Bleach
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Template:Alternateuses Image:Bleach-bottle.jpg 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.
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Types of bleach
Household 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.
Hair bleach contains H2O2 (hydrogen peroxide), which gives off oxygen radicals as it decomposes. Oxygen and chlorine radicals both have comparable bleaching effects.
Chlorine bleach is often used with laundry detergent and is also commonly used as a disinfectant by homemakers and janitors. Mixing bleach and cleaners containing ammonia, or using bleach to clean up urine can create toxic chloramine gases and an explosive called nitrogen trichloride.
Not all bleaches have to be of oxidizing nature. Sodium dithionite is used as a powerful reducing agent in some bleaching formulas.
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 (acetone peroxide, benzoyl peroxide, etc.) and other agents (e.g. bromates) are used as flour bleaching and maturing agents.
How bleaches work
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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.Template:Ref label
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. Template:Ref label
History
[[Category:{{{1|}}} articles with sections needing expansion]]Bleaching of linens was known in the ancient world among the Greeks, Romans, Egyptians and Phoenicians but it is not entirely clear how this was accomplished. Before modern bleaches were developed cloth was often whitened or lightened through a process of repeated boilings and soakings in acidic and akaline substances including lye, urine, potash, pearl ash, sulphuric acid and buttermilk. Linen cloth was often bleached through exposure to sunlight. The active ingredient in household bleach, sodium hypochlorite, was discovered by the French chemist Claude Louis Berthollet in 1785. Berthollet's bleach was formed from a caustic chlorine-potash solution, and by 1789 it was sold as "eau de Javelle" after his chemical plant. Later on, during the 19th century, Louis Pasteur recognized its potential as a disinfectant.
Hazards
A problem with chlorine is that it reacts with organic material to form trihalomethanes like chloroform, which is a well known carcinogen. But 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.
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 generic mustard gas when bleach comes into contact with ammonia.
Bibliography
E.R. Trotman. Textile Scouring and Bleaching. London: Charles Griffin & Co., 1968.