Classes of bleaches::Bleach


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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 (acidic) favors chlorine,<ref name="c&w">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref>

Cl2 + H2O <math>\rightleftharpoons</math> 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Sodium hypochlorite and ammonia react to form a number of products, depending on the temperature, concentration, and how they are mixed.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> The main reaction is chlorination of ammonia, first giving chloramine (NH2Cl), then dichloramine (NHCl2) and finally nitrogen trichloride (NCl3). 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:<ref name="c&w"/>

2 NH2Cl + N2H4 → 2 NH4Cl + N2

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.{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }} Salts particularly effective, in this regard, include MgCl2, CaCl2, FeCl3 and, to a lesser 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 (and old, see Watt's Dictionary of Chemistry{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }}) equilibrium reaction to the right:

CaCl2 + 2 HClO = Ca(OH)2 + 2 Cl2

Sodium hypochlorite

{{#invoke:main|main}} 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<ref name=len>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Cl2 (g) + 2 NaOH (aq) → NaCl (aq) + NaClO (aq) + H2O (l)

or by electrolysis of brine (sodium chloride in water).<ref name=len/><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

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.<ref name="Technology Economics Program">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Bleaching powder


Bleaching powder is any of various mixtures of calcium hypochlorite, lime (calcium hydroxide), and calcium chloride.<ref name=Ullmann>Vogt, H.; Balej, J.; Bennett, J. E.; Wintzer, P.; Sheikh, S. A.; Gallone, P.; Vasudevan, S.; Pelin, K. (2010). "Chlorine Oxides and Chlorine Oxygen Acids". Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH. doi:10.1002/14356007.a06_483.pub2.</ref> Also known as "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 a white powder. A purer, more stable form of calcium hypochlorite is called HTH or high test hypochlorite. Bleaching tablets 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".<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Percent active chlorine in these materials ranges from 20% for bleaching powder to 70% for HTH.



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 unstable gas and is used 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 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<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref>

2 NaClO3 + 2 HX + "R" → 2 NaX + 2 ClO2 + "RO" + H2O

where "R" is the reducing agent and "RO" is the oxidized form.<ref></ref>

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

{{#invoke:main|main}} 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 (see above) and sodium carbonate. It is generally considered to be an eco-friendly cleaning agent.

Sodium perborate

{{#invoke:main|main}} Sodium perborate, Na2H4B2O8, is made by reacting borax with sodium hydroxide to give sodium metaborate (NaBO2) which is then reacted with hydrogen peroxide to give hydrated sodium perborate.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

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 (B(OOH)(OH)3), which is activated for nucleophilic oxidation.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Miscellaneous bleaches

Peracetic acid and ozone are used in the manufacture of paper products, especially newsprint and white Kraft paper.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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

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

which is used in bleaching wood pulp, cotton, wool, leather and clay.<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref>

Bleach sections
Intro   History    Mechanism of action  Classes of bleaches   Environmental impact    Disinfection   Color safe bleach   See also    References    Further reading    External links   

Classes of bleaches
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