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Bleaching Powder

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Malavika Mallika

on 6 July 2014

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Transcript of Bleaching Powder

Bleaching Powder

Introduction on Bleaching Powder
Bleach refers to a number of chemicals which remove colour, whiten or disinfect, often by oxidation.
The bleaching process has been known for millennia, 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 assodium dithionite and sodium borohydride.
Bleaches are used as household chemicals to whiten clothes and remove stains and as disinfectants, primarily in the bathroom and kitchen. Many bleaches have strong bactericidal properties, and are used for disinfecting and sterilizing and thus are used in swimming pool sanitation to control bacteria, viruses and algae and in any institution where sterile conditions are needed. They are also used in many industrial processes, notably in the bleaching of wood pulp. Bleach is also used for removing mildew, killing weeds and increasing the longevity of flowers.

Uses of Bleaching Powder
It is used
(i) as a disinfectant and germicide especially in the sterilization of drinking water.

(ii) for manufacture of chloroform.
(iii) for making wool unshrinkable.

(iv) as an oxidising agent in industry.
(v) mainly as bleaching agent for cotton, linen and wood pulp.
1. It is a pale yellow powder. It has strong smell of chlorine. It is soluble in water but a clear solution is never formed due to the presence of impurities.

2. On long standing it undergoes auto-oxidation into calcium chlorate and calcium chloride.
auto oxidation of bleaching chloride

3. In the presence of cobalt chloride it looses its oxygen.
action of bleaching chloride with cobalt chloride

4. On account of the formation of nascent oxygen, it shows oxidising and bleaching properties.
formation of nascent oxygen

5. It loses its chlorine by the action of dilute acids (in excess).

formation of available chlorine

The amount of chlorine obtained from a sample of bleaching powder by treatment with excess of dilute acids or carbon dioxide is called available chlorine. A good sample of bleaching powder contains 35 38% available chlorine.

Types of Bleaching Powder
There are four types of Bleaches:-
Chlorine-based Bleaches
Peroxide-based Bleaches
Miscellaneous Bleaches
Reducing Bleaches
Manufacturing Process
The manufacture of bleaching powder is carried out in Backmann's plant as follows:

It consists of a vertical cast-iron tower. The tower is provided with a hopper at the top, two inlets near the base (one for chlorine and other for hot air) and an exit for waste gases near the top.
The tower is fitted with eight shelves at different heights each equipped with rotating rakes. The slaked lime is introduced through the hopper and it comes in contact with chlorine, which slowly moves upwards. Bleaching powder is collected in a barrel at the base. The chlorine used in the manufacture of bleaching powder should be dilute and the temperature should be maintained below 40oC.
Quiz Time!!!!
Only the qroups asked the questions should answer them.
All the best :D
HISTORY (cont.....)
HISTORY (cont...)
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 had been used in Europe since the early twentieth century, but did not become popular in North America until the 1980s.
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.
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 likesodium 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.
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.

IIt is manufactured by the action of chlorine on dry slaked lime, Ca(OH)2 at 40oC.

This is the Odling view about its formation. There is another view proposed by Clifford according to which bleaching powder is a mixture of calcium hypochlorite and basic calcium chloride.

Done By:
Malavika M.

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. Modern bleaches resulted from the work of 18th century scientists including Swedish chemist Carl Wilhelm Scheele, who discovered chlorine, French scientists Claude Berthollet, who recognized that chlorine could be used to bleach fabrics and who first made sodium hypochlorite (Eau de Javel, or Javel water, named after a quarter in Paris where it was produced) and Antoine Germain Labarraque, who discovered the disinfecting ability of hypochlorites. Scottish chemist and industrialistCharles Tennant first produced a solution of calcium hypochlorite, then solid calcium hypochlorite (bleaching powder).
Miscellaneous Bleaches
Peracetic acid and ozone are used in the manufacture of paper products, especially newsprint and white Kraft paper. In the food industry, some organic peroxides (benzoyl peroxide, etc.) and other agents (e.g.,bromates) are used as flour bleaching and maturing agents.







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