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.
Potassium 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 potash lye. The resulting liquid, known as "Eau de Javel" ("Javel water"), was a weak solution of potassium hypochlorite. Antoine Labarraque replaced potash lye by the cheaper soda lye, thus obtaining sodium hypochlorite (Eau de Labarraque). 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.
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 slowly decomposes, releasing sodium, hydroxide, and chloride ions, and molecular oxygen:
- 4 NaClO + 2 H2O → 4 Na+ + 4 OH− + 4 Cl- + 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 slow the decomposition 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.
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, especially those caused by the tannins in tea.
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; the 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. Liquids containing sodium hypochlorite as the main active component are also used for household cleaning and disinfection, for example toilet cleaners. Some cleaners are formulated to be thick so as not to drain quickly from vertical surfaces (as of a toilet bowl).
See hypochlorous acid for a discussion of the mechanism for disinfectant action.
|This section requires expansion. (June 2013)|
Sodium hypochlorite has deodorising properties
In drinking water systems, swimming pools, etc., sodium hypochlorite is widely used for chlorination (see Water chlorination). Hypochlorites are an alternative to chlorine gas, which is difficult to handle in many contexts. Also, accidents involving chlorine gas are more serious than accidents involving hypochlorites. Chlorination usually produces small quantities of harmful byproducts. Hypochlorites are very similar to chlorine gas in this regard.
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 (chemical warfare nerve gas) 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 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 Pollution Abatement System to ensure that no nerve agent is released in furnace flue gas.
Reduction of skin damage
Dilute bleach baths have been used for decades to treat moderate to severe eczema in humans, but it has not been clear why they work. According to work published by researchers at the Stanford University School of Medicine in November 2013, a very dilute (0.005%) solution of sodium hypochlorite in water was successful in treating skin damage with an inflammatory component caused by radiation therapy, excess sun exposure or ageing in laboratory mice. Mice with radiation dermatitis given daily 30-minute baths in bleach solution experienced less severe skin damage and better healing and hair regrowth than animals bathed in water. A molecule called nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) is known to play a critical role in inflammation, ageing and response to radiation. The researchers found that if NF-kB activity was blocked in elderly mice by bathing them in bleach solution, the animals' skin began to look younger, going from old and fragile to thicker, with increased cell proliferation. The effect diminished after the baths were stopped, indicating that regular exposure was necessary to maintain skin thickness.
Sodium hypochlorite is a strong oxidizer. Oxidation reactions are corrosive, solutions burn skin and cause eye damage, especially 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).
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).
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]
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.
Chlorination of drinking water can oxidize organic contaminants, producing chloroform and other trihalomethanes, which are carcinogenic, and many hundreds of possible disinfection by-products, the vast majority of which are not monitored.
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.
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.
- Helmut Vogt, Jan Balej, John E. Bennett, Peter Wintzer, Saeed Akhbar Sheikh, Patrizio Gallone (2007), "Chlorine Oxides and Chlorine Oxygen Acids", Ullman's Encyclopedia of Industrial Chemistry (7th ed.), Wiley, p. 2
- "How Products Are Made Volume 2". may 2011.
- Bretherick’s Handbook of Reactive Chemical Hazards, 7th Edition; vol. 1, pg 1433
- Univar, sodium hypochlorite safety sheet
- Metcalf & Eddy, Inc (1991). Wastewater Engineering: Treatment, Disposal, & Reuse 3rd Edition; pg 497
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- Lynn R. Marotz, Health, Safety, and Nutrition for the Young Child, pp. 126-127, Cengage Learning, 2011 ISBN 1111298378.
- G. A. Mirafzal and A. M. Lozeva (1998). "Phase transfer catalyzed oxidation of alcohols with sodium hypochlorite". Tetrahedron Letters 39 (40): 7263–7266. doi:10.1016/S0040-4039(98)01584-6.
- Smith WT. (1994). Human and Environmental Safety of Hypochlorite. In: Proceedings of the 3rd World Conference on Detergents: Global Perspectives, pp. 183-5.
- 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 (2009). "Clinical efficacy of 5% sodium hypochlorite for removal of stains caused by dental fluorosis". The Journal of clinical pediatric dentistry 33 (3): 187–91. PMID 19476089.
- US Environmental Protection Agency, Learn About Chemicals Around Your House, Toilet Cleaners
- 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.
- Inflammatory skin damage in mice blocked by bleach solution, study finds, Stanford School of Medicine, 15 November 2013
- Bleach baths using Milton Sterilising Fluid for recurrent infected atopic eczema, K Pett, K Batta, C Vlachou, and G Nicholls
- Leung, T. H.; Zhang, L. F.; Wang, J.; Ning, S.; Knox, S. J.; Kim, S. K. (2013). "Topical hypochlorite ameliorates NF-κB–mediated skin diseases in mice". Journal of Clinical Investigation 123 (12): 5361–5370. doi:10.1172/JCI70895. PMID 24231355.
- 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)