Bleach

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. Bleaching powder is calcium hypochlorite.

Many bleaches have strong bactericidal properties, and are used for disinfecting and sterilizing.

Contents 1 Other types of bleaches 1.1 Environmental impact 1.2 Chemical interactions 2 Dilution 3 Chemistry 4 Mechanism of bleach action 5 Antimicrobial efficacy 6 See also 7 References 8 Further reading

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.

Environmental impact

Bleach is highly toxic to fish and invertebrates. In confined spaces, fish will attempt to swim away from the source.

High levels of 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, in this case trichloroacetic acid). Most AOX go into the sewer with wash water.

Chemical interactions

Hypochlorite and chlorine are in equilibrium in water; the position of the equilibrium is pH dependent and low pH (acidic) favors chlorine,^[2]

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.^[3]

Sodium hypochlorite and ammonia react to form a number of products, depending on the temperature, concentration, and how they are mixed.^[4] 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 the eyes and lungs and are toxic above certain concentrations.

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:^[2]

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 and 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.^[5] However, respiratory risk from chlorine and highly toxic chlorinated byproducts still exists.

A recent European study 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).^[6] 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,^[7] 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,^[8] almost 140 times higher;
• The OSHA highest allowable peak concentration (5 minute exposure for five minutes in a 4-hour period) is 200 ppm,^[8] 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.

Dilution

Bleach is sold extremely concentrated and must be diluted to be used safely when disinfecting surfaces and when used to treat drinking water. When disinfecting most surfaces, 1 part bleach to 9 parts water is sufficient for sanitizing. In an emergency, drinking water can be treated: Ratio of bleach to water for purification: 2 drops of bleach per litre of water or 8 drops of bleach per gallon (4L) of water; 1/2 teaspoon bleach per five gallons (19L) of water. If water is cloudy, double the recommended dosages of bleach. Additional bleach will not kill more bacteria and can endanger health.^[9]

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:

2ClO^- 2Cl^- + O₂

Mechanism of bleach action

Color in most dyes and pigments are 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, particularly sodium hypochlorite, owes to the nature of its chemical reactivity with microbes. Rather than acting in an inhibitory or toxic fashion in the manner of antibiotics, bleach quickly reacts with microbial cells to irreversibly denature and destroy many pathogens. Bleach, particularly sodium hypochlorite has been shown to react with a microbe's heat shock proteins, stimulating their role as intra-cellular chaperone and causing the bacteria to form into clumps (much like an egg that has been boiled) that will eventually die off. In some cases, bleach's base acidity compromises a bacterium's lipid membrane, a reaction similar to popping a balloon. The range of micro-organisms effectively killed by bleach (particularly sodium hypochlorite) is extensive, making it an extremely versatile disinfectant.

In response to infection, the human immune system will produce a strong oxidizer, hypochlorous acid, to kill bacterial invaders.

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.