Bleaching active chlorine

Bleich-beize, /. bleaching mordant, -chlor, n. chlorine for bleaching active chlorine. 

In solutions, the concentration of available chlorine in the form of hypochlorite or hypochlorous acid is called free-available chlorine. The available chlorine in the form of undissociated A/-chloro compounds is called combined-available chlorine. Several analytical methods can be used to distinguish between free- and combined-available chlorine (8). Bleaches that do not form hypochlorite in solution like chlorine dioxide and nonchlorine bleaches can be characterized by thek equivalent available chlorine content. This can be calculated from equation 5 by substituting the number of electrons accepted divided by two for the number of active chlorine atoms. It can also be measured by iodomettic titration. 

The chloroisocyanurates can be used in the bleaching of cotton, synthetics, and their blends they do, however, attack proteinaceous fibers, such as silk or wool, presumably via active chlorine reaction with the peptide (amide) linkage. However, the chloroisocyanurates can be used as shrink-proofing agents in wool finishing (131), (see Textiles Wool). The same action of chlorine upon proteins contributes to the effectiveness of chloroisocyanurates in automatic dishwashers. 

Earlier formulations contained mainly chlorine bleach, metasiUcates, triphosphate, and nonionic surfactants. Modem manufacturers have switched to more compHcated formulations with disiUcates, phosphates or citrate, phosphonates, polycarboxylates, nonionic surfactants, oxygen bleach, bleach activator, and enzymes. The replacement of metasiUcates by disilicates lowers pH from approximately 12 to 10.5, at 1 g ADD/L water. The combined effect of decreased pH, the absence of hypochlorite, and the trend toward lower wash temperatures has paved the way for the introduction of enzymes into ADDs. Most ADD brands in Europe are part of the new generation of ADD products with enzymes. The new formulations are described in the patent hterature (55—57). 

Despite its technical usefulness, hypochlorite bleaching faces severe environmental pressures because it yields A OX values well in excess of permitted levels. The A OX value observed increases with the active chlorine content of the bleach liquor (Figure 10.31) and with the time of treatment (Figure 10.32). 

Household bleach is not an efficient means of decontaminating large quantities of V-series agents. In addition to limited solubility in commercial bleach due to the high pH, a minimum 10-fold excess of active chlorine to nerve agent is required to ensure destruction of the agent. 

Fig. 3.68. Analytical HPLC chromatograms with detection of diode array of 4.7 x 10"5mol/l of R3R dye curve (1) before and curve (2) after 180 min of photoelectrocatalysis on the Ti02 thin-film electrode biased at +1.0 V in NajSCT, 0.025 mol/l. Curve (4) before and curve (3) after photoelectrocatalysis in NaCl 0.022 mol/l and curve (5) after bleaching of 4.7 X 10-5 mol/l of R3R dye submitted to a chemical treatment by active chlorine addition. The mobile phase was methanol-water 80 20 per cent with a flow rate of 1 ml/min and controlled temperature at 30°C. The column was a Shimpack (Shimadzu) CLC-ODS, 5 /an (250 mm X 4.6 mm). Reprinted with permission from P. A. Cameiro el al. [138].

From these equations it is evident that 1 mole of chlorine has the same oxidation power as one gram-ion of CIO- (i. e. one mole of NaCIO or half a mole of Ca(C10)2), half a inole of NaC102. or finally, two fifths of a mole of chlorine dioxide. For example, one gram-molecule of pure calcium hypochlorite of 142.99 grams in weight has the same bleaching power as two moles of chlorine weighing 141.84 g. The content of active chlorine in this substance thus equals 141.84/142.99 = 99.19 per cent. 

The finished bleaching powder usually contains 36 to 39 per cent total chlorine, 35 to 37 per cent active chlorine, 2 to 4 per cent calcium chloride, 0.5 per cent calcium chlorate, 2 to 3 per cent calcium carbonate, 8 to 16 per cent free calcium hydroxide, 0.05 to 0.15 per cent iron, 5 to 8 per cent moisture and 12 to 18 per cent total water. The product is not stable and the chlorine content gradually decreases. After being stored for 70 days, the average active chlorine loss amounts to some 11 per cent, after 120 days it reaches some 14 per cent. In order to reduce losses it is necessary to store the bleaching powder in a cool and dry place. 

Bleaching powder manufactured in chambers or in Hasenclever s apparatus is not stable and in the course of time the active chlorine content diminishes due to the following reactions ... 

A stability test is made after 2 hours drying at 100 °C hi order to ascertain the drop in active chlorine content. A stable product manufactured in Moore s equipment shows a loss of 3.5 per cent while an unstable product about 14 per cent. From this we can see that although decomposition is suppressed to a considerable extent in the case of a stable product, the active chlorine loss is still noticeable. The so called superstable bleaching powder is prepared by the addition of quick lime to the stable product. A stability test then shows active chlorine loss to be less than 0.75 per cent. 

Efforts to produce a bleaching powder in a more concentrated and stable form resulted in the manufacture of calcium hypochlorite Ca(0 Cl)2. In pure form it contains 99.2 per cent active chlorine. It is manufactured in various ways and sold commercially under different names, e. g. Perchloron produced by I. G. Far ben, H. T. H. by Mathieson Alkali Works, Pittchlor by Pittsburg Plate Glass Co., Columbia. Chemical Division. 

The advantage of calcium hypochlorite, compared to standard bleaching powder, is a higher content of active chlorine. Moreover, it forms clear solutions in water and is much more stable particularly at higher temperatures. After 5 years storage in tropical countries, chlorine content will not drop below 55 per cent. Another advantage of the calcium hypochlorite is that it is less hygroscopic due to a lower content of calcium chloride. 

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