Everything about Bleach totally explained
A
bleach is a
chemical that removes color or whitens, often via
oxidation. Common chemical bleaches include household "chlorine bleach", a solution of approximately 3-6%
sodium hypochlorite (NaOCl), 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.
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 (for example
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
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,
Cl
2 + H
2O
H
+ + Cl
- + HClO
Chlorine is a respiratory irritant that 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 U.S.
Sodium hypochlorite and
ammonia react to form a number of products, depending on the temperature, concentration, and how they're mixed. . The main reaction is chlorination of ammonia, first giving
chloramine (NH
2Cl), then NHCl
2 and finally
nitrogen trichloride (NCl
3). These materials are very irritating to eyes and lungs and are toxic above certain concentrations.
NH
3 + NaOCl --> NaOH + NH
2Cl
NH
2Cl + NaOCl --> NaOH + NHCl
2
NHCl
2 + NaOCl --> NaOH + NCl
3
Additional reactions produce
hydrazine, in a variation of the
Olin Raschig process.
NH
3 + NH
2Cl + NaOH --> N
2H
4 + NaCl + H
2O
The hydrazine generated can further react with the
monochloramine in an
exothermic reaction: However, respiratory risk from chlorine and highly toxic chlorinated byproducts still
Chemistry
The process of bleaching can be summarised in the following set of chemical reaction:
Cl
2(aq) + H
2O(l)
H
+(aq) + Cl
-(aq) + HClO(aq)
The H
+ ion of the hypochlorous acid then dissolves into solution, and so the final result is effectively:
Cl
2(aq) + H
2O(l)
2H
+(aq) + Cl
-(aq) + ClO
-(aq)
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 doesn't contain a chromophore, or contains a chromophore that doesn't 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.
Further Information
Get more info on 'Bleach'.
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