Residual chlorine

This is an indirect method of analysis because the chlorine-containing species do not react with the titrant. Instead the total chlorine residual oxidizes l to l3 , and the amount of 13 is determined by the redox titration with Na282 03. 

One of the most important applications of redox titrimetry is in evaluating the chlorination of public water supplies. In Method 9.3 an approach for determining the total chlorine residual was described in which the oxidizing power of chlorine is used to oxidize R to 13 . The amount of 13 formed is determined by a back titration with 8203 . 

The methods described earlier for determining the total, free, or combined chlorine residual also are used in establishing the chlorine demand of a water supply. The chlorine demand is defined as the quantity of chlorine that must be added to a water supply to completely react with any substance that can be oxidized by chlorine while also maintaining the desired chlorine residual. It is determined by adding progressively greater amounts of chlorine to a set of samples drawn from the water supply and determining the total, free, or combined chlorine residual. 

In the DPD colorimetric method for the free chlorine residual, which is reported as parts per million of CI2, the oxidizing power of free chlorine converts the colorless amine N,N-diethyl-p-phenylenediamine to a colored dye that absorbs strongly over the wavelength range of 440-580 nm. Analysis of a set of calibration standards gave the following results... 

A sample from a public water supply is analyzed to determine the free chlorine residual, giving an absorbance of 0.113. What is the free chlorine residual for the sample in parts per million CI2 ... 

The organo chlorine compounds are more expensive than inorganic chlorine compounds, but offer improved stabiUty against photolytic breakdown ia swimming pools (21). Swimming pool sanitation is generally accompHshed with 1—3 ppm free chlorine residual (see CHLORAMINES AND BROMAMINES Water, treati nt of swifting pools, spas, and hot tubs). ... 

Most microbes in cooling systems can be controlled by chlorine or bromine treatment if exposed to a sufficient residual for a long enough time. A free chlorine residual of 0.1—0.5 ppm is adequate to control bulk water organisms if the residual can be maintained for a sufficient period of time. 

The goal of filtration in the modem municipal treatment plant is a maximum of 0.1 ntu (nephelometric turbidity unit), which ensures a sparkling, clear water (8). Freedom from disease organisms is associated with freedom from turbidity, and complete freedom from taste and odor requites no less than such clarity. The National Interim Primary Drinking Water Regulations (NIPDWR) requite that the maximum contaminant level for turbidity at the point of entry into the distribution system be 1.0 ntu unless it can be shown that levels up to 5 ntu do not interfere with disinfection, interfere with the maintenance of a chlorine residual in the distribution system, nor interfere with bacteriological analyses. 

Ga.s Eeeders. Chlorine gas is usually fed from a chlorine cylinder equipped with a pressure gauge, reducing valve, regulating valve, feed-rate indicator, and aspirator-type injector for dissolving the chlorine gas in water. The feeder can be manually, or more desirably automatically, controlled utili2ing continuous amperometric or potentiometric measurement of the free chlorine residual. The chlorine solution is normally introduced into the return line to the filter. 

A typical reactor operates at 600—900°C with no catalyst and a residence time of 10—12 s. It produces a 92—93% yield of carbon tetrachloride and tetrachloroethylene, based on the chlorine input. The principal steps in the process include (/) chlorination of the hydrocarbon (2) quenching of reactor effluents 3) separation of hydrogen chloride and chlorine (4) recycling of chlorine to the reactor and (i) distillation to separate reaction products from the hydrogen chloride by-product. Advantages of this process include the use of cheap raw materials, flexibiUty of the ratios of carbon tetrachloride and tetrachloroethylene produced, and utilization of waste chlorinated residues that are used as a feedstock to the reactor. The hydrogen chloride by-product can be recycled to an oxychlorination unit (30) or sold as anhydrous or aqueous hydrogen chloride. 

There are three basic terms used in the chlorination process chlorine demand, chlorine dosage and chlorine residual. Chlorine demand is the amount of chlorine which will reduced or consumed in the process of oxidizing impurities in the water. Chlorine dosage is the amount of chlorine fed into the water. Chlorine residual is the amount of chlorine still remaining in water after oxidation takes place. For example, if a water has 2.0 ppm chlorine demand and is fed into the water in a chlorine dosage of 5.0 ppm, the chlorine residual would be 3.0 ppm. 

The effectiveness of chlorine residuals increase with higher temperatures within the normal water temperature range. 

In order to ensure the destruction of pathogens, the process of chlorination must achieve certain control of at least one factor and, preferably two, to compensate for fluctuations that occur. For this reason, some authorities on the subject stress the fact that the type and concentration of the chlorine residual must be controlled to ensure adequate disinfection. Only this way, they claim, can chlorination adequately take into account variations in temperature, pH, chlorine demand and types of organisms in the water. While possible to increase minimum contact times, it is difficult to do so. Five to ten minutes is normally all the time available with the type of pressure systems normally used for small water supplies. Many experts feel that satisfactory chlorine residual alone can provide adequate control for disinfection. In their opinion, superchlorination-dechlorination does the best job. Briefly, what is this technique and how does it operate ... 

Table 3 gives recommended ranges of chlorine dosages for disinfection of various wastewaters. Recommended minimum bactericidal chlorine residuals are given in Table 4. Data in Table 4 are based on water temperatures between 20 C to 25 C after a 10-minute contact for free chlorine and a 60 minute contact for combined available chlorine. 

The minimum residuals required for cyst destruction and inactivation of viruses are much greater. Although chlorine residuals in Table 4 are generally adequate, surface waters from polluted waterways are usually treated with much heavier chlorine dosages. Ordinary chlorination will destroy all strains of coli, aerogenes, pyocyaneae, typhsa, and dysenteria. 

The toxicity of chlorine residuals to aquatic life has been well documented. Studies indicate that at chlorine concentrations in excess of 0.01 mg/1, serious hazard to marine and estuarine life exists. This has led to the dechlorination of wastewaters before they are discharged into surface water bodies. In addition to being toxic to aquatic life, residuals of chlorine can produce halogenated organic compounds that are potentially toxic to man. Trihalomelhanes (chloroform and bromoform), which are carcinogens, are produced by chlorination. 

One of the most widely established processes using SCCO2 is the decaffeination of coffee. Prior to widespread use of this process in the 1980s the preferred extraction solvent was dichloromethane. The potential adverse health effects of chlorinated materials were realized at this time and, although there was no direct evidence of any adverse health effects being caused by any chlorinated residues in decaffeinated coffee there was always the risk, highlighted in some press scare stories. Hence the current processes offer health, environmental and economic advantages. 

From the chlorination residue, iron can be removed by making use of the following reaction at 420-450 °C ... 

Recently it was shown that when DDT, benzene hexachloride, or toxaphene is fed or applied to cattle, such organic chlorine residue as may be present in the fatty tissues consists essentially of unchanged insecticide. Carter (12) demonstrated their presence by separating the fats and other oxygenated products with sulfuric acid-sodium sulfate mixture and determining total chlorine. In experiments with DDT Schechter (46) demonstrated its presence in fatty tissue and in butterfat by the Schechter-Haller colorimetric method (47). The residues were then tested for toxicity to houseflies in comparison with the known insecticides of the same concentration. In both cases the known insecticide gave the same mortality as the residue. 

It is also shown that organic-chlorine residues on alfalfa hay resulting from insecticide applications of toxaphene and the organic-chlorine content of beef fat from animals fed alfalfa hay containing toxaphene residues or sprayed with benzene hexachloride or DDT approximate a true measure of the amounts of these compounds present. 

The practical application of these observations is to minimise the effect of iodate by rapidly carrying out the iodometric titration of chlorine residual in seawater at pH 4. Moreover, if desired, a titration correction curve can be generated using iodate at the specific concentration of iodide in the sample in question, as there appears to be a complete conversion of seawater iodide to iodate in the presence of excess chlorine. 

Until progress can be made in development of practical and affordable online contaminant monitoring and surveillance systems, most chemical industrial facilities must use other approaches to contaminant monitoring and surveillance. This includes monitoring data of physical and chemical contamination surrogates, pressure change abnormalities, free and total chlorine residual, temperature, dissolved oxygen, and conductivity. 

Site number Date sampled Type of source Source-water quality Chlorine Free chlorine Residual (mg/L) Plant Finish Water concentration ( g/L) ... 

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