Bleaching powders

Continuous decomposition of bleaching powder during storage is its chief drawback. Being hygroscopic, it absorbs moisture from atmosphere and evolves chlorine  

Due to this deterioration on standing, a sample of bleaching powder may always contain less amount of chlorine than what is expected and therefore, before use, every sample must be analysed for its effective or available chlorine. 

1 Determination oflhe percentage oJ A vailahle Chlorine in a given sample of bleaching powder by Bunsen s method 

The bleaching powder solution or suspension is treated with an excess of KI solution and then acidified with dilute H2SO4  

The liberated iodine is titrated witli a standard solution of sodium thiosulphate, using starch solution as indicator near the end-point  

The reaction mechanism accompanying the formation of the bleaching powder is not yet positively known. According to Ditz s opinion, the basic hypochlorite CaO. Ca Cl (OC1). H20 is formed at lower temperature in accordance with the following reaction- 

At higher temperatures tile compound Ca Cl (OCl). H20 will arise  

The calcium hydroxide which is simultaneously formed will in turn react with the chlorine according to the first reaction, so that the final product will be a mixture of the compounds 

By his theory Ditz accounts for the formation of free reaction water, further for the presence of hydrate water, for the variable composition of the bleaching 

According to Bunn and Clark, a mixture of basic hypochlorite Ca(OCl)2. 2 Ca(OH)2 with basic calcium chloride CaCl2. Ca(OH)2. HaO is formed during the first chlorination stage. As the reaction proceeds the basic hypochlorite will be further chlorinated and converted into a less basic salt 3 Ca(OCl)2.2 Ca(OH)2. 2 H20, while the composition of the basic calcium chloride remains unchanged. 

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CH2CI-CO-CH3. Colourless lachrymatory liquid b.p. 119°C. Manufactured by treating propanone with bleaching powder or chlorine. It is used as a tear gas and is usually mixed with the more potent bromoacetone. chloro acids Complex chloroanions are formed by most elements of the periodic table by solution of oxides or chlorides in concentrated hydrochloric acid. Potassium salts are precipitated from solution when potassium chloride is added to a solution of the chloro acid, the free acids are generally unstable. 

C4H8CI2S, (CH2CICH2)2S. Colourless oily liquid with a faint garlic-like odour m.p. 13-I4 C, b.p. 215-217 C. Manufactured by treating S2CI2 with ethene at 30-35°C. Decomposed violently by bleaching powder. It is a powerful vesicant and poison, and causes... 

Acetaldehyde and also many ketones, such as acetone, containing the CH3CO-group behave similarly when treated with bleaching powder, chlorination of the CHsCO - group being immediately followed by fission of the molecule by the alkali present in the bleaching powder ... 

Yields aniline (violet and then brown coloration with bleaching powder solution). 

Heating with soda-dime. See Section 3, p. 327, Condense the aniline which distils over in a test-tube, dilute with water and add bleaching powder (or NaClO) solution. Note the iolet coloration. 

No colour reaction with bleaching powder cf. amines). 

Bleaching Powder. Shake a mixture of 125 g. of bleaching powder and I litre of water at intervals for 2 hours, then filter. 

Acetone (R = CHj) when treated with sodium hypochlorite or bleaching powder solution yields chloroform, probably in accordance with the following mechanism ... 

Dichloranrine-T (p-toluenesulphondichloramide). Prepare about 200 ml. of a saturated solution of calcium hjrpochlorite by grinding a fresh sample of bleaching powder with water and filtering with shght suction. Dissolve 5 g. of p-toluenesulphonamide in as small a volume of the calcium hypochlorite solution as possible (about 150 ml.) and filter the solution if necessary. Cool in ice, and add about 50 ml. of a mixture of equal volumes of glacial acetic acid and water slowly and with stirring until precipitation is complete. The dichloramine T separates out first as a fine emulsion, which rapidly forms colourless crystals. Filter the latter... 

The copper-chelating abihty of sahcylaldoxime has been used to remove copper from brine in a seawater desalination plant effluent. A carbon—sorbate bed produced by sorption of the oxime on carbon proved to be extremely effective in the continuous process (99). In another apphcation, the chelating abihty of sahcylaldoxime with iron and copper was used to stabilize bleaching powders containing inorganic peroxide salts (100). 

After World War I, other chlohne-based bleaches were developed. In 1921 the use of chlorine dioxide for bleaching fibers was reported followed by the development of the commercial process for large-scale production of sodium chlorite. In 1928 the first dry calcium hypochlorite containing 70% available chlorine was produced in the United States. This material largely replaced bleaching powder as a commercial bleaching agent. 

Historically, bleaching powder and tropical bleach were significant sources of available chlorine but very Httle are used today. This is because of the greater availabiUty of sodium hypochlorite solutions and the development of calcium hypochlorite. They are stiU used to sanitize fields, drainage ditches, and reservoirs where its insoluble portion is not important. And, they are important sources of available chlorine within some less developed tropical countries. 

This product, called chloride of lime [67560-00-3] or bleaching powder, has about 35% available chlorine. 

Calcium hypochlorite is the principal commercial soHd hypochlorite it is produced on a large scale and marketed as a 65—70% product containing sodium chloride and water as the main diluents. A product with a significantly higher available chlorine, av CI2, (75—80%) has been introduced by Olin. Calcium hypochlorite is also manufactured to a smaller extent as a hemibasic compound (- 60% av Cl ) and to a lesser extent in the form of bleaching powder (- 35% av CI2). Lithium hypochlorite is produced on a small scale and is sold as a 35% assay product for specialty appHcations. Small amounts of NaOCl ate employed in the manufacture of crystalline chlorinated ttisodium phosphate [56802-99-4]. 

Because of the simplicity of the manufactuting process it is cheaper than neutral calcium hypochlorite, and because of its higher av CI2 and better stabiHty, it is a superior alternative to bleaching powder [64175-94-6J. 

Applications. Because of its high lime content, the use of hemibasic calcium hypochlorite ia geaeral sanitatioa is limited. It is used primarily ia Japan and lesser developed countries as an alternative to bleaching powder. 

Bleaching Powder. This material, known siace 1798, is made by chlorination of slightly moist hydrated lime, calcium hydroxide [1305-62-0] Ca(OH)2- It has the empirical formula Ca(OCl)2 CaCl2 Ca(OH)2 2H20. Its compositioa, loag a subject of coatroversy, was estabHshed by phase studies, microscopy, and x-ray diffraction techniques (241). The initial chlorination products are monobasic calcium chloride [14031-38-4] and dibasic calcium hypochlorite [12394-14-8] ... 

H2O) made by the older chamber process and was suitable for use ia tropical climates. Ia less developed or remote areas, bleaching powder may stiU be prepared oa a small scale by generating CI2 from Ma02, NaCl, and H2SO4, and treating it with hydrated lime placed on shelves ia an absorption chamber (245). 

Economic Aspects. The production of bleaching powder has been steadily declining. Peak U.S. production was 133,400 metric tons ia 1923 it decreased to 23,600 t ia 1955 and has not been reported siace. Imports, averagiag 1160 t duriag 1980—1987, came from ladia, Spaia, the Uoited Kiagdom, Germany, and Canada (220). It is probably also manufactured ia some less developed couatries. 

Observable Characteristics - Physical State (as normally shipped) Solid Color White Odor Like bleaching powder. 

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