Chloramines are derivatives of ammonia by substitution of one, two or three hydrogen atoms with chlorine atoms: monochloramine (chloroamine, NH2Cl), dichloramine (NHCl2>), and nitrogen trichloride (NCl3). The term chloramine also refers to a family of organic compounds with the formulas R2NCl and RNCl2 (where R is an organic group).
Monochloramine (chloramine) is an inorganic compound with the formula NH2Cl. It is an unstable colorless liquid at its melting point of âˆ'66 °C (âˆ'87 °F), but it is usually handled as a dilute aqueous solution, in which form it is sometimes used as a disinfectant. Chloramine's boiling point is 24 °C (75 °F). It is listed as a tumorigen and mutagen.
The wholesale cost in the developing world is about 13.80 to 18.41 U.S. dollars per 500Â grams.
Water treatment
Chloramine is used as a disinfectant for water because it is less aggressive than chlorine and more stable against light than hypochlorites.
Drinking water disinfection
NH2Cl is commonly used in low concentrations as a secondary disinfectant in municipal water distribution systems as an alternative to chlorination. This application is increasing. Chlorine (referred to in water treatment as free chlorine) is being displaced by chloramineâ€"to be specific monochloramineâ€"which is much more stable and does not dissipate as rapidly as free chlorine. NH2Cl also has a much lower, however still present, tendency than free chlorine to convert organic materials into chlorocarbons such as chloroform and carbon tetrachloride. Such compounds have been identified as carcinogens and in 1979 the United States Environmental Protection Agency began regulating their levels in U.S. drinking water.
Some of the unregulated byproducts may possibly pose greater health risks than the regulated chemicals.
Adding chloramine to the water supply may increase exposure to lead in drinking water, especially in areas with older housing; this exposure can result in increased lead levels in the bloodstream, which may pose a significant health risk.
Swimming pool disinfection
In swimming pools, chloramines are formed by the reaction of free chlorine with organic substances, such as urine and sweat. Chloramines, compared to free chlorine, are both less effective as a sanitizer and, if not managed correctly, more irritating to the eyes of swimmers. Chloramines are also responsible for the reported "chlorine" smell of swimming pools. Some pool test kits designed for use by homeowners are not able to distinguish free chlorine and chloramines, which can be misleading and lead to non-optimal levels of chloramines in the pool water. There is also evidence that exposure to chloramine can contribute to respiratory problems, including asthma, among swimmers. Respiratory problems related to chloramine exposure are common and prevalent among competitive swimmers.
Safety
US EPA drinking water quality standards limit chloramine concentration for public water systems to 4 parts per million (ppm) based on a running annual average of all samples in the distribution system. In order to meet EPA-regulated limits on halogenated disinfection by-products, many utilities are switching from chlorination to chloramination. While chloramination produces fewer regulated total halogenated disinfection by-products, it can produce greater concentrations of unregulated iodinated disinfection byproducts and N-nitrosodimethylamine. Both iodinated disinfection by-products and N-nitrosodimethylamine have been shown to be genotoxic.
Synthesis and chemical reactions
NH2Cl is a highly unstable compound in concentrated form. Pure NH2Cl decomposes violently above âˆ'40 °C (âˆ'40 °F). Gaseous chloroamine at low pressures and low concentrations of chloroamine in aqueous solution are thermally slightly more stable. Chloroamine is readily soluble in water and ether, but less soluble in chloroform and carbon tetrachloride.
Production
In dilute aqueous solution, chloroamine is prepared by the reaction of ammonia with sodium hypochlorite:
- NH3 + ClOâˆ' â†' NH2Cl + OHâˆ'
This is also the first step of the Raschig hydrazine synthesis. The reaction has to be carried out in a slightly alkaline medium (pH 8.5â€"11). The acting chlorinating agent in this reaction is hypochlorous acid (HOCl), which has to be generated by protonation of hypochlorite, and then reacts in a nucleophilic substitution of the hydroxyl against the amino group. The reaction occurs quickest at around pH 8. At higher pH values the concentration of hypochlorous acid is lower, at lower pH values ammonia is protonated to form ammonium ions NH+
4, which do not react further.
The chloroamine solution can be concentrated by vacuum distillation and by passing the vapor through potassium carbonate which absorbs the water. Chloroamine can be extracted with ether.
Gaseous chloroamine can be obtained from the reaction of gaseous ammonia with chlorine gas (diluted with nitrogen gas):
- 2 NH3(g) + Cl2(g) ⇌ NH2Cl(g) + NH4Cl(s)
Pure chloroamine can be prepared by passing fluoroamine through calcium chloride:
- 2 NH2F + CaCl2 â†' 2 NH2Cl + CaF2
Decomposition
The covalent Nâˆ'Cl bonds of chloroamines are readily hydrolyzed with release of hypochlorous acid:
- RR′NCl + H2O ⇌ RR′NH + HOCl
The quantitative hydrolysis constant (K value) is used to express the bactericidal power of chloroamines, which depends on their generating hypochlorous acid in water. It is expressed by the equation below, and is generally in the range 10âˆ'4 to 10âˆ'10 (2.8×10âˆ'10 for monochloroamine):
In aqueous solution, chloroamine slowly decomposes to dinitrogen and ammonium chloride in a neutral or mildly alkaline (pH ≤ 11) medium:
- 3 NH2Cl â†' N2 + NH4Cl + 2 HCl
However, only a few percent of a 0.1Â M chloroamine solution in water decomposes according to the formula in several weeks. At pH values above 11, the following reaction with hydroxide ions slowly occurs:
- 3 NH2Cl + 3 OHâˆ' â†' NH3 + N2 + 3 Clâˆ' + 3 H2O
In an acidic medium at pH values of around 4, chloroamine disproportionates to form dichloroamine, which in turn disproportionates again at pH values below 3 to form nitrogen trichloride:
- 2 NH2Cl + H+ ⇌ NHCl2 + NH+
4 - 3 NHCl2 + H+ ⇌ 2 NCl3 + NH+
4
At low pH values, nitrogen trichloride dominates and at pH 3â€"5 dichloroamine dominates. These equilibria are disturbed by the irreversible decomposition of both compounds:
- NHCl2 + NCl3 + 2 H2O â†' N2 + 3 HCl + 2 HOCl
Reactions
In water, chloroamine is pH-neutral. It is an oxidizing agent (acidic solution: E° = âˆ'1.48 V, in basic solution E° = âˆ'0.81 V):
- NH2Cl + 2 H+ + 2 eâˆ' â†' NH+
4 + Clâˆ'
Reactions of chloroamine include radical, nucleophilic, and electrophilic substitution of chlorine, electrophilic substitution of hydrogen, and oxidative additions.
Chloroamine can, like hypochlorous acid, donate positively charged chlorine in reactions with nucleophiles (Nuâˆ'):
- Nuâˆ' + NH3Cl+ â†' NuCl + NH3
Examples of chlorination reactions include transformations to dichloroamine and nitrogen trichloride in acidic medium, as described in the decomposition section.
Chloroamine may also aminate nucleophiles (electrophilic amination):
- Nuâˆ' + NH2Cl â†' NuNH2 + Clâˆ'
The amination of ammonia with chloroamine to form hydrazine is an example of this mechanism (the Raschig process):
- NH2Cl + NH3 + NaOH â†' N2H4 + NaCl + H2O
Chloramine electrophilically aminates itself in neutral and alkaline media to start its decomposition:
- 2 NH2Cl â†' N2H3Cl + HCl
The chlorohydrazine (N2H3Cl) formed during self-decomposition is unstable and decomposes itself, which leads to the net decomposition reaction:
- 3 NH2Cl â†' N2 + NH4Cl + 2 HCl
Monochloramine oxidizes sulfhydryls and disulfides in the same manner as hypochlorous acid, but only possesses 0.4% of the biocidal effect of HClO.
Removing from water
Chloramines should be removed from water for dialysis, aquariums, hydroponic applications, and homebrewing beer. Chloramines can interfere with dialysis, can hurt aquatic animals, and can give homebrewed beer a medicinal taste by forming chlorophenols. In hydroponic applications, it will stunt the growth of plants.
When a chemical or biological process that changes the chemistry of chloramines is used, it falls under reductive dechlorination. Other techniques use physicalâ€"not chemicalâ€"methods for removing chloramines.
Dialysis
Chloramine must be removed from the water prior to use in kidney dialysis machines, as it would come in contact with the bloodstream across a permeable membrane. However, since chloramine is neutralized by the digestive process, kidney dialysis patients can still safely drink chloramine-treated water.
Ultraviolet light
The use of ultraviolet light for chlorine or chloramine removal is an established technology that has been widely accepted in pharmaceutical, beverage, and dialysis applications. UV is also used for disinfection at aquatic facilities .
Superchlorination
Chloramine can be removed from tap water by treatment with superchlorination (10Â ppm or more of free chlorine, such as from a dose of sodium hypochlorite bleach or pool sanitizer) while maintaining a pH of about 7 (such as from a dose of hydrochloric acid). Hypochlorous acid from the free chlorine strips the ammonia from the chloramine, and the ammonia outgasses from the surface of the bulk water. This process takes about 24 hours for normal tap water concentrations of a few ppm of chloramine. Residual free chlorine can then be removed by exposure to bright sunlight for about 4 hours.
Ascorbic acid and sodium ascorbate
Ascorbic acid (vitamin C) and sodium ascorbate completely neutralize both chlorine and chloramine, but degrade in a day or two, which makes them usable only for short-term applications. SFPUC determined that 1000 mg of vitaminC tablets, crushed and mixed in with bath water, completely remove chloramine in a medium-size bathtub without significantly depressing pH.
Activated carbon
Activated carbon has been used for chloramine removal long before catalytic carbon became available; standard activated carbon requires a very long contact time, which means a large volume of carbon is needed. For thorough removal, up to four times the contact time of catalytic carbon may be required.
Most dialysis units now depend on granular activated carbon (GAC) filters, two of which should be placed in series so that chloramine breakthrough can be detected after the first one, before the second one fails. Additionally, sodium metabisulfite injection may be used in certain circumstances.
Campden tablets
Home brewers use reducing agents such as sodium metabisulfite or potassium metabisulfite (both proprietary sold as Campden tablets) to remove chloramine from brewing fermented beverages. However, residual sodium can cause off flavors in beer so potassium metabisulfite is preferred.
Sodium thiosulfate
Sodium thiosulfate is used to dechlorinate tapwater for aquariums or treat effluent from wastewater treatments prior to release into rivers. The reduction reaction is analogous to the iodine reduction reaction. Treatment of tapwater requires between 0.1 and 0.3 grams of pentahydrated (crystalline) sodium thiosulfate per 10Â L of water. Many animals are sensitive to chloramine, and it must be removed from water given to many animals in zoos.
Other methods
Chloramine, like chlorine, can be removed by boiling and aging. However, time required to remove chloramine is much longer than that of chlorine. The time required to remove half of the chloramine (half-life) from 10 US gallons (38 l; 8.3 imp gal) of water by boiling is 26.6 hours, whereas the half-life of free chlorine in boiling 10 gallons of water is only 1.8 hours.
Organic chloramines
A variety of organic chloramines are known and proven useful in organic synthesis. Examples include N-chloromorpholine (ClN(CH2CH2)2O), N-chloropiperidine, and N-chloroquinuclidinium chloride.
Reduction of organic chloramines
Chloramines are often an unwanted side-product of oxidation reactions of organic compounds (with amino groups) with bleach. The reduction of chloramines back into amines can be carried out through a mild hydride donor. Sodium borohydride will reduce chloramines, but this reaction is greatly sped up with acid catalysis.
See also
- Disinfection
- Disinfection by-products
- Water treatment
- Pathogen
References
External links
- "Chlorinated drinking water", IARC Monograph (1991)
- EPA Maximum Contaminant Levels
- WebBook page for NH2Cl
- Chlorine and chloramines in the freshwater aquarium