Bleaching powder, available chlorine

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]. 

The available chlorine refers to the chlorine liberated by the action of dilute acids on the hypochlorite, and is expressed as the percentage by weight in the case of bleaching powder. Commercial bleaching powder contains 36-38 per cent of available chlorine. 

Sodium hypochlorite is commercially available as an alkaline solution, normally containing the equivalent of 12-14% available chlorine. However, this is so unstable that analytical testing of its strength is always necessary before use. Calcium hypochlorite (bleaching powder), stabilised by adding lime, has been used in the past but this product is no longer used in textile bleaching. 

If no fresh bleaching powder is available, a hypochlorite solution is prepared by passing chlorine into an ice-cooled solution of 15 g. of sodium hydroxide in 250 c.c. of water, until the increase in weight is 14 g. 

Iodometry is an indirect procedure based on the aforesaid reversible reaction whereby the assay of oxidizing agents, for instance available chlorine in bleaching powder, cupric and ferric salts may be carried out by reducing them with an excess potassium iodide thereby liberating an equivalent quantity of iodine which can be estimated using a standard solution of thiosulphate. 

Chlorinated lime or bleaching powder, CaOCl2, contains about 30% w/w of available chlorine. 

Example. —20 grms. of bleaching powder were made up with water to a litre, and 25 c.e. were titrated with fV-standard sodium arsenite soln., i.e,. a soln. of such a concentration that 1 c.c. corresponded with 0 "00355 grm. of chlorine. After the addition of 46 c.c. of tbe standard soln., a drop of the liquid being titrated gives no coloration with starch and potassium iodide. Here 46 x0"00355=0"163 grm. of available chlorine was contained in 25 c.c. of the given soln. or 6"5 grms. per litre of the soln., i.e. in 20 grms. of the sample. Hence the sample contained 32 "5 per cent, of available chlorine. 

G. Lunge also gives a table of specific gravities of soln. of this sample of bleaching powder for different amounts of available chlorine in grams per litre ... 

However carefully prepared, bleaching powder always contains an excess of lime at any rate, a deposit of slaked lime is always found as a residue when bleaching powder is extracted with water. Since the available chlorine in commercial bleaching powder usually ranges between 36 and 38 per cent., and under the very best conditions a maximum of about 43 5 per cent, of available chlorine is taken up, it is inferred that the calcium hydroxide, Ca(0H)2, is not completely sat. with chlorine, and that calcium hydroxide is present—either free or combined. 

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