Bleach

Bleach is the generic name for any chemical product that is used industrially or domestically to remove color from fabric or fiber 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, some are reducing agents.
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 and acid baths. 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 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 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. His work greatly improved medical practice, public health, and the sanitary conditions in hospitals, slaughterhouses, and all industries dealing with animal products.
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.

Mechanism of action

Whitening

Colors of natural organic materials typically arise from organic pigments, 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. This is the mechanism of bleaches based on chlorine but also of oxygen-anions which react through the initial nucleophilic attack.
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. This is the mechanism of bleaches based on sulfur dioxide.

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 a white and typically very faded blue.

Antimicrobial efficacy

The broad-spectrum effectiveness of most bleaches is due to their general chemical reactivity against organic compounds, rather than the selective inhibitory or toxic actions of antibiotics. They irreversibly denature or destroy many proteins, including all prions, making them extremely versatile disinfectants.
Hypochlorite bleaches in low concentration were also found to attack bacteria by interfering with heat shock proteins on their walls. According to 2013 Home Hygiene and Health report, using bleach, whether chlorine- or peroxide-based, significantly increases germicidal efficiency of laundry even at low temperatures, which makes it possible to eliminate viruses, bacteria, and fungi from a variety of clothing in a home setting.

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.
The most common chlorine-based bleaches are the following:

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.
Bleaching powder, usually a mixture of calcium hypochlorite, calcium hydroxide, and calcium chloride in variable amounts. 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.
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.
Chlorine dioxide. 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.

Other examples of chlorine-based bleaches, used mostly as disinfectants, are monochloramine, halazone, and sodium dichloroisocyanurate.

Peroxide-based bleaches

es are characterized by the peroxide chemical group, namely two oxygen atoms connected by a single bond,. 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. 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 acid. Generated in situ by some laundry detergents, and also marketed for use as industrial and agricultural disinfection and water treatment.
Benzoyl peroxide. It is used in topical medications for acne and to bleach flour.
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 and other persulfate salts. It, alongside ammonium and sodium persulfate, is common in hair-lightening products.
Permanganate salts such as potassium permanganate.

In the food industry, other oxidizing products such as bromates are used as flour bleaching and maturing agents.

Reducing bleaches

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.
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.
Thus is used in bleaching wood pulp, cotton, wool, leather and clay.