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 water, it partially splits into the sodium cation Na⁺ and the hypochlorite anion ClO^-, while a substantial portion hydrolyses into sodium hydroxide and hypochlorous acid. The oxidizing power 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, owing to the equilibrium between hypochlorous acid and the hypochlorite anion, the hypochlorous acid molecules, 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, this inactivating 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.

Production

Sodium hypochloride is commercially produced by absorbing chlorine gas in cold sodium hydroxide solution:

2 NaOH + Cl₂ = NaCl + NaClO + H₂O

The resulting solution still contains free sodium hydroxide, because the reaction with chlorine gas an sodium hypochlorite is also possible, and would result in a product of lower quality (increasing chlorate content). The commercial solutions always contain significant amounts of sodium chloride (common salt) as the main byproduct, as can be seen in the equation above.
Sodium hypochlorite is thermally instable, and decomposes into a mixture of sodium chloride and sodium chlorate, for example upon boiling of the solution.

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.

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 content of "active chlorine" may be much higher.

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, 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 off 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 bacteria and some 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. However, the alkalinity of the sodium hypochlorite solution also causes the 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 questionable.

For disinfection of drinking water, often but wrongly called "purification", 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. The use as a water disinfectant always results in the formation of toxic compounds such as trihalomethanes (THM) and trihaloacetic acids, aside from toxic chlorate that is always present at varying levels in commercial hypochlorite. One should always weigh the infection risk against the risk of consuming these chemicals.

Cautions

Hypochlorite is a strong oxidant, and the technical products are corrosive and can burn skin and cause eye damage, particularly in concentrated forms. Hypochlorite must not be mixed with organic materials (e.g. dirt), the resulting trihalomethanes are carcinogenic. The extent of the hazard thus created is the subject of disagreement.

Bleach should never be mixed with other household cleaners, especially not with ones containing acid, since chlorine gas can form. It sould also never be mixed with anything containing ammonia, since chloramine gas can form. Urine contains ammonia, so bleach should not be used to clean it. Both chlorine gas and chloramine gas are highly toxic. It also reacts rather violently with Hydrogen Peroxide H2O2

Reference numbers

The CAS number of sodium hypochlorite is 7681-52-9 and the UN number of sodium hypochlorite solution is 1791.