Chlorine combined available

Water Quality Maintenance. In addition to controlling algae and microorganisms such as bacteria, proper swimming pool maintenance requires control of free and combined available chlorine, pH, alkalinity, hardness, and saturation index. Ranges for various swimming pool parameters (Table 2) are recommended by The National Spa and Pool Institute (14). 

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. 

Hypochlorous acid and hypochlorite ion are known as free available chlorine. The chloramines are known as combined available chlorine and are slower than free chlorine in killing microorganisms. For identical conditions of contact time, temperature, and pH in the range of 6 to 8, it takes at least 25 times more combined available chlorine to produce the same germicidal efficiency. The difference in potency between chloramines and HOCl can be explained by the difference in their oxidation potentials, assuming the action of chloramine is of an electrochemical nature rather than one of diffusion, as seems to be the case for HOCl. 

It has also been demonstrated that the germicidal effectiveness of free and combined chlorine is markedly diminished with decreasing water temperature. In any situation in which the effects of lowered temperature and high pH value are combined, reduced efficiency of free chlorine and chloramines is marked. These factors directly affect the exposure time needed to achieve satisfactory disinfection. Under the most ideal conditions, the contact time needed with free available chlorine may only be on the order of a few minutes combined available chlorine under the same conditions might require hours. 

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. 

Combined available chlorine The concentration of chlorine which is combined with ammonia (NH3) as chloramine or as other chloro derivatives, yet is still available to oxidize organic matter. 

What would lead you to believe that monochloramine and dichloramine are not regarded as very toxic, at least in an unconcentrated form How are these compounds used How are they related to combined available chlorine ... 

Chlorine is abundantly available in NaCl and in saltwater. Hence, the quantity of chlorine combined in these natural sources is enormous. The Great Salt Lake contains 23% salt, and the Dead Sea contains about 30%. Chlorine also occurs in a few minerals, but the abundance of naturally occurring salt water makes these of little importance. 

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 ... 

Combined available chlorine = [NH2C1] + 2[NHC12] + 3[NC13]... 

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. ... 

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). 

The chemical form in which chlorine is present in wastewater is directly related to pH. The first reaction of chlorine is with ammonia (combined available chlorine). However, this is a small portion of the chlorine added for this process. Most of the chlorine (free available chlorine) ends up as either hydrochloric acid, HCl, or hypochlorous acid, HOCl. The HOCl subsequently breaks down into nascent oxygen, O, and HCl. Below pH 5, molecular chlorine, CI2, appears in solution and increases in concentration with decreasing pH. The equations for the reaction of free available chlorine in water can be summarized as follows ... 

If DPD is employed for determination of chlorine, it should be noted that in order to determine the so-called free available chlorine, a red dye is formed in the presence of chlorine, whereas for detection of the so-called "total chlorine" and/or "combined available chlorine" this reaction does not occur until iodide ions are added. 

Both cuvettes are placed in the comparator, e.g. in the Multicol apparatus, and the colour disk turned until the colours are equal. The value measured is multiplied by the factor 1.9, giving the content of mg CIO2/I water (see also the sections on the determination of total chlorine, free available chlorine and combined available chlorine). 

Combined (available) chlorine total available chlorine in the form of undissociated N-chloro compounds that can hydrolyze according to reaction [III]. 

For polluted waters or chlorinated waste waters where residual chlorine is present mainly in the combined form, analytical methods do not need to distinguish between FAC and combined available chlorine. lodometric titration is not recommended because these water samples typically contain small amoimts of reducing agents that can react with, and thereby reduce, the liberated iodine over the course of the titration. In this case, an iodimetric procedme is recommended. Arsenic(III) (as arsenic trioxide, AS2O3) is added to the sample in a known amount that is in excess of the combined chlorine, and unreacted As(III) is titrated with a standard solution of I3" to a starch endpoint (formation of a blue color). 

Other spectrophotometric methods Other methods for the determination of dissolved chlorine include the use of leuco crystal violet and syringaldazine as color-developing reagents. In the leuco crystal violet method, a colorless precursor (leuco crystal violet) is oxidized to crystal violet (reaction [XIX]), which has an absorption maximum at 588 nm. Interference from combined available chlorine can be avoided if the test is carried out within 5 min of indicator addition. Total chlorine determination can be accomplished by the addition of iodide ion to produce hypoiodous acid (FIIO), which reacts instantly with the indicator. Combined available chlorine is determined by difference. 

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. 

Colorimetric-DPD N,N -diethyl phenylenediamine) method. The most common, and the preferred method of measuring free and combined chlorine concentration is the N,N -diethyl phenylenediamine (DPD) method. Chlorine-containing compounds (HOCl, OCl , NH2CI) react with the DPD indicator to form a pink color. Chloride ions do not react or interfere with the test. In a two-stage process free available chlorine (FAC) and total chlorine (TC) are determined separately. Combined available chlorine (CAC) is calculated from the equation ... 

The two chemical species formed by chlorine in water, HOCl and OCl , are known as free available chlorine and are very effective in kilting bacteria and other pathogens. In the presence of ammonia, HOCl reacts with ammonium ion to produce monochloramine (NH2CI), dichloramine (NHCI2), and trichloramine (NCI3), three species collectively called combined available chlorine. Although weaker disinfectants than chlorine and hypochlorite, the chloramines persist in water distribution systems to provide residual disinfection. 

The chloramines are called combined available chlorine. Chlorination practice frequently provides for formation of combined available chlorine which, although a weaker disinfectant than free available chlorine, is more readily retained as a disinfectant throughout the water distribution system. Too much ammonia in water is considered undesirable because it exerts excess demand for chlorine. 

Distinguish between free available chlorine and combined available chlorine in water disinfection. 

The most obvious feature of these C chemical shifts is that the closer the carbon is to the electronegative chlorine the more deshielded it is Peak assignments will not always be this easy but the correspondence with electronegativity is so pronounced that spec trum simulators are available that allow reliable prediction of chemical shifts from structural formulas These simulators are based on arithmetic formulas that combine experimentally derived chemical shift increments for the various structural units within a molecule... 

The recent (since 1993) tightness of the chlorine market, which was accompanied by rising chlorine prices, sparked interest in alternative sources of chlorine, ie, HCl and EDC, where these are available. Some plants no longer operate in a strictly balanced mode, but instead operate with more than half of thek EDC made from oxychlorination (owing to importation of HCl or EDC as a chlorine source, thus bypassing dkect chlorination). The ideal situation is one in which the plant can adapt to any feed combination, allowing operation at the optimum mix of feedstocks as determined by minimization of the sum of raw material and operating costs. 

In reahty the chemistry of breakpoint chlorination is much more complex and has been modeled by computer (21). Conversion of NH/ to monochloramine is rapid and causes an essentially linear increase in CAC with chlorine dosage. Further addition of chlorine results in formation of unstable dichloramine which decomposes to N2 thereby causing a reduction in CAC (22). At breakpoint, the process is essentially complete, and further addition of chlorine causes an equivalent linear increase in free available chlorine. Small concentrations of combined chlorine remaining beyond breakpoint are due primarily to organic chloramines. Breakpoint occurs slightly above the theoretical C1 N ratio (1.75 vs 1.5) because of competitive oxidation of NH/ to nitrate ion. Organic matter consumes chlorine and its oxidation also increases the breakpoint chlorine demand. Cyanuric acid does not interfere with breakpoint chlorination (23). 

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