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

Contents

Other types of bleach

Hazards

History

Chemistry

How bleaches work

See also

References

Further reading

External links

Other types of bleach

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.
Peracetic acid is used in paper 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.

Hazards

Since bleaches are strong oxidizing or reducing agents, they can be quite hazardous.
One of the most common dangers involving household bleach is the mixing of 'sodium hypochlorite' with ammonia. Several reactions can occur, producing a variety of dangerous compounds:

★ 2 NaOCl + 2 NH₃ --> 2 NaONH₃ + Cl₂^[1]
:This reaction liberates elemental chlorine, which is a respiratory irritant. It also attacks mucous membranes and burns the skin. As little as 3.5 ppm can be detected as an odour, 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 - 40 hour week) by OSHA in the US.^[2]

★ 3 NaOCl + NH₃ --> 3 NaOH + NCl₃
:This reaction produces nitrogen trichloride, a toxic, explosive compound.

★ NH₃ + NaOCl --> NaOH + NH₂Cl
:NH₃ + NH₂Cl + NaOH --> N₂H₄ + NaCl + H₂O
:These reactions produce hydrazine, a component of some rocket fuels, in a variation of the Olin Raschig process. The hydrazine generated can further react with the monochloramine in an extremely exothermic reaction:

★ 2 NH₂Cl + N₂H₄ --> 2 NH₄Cl + N₂
For these reasons, some consumers avoid the use of bleach.
Industrial bleaching agents can also be hazardous. For example, the use of elemental chlorine in industrial processes such as paper bleaching produces organochlorine-persistent organic pollutants, including dioxins. To mitigate the dioxin production, 80% of woodpulp is now bleached with chlorine dioxide, reducing the dioxin generation under detectable levels.

History

Chlorine was first characterized by the Swedish chemist Carl Wilhelm Scheele in 1774 (as an adherent of the Phlogiston theory, he called it "dephlogisticated marine acid"). French chemist Claude Louis Berthollet, noting the bleaching properties of chlorine, invented hypochlorite bleach in 1789. In French, bleach is known as ''Eau de Javel'', after the village where it was manufactured.
Several alternatives to bleach have recently appeared in industrialized countries. These substances are touted as being less toxic, and the use of bleach as a stain remover has become less popular in the United States. However, due to the recent upsurge of illness due to methicillin-resistant ''Staphylococcus aureus'' (known as MRSA) and other bacterial pathogens susceptible to bleach, the bleach industry has recovered somewhat, and the use of bleach as a disinfectant is increasing in a variety of industrial and commercial, as well as household settings.

Chemistry

The process of bleaching can be summarised in the following set of chemical reactions:
Cl₂(aq) + H₂O(l) $$
ightleftharpoons H⁺(aq) + Cl^-(aq) + HClO(aq)
The H⁺ ion of the hypochlorous acid then dissolves into solution, and so the final result is effectively:
Cl₂(aq) + H₂O(l) $$
ightleftharpoons 2H⁺(aq) + Cl^-(aq) + ClO^-(aq)

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.^[2]
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 light and typically very faded blue spectrums.^[2]

See also

★ Household chemicals

★ Tooth bleaching

References

1. BBC - h2g2 - The Dangers of Mixing Bleach and Ammonia - A795611
2.
3.
4.

Further reading

★ E.R. Trotman. Textile Scouring and Bleaching. London: Charles Griffin & Co., 1968.

★ Dr. Bailey Bodkins. Bleach. Philedelphia: Virginia Printing Press 1995.

★ http://livre.inventeur.info/book_english/index-section.php3?cat_id=23

External links

★ Washington Post's ''A Sanitary History Of Household Bleach''