Bleach

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A bleach is a chemical that removes colour or whitens, often via oxidation. Common chemical bleaches include household "chlorine bleach", a solution of approximately 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.

One of the most well known manufacturers of Bleach is Clorox.

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 have to be of 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.

Hazards and concerns

Since bleaches are strong oxidizing agents, they can be quite hazardous, especially when reacted with other common household chemicals.

Mixing sodium hypochlorite with acids like vinegar or drain cleaners containing sodium bisulfate (sodium hydrogen sulfate), or even lemon juice can release chlorine. Hypochlorite and chlorine are in equilibrium in water, the position of the equilibrium is pH dependant and low pH (acidic) favors chlorine,<ref name="c&w">Modèle:Cite book</ref>

Cl₂ + H₂O <math>\rightleftharpoons</math> H⁺ + Cl^- + HClO

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Sodium hypochlorite and ammonia react to form a number of products, depending on the temperature, concentration, and how they are mixed. <ref>«  »</ref>. The main reaction is chlorination of ammonia, first giving chloramine (NH₂Cl), then NHCl₂ and finally nitrogen trichloride (NCl₃). These materials are very irritating to 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:<ref name="c&w"/>

2 NH₂Cl + N₂H₄ --> 2 NH₄Cl + N₂

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Chemistry

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

Cl₂(aq) + H₂O(l) <math>\rightleftharpoons</math> 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) <math>\rightleftharpoons</math> 2H⁺(aq) + Cl^-(aq) + ClO^-(aq)

How bleaches work

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.

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See also

• Household chemicals
• Tooth bleaching
• Bleaching of wood pulp
• Bleachfield

References

Further reading

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

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External links

Modèle:Wiktionary c3.org/chlorine_knowledge_center/070397bleach.html American Chemistry Council, Chlorine Chemistry Division]//c3.org/chlorine_knowledge_center/070397bleach.html American Chemistry Council, Chlorine Chemistry Division]de:Bleichmittel es:Lejía fa:آب ژاول fr:Eau de Javel it:Candeggina nl:Chloorbleek ja:漂白剤 pt:lixívia simple:Bleach zh:漂白劑