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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 or sodium percarbonate. 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, and should be used with care.
Prior to chemical bleaches, Fuller's Earth (Montmorillonite), river mud, and old urine (which contains ammonia) were used as whitening and cleaning agents. Later wood ashes, and soda were used. This type of cleaning was called fulling, and typically got out dirt and grease making clothing 'white'.
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, ozone and hydrogen peroxide and oxygen are used in bleaching sequences in the pulp industry to produce totally chlorine free (TCF) paper.
Not all bleaches are hazardous and have an oxidizing nature. Sodium dithionite is used as a powerful reducing agent in some bleaching formulas. It is commonly used to bleach wood pulp used to make newsprint.
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.
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.
Sodium hypochlorite's anti-bacterial mechanism works by causing "shock" proteins to aggregate, which then fall off.
[6 ธ.ค. 2551 15:52]