Bleach is the generic name for any chemical product that is used industrially or domestically to remove color from (i.e. to whiten) fabric, fiber, or food (in a process called bleaching) or to disinfect after cleaning. It often refers specifically to a dilute solution of sodium hypochlorite, also called "liquid bleach".
Many bleaches have broad-spectrum bactericidal properties, making them useful for disinfecting and sterilizing. Liquid bleach is one of the few compounds capable of fully annihilating DNA, making it commonplace for sanitizing laboratory equipment. They are used in swimming pool sanitation to control bacteria, viruses, and algae and in many places where sterile conditions are required. They are also used in many industrial processes, notably in the bleaching of wood pulp. Bleaches also have other minor uses, such as removing mildew, killing weeds, and increasing the longevity of cut flowers.
Bleaches work by reacting with many colored organic compounds, such as natural pigments, and turning them into colorless ones. While most bleaches are oxidizing agents (chemicals that can remove electrons from other molecules), some are reducing agents (that donate electrons).
Chlorine, a powerful oxidizer, is the active agent in many household bleaches. Since pure chlorine is a toxic corrosive gas, these products usually contain hypochlorite, which releases chlorine. "Bleaching powder" usually refers to a formulation containing calcium hypochlorite.
Oxidizing bleaching agents that do not contain chlorine are usually based on peroxides, such as hydrogen peroxide, sodium percarbonate, and sodium perborate. These bleaches are called "non-chlorine bleach", "oxygen bleach", or "color-safe bleach".
Reducing bleaches have niche uses, such as sulfur dioxide, which is used to bleach wool, either as gas or from solutions of sodium dithionite, and sodium borohydride.
Bleaches generally react with many other organic substances besides the intended colored pigments, so they can weaken or damage natural materials such as fibers, cloth, and leather, and intentionally applied dyes, such as the indigo of denim. For the same reason, ingestion of the products, breathing of the fumes, or contact with skin or eyes can cause bodily harm and damage health.
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. In the 17th century, there was a significant cloth bleaching industry in Western Europe, using alternating alkaline baths (generally lye) and acid baths (such as lactic acid from sour milk, and later diluted sulfuric acid). The whole process lasted up to six months.
Chlorine-based bleaches, which shortened that process from months to hours, were invented in Europe in the late 18th century. German-Swedish chemist Carl Wilhelm Scheele discovered chlorine in 1774, and in 1785 Savoyard scientist Claude Berthollet recognized that it could be used to bleach fabrics. Berthollet also discovered sodium hypochlorite, which became the first commercial bleach, named Eau de Javel ("Javel water") after the borough of Javel, near Paris, where it was produced.
Scottish chemist and industrialist Charles Tennant proposed in 1798 a solution of calcium hypochlorite as an alternative for Javel water, and patented bleaching powder (solid calcium hypochlorite) in 1799. Around 1820, French chemist Antoine Germain Labarraque discovered the disinfecting and deodorizing ability of hypochlorites and was instrumental in popularizing their use for such purpose.Scott, James, transl. (1828). On the disinfecting properties of Labarraque's preparations of chlorine Published by S. Highley. His work greatly improved medical practice, public health, and the sanitary conditions in hospitals, slaughterhouses, and all industries dealing with animal products.Labarraque, Antoine-Germain, Nouvelle biographie générale, volume 28 (1859), columns 323-324.
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 has been used in Europe since the early twentieth century, and became popular in North America in the 1980s.
Chlorine-based bleaches have been used since the late 18th century to whiten cotton and linen clothes, removing either the natural fiber color or stains of sweat or other organic residues. They are still used in households for laundry and to remove organic stains (such as mildew) on surfaces.
Types of bleaches
Most industrial and household bleaches belong to three broad classes: Chlorine-based bleaches, whose active agent is chlorine, usually from the decomposition of some chlorine compound such as hypochlorite or chloramine. Peroxide-based bleaches, whose active agent is oxygen, almost always from the decomposition of a peroxide compound such as hydrogen peroxide. Sulfur dioxide-based bleaches, whose active agent is sulfur dioxide, possibly from the decomposition of some oxosulfur anion.
Chlorine-based bleaches
Chlorine-based bleaches are found in many household "bleach" products, as well as in specialized products for hospitals, public health, water chlorination, and industrial processes. The grade of chlorine-based bleaches is often expressed as percent active chlorine. One gram of 100% active chlorine bleach has the same bleaching power as one gram of elemental chlorine.
Some common chlorine-based bleaches are the following:
Chlorine gas (). It is used as a disinfectant in water treatment, especially to make drinking water and in large public swimming pools. It was used extensively to bleach wood pulp, but this use has decreased significantly due to environmental concerns. Sodium hypochlorite, . This chlorine-releasing compound is the most common bleaching and disinfection compound. A dilute (3–6%) aqueous solution in water, historically known as Eau de Labarraque or "Labarraque's water", is widely marketed as a household cleaning product, under the name "liquid bleach" or simply "bleach". More concentrated solutions are used to disinfect drinking water and as bleaching agents in industrial processes. A more diluted solution (up to 0.5%) has been in use since 1915 to cleanse and disinfect wounds, under the name of Dakin's solution. Sodium hypochlorite, usually as a 3–6% solution in water, usually called "liquid bleach" or just "bleach". Historically called "Javel water" (). It 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. Calcium hypochlorite, . This product, known as "bleaching powder" or "chlorinated lime", it is used in many of the same applications as sodium hypochlorite, but is more stable and contains more available chlorine. It is usually marketed as a white powder that contains, besides the hypochlorite, also calcium hydroxide ("lime") and calcium chloride . A purer, more stable form of calcium hypochlorite is called HTH or high test hypochlorite. It is also available as bleaching tablets that contain calcium hypochlorite and 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". Percent active chlorine in these materials ranges from 20% for bleaching powder to 70% for HTH. Sold as a white powder or in tablets, it is used in many of the same applications as sodium hypochlorite but is more stable and contains more available chlorine. Potassium hypochlorite, . This was the first chlorine-based bleaching agent, which became available around 1785 under the name Eau de Javel or "Javel water". It is no longer commonly used, having been superseded by the cheaper sodium analog. Chloramine, . This chemical is commonly handled as a dilute aqueous solution. It is used as an alternative to chlorine and sodium hypochlorite for disinfection of drinking water and swimming pools. Chloramine-T, or tosylchloramide sodium salt, . This solid compound is available in tablet or powder form, and used in medical establishments to disinfect surfaces, equipment, and instruments. Sodium dichloroisocyanurate . This solid compound, available as tablets, is widely used as a disinfectant, to sterilize drinking water, swimming pools, tableware, farming installations, and air; and as an industrial deodorant. It is also used for bleaching textiles. Halazone, or 4-((dichloroamino)sulfonyl)benzoic acid, . This compound was for a while used to disinfect drinking water in field situations, but has largely been replaced in that use by sodium dichloroisocyanurate. This unstable gas is generated in situ or stored as dilute aqueous solutions. It finds large-scale applications for the bleaching of wood pulp, fats and oils, cellulose, flour, textiles, beeswax, skin, and in a number of other industries.
Chlorine dioxide, . This is an unstable gas, which is usually prepared in situ or stored as dilute aqueous solutions. Despite these limitations it finds large-scale applications for the bleaching of wood pulp, fats and oils, cellulose, flour, textiles, beeswax, skin, and in a number of other industries. It has also been used for chlorination of tap water.
Peroxide-based bleaches Peroxide-based bleaches are characterized by the peroxide chemical group, namely two oxygen atoms connected by a single bond, (–O–O–). This bond is easily broken, giving rise to very reactive oxygen species, which are the active agents of this type of bleach.
The main products in this class are the following: Hydrogen peroxide (). It is used, for example, to bleach wood pulp, hair and teeth, or to prepare other bleaching agents such as perborates, percarbonates, peracids, etc. Sodium percarbonate (), an adduct of hydrogen peroxide and sodium carbonate ("soda ash" or "washing soda", ). Dissolved in water, it yields a solution of the two products, that combines the degreasing action of the carbonate with the bleaching action of the peroxide. Sodium perborate (). Dissolved in water it forms some hydrogen peroxide, but also the perborate anion () which can perform nucleophilic oxidation. Peracetic (peroxoacetic) acid (). Generated in situ by some laundry detergents, and also marketed for use as industrial and agricultural disinfection and water treatment.V. Namboodiri and A. Garg (2017): "Evaluation of Combined Peracetic acid and UV treatment for Disinfection of Secondary Wastewater Effluent". document EPA/600/R-17/172, National Risk Management Research Laboratory, U.S. Environmental Protection Agency, Benzoyl peroxide (). It is used in topical medications for acne and to bleach flour.(2004) "Benzoyl peroxide" FAO Publication FNP 52 Addendum 12. Ozone (). While not properly a peroxide, its mechanism of action is similar. It is used in the manufacture of paper products, especially newsprint and white kraft paper. Potassium persulfate (K2 S2O8) and other persulfate salts. It, alongside ammonium and sodium persulfate, is common in hair-lightening products. Permanganate salts such as potassium permanganate (KMnO4).
In the food industry, other oxidizing products such as bromates are used as flour bleaching and maturing agents.
Reducing bleaches Sodium dithionite (also known as sodium hydrosulfite) 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