Sodium hypochlorite is a chemical compound with the formula NaClO. It is composed of a sodium cation (Na+
) and a hypochlorite anion (ClO−
); it may also be viewed as the sodium salt of hypochlorous acid. It is commonly known as bleach or liquid bleach, is frequently used as a disinfectant or a bleaching agent.
Sodium hypochlorite was first produced in 1789 by Claude Louis Berthollet in his laboratory on the quay Javel in Paris, France, by passing chlorine 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 after World War I under the trade names "Eusol" and "Dakin's Solution".
Near the end of the nineteenth century, E. S. Smith patented the chloralkali process: a method of producing sodium hypochlorite involving the electrolysis of brine to produce sodium hydroxide and chlorine gas, which then mixed to form sodium hypochlorite.[unreliable source?] 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 (named after Hooker Chemicals, now Occidental Petroleum), is the only large scale industrial method of sodium hypochlorite production. In the process, sodium hypochlorite (NaClO) and sodium chloride (NaCl) are formed when chlorine is passed into cold and dilute sodium hydroxide solution. It is prepared industrially by electrolysis with minimal separation between the anode and the cathode. The solution must be kept below 40 °C (by cooling coils) to prevent the undesired formation of sodium chlorate.
- Cl2 + 2 NaOH → NaCl + NaClO + H2O
Hence, chlorine is simultaneously reduced and oxidized; this process is known as disproportionation.
The commercial solutions always contain significant amounts of sodium chloride (common salt) as the main by-product, as seen in the equation above.
Packaging and sale
Like many hypochlorites, anhydrous NaClO obtained by desiccation of the pentahydrate will decompose violently on heating or friction, however, it is more stable in cold dilute solutions.
Household bleach sold for use in laundering clothes is a 3-8% solution 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 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.
Sodium hypochlorite 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:
- NaClO + CH3COOH → HClO + CH3COONa
It decomposes when heated 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↑
When dissolved in water it will slowly decompose, releasing chlorine, oxygen and sodium and hydroxide ions.
- 4 NaClO + 2 H2O → 4 Na+ + 4 OH− + 2 Cl2 + O2
Sodium hypochlorite reacts with most nitrogen compounds to form volatile chloramines, dichloramines, and nitrogen trichloride:
- NH3 + NaOCl → NH2Cl + NaOH
- NH2Cl+ NaOCl → NHCl2 + NaOH
- NHCl2 + NaOCl → NCl3 + NaOH
- NaClO + Zn → ZnO + NaCl
Household bleach is, in general, a solution containing 3-8% sodium hypochlorite and 0.01-0.05% sodium hydroxide; the sodium hydroxide is used to delay the breakdown of sodium hypochlorite into sodium chloride and sodium chlorate.
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 (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.
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 and some viruses, and is often the disinfectant of choice in cleaning surfaces in hospitals (primarily in the United States). The solution is corrosive, and needs to be thoroughly removed afterwards, so the bleach disinfection is sometimes followed by an ethanol disinfection.
See Hypochlorous acid for a discussion of the mechanism for disinfectant action.
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Sodium hypochlorite has destaining properties. Amongst other applications, it can be used to remove mold stains, dental stains caused by fluorosis, and stains on crockery, in particular those caused by the tannins in tea.
Sodium hypochlorite has been used for the disinfection of drinking water or water systems. 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.
Additionally, transport and handling safety concerns have directed public opinion towards the use of sodium hypochlorite rather than chlorine gas in water treatment, which represents a significant market expansion potential.[self-published source?]
Sodium hypochlorite solutions have been used to treat dilute cyanide wastewater, such as electroplating wastes. In batch treatment operations, sodium hypochlorite has been used to treat more concentrated cyanide wastes, such as silver cyanide plating solutions. Toxic cyanide is oxidized to cyanate (OCN−) that is not toxic, idealized as follows:
- CN− + OCl− → CNO− + Cl−
Sodium hypochlorite is commonly used as a biocide in industrial applications to control slime and bacteria formation in water systems used at power plants, pulp and paper mills, etc. in solutions typically of 10%-15% by weight.
Sodium hypochlorite is now used in endodontics 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'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%.
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.
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).
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 and saponification of skin oils and destruction of tissue. The slippery feel of bleach on skin is due to this process. Trichloramine, the gas that is in swimming pools can cause atopic asthma.[verification needed]
Sodium thiosulfate (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 cleaner with sodium hypochlorite bleach generates chlorine gas. Mixing with ammonia solutions (including urine) produces chloramines. Mixtures of other cleaning agents and or organic matter can result in a gaseous reaction that can cause acute lung injury.
- NH4OH + NaClO → NaOH + NH2Cl + H2O
- 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 and carbon tetrachloride. 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. The authors suggested that using these cleaning products may significantly increase the cancer risk.
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- A.I.S.E. Hypochlorite Scientific Dossier. "This Support Dossier deals with information on the environmental and human safety evaluation of hypochlorite, and on its benefits as a disinfecting, deodorising and stain removing agent."
- Cárdenas Flores A, Flores Reyes H, Gordillo Moscoso A, Castanedo Cázares JP, Pozos Guillén Ade J.: Clinical efficacy of 5% sodium hypochlorite for removal of stains caused by dental fluorosis, J Clin Pediatr Dent. 2009 Spring;33(3):187-91
- Intratec Solutions (2013). Technology Economics: Sodium Hypochlorite Chemical Production (Technical report). Houston, TX: Intratec Solutions. ISBN 978-1-483-95119-5.
- Zehnder M et al. (2002). "Tissue dissolving capacity and antibacterial effect of buffered and unbuffered hypochlorite solutions". Oral Surg Oral Med Oral Pathol Oral Radio Endodon 94 (6): 756–62. doi:10.1067/moe.2002.128961. PMID 12464903.
- Odabasi, M., “Halogenated Volatile Organic Compounds from the Use of Chlorine-Bleach- Containing Household Products”, Environmental Science & Technology 42, 1445-1451, (2008). Available at: http://pubs.acs.org/journals/esthag/
- Odabasi, M., “Halogenated Volatile Organic Compounds from the Use of Chlorine-Bleach- Containing Household Products, Slide presentation (2008). Available at: http://www.slideworld.org/ViewSlides.aspx?URL=5092
||This section includes a list of references, related reading or external links, but the sources of this section remain unclear because it lacks inline citations. (July 2013)|
- Jones, F.-L. (1972). "Chlorine poisoning from mixing household cleaners". J. Am. Med. Assoc. 222 (10): 1312. doi:10.1001/jama.222.10.1312.
- Institut National de Recherche et de Sécurité. (2004). "Eaux et extraits de Javel. Hypochlorite de sodium en solution". Fiche toxicologique n° 157, Paris.
- International Chemical Safety Card 0482 (solutions<10% active Cl)
- International Chemical Safety Card 1119 (solutions >10% active Cl)
- Institut national de recherche et de sécurité (in French)
- Home and Leisure Accident Statistics 2002 (UK RoSPA)
- Emergency Disinfection of Drinking Water (United States Environmental Protection Agency)
- Chlorinated Drinking Water (IARC Monograph)
- NTP Study Report TR-392: Chlorinated & Chloraminated Water (US NIH)
- Guidelines for the Use of Chlorine Bleach as a Sanitizer in Food Processing Operations (Oklahoma State University)