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To bleach something is to remove or lighten its color; a "bleach" is a chemical that can produce these effects, often via oxidization. Common chemical bleaches include sodium hypochlorite, or "chlorine bleach," and "oxygen bleach," which contains hydrogen peroxide or a peroxide-releasing compound such as sodium perborate or sodium percarbonate. "Bleaching powder" is calcium hypochlorite. Bleaching may be a preliminary step in the process of dyeing.
Types of bleach
Household bleach, also known as chlorine bleach or sodium hypochlorite (NaOCl) is used in the home for whitening clothes, removing stains, and disinfecting. This is because sodium hypochlorite yields chlorine radicals—oxidizing agents readily reacting with many substances.
Chlorine bleach is often used with laundry detergents and is also commonly used as a disinfectant. Mixing bleach and cleaners containing ammonia, or using bleach to clean up urine can create toxic chloramine gases and an explosive called nitrogen trichloride.
Hair bleach contains H2O2 (hydrogen peroxide), which gives off oxygen radicals as it decomposes. Oxygen and chlorine radicals both have comparable bleaching effects.
Various other peroxide yielding chemicals are used as bleaching additives. Sodium perborate, sodium percarbonate, sodium persulfate, sodium perphosphate, sodium persilicate, their ammonium, potassium and lithium analogs, calcium peroxide, zinc peroxide, sodium peroxide, carbamide peroxide, and others are commonly used in detergents, toothpastes, and other products.
Chlorine dioxide is used for the bleaching of wood pulp, fats and oils, cellulose, flour, textiles, beeswax, 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.
Not all bleaches have to be of oxidizing nature. Sodium dithionite is used as a powerful reducing agent in some bleaching formulas.
How bleaches work
Color in most dyes and pigments is produced by molecules, such as beta carotene, that contain moieties (pieces) known as 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.
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.
Hazards
A problem with chlorine is that it reacts with organic material to form trihalomethanes like chloroform, which is a well known carcinogen. The benefit of using chlorine to kill the germs in drinking water far outweighs any risk from the tiny trace of chloroform in treated drinking water. However, the use of bleach in industrial processes such as paper bleaching, with its attendant production of organochlorine-persistent organic pollutants (including dioxins), does not have any such clear benefit.
Chlorine is a respiratory irritant. It also attacks mucus membranes and burns the skin. As little as 3.5 ppm can be detected as an odor, and 1000 ppm is likely to be fatal after a few deep breaths. Exposure to chlorine should not exceed 0.5 ppm (8-hour time-weighted average - 40 hour week).
Another hazard is the formation of dangerous chlorine gas when bleach comes into contact with ammonia.
Bibliography
E.R. Trotman. Textile Scouring and Bleaching. London: Charles Griffin & Co., 1968.
References
External links
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"Bleach"