Sodium hypochlorite

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Sodium hypochlorite
Image:Sodium-hypochlorite.png
General
Other names	Sodium chlorate(I)
Molecular formula	NaOCl
Molar mass	74.44 g/mol
Appearance	white solid
CAS number	[7681-52-9]
Properties
Density and phase	?
Solubility in water	Fully miscible
Melting point	18 °C Pentahydrate
Boiling point	? °C Decomposes
Acidity (pK_a)	?
Basicity (pK_b)	?
Hazards
EU classification	Corrosive (C) Dangerous for the environment (N)
R-phrases	R31, R34, R50
S-phrases	S1/2, S28, S45, S50, S61
NFPA 704
Related compounds
Other anions	Sodium chloride Sodium chlorite Sodium chlorate Sodium perchlorate
Other cations	Lithium hypochlorite Calcium hypochlorite
Related compounds	Hypochlorous acid
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Sodium hypochlorite is a chemical compound with the formula NaOCl. 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.

1 Production
2 Packaging and sale
3 Uses
4 Mechanism of action
5 Cautions
6 See also
7 Bibliography
8 External links

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Production

Sodium hypochlorite may be prepared by absorbing chlorine gas in cold sodium hydroxide solution:

2NaOH + Cl₂ ⇌ NaCl + NaOCl + H₂O

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 electrolysis of sodium chloride solution without any separation between the anode and 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.

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Packaging and sale

Household bleach sold 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 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.

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Uses

In 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 % 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 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.

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. The alkalinity of the sodium hypochlorite solution also causes the precipitation of minerals such as calcium carbonate, so that the shock chlorination is often accompanied by a clogging effect. The precipitate also preserves bacteria, making this practice 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 EPA recommends the 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. Historicaly, Henry Drysdale Dakin's solution (0.5%) had been used. Nowadays, 2.5-5.25% solutions are being used.

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.

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Mechanism of action

Like all hypochlorites, sodium hypochlorite is a salt of hypochlorous acid, HClO. Sodium hypochlorite is a colorless, transparent liquid. 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 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.

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Cautions

Hypochlorite 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 trihalomethanes (also called haloforms) are 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 chlorine gas. It should also never be mixed with anything containing ammonia, since chloramine gas can be gained from this combination. Urine contains 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).

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Bibliography

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Institut National de Recherche et de Sécurité. (2004). "Eaux et extraits de Javel. Hypochlorite de sodium en solution". Fiche toxicologique n° 157, Paris.

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