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For other uses, see Bleach (disambiguation)
"Discoloration" redirects here. For discoloration of paper, see Foxing.

Bleach refers to a number of chemicals which remove colour, whiten or disinfect, often by oxidation.

The bleaching process has been known for millennia,[1] 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.





How bleaches work[edit]


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:



Antimicrobial efficacy[edit]



Classes of bleaches[edit]

Chlorine-based bleaches[edit]


Chemical interactions[edit]


Cl2 + H2O \rightleftharpoons 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 (8-hour time-weighted average—38 hour week) by OSHA in the U.S.[14]


NH3 + NaOCl → NaOH + NH2Cl

NH2Cl + NaOCl → NaOH + NHCl2

NHCl2 + NaOCl → NaOH + NCl3


NH3 + NH2Cl + NaOH → N2H4 + NaCl + H2O


2 NH2Cl + N2H4 → 2 NH4Cl + N2

Atmospheric carbon dioxide and water react with bleaching powder (CaCl(OCl)) 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.[citation needed]

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.[citation needed] 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



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[citation needed] (which it should be if one subscribes to the action of atomic oxygen on HCl). 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[edit]

Main article: Sodium hypochlorite

Sodium hypochlorite is the most commonly encountered bleaching agent, usually as a dilute (3-6%) 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[16]



2 Cl → Cl2 + 2 e
Cl2 + H2O ↔ HClO + Cl + H+



Calcium hypochlorite[edit]

Main article: 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 (calcium oxide) is known as "tropical bleach" .[19] Percent active chlorine in these materials ranges from 20% for bleaching powder to 70% for HTH.


Main article: Chlorine

Chlorine is produced by the electrolysis of sodium chloride.

2 NaCl + 2 H2O → Cl2 + H2 + 2 NaOH


Chlorine dioxide[edit]

Main article: 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



where "R" is the reducing agent and "RO" is the oxidized form.[21]

Peroxide-based bleaches[edit]


Hydrogen peroxide[edit]

Main article: Hydrogen peroxide


Sodium percarbonate[edit]

Main article: Sodium percarbonate


2Na2CO3 + 3H2O2→2Na2CO3.3H2O2


Sodium perborate[edit]

Main article: Sodium perborate


Na2B4O7 + 2 NaOH → 4 NaBO2 + H2O

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 (B(OOH)(OH)3-), which is activated for nucleophilic oxidation.[23]

Miscellaneous bleaches[edit]

Peracetic acid and ozone are used in the manufacture of paper products, especially newsprint and white Kraft paper.[24] In the food industry, some organic peroxides (benzoyl peroxide, etc.) and other agents (e.g., bromates) are used as flour bleaching and maturing agents.

Reducing bleaches[edit]


2 NaHSO3 + Zn → Na2S2O4 + Zn(OH)2


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.[25]

Environmental impact[edit]


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.[27] However, respiratory risk from chlorine and highly toxic chlorinated byproducts still exists.


  • The OSHA highest allowable peak concentration (5 minute exposure for five minutes in a 4-hour period) is 200 ppm,[30] twice as high as the reported highest peak level (from the headspace of a bottle of a sample of bleach plus detergent).

Further studies of the use of these products and other possible exposure routes (i.e., dermal) 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.


Sodium hypochlorite solution, 3–6%, (common household bleach) 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.[31][32]

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 (5-6% sodium hypochlorite solution) per liter of water or 8 drops of bleach per gallon (3.79L) of water; 1/2 teaspoon bleach per five gallons (19L) 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.[33][34][35][36] 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 (21 CFR Part 178) 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 (ppm) available chlorine (for example, one tablespoon of typical household bleach containing 5.25% sodium hypochlorite, per gallon of water). If higher concentrations are used, the surface must be rinsed with potable water after sanitizing.



Treatment of Gingivitis [38]


Color safe bleach[edit]

Color safe bleach is a chemical that uses hydrogen peroxide as the active ingredient (to help remove stains) rather than sodium hypochlorite or chlorine.[39] It also has chemicals in it that help brighten colors.[40] 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.[40]

See also[edit]


  1. ^ a b c Public Domain Chisholm, Hugh, ed. (1911). "Bleaching". Encyclopædia Britannica 4 (11th ed.). Cambridge University Press. pp. 49–55. 
  2. ^
  4. ^ L. J. Thénard (1818). "Observations sur des nouvelles combinaisons entre l'oxigène et divers acides". Annales de chimie et de physique, 2nd series 8: 306–312. 
  6. ^ Milne, Neil (1998). "Oxygen bleaching systems in domestic laundry". J. Surfactants and Detergents 1 (2): 253–261. doi:10.1007/s11743-998-0029-z. Retrieved 8 May 2012. 
  7. ^ Field, Simon Q (2006). "Ingredients -- Bleach". Science Toys. Retrieved 2006-03-02. 
  9. ^ a b Jakob, U.; J. Winter, M. Ilbert, P.C.F. Graf, and D. Özcelik (14 November 2008). "Bleach Activates A Redox-Regulated Chaperone by Oxidative Protein Unfolding". Cell (Elsevier) 135 (4): 691–701. doi:10.1016/j.cell.2008.09.024. PMC 2606091. PMID 19013278. Retrieved 2008-11-19. 
  10. ^ Harrison, J. E., and J. Schultz. 1976. Studies on the chlorinating activity of myeloperoxidase. Journal of Biological Chemistry volume 251, pages 1371-74.
  11. ^ Thomas, E. L. 1979. Myeloperoxidase, hydrogen peroxide, chloride antimicrobial system: Nitrogen-chlorine derivatives of bacterial components in bactericidal action against Escherichia coli. Infect. Immun. 23:522-531.
  12. ^ Albrich, J. M., C. A. McCarthy, and J. K. Hurst. 1981. Biological reactivity of hypochlorous acid: Implications for microbicidal mechanisms of leukocyte myeloperoxidase. Proc. Natl. Acad. Sci. USA 78:210-214.
  14. ^ Occupational Safety & Health Administration (2007). and peroxide/recognition.html "OSHA – Chlorine". OSHA. Retrieved 2007-08-26. 
  15. ^ Rizk-Ouaini, Rosette; Ferriol, Michel; Gazet, Josette; Saugier-Cohen Adad, Marie Therese (1986). "Oxidation reaction of ammonia with sodium hypochlorite. Production and degradation reactions of chloramines". Bulletin de la Societe Chimique de France 4: 512–21. doi:10.1002/14356007.a02_143.pub2. ISBN 3-527-30673-0. 
  19. ^ "Calcium Hypochlorite: Different forms of calcium hypochlorite". World Health Organization. Retrieved 27 May 2012. 
  22. ^ "Sodium Perborate REACH Consortium". Retrieved 2012-06-07. 
  24. ^ "Ozo formulas". Ozone Information. 
  27. ^ "ECF: The Sustainable Technology". Alliance for Environmental Technology. Retrieved 2007-09-19. 
  28. ^ Odabasi, M., “Halogenated Volatile Organic Compounds from the Use of Chlorine-Bleach- Containing Household Products”, Environmental Science & Technology 42, 1445-1451, (2008). Available at: Environmental Science & Technology (ACS Publications)
  31. ^ Dvorak, Glenda (February 2005). "Disinfection". Center for Food Security and Public Health. Ames, IA: Center for Food Security and Public Health, Iowa State University. p. 12. Retrieved 7 February 2011. 
  33. ^ Washington State Department of Health: How to Purify Your Drinking Water
  34. ^ "Personal Preparation and Storage of Safe Water". Centers for Disease Control. Retrieved 2012-01-03. 
  35. ^ "Guidelines for Managing Water Supplies". U.S. Federal Emergency Management Agency. Retrieved 2012-01-03. 
  36. ^ "Clorox Regular Bleach FAQ". The Clorox Company. Retrieved 2012-01-03. 
  37. ^ KAM Scientific
  39. ^ Dr. Laundry archive
  40. ^ a b Non Chlorine Bleach – Stain Fighter & Color Booster Liquid | Clorox

Further reading[edit]


External links[edit]