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Sodium hypochlorite
Sodium hypochlorite is a chemical compoundwith the formulaNaClO. A solution of sodium hypochlorite is frequently used as a disinfectantand as a bleaching agent; indeed, often it is simply called "bleach", though other chemicals are sometimes given that name as well.
ProductionSodium hypochlorite may be prepared by absorbing chlorinegas in cold sodium hydroxidesolution:
Sodium hydroxide and chlorine are commercially produced by the chloralkali process, and there is no need to isolate them to prepare sodium hypochlorite. Hence NaClO is prepared industrially by the electrolysisof sodium chloridesolution without any separation between the anodeand the cathode. The solution must be kept below 40 °C (by cooling coils) to prevent the formation of sodium chlorate. The commercial solutions always contain significant amounts of sodium chloride (common salt) as the main byproduct, as seen in the equation above. Packaging and saleHousehold bleach sold for use in laundering clothes is a 3-6% solutionof 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. High-test hypochlorite (HTH) is sold for chlorination of swimming pools and contains approximately 30% sodium hypochlorite. The crystalline salt is also sold for the same use; this salt usually contains less than 50% of sodium hypochlorite. However, the level of "active chlorine" may be much higher. UsesIn household bleach form, sodium hypochlorite is used for removal of stains from laundry. It is particularly effective on cotton fiber, which stains easily but bleaches well. 50 to 250 ml per load is usually recommended for a standard-size washer. Hot water increases the activity of the bleach, owing to the thermal decomposition of hypochlorite which ultimately generates environmentally-undesirable chlorate. A weak solution of 1 percent 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. A 1 in 5 dilution of household bleach with water (1 part bleach to 4 parts water) is effective against many bacteriaand some viruses, and is often the disinfectant of choice in cleaning surfaces in hospitals. The solution is corrosive, and needs to be thoroughly removed afterwards, so the bleach disinfection is sometimes followed by an ethanoldisinfection. For shock chlorinationof wells or water systems, a 2% solution of household bleach is used. For larger systems, HTH 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 (lime), so that the shock chlorination is often accompanied by a clogging effect. The precipitate also preserves bacteria, making this practise somewhat less effective. Sodium hypochlorite has been used for the disinfection of drinking water, at 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 US EPArecommendsthe use of 2 drops of 5%ac household bleach per quart of water. If the treated water doesn't 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. It is also used in dentistry, during root canal treatment, disinfecting the canal and disolving any remaining pulp tissue. 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. A well-mixed solution is fully treated when an excess of chlorine is detected. More concentrated cyanide solutions are much more difficult to dispose of. Mechanism of actionLike all hypochlorites, sodium hypochlorite is a saltof hypochlorous acid, HClO. Sodium hypochlorite is a colorless, transparent liquid. In water, it partially splits into the sodium cationNa+ and the hypochloriteanionClO-, while a substantial portion hydrolysesinto sodium hydroxideand hypochlorous acid. The oxidizingpower of the latter and of the hypochlorite anion cause the bleaching effect. Its negative charge, however, prevents it from diffusing through the cell walls of bacteria and microbes, making it a poor disinfectant. However the hypochlorous acid molecules that exist in equilibrium with the hypochlorite anion, due to their neutral charge and small size, easily diffuse through the cell walls of bacteria. This changes the oxidation-reduction potential (ORP) of the cell, and inactivates the enzyme triosephosphate dehydrogenase. Triosephosphate dehydrogenase (or glyceraldehyde-3-phosphate dehydrogenase/GAPDH) is essential for the digestion of glucose, but is particularly sensitive to oxidising agents. Its inactivation effectively destroys the micro-organism's ability to function. CautionsHypochlorite is a strong oxidizer, and the products of the oxidation reactions are corrosive, and can burn skin and cause eye damage, particularly when used in concentrated forms. Hypochlorite must not be mixed with organic materials (e.g. dirt), as the resulting trihalomethanesare carcinogenic. The extent of the hazard thus created is a subject of disagreement. Bleach should never be mixed with other household cleaners, especially not with ones containing acid, since this results in the generation of chlorinegas. It should also never be mixed with anything containing ammonia, since chloraminegas can be gained from this combination. Urinecontains ammonia, so bleach should not be used to clean urine spills. Both chlorine gas and chloramine gas are highly toxic. Bleaches also react rather violently with hydrogen peroxide. It is estimated that there are about 3300 accidents needing hospital treatment caused by sodium hypochlorite solutions each year in British homes (RoSPA, 2002). See also
Bibliography
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External links
es:Hipoclorito de sodio fr:Eau de Javel nl:Bleekloog ja:次亜塩素酸ナトリウム pt:Água sanitária Categories: Sodium compounds| Hypochlorites| Antiseptics| Disinfectants| Bleaches
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