Bleach refers to a number of chemicals which
remove colour, whiten or disinfect, often by oxidation. The bleaching process has been known for millennia,
but the chemicals currently used for bleaching resulted from the work of several 18th century
scientists. Chlorine is the basis for the most commonly
used bleaches, for example, the solution of sodium hypochlorite, which is so ubiquitous
that most simply call it “bleach”, and calcium hypochlorite, the major compound in “bleaching
powder”. Oxidizing bleaching agents that do not contain
chlorine most often are based on peroxides, such as hydrogen peroxide, sodium percarbonate
and sodium perborate. While most bleaches are oxidizing agents,
some are reducing agents such as sodium dithionite and sodium borohydride. Bleaches are used as household chemicals to
whiten clothes and remove stains and as disinfectants, primarily in the bathroom and kitchen. Many bleaches have strong bactericidal properties,
and are used for disinfecting and sterilizing and thus are used in swimming pool sanitation
to control bacteria, viruses and algae and in any institution where sterile conditions
are needed. They are also used in many industrial processes,
notably in the bleaching of wood pulp. Bleach is also used for removing mildew, killing
weeds and increasing the longevity of flowers. History
The earliest form of bleaching involved spreading fabrics and cloth out in a bleachfield to
be whitened by the action of the sun and water. Modern bleaches resulted from the work of
18th century scientists including Swedish chemist Carl Wilhelm Scheele, who discovered
chlorine, French scientists Claude Berthollet, who recognized that chlorine could be used
to bleach fabrics and who first made sodium hypochlorite and Antoine Germain Labarraque,
who discovered the disinfecting ability of hypochlorites. Scottish chemist and industrialist Charles
Tennant first produced a solution of calcium hypochlorite, then solid calcium hypochlorite. Louis Jacques Thénard first produced hydrogen
peroxide in 1818 by reacting barium peroxide with nitric acid. Hydrogen peroxide was first used for bleaching
in 1882, but did not become commercially important until after 1930. Sodium perborate as a laundry bleach had been
used in Europe since the early twentieth century, but did not become popular in North America
until the 1980s. How bleaches work
Whitening Colors in most dye and pigments are produced
by molecules which contain chromophores, such as beta carotene. 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. Antimicrobial efficacy
The broad-spectrum effectiveness of bleach, particularly sodium hypochlorite, is owed
to the nature of its chemical reactivity with microbes. Rather than acting in an inhibitory or toxic
fashion in the manner of antibiotics, bleach quickly reacts with microbial cells to irreversibly
denature and destroy many pathogens. Bleach, particularly sodium hypochlorite,
has been shown to react with a microbe’s heat shock proteins, stimulating their role as
intra-cellular chaperone and causing the bacteria to form into clumps that will eventually die
off. In some cases, bleach’s base acidity compromises
a bacterium’s lipid membrane, a reaction similar to popping a balloon. The range of micro-organisms effectively killed
by bleach is extensive, making it an extremely versatile disinfectant. The same study found that at low sodium hypochlorite
levels, E. coli and Vibrio cholerae activate a defense mechanism that helps protect the
bacteria, though the implications of this defense mechanism have not been fully investigated. In response to infection, the human immune
system will produce a strong oxidizer, hypochlorous acid, which is generated in activated neutrophils
by myeloperoxidase-mediated peroxidation of chloride ions, and contributes to the destruction
of bacteria. Classes of bleaches
Chlorine-based bleaches Chlorine-based bleaches are found in many
household cleaners. The concentration of chlorine-based bleaches
is often expressed as percent active chlorine where one gram of a 100% active chlorine bleach
has the same bleaching power as one gram of chlorine. These bleaches can react with other common
household chemicals like vinegar or ammonia to produce toxic gases. Labels on sodium hypochlorite bleach warn
about these interactions. Chemical interactions
Mixing a hypochlorite bleach with an acid can liberate chlorine gas. Hypochlorite and chlorine are in equilibrium
in water; the position of the equilibrium is pH dependent and low pH favors chlorine,
Cl2 + H2O H+ + Cl− + HClO Chlorine is a respiratory irritant that attacks
mucous membranes and burns the skin. As little as 3.53 ppm can be detected as an
odor, and 1000 ppm is likely to be fatal after a few deep breaths. Exposure to chlorine has been limited to 0.5
ppm 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 are mixed. The main reaction is chlorination of ammonia,
first giving chloramine, then dichloramine and finally nitrogen trichloride. These materials are very irritating to the
eyes and lungs and are toxic above certain concentrations; nitrogen trichloride is also
a very sensitive explosive. 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 react with more chloramine in an exothermic reaction to produce
ammonium chloride and nitrogen gas: 2 NH2Cl + N2H4 → 2 NH4Cl + N2
Atmospheric carbon dioxide and water react with bleaching powder) to release hypochlorous
acid which gives a characteristic smell to the bleaching powder. Hypochlorous acid decomposes readily to atomic
oxygen. This atomic oxygen acts as bleaching agent
through oxidation. 2CaCl(OCl) + H2O + CO2 → CaCO3 + CaCl2 +
2HClO HClO → HCl + [O]
2HCl + [O] → H2O + Cl2 However, the place of atomic oxygen in accounting
for the formation of chlorine is not as plausible as another theory based on the so-called ‘chloride
system’ employed in modern hydrometallurgy to dissolve ores with weak acids in highly
ionic and concentrated salt solutions. Salts particularly effective, in this regard,
include MgCl2, CaCl2, FeCl3 and, to a less extent the mono-valent NaCl. This is, in effect, an application of the
non-common ion theory, or as discussed in Wikipedia under Solubility Equilibrium as
the ‘salt effect’. With respect to Bleaching powder, which has
been described as a compound salt of the form Ca(ClO)2.CaCl2.Ca(OH)2.xH2O, the presence
of CaCl2 in very concentrated solutions can greatly increase the ‘activity level’ of weak
acids. So, in this particular proposed application,
H2CO3 from CO2 and moisture on the Bleaching powder, acts on the CaCl2 to release some
HCl which acts on the HClO releasing Chlorine: HClO + HCl → H2O + Cl2
or, the increasing acidity creates more HClO which moves the following known equilibrium
reaction to the right: CaCl2 + 2 HClO=Ca(OH)2 + 2 Cl2
Now, the strength of the particular application of this theory is that a similar release of
Chlorine is not as easily observed with concentrated NaClO solutions. As the latter Chlorine bleach also contains
NaCl, and as the NaCl is not quite as effective as previously noted as, for example, with
CaCl2, the ionic strength is not as great for noticeable Chlorine formation. Sodium hypochlorite Sodium hypochlorite is the most commonly encountered
bleaching agent, usually as a dilute solution in water. This solution of sodium hypochlorite, commonly
referred to as simply “bleach”, was also one of the first mass-produced bleaches. It is produced by passing chlorine gas through
a dilute sodium hydroxide solution Cl2 + 2 NaOH → NaCl + NaClO + H2O
or by electrolysis of brine. 2 Cl− → Cl2 + 2 e−
Cl2 + H2O ↔ HClO + Cl− + H+ The dilute solution of sodium hypochlorite
is used in many households to whiten laundry, disinfect hard surfaces in kitchens and bathrooms,
treat water for drinking and keep swimming pools free of infectious agents. Moreover, due to transport and handling safety
concerns, the use of sodium hypochlorite is preferred over chlorine gas in water treatment,
which represents a significant market expansion potential. Calcium hypochlorite Calcium hypochlorite, also known as chloride
of lime, is made by reacting chlorine with calcium hydroxide:
2Cl2 + 2Ca(OH)2 → Ca(ClO)2 + CaCl2 + 2H2O It is used in many of the same applications
as sodium hypochlorite, but has the advantages of being more stable and containing more available
chlorine. Calcium hypochlorite is the active ingredient
in bleaching powder or “chlorinated lime”, which is usually a white powder containing
calcium hypochlorite, calcium hydroxide and calcium chloride. A purer, more stable form of calcium hypochlorite
is called HTH or high test hypochlorite. Bleaching tablets contain calcium hypochlorite
plus other ingredients to prevent the tablets from crumbling. A supposedly more stable mixture of calcium
hypochlorite and quicklime is known as “tropical bleach” . Percent active chlorine in these
materials ranges from 20% for bleaching powder to 70% for HTH. Chlorine Chlorine is produced by the electrolysis of
sodium chloride. 2 NaCl + 2 H2O → Cl2 + H2 + 2 NaOH
Chlorine is used to prepare sodium and calcium hypochlorites. It is used as a disinfectant in water treatment,
especially to make drinking water and in large public swimming pools . Chlorine was used
extensively to bleach wood pulp, but this use has decreased significantly due to environmental
concerns. Chlorine dioxide Chlorine dioxide, ClO2, is an explosive gas
and must be used where it is made or shipped and stored as dilute aqueous solutions. Despite these limitations it finds applications
for the bleaching of wood pulp, fats and oils, cellulose, flour, textiles, beeswax, skin,
and in a number of other industries. It can be prepared by oxidizing sodium chlorite
with chlorine 2 NaClO2 + Cl2 → 2 ClO2 + 2 NaCl
but more commonly it is prepared by reducing sodium chlorate with a suitable reducing agent
like methanol, hydrogen peroxide, hydrochloric acid, or sulfur dioxide
2 NaClO3 + 2 HX + “R” → 2 NaX + 2 ClO2 + “RO” + H2O
where “R” is the reducing agent and “RO” is the oxidized form. Peroxide-based bleaches
After chlorine-based bleaches, the peroxide bleaches are most commonly encountered. Peroxides are compounds that contain an oxygen-oxygen
single bond, O-O. This is a fairly weak bond so reactions of
peroxides often involve breaking this bond, giving very reactive oxygen species. Most peroxide bleaches are adducts of hydrogen
peroxide. They contain hydrogen peroxide, HOOH in combination
with another material like sodium carbonate or urea. An exception is sodium perborate, which has
a cyclic structure containing two O-O single bonds. All peroxide-based bleaches release hydrogen
peroxide when dissolved in water. Peroxide bleaches are often used with catalysts
and activators, e.g., tetraacetylethylenediamine or sodium nonanoyloxybenzenesulfonate. Hydrogen peroxide Hydrogen peroxide is produced in very large
amounts by several different processes. Its action as an oxidizer is why it is made
and used in such large quantities. It is used by itself as a bleaching agent,
for example to bleach wood pulp, hair and so on, or to prepare other bleaching agents
like the perborates, percarbonates, peracids, etc. Sodium percarbonate Sodium percarbonate is produced industrially
by reaction of sodium carbonate and hydrogen peroxide, followed by crystallization. Also, dry sodium carbonate may be treated
directly with concentrated hydrogen peroxide solution. 2Na2CO3 + 3H2O2→2Na2CO3.3H2O2
Dissolved in water, it yields a mixture of hydrogen peroxide and sodium carbonate. It is generally considered to be an eco-friendly
cleaning agent. Sodium perborate Sodium perborate, Na2H4B2O8, is made by reacting
borax with sodium hydroxide to give sodium metaborate which is then reacted with hydrogen
peroxide to give hydrated sodium perborate. Na2B4O7 + 2 NaOH → 4 NaBO2 + H2O
2 NaBO2 + 2 H2O2 + 6 H2O → [NaBO2(OH)2 x 3 H2O]2
Sodium perborate is useful because it is a stable, source of peroxide anions. When dissolved in water it forms some hydrogen
peroxide, but also perborate anion(OH)3-), which is activated for nucleophilic oxidation. Miscellaneous bleaches
Peracetic acid and ozone are used in the manufacture of paper products, especially newsprint and
white Kraft paper. In the food industry, some organic peroxides
and other agents are used as flour bleaching and maturing agents. Reducing bleaches
Sodium dithionite is one of the most important reductive bleaching agents. It is a white crystalline powder with a weak
sulfurous odor. It can be obtained by reacting sodium bisulfite
with zinc 2 NaHSO3 + Zn → Na2S2O4 + Zn(OH)2
It is used as such in some industrial dyeing processes to eliminate excess dye, residual
oxide, and unintended pigments and for bleaching wood pulp. Reaction of sodium dithionite with formaldehyde
produces Rongalite, Na2S2O4 + 2 CH2O + H2O → NaHOCH2SO3 + NaHOCH2SO2
which is used in bleaching wood pulp, cotton, wool, leather and clay. Environmental impact
A Risk Assessment Report conducted by the European Union on sodium hypochlorite conducted
under Regulation EEC 793/93 concluded that this substance is safe for the environment
in all its current, normal uses. This is due to its high reactivity and instability. Disappearance of hypochlorite is practically
immediate in the natural aquatic environment, reaching in a short time concentration as
low as 10−22 μg/L or less in all emission scenarios. In addition, it was found that while volatile
chlorine species may be relevant in some indoor scenarios, they have negligible impact in
open environmental conditions. Further, the role of hypochlorite pollution
is assumed as negligible in soils. Industrial bleaching agents can also be sources
of concern. For example, the use of elemental chlorine
in the bleaching of wood pulp produces organochlorines and persistent organic pollutants, including
dioxins. According to an industry group, the use of
chlorine dioxide in these processes has reduced the dioxin generation to under detectable
levels. However, respiratory risk from chlorine and
highly toxic chlorinated byproducts still exists. A recent European study indicated that sodium
hypochlorite and organic chemicals contained in several household cleaning products can
react to generate chlorinated volatile organic compounds. These chlorinated compounds are emitted during
cleaning applications, some of which are toxic and probable human carcinogens. The study showed that indoor air concentrations
significantly increase during the use of bleach containing products. The increase in chlorinated volatile organic
compound concentrations was the lowest for plain bleach and the highest for the products
in the form of “thick liquid and gel”. The significant increases observed in indoor
air concentrations of several chlorinated VOCs indicate that the bleach use may be a
source that could be important in terms of inhalation exposure to these compounds. While the authors suggested that using these
cleaning products may significantly increase the cancer risk, this conclusion appears to
be hypothetical: The highest level cited for concentration
of carbon tetrachloride is 459 micrograms per cubic meter, translating to 0.073 ppm,
or 73 ppb. The OSHA-allowable time-weighted average concentration
over an eight-hour period is 10 ppm, almost 140 times higher;
The OSHA highest allowable peak concentration is 200 ppm, twice as high as the reported
highest peak level. Further studies of the use of these products
and other possible exposure routes may reveal other risks. Though the author further cited ozone depletion
greenhouse effects for these gases, the very low amount of such gases, generated as prescribed,
should minimize their contribution relative to other sources. Disinfection
Sodium hypochlorite solution, 3–6%, must be diluted to be used safely when disinfecting
surfaces and when used to treat drinking water. When disinfecting most surfaces, 1 part liquid
household bleach to 100 parts water is sufficient for sanitizing. Stronger or weaker solutions may be more appropriate
to meet specific goals, such as killing resistant viruses or sanitizing surfaces that will not
be in contact with food. See references for more information. In an emergency, drinking water should be
treated by boiling for 1–3 minutes, longer at higher altitudes. If boiling is not possible, water can be chemically
treated with a ratio of 2 drops of plain liquid household bleach per liter of water or 8 drops
of bleach per gallon of water; 1/2 teaspoon bleach per five gallons of water. Do not use powdered bleach, or bleach with
scents, cleaners or other additives. Do not collect water for treatment from flood
waters or other potentially contaminated sources. If water appears dirty or cloudy, let it settle
and/or filter the water before adding the bleach. Let treated water stand covered for 30 minutes. If water is still cloudy after filtering,
double the amount of bleach used. If the water is very cold, either warm it
before treatment or double the treatment time. Treated water should still have a slight bleach
odor after treatment. If it does not, repeat the treatment. If no bleach odor is evident after a second
treatment, discard the water and find a better water source. Inappropriate dilutions of bleach can endanger
your health. A weak solution of 2% household bleach in
warm water is used to sanitize smooth surfaces prior to brewing of beer or wine. Surfaces must be rinsed to avoid imparting
flavors to the brew; these chlorinated byproducts of sanitizing surfaces are also harmful. US Government regulations allow food processing
equipment and food contact surfaces to be sanitized with solutions containing bleach,
provided that the solution is allowed to drain adequately before contact with food, and that
the solutions do not exceed 200 parts per million available chlorine. If higher concentrations are used, the surface
must be rinsed with potable water after sanitizing. A 1-in-5 dilution of household bleach with
water is effective against many bacteria and some viruses, and is often the disinfectant
of choice in cleaning surfaces in hospitals. The solution is corrosive, and needs to be
thoroughly removed afterwards, so the bleach disinfection is sometimes followed by an ethanol
disinfection. Even “scientific-grade”, commercially produced
disinfection solutions such as Virocidin-X usually have sodium hypochlorite as their
sole active ingredient, though they also contain surfactants and fragrances. See Hypochlorous acid for a discussion of
the mechanism for disinfectant action. Treatment of Gingivitis
Diluted sodium hypochlorite at a rate of 2000–1 may represent an efficacious, safe and affordable
antimicrobial agent in the prevention and treatment of periodontal disease. Color safe bleach
Color safe bleach is a chemical that uses hydrogen peroxide as the active ingredient
rather than sodium hypochlorite or chlorine. It also has chemicals in it that help brighten
colors. Hydrogen peroxide is also used for sterilization
purposes and water treatment, but its disinfectant capabilities may be limited due to the concentration
in the colorsafe bleach solution as compared to other applications. See also
Percent active chlorine Bleachfield
Household chemicals Tooth bleaching
References Further reading
Bodkins, Dr. Bailey. Bleach. Philadelphia: Virginia Printing Press, 1995. Trotman, E.R. Textile Scouring and Bleaching. London: Charles Griffin & Co., 1968. ISBN 0-85264-067-6. External links
Bleach in Britannica Bleach

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