TheInfoList

Sodium hypochlorite is a chemical compound with the formula NaClO. It is composed of a sodium cation (Na+
) and a hypochlorite anion (ClO
); it may also be viewed as the sodium salt of hypochlorous acid. When dissolved in water it is commonly known as bleach or liquid bleach.[1] Sodium hypochlorite is practically and chemically distinct from chlorine, but may be converted into it by the addition of acid.[2] Sodium hypochlorite is frequently used as a disinfectant or a bleaching agent. Hypochlorite solutions liberate toxic gases such as chlorine when acidified or heated. The reaction with ammonia or with substances that can generate ammonia can produce chloramines which are also toxic and have explosive potential.

## Uses

### Bleaching

Household bleach is, in general, a solution containing 3–8% sodium hypochlorite, by weight, and 0.01–0.05% sodium hydroxide; the sodium hydroxide is used to slow the decomposition of sodium hypochlorite into sodium chloride and sodium chlorate.[3]

### Cleaning

Sodium hypochlorite has destaining properties.[4] Among other applications, it can be used to remove mold stains, dental stains caused by fluorosis,[5] and stains on crockery, especially those caused by the tannins in tea. It has also been used in laundry detergents and as a surface cleaner.

Its bleaching, cleaning, deodorizing and caustic effects are due to oxidation and hydrolysis (saponification). Organic dirt exposed to hypochlorite becomes water-soluble and non-volatile, which reduces its odor and facilitates its removal.

### Disinfection

Sodium hypochlorite in solution exhibits broad spectrum anti-microbial activity and is widely used in healthcare facilities in a variety of settings.[6] It is usually diluted in water depending on its intended use. "Strong chlorine solution" is a 0.5% solution of hypochlorite (containing approximately 5000 ppm free chlorine) used for disinfecting areas contaminated with body fluids, including large blood spills (the area is first cleaned with detergent before being disinfected).[6][7] It may be made by diluting household bleach as appropriate (normally 1 part bleach to 9 parts water).[8] Such solutions have been demonstrated to inactivate both C. difficile[6] and HPV.[9] "Weak chlorine solution" is a 0.05% solution of hypochlorite used for washing hands, but is normally prepared with calcium hypochlorite granules.[7]

US government regulations allow food processing equipment and food contact surfaces to be sanitized with solutions containing bleach, provided that the solution is allowed to drain adequately before contact with food, and that 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).[10] If higher concentrations are used, the surface must be rinsed with potable water after sanitizing.

A similar concentration of bleach in warm water is used to sanitize surfaces prior to brewing of beer or wine. Surfaces must be rinsed with sterilized (boiled) water to avoid imparting flavors to the brew; the chlorinated byproducts of sanitizing surfaces are also harmful. The mode of disinfectant action of sodium hypochlorite is similar to that of hypochlorous acid.

Solutions containing more than 500 ppm available chlorine are corrosive to some metals, alloys and many thermoplastics (such as acetal resin) and need to be thoroughly removed afterwards, so the bleach disinfection is sometimes followed by an ethanol disinfection. Liquids containing sodium hypochlorite as the main active component are also used for household cleaning and disinfection, for example toilet cleaners.[11] Some cleaners are formulated to be thick so as not to drain quickly from vertical surfaces, such as the inside of a toilet bowl.

Neutrophils of the human immune system produce small amounts of hypochlorite inside phagosomes, which digest bacteria and viruses.

### Deodorizing

Sodium hypochlorite has deodorizing properties, which go hand in hand with its cleaning properties.[4]

### Water treatment

Sodium hypochlorite solutions have been used to treat dilute cyanide waste water, such as electroplating wastes. In batch treatment operations, sodium hypochlorite has been used to treat more concentrated cyanide wastes, such as silver cyanide plating solutions. Toxic cyanide is oxidized to cyanate (OCN) that is not toxic, idealized as follows:

CN + OCl → OCN + Cl

Sodium hypochlorite is commonly used as a biocide in industrial applications to control slime and bacteria formation in water systems used at power plants, pulp and paper mills, etc., in solutions typically of 10–15% by weight.

### Endodontics

Sodium hypochlorite is the medicament of choice due to its efficacy against pathogenic organisms and pulp digestion in endodontic therapy. Its concentration for use varies from 0.5% to 5.25%. At low concentrations it dissolves mainly necrotic tissue; at higher concentrations it also dissolves vital tissue and additional bacterial species. One study has shown that Enterococcus faecalis was still present in the dentin after 40 minutes of exposure of 1.3% and 2.5% sodium hypochlorite, whereas 40 minutes at a concentration of 5.25% was effective in E. faecalis removal.[12] In addition to higher concentrations of sodium hypochlorite, longer time exposure and warming the solution also increases its effectiveness in removing soft tissue and bacteria within the root canal chamber.[12] 2% is a common concentration as there is less risk of an iatrogenic hypochlorite incident.[13] A hypochlorite incident is an immediate reaction of severe pain, followed by edema, haematoma, and ecchymosis as a consequence of the solution escaping the confines of the tooth and entering the periapical space. This may be caused by binding or excessive pressure on the irrigant syringe, or it may occur if the tooth has an unusually large apical foramen.[14]

### Nerve agent neutralization

At the various nerve agent (chemical warfare nerve gas) destruction facilities throughout the United States, 50% sodium hypochlorite is used to remove all traces of nerve agent or blister agent from Personal Protection Equipment after an entry is made by personnel into toxic areas. 50% sodium hypochlorite is also used to neutralize any accidental releases of nerve agent in the toxic areas. Lesser concentrations of sodium hypochlorite are used in similar fashion in the Pollution Abatement System to ensure that no nerve agent is released in furnace flue gas.

### Reduction of skin damage

Dilute bleach baths have been used for decades to treat moderate to severe eczema in humans,[15][16] but it has not been clear why they work. According to work published by researchers at the Stanford University School of Medicine in November 2013, a very dilute (0.005%) solution of sodium hypochlorite in water was successful in treating skin damage with an inflammatory component caused by radiation therapy, excess sun exposure or aging in laboratory mice. Mice with radiation dermatitis given daily 30-minute baths in bleach solution experienced less severe skin damage and better healing and hair regrowth than animals bathed in water. A molecule called nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) is known to play a critical role in inflammation, aging, and response to radiation. The researchers found that if NF-κB activity was blocked in elderly mice by bathing them in bleach solution, the animals' skin began to look younger, going from old and fragile to thicker, with increased cell proliferation. The effect diminished after the baths were stopped, indicating that regular exposure was necessary to maintain skin thickness.[15][17]

## Safety

Sodium hypochlorite is a strong oxidizer. Oxidation reactions are corrosive. Solutions burn the skin and cause eye damage, especially 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).

Mixing bleach with some household cleaners can be hazardous. For example, mixing an acid cleaner with sodium hypochlorite bleach generates toxic chlorine gas. Mixing bleach with amines (for example, cleaning products containing ammonia or related compounds and biological materials such as urine) produces nitrogen trichloride.[18] This gaseous product can cause acute lung injury. Chronic exposure, for example, from the air at swimming pools where chlorine is used as the disinfectant, can lead to the development of atopic asthma.[19]

Bleach can react violently with hydrogen peroxide and produce oxygen gas:

H2O2(aq) + NaOCl(aq) → NaCl(aq) + H2O(l) + O2(g)

It is estimated that there are about 3300 accidents needing hospital treatment caused by sodium hypochlorite solutions each year in British homes (RoSPA, 2002).

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.

A European study, published in 2008, indicated that sodium hypochlorite and organic chemicals (e.g., surfactants, fragrances) contained in several household cleaning products can react to generate chlorinated volatile organic compounds (VOCs).[20] These chlorinated compounds are emitted during cleaning applications, some of which are toxic and probable human carcinogens. The study showed that indoor air concentrations significantly increase (8–52 times for chloroform and 1–1170 times for carbon tetrachloride, respectively, above baseline quantities in the household) during the use of bleach containing products. The increase in chlorinated volatile organic compound concentrations was the lowest for plain bleach and the highest for the products in the form of "thick liquid and gel." The significant increases observed in indoor air concentrations of several chlorinated VOCs (especially carbon tetrachloride and chloroform) indicate that the bleach use may be a source that could be important in terms of inhalation exposure to these compounds. The authors suggested that using these cleaning products may significantly increase the cancer risk.[20]

One major concern arising from sodium hypochlorite use is that it tends to form chlorinated organic compounds, some of which are carcinogenic. This can occur during household storage and use as well during industrial use.[3] For example, when household bleach and wastewater were mixed, 1–2% of the available chlorine was observed to form organic compounds.[3] As of 1994, not all the byproducts had been identified, but identified compounds include chloroform and carbon tetrachloride.[3] The estimated exposure to these chemicals from use is estimated to be within occupational exposure limits.[3]

## Production

Potassium hypochlorite was first produced in 1789 by Claude Louis Berthollet in his laboratory on the Quai de Javel in Paris, France, by passing chlorine gas through a solution of potash lye. The resulting liquid, known as "Eau de Javel" ("Javel water"), was a weak solution of potassium hypochlorite. Antoine Labarraque replaced potash lye by the cheaper soda lye, thus obtaining sodium hypochlorite (Eau de Labarraque).[21] However, this process was not very efficient, and alternative production methods were sought. One such method involved the extraction of chlorinated lime (known as bleaching powder) with sodium carbonate to yield low levels of available chlorine. This method was commonly used to produce hypochlorite solutions for use as a hospital antiseptic that was sold after World War I under the names "Eusol", an abbreviation for Edinburgh University Solution Of (chlorinated) Lime – a reference to the university's pathology department, where it was developed[22] – and "Dakin's Solution." The UK's National Institute for Health and Care Excellence in October 2008 recommended the preparation should not be used in routine wound care.[23]

Near the end of the nineteenth century, E. S. Smith patented the chloralkali process: a method of producing sodium hypochlorite involving the electrolysis of brine to produce sodium hydroxide and chlorine gas, which then mixed to form sodium hypochlorite.[24] Both electric power and brine solution were in cheap supply at the time, and various enterprising marketers took advantage of the situation to satisfy the market's demand for sodium hypochlorite. Bottled solutions of sodium hypochlorite were sold under numerous trade names.

Today, an improved version of this method, known as the Hooker process (named after Hooker Chemicals, acquired by Occidental Petroleum), is the only large scale industrial method of sodium hypochlorite production. In the process, sodium hypochlorite (NaClO) and sodium chloride (NaCl) are formed when chlorine is passed into cold dilute sodium hydroxide solution. It is prepared industrially by electrolysis with minimal separation between the anode and the cathode. The solution must be kept below 40 °C (by cooling coils) to prevent the undesired formation of sodium chlorate.

Cl2(g) + 2 NaOH(aq) → NaCl(aq) + NaOCl(aq) + H2O(l)

Hence, chlorine is simultaneously reduced and oxidized; this process is known as disproportionation.

Commercial solutions always contain significant amounts of sodium chloride (common salt) as the main by-product, as seen in the equation above.

### Other production routes

Sodium hypochlorite can be easily produced for research purposes by reacting ozone with salt.

NaCl + O3 → NaClO + O2

This reaction happens at room temperature and can be helpful for oxidizing alcohols.

## Packaging and sale

Like many hypochlorites, anhydrous NaClO obtained by desiccation of the pentahydrate will decompose violently on heating or friction.[25] However, it is more stable in cold dilute solutions.

Household bleach sold for use in laundering clothes is a 3–8% solution of sodium hypochlorite at the time of manufacture. Strength varies from one formulation to another and gradually decreases with long storage.

A 10–25% solution of sodium hypochlorite is, according to Univar's safety sheet, supplied with synonyms or trade names bleach, Hypo, Everchlor, Chloros, Hispec, Bridos, Bleacol, or Vo-redox 9110.[26]

A 12% solution is widely used in waterworks for the chlorination of water, and a 15% solution is more commonly[27] used for disinfection of waste water in treatment plants. Sodium hypochlorite can also be used for point-of-use disinfection of drinking water.[28]

Dilute solutions (50 ppm to 1.5%) are found in disinfecting sprays and wipes used on hard surfaces.[29][30]

## Reactions

All hypochlorite solutions are fairly alkaline. As salts of a weak acid, hypochlorous acid, hypochlorites ionize water to produce hydroxide anions, thus increasing pH:

NaOCl(aq) ⇌ Na⁺(aq) + OCl⁻(aq)
OCl⁻(aq) + H₂O(l) ⇌ HOCl(aq) + OH⁻(aq)

Sodium hypochlorite reacts with strong acids such as hydrochloric acid to release chlorine gas:

NaClO(aq) + 2 HCl(aq) → Cl2(g) + H2O(l) + NaCl(aq)

It reacts with weak acids, such as acetic acid, to release hypochlorous acid (at pH < 2, chlorine gas evolves):

NaClO + CH3COOH → HClO + CH3COONa

It decomposes (autoxidizes) when heated, forming sodium chlorate and sodium chloride:

3 NaOCl(aq) → NaClO3(aq) + 2 NaCl(aq)

Sodium hypochlorite decomposes with increasing temperature and under the influence of light and such metals as copper, nickel, or cobalt:

2 OCl⁻(aq) → 2 Cl⁻(aq) + O₂(g)

For this reason the more stable calcium hypochlorite or a complex of hypochlorite with cyanuric acid (Trichloroisocyanuric acid) is used in certain applications.

If not properly stored in airtight containers, sodium hypochlorite reacts with carbon dioxide to form sodium carbonate:

2 NaOCl(aq) + CO₂(g) → Na₂CO₃(aq) + Cl₂(g)

Depending on the pH of solution the hypochlorous acid dissociates to form hypochlorite ion and hydrogen ion.

HOCl ⇌ H+ + OCl

The undissociated (nonionized) hypochlorous acid is believed to react with and inactivate bacterial and viral enzymes.

In NaClO solutions, the following species are thought to be present when the system is in equilibrium.[31]

HOCl ⇌ H+ + OCl
HOCl + Cl + H+ ⇌ Cl2 + H2O

The ratio Cl2 : HOCl : OCl is pH dependent.[32] The amount of undissociated (nonionized) HOCl is highest at a pH of about 4. At pH < 2, chlorine gas (Cl₂) evolves. At pH > 7, only hypochlorite anions (OCl⁻) remain.

Sodium hypochlorite reacts with most nitrogen compounds to form volatile chloramines, dichloramines, and nitrogen trichloride:

NH3 + NaClO → NH2Cl + NaOH
NH2Cl+ NaClO → NHCl2 + NaOH
NHCl2 + NaClO → NCl3 + NaOH

In the presence of a phase-transfer catalyst, alcohols are oxidized to the corresponding carbonyl compound.[33]

Heterogeneous reactions of sodium hypochlorite and metals such as zinc proceed slowly to give the metal oxide or hydroxide:

NaClO + Zn → ZnO + NaCl

Homogeneous reactions with metal coordination complexes proceed somewhat faster. This has been exploited in the Jacobsen epoxidation.

## Relationship to free chlorine

When used as a swimming pool disinfectant or for water treatment, the efficacy of the treated water is usually measured in the mass concentration of "free chlorine" or "available chlorine". The usual units are mg/L or, equivalently, ppm (parts per million). The unit of gpl (grams per liter) is also used. The NaClO content of household bleach is generally specified as a mass or weight percent.

The "free chlorine" is a measure of the amount of chlorine gas (Cl2) that would yield the same oxidizing power as the NaClO in solution. As one molecule of NaClO has the same oxidizing power as one molecule of Cl2, for a given mass of NaClO, the mass of "free chlorine" is 1/1.05 times that quantity (the ratio of the molecular weights of the two compounds).[1]

To determine the mass concentration of NaClO or free chlorine in a diluted solution from the NaClO concentration, the density of the solutions must be considered. Since NaClO in solution is hydrophilic, water molecules are attracted to the hypochlorite ion, and the density of the solution is rather higher than might be expected from a simple calculation. Volumes are thus not strictly additive; the volume of a liter of NaClO solution and an equal volume of water will somewhat less than twice the original volume. The density of a solution (ρ(w)) will be a function of the mass fraction of NaClO (w) and tables are given in the OxyChem Handbook.[1]

The masses of NaClO and water will be constant before and after dilution. If wb, ρ(wb) are the mass fraction and density of NaClO in the undiluted solution, and Vb its volume, and w and ρ(w) and V are likewise for the final solution, the masses of NaClO (Mb) and water (Mw) before and after dilution are conserved:

${\displaystyle M_{b}=w_{b}\rho (w_{b})V_{b}=w\rho (w)V}$
${\displaystyle M_{w}=(1-w_{b})\rho (w_{b})V_{b}+\rho _{w}V_{w}=(1-w)\rho (w)V}$

where ρw and Vw are the density and volume of the added water. Solving the above equations for w and V, the final NaClO mass fraction will be:

${\displaystyle w={\frac {V_{b}w_{b}\rho (w_{b})}{V_{b}\rho (w_{b})+V_{w}\rho _{w}}}}$

and the final NaClO mass concentration will be:

${\displaystyle c={\frac {M_{b}}{V}}=w\rho (w)}$

Dividing this by 1.05 will yield the mass concentration of free chlorine. For example, one ml of 5.25 wt% NaClO bleach added to ten liters of water, will yield a NaClO concentration of about 5.76 mg/L NaClO and 5.48 mg/L of free chlorine. Sodium hydroxide (NaOH or lye) is usually added in small amounts to household bleach to slow down the decomposition of NaClO. This will require a small correction to the above calculations.[1]

## Neutralization

Sodium thiosulfate is an effective chlorine neutralizer. Rinsing with a 5 mg/L solution, followed by washing with soap and water, will remove chlorine odor from the hands.[34]

## References

1. ^ a b c d "OxyChem Sodium Hypochlorite Handbook" (PDF). oxy.com. OxyChem.
2. ^ "Pamphlet 96, The Sodium Hypochorite Manual". www.chlorineinstitute.org. The Chlorine Institute.
3. Smith WT. (1994). Human and Environmental Safety of Hypochlorite. In: Proceedings of the 3rd World Conference on Detergents: Global Perspectives, pp. 183–5.
4. ^ a b "Benefits and Safety Aspects of Hypochlorite Formulated in Domestic Products" (PDF). AISE – International Association for Soaps, Detergents and Maintenance Products. March 1997. Archived from the original (PDF) on 30 March 2014. This Support Dossier deals with information on the environmental and human safety evaluation of hypochlorite, and on its benefits as a disinfecting, deodorising and stain removing agent.
5. ^ Cárdenas Flores, A; Flores Reyes, H; Gordillo Moscoso, A; Castanedo Cázares, JP; Pozos Guillén Ade, J (2009). "Clinical efficacy of 5% sodium hypochlorite for removal of stains caused by dental fluorosis". The Journal of clinical pediatric dentistry. 33 (3): 187–91. doi:10.17796/jcpd.33.3.c6282t1054584157. PMID 19476089.
6. ^ a b c Rutala, William A.; Weber, David J. (15 February 2017) [2008]. "Guideline for Disinfection and Sterilization in Healthcare Facilities" (PDF). www.cdc.gov. Retrieved 2017-08-29.
7. ^ a b "For General Healthcare Settings in West Africa: How to Prepare and Use Chlorine Solutions". Ebola Hemorrhagic Fever. Centers for Disease Control and Prevention. Retrieved 2016-04-27.
8. ^ https://www.cdc.gov/vhf/ebola/pdf/cleaning-handwashing-5percent-liquid-bleach.pdf
9. ^ Meyers, Jordan; Ryndock, Eric; Conway, Michael J.; Meyers, Craig; Robison, Richard (2014-06-01). "Susceptibility of high-risk human papillomavirus type 16 to clinical disinfectants". Journal of Antimicrobial Chemotherapy. 69 (6): 1546–1550. doi:10.1093/jac/dku006. ISSN 0305-7453. PMC . PMID 24500190.
10. ^ 21 CFR Part 178
11. ^
12. ^ a b Root Canal Irrigants and Disinfectants. Endodontics. Colleagues for Excellence Published for the Dental Professional Community by the American Association of Endodontists. Winter 2011. http://www.aae.org/uploadedfiles/publications_and_research/endodontics_colleagues_for_excellence_newsletter/rootcanalirrigantsdisinfectants.pdf
13. ^ Torabinejad, Mahmoud, Richard Walton. Endodontics, 4th Edition. Page 265. W.B. Saunders Company, 2008. VitalBook file
14. ^ Complications during root canal irrigation – literature review and case reports M. Hülsmann & W. Hahn. International Endodontic Journal, 33, 186 –193, 2000, http://endoexperience.com/documents/Complicationsduringrootcanalirrigation_literaturereviewandcasereports.pdf
15. ^ a b Conger, Krista (15 November 2013). "Inflammatory skin damage in mice blocked by bleach solution, study finds". Stanford School of Medicine. Archived from the original on 7 December 2013.
16. ^
17. ^ Leung TH, Zhang LF, Wang J, Ning S, Knox SJ, Kim SK (2013). "Topical hypochlorite ameliorates NF-κB–mediated skin diseases in mice". Journal of Clinical Investigation. 123 (12): 5361–5370. doi:10.1172/JCI70895. PMC . PMID 24231355.
18. ^ Krieger, Gary R.; Sullivan, John B. Jr. (2001). Clinical environmental health and toxic exposures (2 ed.). Philadelphia, Pa. [u.a.]: Lippincott Williams & Wilkins. p. 968. ISBN 9780683080278. Retrieved 30 August 2016.
19. ^ Nickmilder, Marc; Carbonnelle, Sylviane; Bernard, Alfred (2007). "House cleaning with chlorine bleach and the risks of allergic and respiratory diseases in children". Pediatric Allergy and Immunology. 18: 27–35. doi:10.1111/j.1399-3038.2006.00487.x.
20. ^ a b Odabasi, Mustafa (1 March 2008). "Halogenated Volatile Organic Compounds from the Use of Chlorine-Bleach- Containing Household Products". Environmental Science & Technology. 42 (5): 1445–1451. doi:10.1021/es702355u. Lay summary.
21. ^ Helmut Vogt; Jan Balej; John E. Bennett; Peter Wintzer; Saeed Akhbar Sheikh; Patrizio Gallone (2007), "Chlorine Oxides and Chlorine Oxygen Acids", Ullmann's Encyclopedia of Industrial Chemistry (7th ed.), Wiley, p. 2
22. ^ eusol, Oxford English Dictionary.Accessed 3 July 2014.
23. ^
24. ^ Paul May. "Bleach (Sodium Hypochlorite)". University of Bristol. Archived from the original on 13 December 2016. Retrieved 13 December 2016.
25. ^ Bretherick's Handbook of Reactive Chemical Hazards, 7th Edition; vol. 1, pg 1433
26. ^ "SAFETY DATA SHEET Sodium Hypochlorite" (PDF). Univar. 9 August 2007.
27. ^ Wastewater Engineering: Treatment, Disposal, & Reuse (3rd ed.). Metcalf & Eddy, Inc. 1991. p. 497.
28. ^ Daniele S. Lantagne (2008). "Sodium hypochlorite dosage for household and emergency water treatment". E-Journal AWWA. 100 (8).
29. ^ Vieira, Ernest R. (1999). Elementary Food Science. Springer. pp. 381–382. ISBN 0-8342-1657-4.
30. ^ Marotz, Lynn R. (2011). Health, Safety, and Nutrition for the Young Child. Cengage Learning. pp. 126–127. ISBN 1-111-29837-8.
31. ^
32. ^
33. ^ G. A. Mirafzal; A. M. Lozeva (1998). "Phase transfer catalyzed oxidation of alcohols with sodium hypochlorite". Tetrahedron Letters. 39 (40): 7263–7266. doi:10.1016/S0040-4039(98)01584-6.
34. ^ "Standard Methods for the Examination of Water and Wastewater" (PDF). www.umass.edu. American Public Health Association, American Water Works Association, Water Environment Federation. Retrieved 26 Jan 2017.

## Bibliography

• Jones, F.-L. (1972). "Chlorine poisoning from mixing household cleaners". J. Am. Med. Assoc. 222 (10): 1312. doi:10.1001/jama.222.10.1312.
• Institut National de Recherche et de Sécurité. (2004). "Eaux et extraits de Javel. Hypochlorite de sodium en solution." Fiche toxicologique n° 157, Paris.