Significations et usages de Bleach

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A bleach is a chemical that removes colors or whitens, often via oxidation. Common chemical bleaches include household chlorine bleach, a solution of approximately 3–6% sodium hypochlorite (NaClO), and oxygen bleach, which contains hydrogen peroxide or a peroxide-releasing compound such as sodium perborate, sodium percarbonate, sodium persulfate, tetrasodium pyrophosphate, or urea peroxide together with catalysts and activators, e.g., tetraacetylethylenediamine and/or sodium nonanoyloxybenzenesulfonate. To "bleach" something is to apply bleach, sometimes as a preliminary step in the process of dyeing. Bleaching powder is calcium hypochlorite.

Many bleaches have strong bactericidal properties, and are used for disinfecting and sterilizing. Most bleaches are hazardous if ingested or inhaled, and should be used with care.

Other types of bleaches

Chlorine dioxide is used for the bleaching of wood pulp, fats and oils, cellulose, flour, textiles, beeswax, skin, and in a number of other industries.

In the food industry, some organic peroxides (benzoyl peroxide, etc.) and other agents (e.g., bromates) are used as flour bleaching and maturing agents.

Peracetic acid and ozone are used in the manufacture of paper products, especially newsprint and white Kraft paper.^[1]

Two-part bleaches are utilized in the whitening of wood, especially oak.

Human and environmental safety

Studies of human safety and environmental effects associated with household use of sodium hypochlorite bleach have been extensively documented.

• Neither carcinogenesis, mutagenesis, nor teratogenesis are indicated
• There may be minor, temporary effects such as localized skin and eye irritant (localized; potentially increasing with concentration, but this is unlikely to be significant at levels encountered in the household). There are no chronic effects, and while accidental acute effects can be painful, they are mostly reversible
• Hypochlorite imparts no systemic effects; it does not cause sensitization
• Excessive exposure to hair may cause thinning or temporary balding.

Environmental impact

• No emissions of sodium hypochlorite from normal household or institutional use find their way directly to the environment. Sodium hypochlorite degrades quickly, primarily to sodium chloride, during use or in sewage systems. It also decomposes in soil, primarily to salt. Typical use was found to be not harmful to sewage treatment or septic tanks
• Sodium hypochlorite is toxic when undiluted (5% concentration as sold), but is rapidly diluted or decomposed to harmless levels in soil or sewage systems.
• Bleach is highly toxic to fish and invertebrates. In confined spaces, fish will attempt to swim away from the source. In addition, sodium hypochlorite readily disperses and degrades mostly to salt in surface waters, limiting impact.
• Very low levels absorbable organic halides (AOX) can be found during reaction of sodium hypochlorite and soils, including carbon tetrachloride, trihalomethanes (THM, such as chloroform), and trihaloacetic acid (THAA). Most AOX go into the sewer with wash water; amounts emitted to air well below safe limits. Most AOX degrades in sewage treatment like starting soil; wastewater genotoxicity not increased. Remnants are not harmful at levels detected (acute and chronic); no persistent or lipophilic chlorinated compounds were detected. Limited amounts of AOX have been detectable on fabrics below significant effect levels
• Bleach is not a source of dioxin, which only forms below pH 5. The risk of generating dioxin from use of household bleach is non-existent
• Chlorate ion can form during decomposition of sodium hypochlorite, but is readily decomposed during waste treatment
• Perchlorate can also form through decomposition; it is estimated that less than 5 ppb could be released in the wash, and less than 1 ppb could be found after dilution in waste treatment and septic systems. Most prevalent sources of perchlorate contamination in environment found to be blasting agents, military munitions, and fireworks. Massachusetts EPA concluded that normal household discharge of bleaches into municipal sewerage or conventional septic systems should not be an environmental issue

In 2008, the Scientific Committee on Health and Environmental Risks (SCHER) for the European Commission concluded that the Risk Assessment Report (RAR) was of good quality, and agreed with its conclusions. No further study on human health is indicated. ^[2]

Chemical interactions

Hypochlorite and chlorine are in equilibrium in water; the position of the equilibrium is pH dependent and low pH (acidic) favors chlorine,^[3]

Cl₂ + H₂O H⁺ + Cl^- + HClO

Chlorine is a respiratory irritant that attacks mucous membranes and burns the skin. As little as 3.53 ppm can be detected as an odor, and 1000 ppm is likely to be fatal after a few deep breaths. Exposure to chlorine has been limited to 0.5 ppm (8-hour time-weighted average—38 hour week) by OSHA in the U.S.^[4]

Sodium hypochlorite and ammonia react to form a number of products, depending on the temperature, concentration, and how they are mixed.^[5] The main reaction is chlorination of ammonia, first giving chloramine (NH₂Cl), then dichloramine (NHCl₂) and finally nitrogen trichloride (NCl₃). These materials are very irritating to eyes and lungs and are toxic above certain concentrations. Lastly there is bleach containing sodium perchlorate.

NH₃ + NaOCl --> NaOH + NH₂Cl

NH₂Cl + NaOCl --> NaOH + NHCl₂

NHCl₂ + NaOCl --> NaOH + NCl₃

Additional reactions produce hydrazine, in a variation of the Olin Raschig process.

NH₃ + NH₂Cl + NaOH --> N₂H₄ + NaCl + H₂O

The hydrazine generated can further react with the monochloramine in an exothermic reaction:^[3]

2 NH₂Cl + N₂H₄ --> 2 NH₄Cl + N₂

Industrial bleaching agents can also be sources of concern. For example, the use of elemental chlorine in the bleaching of wood pulp produces organochlorines, persistent organic pollutants, including dioxins. According to an industry group, the use of chlorine dioxide in these processes has reduced the dioxin generation to under detectable levels.^[6] However, respiratory risk from chlorine and highly toxic chlorinated byproducts still exists.

An old European study indicated that sodium hypochlorite and organic chemicals (e.g., surfactants, fragrances) contained in mostly household cleaning products can react to generate chlorinated volatile organic compounds (VOCs).^[7] 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. While the authors suggested that using these cleaning products may significantly increase the cancer risk,^[8] this conclusion appears to be hypothetical:

• The highest level cited for concentration of carbon tetrachloride (seemingly of highest concern) is 459 micrograms per cubic meter, translating to 0.073 ppm (part per million), or 73 ppb (part per billion). The OSHA-allowable time-weighted average concentration over an eight-hour period is 10 ppm,^[9] almost 140 times higher;
• The OSHA highest allowable peak concentration (5 minute exposure for five minutes in a 4-hour period) is 200 ppm,^[9] twice as high as the reported highest peak level (from the headspace of a bottle of a sample of bleach plus detergent).

Further studies of the use of these products and other possible exposure routes (i.e., dermal) may reveal other risks. Though the author further cited ozone depletion greenhouse effects for these gases, the very low amount of such gases, generated as prescribed, should minimize their contribution relative to other sources.

Chemistry

The process of bleaching can be summarized in the following set of chemical reactions:

Cl₂(aq) + H₂O(l) H⁺(aq) + Cl^-(aq) + HClO(aq)

The H⁺ ion of the hypochlorous acid then dissolves into solution, and so the final result is effectively:

Cl₂(aq) + H₂O(l) 2H⁺(aq) + Cl^-(aq) + ClO^-(aq)

Hypochlorite tends to decompose into chloride and a highly reactive form of oxygen:

ClO^- Cl^- +1/2 O₂

This oxygen then reacts with organic substances to produce bleaching or antiseptic effects.

Mechanism of bleach action

Color in most dyes and pigments is produced by molecules, such as beta carotene, which contain chromophores. Chemical bleaches work in one of two ways:

• An oxidizing bleach works by breaking the chemical bonds that make up the chromophore. This changes the molecule into a different substance that either does not contain a chromophore, or contains a chromophore that does not absorb visible light.

• A reducing bleach works by converting double bonds in the chromophore into single bonds. This eliminates the ability of the chromophore to absorb visible light.^[10]

Sunlight acts as a bleach through a process leading to similar results: high energy photons of light, often in the violet or ultraviolet range, can disrupt the bonds in the chromophore, rendering the resulting substance colorless. Extended exposure often leads to massive discoloration usually reducing the colors to white and typically very faded blue spectrums.^[11]

Sodium hypochlorite's anti-bacterial mechanism works by causing proteins to aggregate.^[12][13]

Antimicrobial efficacy

The broad-spectrum effectiveness of bleach, for example sodium hypochlorite, owes to the nature of the chemical reactivity of the bleach with the microbes. Rather than act in an inhibitory or specific toxic fashion in the manner of antibiotics, the reaction with the microbial cells quickly and irreversibly denatures, and often destroys the pathogen. Specifically, with sodium hypochlorite it is found that:

• the bleach attacks proteins in bacteria, causing them to clump up much like an egg that has been boiled,

• when exposed to bleach, the heat shock protein of bacteria become active in an attempt to protect other proteins in the bacteria from losing their chemical structure, forming clumps that would eventually die off, and

• the human immune system produces hypochlorous acid in response to infection to kill bacterial invaders

• the lipid membrane of the bacteria is destroyed, as if popped like a balloon, as a result of the bleach being very basic.

As noted, the range of micro-organisms effectively killed by bleach (in particular by sodium hypochlorite) is extensive, making it extremely versatile.

See also

• Disinfectant
• Household chemicals
• Tooth bleaching
• Bleaching of wood pulp
• Bleachfield
• Bleaching of the Great Barrier Reef

References

Further reading

• Bodkins, Dr. Bailey. Bleach. Philadelphia: Virginia Printing Press, 1995.
• Trotman, E.R. Textile Scouring and Bleaching. London: Charles Griffin & Co., 1968. ISBN 0852640676.

External links

• American Chemistry Council, Chlorine Chemistry Division
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