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Sodium hypochlorite is a chemical compound with the formula NaClO. A solution of sodium hypochlorite is frequently used as a disinfectant and as a bleaching agent; indeed, often it is simply called "bleach" though other chemicals are sometimes given that name as well.
Sodium hypochlorite (like all hypochlorites) is a salt of hypochlorous acid, HClO. In solution, it splits into the sodium cation Na+ and the hypochlorite anion ClO-. The oxidizing power of the latter causes the bleaching effect. Its negative charge, however, prevents it from diffusing through the cell walls of bacteria and microbes, making it a poor disinfectant. Hypochlorous acid molecules, due to their neutral charge and small size, easily diffuse through the cell walls of bacteria, where they then react with the enzyme triosephosphate dehydrogenase. Triosephosphate dehydrogenase (or glyceraldehyde-3-phosphate dehydrogenase/GAPDH) is an enzyme essential for the digestion of glucose, but is particularly sensitive to oxidising agents. This effectively destroys the micro-organism's ability to function.
Packaging and sale
Household bleach sold to consumers for use in laundering clothes is a 3-6% solution of sodium hypochlorite at the time of manufacture. Strength varies from one formulation to another and decreases with storage.
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.
Uses
In household bleach form, hypochlorite is used for removal of stains from laundry. It is particularly effective on cotton fiber, which stains easily but bleaches well. Use 50 to 250 ml per load for a standard-size washer. Hot water increases the activity of the bleach.
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 off flavors to the brew.
A 1 in 5 dilution of household bleach with water (1 part bleach to 4 parts water) is effective against many bacteria and viruses, and is often the disinfectant of choice in cleaning hospitals. The solution is corrosive however, and needs to be thoroughly removed afterwards; sometimes the bleach disinfection is therefore followed by an ethanol disinfection.
For shock chlorination of 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.
For purification of drinking water, about 1 liter household bleach per 4000 liters of water is used. Exact amounts required depend on 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.
Cautions
Hypochlorite is corrosive and can burn skin and cause eye damage, particularly in concentrated forms. Chloroform compounds formed when hypochlorite is mixed with organic materials (e.g. dirt) are carcinogenic. The extent of the hazard thus created is the subject of disagreement.
Bleach should never be mixed with urine or with other household cleaners, especially not with ones containing acid, since chlorine gas can form; or ammonia, since chloramine gas can form. Both chlorine gas and chloramine gas are highly toxic.
Reference numbers
The CAS number of sodium hypochlorite is 7681-52-9 and the UN number of sodium hypochlorite solution is 1791.
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