Breakpoint chlorination reactions

Fig. 2. Graphical representation of the breakpoint chlorination reaction. The straight line at the left shows that chlorine residual is proportional to dosage in pure water. When impurities are present, they exert a chlorine demand (US EPA).

The maximum amount of ammonia that will be used in the experiments is 12.5 mg as N/liter (0.89 X 10 moles/liter). Select an appropriate acid-conjugate base pair and determine the concentration of it that will control the pH to within 0.5 units of 8.0 during the reaction. Neglect ionic strength effects the temperature = 25°C. Determine the buffer intensity of this solution. (Note There is a detailed discussion of the breakpoint chlorination reaction in Chapter 7.)... 

During superchlorination or shock treatment, ammonium ion is oxidized to nitrogen by breakpoint chlorination which is represented by the simplified reaction sequence... 

Some nitrate is also formed, thus the HOCl/NH stoichiometry is greater than theoretical, ie, - 1.7. This reaction, commonly called breakpoint chlorination, involves intermediate formation of unstable dichloramine and has been modeled kinetically (28). Hypobromous acid also oxidizes ammonia via the breakpoint reaction (29). The reaction is virtually quantitative in the presence of excess HOBr. In the case of chlorine, Htde or no decomposition of NH occurs until essentially complete conversion to monochloramine. In contrast, oxidation of NH commences immediately with HOBr because equihbrium concentrations of NH2Br and NHBr2 are formed initially. As a result, the typical hump in the breakpoint curve is much lower than in the case of chlorine. 

This reaction is called the breakpoint chlorination the combined chlorine level, which was rising as more chlorine was added, now drops suddenly. Free chlorine then rises without a corresponding rise in combined chlorine. This indicates that the pool pollution has been successfully oxidized by chlorine. 

Figure 11.13. General scheme of breakpoint chlorination difference between total residual Cl and chlorine dose reflects chlorine demand, primarily from ammonium and amines. Before breakpoint, most Cl is in combined forms, primarily mono- and dichloramine after the breakpoint, the combined residual consists of slow-reacting organic chloramines. Added Cl remains in free form after the breakpoint. Sharpness of breakpoint and minimum observed Cl concentration depend on pH, temperature, and time of reaction. Loss of residual Cl at breakpoint is caused by oxidation of di- and trichloramines to Nj according to reactions 33a and 33b and other reactions. (Adapted from Brezonik, 1994.)...

An experiment is to be conducted on breakpoint chlorination (the oxidation of ammonia by chlorine) in which it is desired to maintain the pH constant within 0.5 units of the initial pH of 8. Make the assumption that all of the ammonia is in the NH4 form (a reasonable assumption, since the pffg of NH4 is 9.3 and the pH of interest is pH 8). The breakpoint reaction between CI2 and NH3 proceeds as follows ... 

Saunier and Selleck examined this model and experimentally determined the rate constants for each of the reactions in Table 7-10. They concluded that the model fits breakpoint chlorination results. They suggested, but did not prove, that hydroxylamine (NHaOH) and possibly hydrazine (N2H4) could be intermediates in the breakpoint reaction— perhaps the hypothetical NOH of Wei and Morris. 

Chloramines resulting from the reaction of chlorine with nitrogen-containing compounds (ammonia, urea) are the main chemicals responsible for chlorine smell and eye/skin irritation in swimming pools. Saltwater chlorination lowers the chloramine level by a mechanism similar to breakpoint chlorination. The excess concentration of chlorine relative to ammonia close to the electrode surface results in chloramine decomposition prior to injection of treated water into the pool [7]. In the presence of organics, saltwater chlorination, as all chlorine-based disinfections, can generate unhealthy halogenated by-products, in particular trihalomethanes and haloacetic acids. 

Breakpoint chlorination. Breakpoint chlorination is a historical concept where combined chlorine is reoxidized to hypochlorous acid by the addition of an excess concentration of hypochlorous acid. These are collectively referred to as combined chlorine or combined available (CAC) under the assumption that the chlorine can be re-liberated. The model used for nearly all literature cites the interaction between hypochlorous acid and ammonia. In recreational water the amount of hypochlorous acid used for a breakpoint treatment is normally ten times the concentration of the combined chlorine. However, the reaction between hypochlorous acid and more complex nitrogen compounds is not fully reversible. White (1986) showed that breakpoint water containing a mixture of combined chlorine from organic and simple ammonia failed to display the elassic dip of the breakpoint reaction. These waters displayed a plateau concentration below which no further reduction in combined chlorine occurred. The nitrogen compounds in recreational water are introduced in bather waste and from the environment and contain mostly amino acids, peptides, and proteins with little free ammonia. Practical experience has shown that this method will reduce, but not eliminate, the combined chlorine. If repeated breakpoint treatments fail to reduce the combined chlorine to the target level (0.02 to 0.05 ppm CAC) alternate treatments such as oxidation with a potassium monopersulfate or partial water replacement to dilute the chloramines must be used. 

The chemistry of chlorine discussed in this section includes hydrolysis and optimum pH range of chlorination, expression of chlorine disinfectant concentration, reaction mediated by sunlight, reactions with inorganics, reactions with ammonia, reactions with organic nitrogen, breakpoint reaction, reactions with phenols, formation of trihalomethanes, acid generation, and available chlorine. 

Breakpoint reactions. Figure 17.1 shows the status of chlorine residual as a function of chlorine dosage. From zero chlorine applied at the beginning to point A, the applied chlorine is immediately consumed. This consumption is caused by reducing species snch as Fe Mn, H2S, and NO2. The reactions of these substances on HOCl have been discussed previously. As shown, no chlorine residual is produced before point A. 

As shown by the downward swing of the curve, the reactions that occur between point B and the breakpoint are all breakdown reactions. Snbstances that have been formed before reaching point B are destroyed in this range of dosage of chlorine. In other words, the chloro-organics that have been formed, the organic chloramines that have been formed, the ammonia chloramines that have formed, and all other snbstances that have been formed from reactions with compounds such as phenols and fnlvic acids are all broken down within this range. These breakdown reactions have been collectively called breakpoint reactions. 

The breakpoint reactions only break down the decomposable fractions of the respective substances. All the nondecomposables will remain after the breakpoint. This will include, among other nondecomposables, the residual organic chloramines, residual chloro-organic compounds, and residual ammonia chloramines. As we have learned, the trichloramine fraction that comes from ammonia chloramines has to be very small at this point to be of value as a disinfectant. All the substances that could interfere with disinfection and all decomposables wonld have already been destroyed, therefore, any amount of chlorine applied beyond the breakpoint will appear as free chlorine residual. 

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... 

Trichloramine occurs in appreciable concentrations only at pH < 3.5. At very high relative HOCl concentrations, combined chlorine species disappear by a complex process referred to as the breakpoint reaction, characterized by decreasing concen-... 

Reactions with ammonia and organic compounds. Hypobromous acid reacts with amines to form bromamines in a manner similar to ehorine. However bromamines are efficacious as sanitizers in recreational water, unlike chloramines which are biocidal, but of limited value due to their slow rate of kill. Additionally, bromamines are not irritating to the skin and eyes and do not possess a strong odor. Breakpoint bromination is not required. Re-oxidation of the inactive bromide ions will re-convert the salt to biocidal form (HOBr/OBr ). These qualities make bromine biocides much more favorable products for use in spas, relative to chlorine. Bromine products are primarily used for treating indoor pools and spas. Chemicals used for the neutralization of hypobromous acid are identieal to those used for hypochlorous acid. 

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