Combined chlorine residuals

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. 

Therefore, one can say that chloramines produce a combined available chlorine (or combined chlorine residual). Here, each chloramine must be proportionally taken into account according to the factor with which they take 2e as follows ... 

An examination of the nature of the chlorine residual present prior to the breakpoint will reveal that it is composed almost entirely of chloramines. In the jargon of water chemistry these are referred to as "combined chlorine residual" as opposed to CI2, OCl , and HOCl which are known as "free chlorine residuals."... 

Chlorination Addition of chlorine in the form of free chlorine or sodium hypochlorite. Chlorine, hypo-chlorous acid and hypochlorite ions are called free chlorine residuals, whereas the chloramines are referred to as combined chlorine residuals. ... 

An overdose of chlorine not only increases the cost of disinfection but imparts an unpleasant taste to drinking water and is also injurious to health. For efficient disinfcctiou of domestic water supplies, swimming pools and treatment of waste waters, measurement of both free and combined chlorine residuals is essential. 

When KI is added to water at pH 3-4, both free and combined chlorine residuals react to oxidise iodide to free iodine ... 

Determination of Free Chlorine Residuals and Combined Chlorine Residuals (Chloramine and dichloramine) in a treated water (DPD-Ferrous Titrimetric Method)... 

The pH of the reaction mixture should be adjusted between 3 and 4 using acetic acid. At neutral pH, some forms of combined chlorine residuals do... 

When a small amount of chlorine is applied to a water sample containing reducing materials (Fe, etc.), it is readily used up and so there is no residual. This stage is shown by part AB of curve II (for impure water). As the amount of chlorine applied is increased, it starts reacting with organics and also with ammonia which produces chloramines (reactions 3.70 3.71). Mono - and dichloramines arc disinfectants and are determined as combined chlorine residuals. Part BC of the curve thus shows an increase in the amounts of these chloramines. Any further increase in chlorine dosage decomposes the chloramines possibly via the following reactions ... 

The amount of the combined chlorine residuals thus decreases along the curve CD. Point D indicates almost complete decomposition of chloramines. At this stage, known as Break-point Chlorination, there is marked decrease in the residual chlorine ( a minimum in the curve). A higher dosage of chlorine appears as free chlorine residuals and total chlorine residuals start increasing. This is shown by the part DE of the curve. 

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. 

Total chlorine is the sum of free and combined chlorine (defined as the residual chlorine existing in water in chemical combination with ammonia or organic amines). Total chlorine reacts with V,V-(l-naphthyl)-l,2-ethylendiamine hydrochloride and potassium iodide the compound formed is titrated as stated above. 

Then, after the process of disinfection, and after the reactions with reduced organic and inorganic matter (including NH3) take place, the total available chlorine (or total chlorine residual) is given by the sum of the free available chlorine and the combined available chlorine. ... 

Example 17.8 Referring to Figure 17.1, if a dosage of 1.8 mg/L is administered, determine the amount free chlorine residual that results, the amount of combined residual that results, and the amount of combined ammonia chloramine residual that results. Also, determine the amount of organic chloramine residual that results. 

Phenylarsine oxide is used as a titrant for the direct and indirect determination of residual chlorine and ozone in water and wastewater. Preliminary investigations on the direct measurement of PAO by differential pulse polarography (DPP) indicate that this technique is a promising method for lowering the detection limits in the indirect measurement of these oxidants. The control of pH is a necessary consideration in free and combined chlorine analysis with as well as the stability and measurement of... 

The chlorine residual may be either a free available residual, a combined available residual, or a combination of the two. Free available chlorine refers to the total concentration of hypochlorous acid and hypochlorite ions. Combined available chlorine is the total concentration of mono- and dichloramines, plus nitrogen trichloride and organic nitrogen chlorine-containing compounds (1). 

Approximately 0.99 mg of calcium thiosulfate is required to neutralize 1 mg of residual chlorine at pH 7.35. At pH 11,0.45 mg/L of calcium thiosulfate is sufficient to neutralize 1 mg of chlorine residual. On a weight basis, approx 1.30 parts are needed per part of chlorine at pH 6.5. Although calcium thiosulfate is reported to neutralize combined chlorine effectively, the reactions involved are not currently known. Chlorine neutralization produces HCl and H2SO4 that may result in lower pH. It does not scavenge oxygen significantly, and does not produce SO2. 

Chloramine formation and oxidation of ammonia by chlorine combine to create a unique dose-residual curve for the addition of chlorine to ammonia-containing solutions (Fig. 7-24). As the chlorine dose increases, the chlorine residual at first rises to a maximum at a [CI2] dose to [NH3] molar ratio of about 1.0. As the chlorine dose is increased further, the chlorine residual falls to a value close to zero. The chlorine dose corresponding to this minimum is called the "breakpoint" dose, and it occurs at a molar ratio of 1.5 1 to 2 1, depending upon solution conditions. The primary reaction that causes the residual chlorine concentration to decrease and thus to form the breakpoint is the breakpoint reaction, which can be represented as... 

As the chlorine dose is increased in excess of that required to produce the breakpoint, the increase in residual chlorine will be approximately equal to the excess. If we were to examine the nature of the chlorine residual after the breakpoint, we would find that it is largely free chlorine with some combined chlorine. 

In solution, HOCl and hypochlorite concentrations are commonly determined together by standard water analysis methods (AWWA, 1989). The sum of their concentrations is referred to as free available chlorine and is usually reported in parts per million (1 ppm free available chlorine = 1.4x 10" M). Detectable levels of free and combined (nitrogenous) available chlorine in treated water constitute a chlorine residual. In normal practice in North America, water utilities attempt to adjust chlorination levels to a small chlorine residual (2x 10" M or less) that is sufficient to survive throughout the drinking water distribution system all the way to the user s water taps. To achieve this level in the distribution lines, it may be necessary to add as much as 10 times higher amounts to the water in the plant, depending on the amount of reactive material in the source water and in the lines. 

These findings would lend support to observed chlorine residuals and the dominance of NH3 ions in rainfall. Furthermore, there are several major emitters of chlorine gas in western New York State, and long-range transport of chlorine and its reaction products cannot be ruled out. One hypothesis is that industrial emissions of this gas combine with NH3 in the atmosphere to form NHxCly compounds or chloramines which are subsequently removed by atmospheric washout. However, further work is needed to characterize the microcontaminants and assess their impact on the aquatic environment. 

As the residual chlorine in wastewater is usually combined, iodide is often added to liberate iodine, which is more active than the combined chlorine. Such liberation is necessary in view of the presence of amines, ammonia, organic nitrogen, and other organic compounds in wastewaters that tend to combine with a large proportion of the free residual chlorine. The losses of the liberated iodine at pH < 4 are relatively small, but it is preferable to add excess... 

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