is a colorless crystalline compound that is more familiar as the pentahydrate, Na2S2O3•5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulfite or “hypo.”...

 (thio) is an effective chlorine neutralizer. Rinsing with a 5 mg/L solution, followed by washing with soap and water, quickly removes chlorine odor from the hands.

Mixing bleach with some household cleaners can be hazardous. For example, mixing an acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

 cleaner with sodium hypochlorite bleach generates chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 gas. Mixing with ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 solutions (including urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...

) produces chloramine
Chloramine
Chloramines are derivatives of ammonia by substitution of one, two or three hydrogen atoms with chlorine atoms. Monochloramine is an inorganic compound with the formula NH2Cl. It is an unstable colourless liquid at its melting point of -66° temperature, but it is usually handled as a dilute...

s.
NH4OH + NaClO → NaOH + NH2Cl + H2O


Both chlorine gas and chloramine gas are toxic. Bleach can react violently with hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...

 and produce oxygen gas:
H2O2(aq) + NaClO(aq) → NaCl(aq) + H2O(l) + O2(g)


It is estimated that there are about 3300 accidents needing hospital treatment caused by sodium hypochlorite solutions each year in British homes (RoSPA, 2002).

One major concern arising from sodium hypochlorite use is that it tends to form chlorinated organic compounds; this can occur during household storage and use as well during industrial use. For example, when household bleach and wastewater were mixed, 1-2% of the available chlorine was observed to form organic compounds. As of 1994, not all the byproducts had been identified, but identified compounds include chloroform
Chloroform
Chloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous...

 and carbon tetrachloride
Carbon tetrachloride
Carbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent...

. The estimated exposure to these chemicals from use is estimated to be within occupational exposure limits.

A recent European study indicated that sodium hypochlorite and organic chemicals (e.g., surfactants, fragrances) contained in several household cleaning products can react to generate chlorinated volatile organic compounds (VOCs). These chlorinated compounds are emitted during cleaning applications, some of which are toxic and probable human carcinogens. The study showed that indoor air concentrations significantly increase (8-52 times for chloroform and 1-1170 times for carbon tetrachloride, respectively, above baseline quantities in the household) during the use of bleach containing products. The increase in chlorinated volatile organic compound concentrations was the lowest for plain bleach and the highest for the products in the form of “thick liquid and gel”. The significant increases observed in indoor air concentrations of several chlorinated VOCs (especially carbon tetrachloride and chloroform) indicate that the bleach use may be a source that could be important in terms of inhalation exposure to these compounds. While the authors suggested that using these cleaning products may significantly increase the cancer risk, this conclusion appears to be hypothetical:
  • The highest level cited for concentration of carbon tetrachloride (seemingly of highest concern) is 459 micrograms per cubic meter, translating to 0.073 ppm (part per million), or 73 ppb (part per billion). The OSHA
    Occupational Safety and Health Administration
    The United States Occupational Safety and Health Administration is an agency of the United States Department of Labor. It was created by Congress of the United States under the Occupational Safety and Health Act, signed by President Richard M. Nixon, on December 29, 1970...

    -allowable time-weighted average concentration over an eight-hour period is 10 ppm, almost 140 times higher;
  • The OSHA
    Occupational Safety and Health Administration
    The United States Occupational Safety and Health Administration is an agency of the United States Department of Labor. It was created by Congress of the United States under the Occupational Safety and Health Act, signed by President Richard M. Nixon, on December 29, 1970...

     highest allowable peak concentration (5-minute exposure for five minutes in a 4-hour period) is 200 ppm, twice as high as the reported highest peak level (from the headspace of a bottle of a sample of bleach plus detergent).


Further studies of the use of these products and other possible exposure routes (i.e., dermal) may reveal other risks. Though the author further cited ozone depletion greenhouse effects for these gases, the very low amount of such gases, generated as prescribed, should minimize their contribution relative to other sources.

External links

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