Available chlorine estimation

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. 

For example, the available chlorine in bleach is estimated as follows ... 

Both general and class-specific equations (chlorinated hydrocarbon) are available for estimating Koc from Kow and appear in Table 8.1. 

Occasionally, the market will swing the other way leaving chlorine in short supply. Fused sodium chloride is commercially electrolyzed in Down s cells to give chlorine and metallic sodium [11]. Sodium production in the U.S.A. has averaged 135,000-150,000 metric tonnes annually since 1968, which represents about a 2-3% contribution to the chlorine supply from this source. In the U.K., it is estimated that as much as 10% of the available chlorine arises from Down s cell technology. Potassium chloride solutions are also electrolyzed for commercial potassium hydroxide, but the contribution to the chlorine supply from this source is even less than from fused sodium chloride electrolysis. 

The commercial solution of sodium hypochlorite contains 14-15% available chlorine, compared to 35-36% in bleaching powder. The concentration of hypochlorite in the bleaching bath generally varies from 1 to 3 g/1 available chlorine. The optimum bleaching conditions, however, depend on the degree of discolouration of the cloth and thus the temperature and time of reaction should be adjusted according to the requirement. The concentration of hypochlorite solution is normally estimated by means of standard thiosulphate or arsenite titration. 

The estimation by above method expresses chlorates as well as hypochlorites as available chlorine. The errors due to chlorates can be avoided by titration with an N/10 solution of sodium arsenite (Na HAsOj)... 

The composition of bleaching powder is by no means constant, and sodium hypochlorite tends to decompose on storage, especially when exposed to light and in hot weather. It is, therefore, often necessary to determine the amount of available chlorine in a delivery or a stock solution. The estimation is based upon the quantitative liberation of iodine from potassium iodide in an acidified solution, the iodine then being titrated with n/10 sodium thiosulphate, using starch as the indicator. 

The available chlorine in sodium hypochlorite varies. Good results in chlorination depend upon scrupulous attention to detail. Therefore each carboy of hypochlorite should be standardized before use, and any stock tank should be tested daily. The available chlorine is estimated by titration with n/10 sodium thiosulphate as described in Chapter 10. A convenient routine is to dilute 25 ml of the hypochlorite to 500 ml in a graduated flask and then titrate 5 ml of the resultant solution with thiosulphate. The g per litre of available chlorine... 

The presence of hypochlorous acid as opposed to the hypochlorite ion, is responsible for the biocidal activity. Hypochlorous acid is estimated to be twenty times more effective as a micro-biocide than the hypochlorite ion. Unfortunately however, in a cooling tower there is a tendency to strip chlorine from the water by the countercurrent flow of the air, so that Equation 14.23 moves to the left and it is likely that by the time the water reaches the basin of the tower all the chlorine will have been lost to the air in addition to reactions with organic matter within the tower. It will be necessary therefore for maximum utilisation of the available chlorine to ensure that the equilibrium of Equation 14.24, is over to the LHS. Since the ionisation process produces hydrogen ions the equilibrium will be affected by the pH of the solution. The effect of pH on HOCl stability is shown on Fig. 14.16 [Marshall and Bott 1988]. In the figure 100% HOCl indicates that the equilibrium is completely to the left in Equation 14.24 and there is no ionisation of the HOCl. From Fig. 14.16 this condition occurs at pH of around 5. At a pH 7.5 hypochlorite ion and acid co-exist in approximately equal amounts. It nll be seen that as the pH approaches 9 the hypochlorous acid content drops to a very low value and the biocidal effect is negligible. Values of pH lower than around 6.5 are not practical because of the potential risk of corrosion. 

The above reaction forms the basis for the estimation of available chlorine in bleaching powder. 

Available chlorine in a sample of bleaching powder is estimated volumetrically. A known mass of bleaching powder as a suspension in water is treated with excess of potassium iodide solution. The solution is then acidified with a strong solution of acetic acid. Iodine is liberated which is titrated against standard sodium thiosulphate solution. 

Describe how the percentage of available chlorine in a given sample of bleaching powder is estimated. 

Sodium hypochlorite solutions are used extensively in swimming pool sanitation and as a bleach in the pulp and textile industries. A less-concentrated product (5% available chlorine) is used in laundries and as household bleach. Consumption statistics for 1982 indicate that 210 x 10 tons of sodium hypochlorite were consumed in the United States alone. World consumption of soium hypochlorite for household use is estimated to be 426 X 10 metric tons annually in 2005. The reaction described in this experiment illustrates the use of liquid bleach (11.5-12.5% available chlorine) as an oxidizing agent in the organic laboratory. 

Chlorinated compounds are all evaluated on their available chlorine content. For estimation the chlorine is liberated with acid in the presence of excess of potassium iodide, from which the chlorine displaces an equivalent of iodine the liberated iodine is then titrated with thiosulphate. 

Mono- and dichlorotoluenes ate used chiefly as chemical iatermediates ia the manufacture of pesticides, dyestuffs, pharmaceuticals, and peroxides, and as solvents. Total annual production was limited prior to 1960 but has expanded greatly siace that time. Chlorinated toluenes ate produced ia the United States, Germany, Japan, and Italy. Siace the number of manufacturers is small and much of the production is utilised captively, statistics covering production quantities ate not available. Worldwide annual production of o- and -chlorotoluene is estimated at several tens of thousands of metric tons. Yearly productions of polychlorotoluene ate ia the range of 100—1000 tons. 

The chlorination of a series of compounds having electron-withdrawing substituents has been studied. The relative rates of chlorination and the isomer distributions are known. The data give a satisfactory correlation with the Hammett equation using (t, but no rate measurement for benzene under precisely comparable conditions is possible. How could you estimate f,f y and fp for chlorination from the available data ... 

SPEGLs or EEGLs are available, IDLH levels provide alternative criteria. However, because IDLH levels were not developed to account for sensitive populations and because they were based on a maximum 30-min exposure period, the EPA suggests that the identification of an effect zone should be based on exposure levels of one-tenth the IDLH level. For example, the IDLH level for chlorine dioxide is 5 ppm. Effect zones resulting from the release of this gas are defined as any zone in which the concentration of chlorine dioxide is estimated to exceed 0.5 ppm. Of course, the approach is conservative and gives unrealistic results a more realistic approach is to use a constant-dose assumption for releases less than 30 min using the IDLH level. 

Many of the quantitative rate data on radical reactions which are to be found in the scientific literature have been obtained by comparison of reaction rates rather than by direct measurement of absolute rate constants (Ingold, 1973). For example, it is a straightforward matter to compare the rate of chlorine abstraction from CC14 by phenyl radicals with the rate of hydrogen abstraction from cyclohexane by the same species, simply by comparing the PhCl/PhH product ratio from a suitable competition experiment (Bridger and Russell, 1963). In contrast, direct measurements of the absolute rates of these reactions have yet to be carried out (although indirect estimates are available). 

These data were measured at or extrapolated to ambient temperature and pH values. The data are discussed in the text. NA = not available. b/ Kq = soil water distribution coefficient (K ) divided by the organic carbon content of the soil, cj Whenever possible, half-life for soil dissipation is derived from the field data half-lives described in the text rather than lab data. As such, it may not represent a true first-order process. Value has been estimated from the equation in ref. 20. e/ Hydrolysis of total residues (aldicarb + sulfoxide + sulfone). pK for p -phthalic acid is 3.5. The chlorine atoms of DCPA should lower the pK to about 2. Conditions optimized for soil metabolism. 

We have limited data on the PCBs, where information is available only on the biphenyls with no more than a few chlorine atoms. We find it necessary to provide answers to pollution arising from PCBs with five to six carbon atoms. How do we use this limited information to construct a plausible estimate of their vapor pressures, water solubilities, and partition coefficients between octanol and water ... 

Limited information is available regarding hepatic effects in animals following oral exposure to chlorine dioxide or chlorite. Daniel et al. (1990) exposed male and female rats to chlorine dioxide in the drinking water for 90 days at concentrations that resulted in estimated doses of 1.9, 3.6, 6.2, or 11.5 mg/kg/day for males and 2.4, 4.6, 8.2, or 14.9 mg/kg/day for females. Significantly depressed mean absolute liver weights were observed in males at doses 3.6 mg/kg/day and females of the 8.2 mg/kg/day dose group. However, these groups also exhibited decreased water consumption. 

But, given that not all bladder cancer victims smoke, work with dyes, or operate drill presses, where could the carcinogens be coming from Chlorine-treated drinking water is a possibility. Epidemiological surveys have revealed that people who drink surface water treated with chlorine are more likely to develop bladder cancer. How much more likely Estimates are that in North America we can link roughly 4,500 cases of bladder cancer annually to chlorinated water. To put this into more understandable terms, about ten out of every thousand men who do not drink chlorinated water will develop bladder cancer if they live to be seventy, while thirteen will do so if they drink chlorinated water for thirty-five years. This, of course, assumes that those who don t drink chlorinated water are consuming water that has been purified by some other technique, an option not available to most people. 

McNiell has developed a useful technique for the determination of unsaturation using chlorine-36. It is not suggested that the technique should replace existing methods but that it is highly suitable for micro analyses when only small samples are available or for estimation of very low unsaturation values where other methods are not sufficiently sensitive. The superiority of the method for the analysis of butyl rubbers has been demonstrated (61) and it has also been used to measure unsaturation of 0.1—0.01 mole-% of polyisobutene (62). 

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