Sodium hypochlorite

Sodium hypochlorite is a chemical compound with the formula NaOCl. Sodium hypochlorite solution, commonly known as bleach, is frequently used as a disinfectant and as a bleaching agent.

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Production

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

2NaOH + Cl₂ → NaCl + NaClO + 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 with minimal 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.

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^{[citation needed]} used in waterworks for the chlorination of water. High-test hypochlorite (HTH) is sold for chlorination of swimming pools and contains approximately 30% calcium hypochlorite. The crystalline salt is also sold for the same use; this salt usually contains less than 50% of calcium hypochlorite. However, the level of "active chlorine" may be much higher.

It can also be found on store shelves present in "Daily Sanitizing Sprays", as the sole active ingredient at 0.0095%.

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. 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 weak organic acids such as acetic (vinegar) 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 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.

US Government regulations (21 CFR Part 178) allow food processing equipment and food contact surfaces to be sanitized with solutions containing bleach provided the solution is allowed to drain adequately before contact with food, and 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.

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 United States Environmental Protection Agency 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 dissolving any remaining pulp tissue. Historically, 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. In batch treatment operations, sodium hypochlorite has been used to treat more concentrated cyanide wastes, such as silver cyanide plating solutions. A well-mixed solution is fully treated when an excess of chlorine is detected.

Mechanism of action

See Hypochlorous acid

Cautions

Sodium hypochlorite is a strong oxidizer. Products of the oxidation reactions are corrosive. Solutions burn skin and cause eye damage, particularly 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).

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.

Sodium thiosulfate (hypo) is an effective chlorine neutralizer. Rinsing with a 5mg/L solution, followed by washing with soap and water, quickly removes chlorine odor from the hands.

Chlorination of drinking water can oxidize organic contaminants, producing trihalomethanes (also called haloforms), which are carcinogenic. The extent of the hazard thus created is a subject of disagreement.

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. Both chlorine gas and chloramine gas are toxic. Bleach can react violently with hydrogen peroxide and produce oxygen gas:^[1]

H₂O_{2 (aqueous)} + NaClO_(aqueous) → NaCl_(aqueous) + H₂O_(liquid) + O_{2 (gas)}

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

• Chlorination
• Water purification

Bibliography

• Jones, F.-L. (1972). "Chlorine poisoning from mixing household cleaners". J. Am. Med. Assoc. 222: 1312.
• Institut National de Recherche et de Sécurité. (2004). "Eaux et extraits de Javel. Hypochlorite de sodium en solution". Fiche toxicologique n° 157, Paris.

References

1. ^ Hydrogen Peroxide + Bleach Explanation. Retrieved on 10 April 2007.