Bleach

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Bleach is the generic name for any chemical product which is used industrially and domestically to whiten clothes, to lighten hair color and to remove stains. More often, the term specifically refers to a dilute solution of sodium hypochlorite, also called "liquid bleach".

Types

Chlorine 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 most common chlorine-based bleaches are:

  • Sodium hypochlorite (NaClO): Available 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 (formerly known as "chlorinated lime"): A mixture of calcium hypochlorite (Ca(ClO)2), calcium hydroxide (lime, Ca(OH)2), and calcium chloride (CaCl2) in variable amounts. Sold as a white powder or in tablets, is used in many of the same applications as sodium hypochlorite, but is more stable and contains more available chlorine.
  • Chlorine gas (Cl2): 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 (ClO2): This unstable gas is generated in situ from sodium chlorite 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.

Peroxide-based bleaches

Peroxide-based bleaches are characterized by the peroxide chemical group, namely two oxygen atoms connected by a single bond, (–O–O–).

The main products in this class are:

  • Hydrogen peroxide itself (H2O2). It is used, for example, to bleach wood pulp and hair or to prepare other bleaching agents like the perborates, percarbonates, peracids, etc.
  • Sodium percarbonate (Na2CO3·1.5H2O2): 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 (Na2H4B2O8): Dissolved in water it forms some hydrogen peroxide, but also the perborate anion (B(OOH)(OH)3-) which can perform nucleophilic oxidation.
  • Peracetic acid (CH3C(O)OOH): Available as solution or generated in situ by some laundry detergents, and also marketed for use as industrial and agricultural disinfection and water treatment.
  • Potassium persulfate (K2S2O8) and other persulfate salts. It, alongside ammonium and sodium persulfate, are common in hair lightening products.
  • Permanganate salts such as Potassium permanganate (KMnO4): Used in quick water disinfecting tablets.
  • Benzoyl peroxide ((C6H5COO)22): It is used in topical medications for acne and to bleach flour.
  • Ozone (O3): 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.

Reducing bleaches

Unlike the chlorine and peroxide bleaches, reducing bleaches rely on the reducing effect.

The most important ones are:

  • Sodium dithionite (also called sodium hydrosulfite): is one of the most important reductive bleaching agents. It is used as such in some industrial dyeing processes to eliminate excess dye, residual oxide, and unintended pigments and for bleaching wood pulp.
  • Sulfur dioxide: Used as reducing bleach for papers and delicate materials such as clothes. This bleaching effect normally does not last very long, as atmospheric oxygen in the atmosphere reoxidizes the reduced dyes, restoring the color.

Availability

Sodium and calcium hypochlorite can be bought in many hardware stores.

Preparation

Sodium hypochlorite bleach can be made by bubbling chlorine gas through a cold solution of sodium hydroxide.

Projects

  • Disinfecting
  • Make chloroform
  • Make chlorates and perchlorates

Handling and safety

Bleaches are corrosive and proper protection must be worn at all times.

References

Relevant Sciencemadness threads