Chloramines are derivatives of ammonia by substitution of one, two or three hydrogen
atoms with chlorine atoms: monochloramine, dichloramine, and nitrogen trichloride. The
term chloramine also refers to a family of organic compounds with the formulas R2NCl
and RNCl2. Monochloramine is an inorganic compound with
the formula NH2Cl. It is an unstable colorless liquid at its melting point of −66 °C,
but it is usually handled as a dilute aqueous solution, in which form it is sometimes used
as a disinfectant.
Synthesis and chemical reactions NH2Cl is a highly unstable compound in concentrated
form. Pure NH2Cl decomposes violently above −40 °C. 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 + OCl- ⟶ NH2Cl + HO-
This is also the first step of the Raschig hydrazine synthesis. The reaction has to be
carried out in a slightly alkaline medium. The acting chlorinating agent in this reaction
is hypochloric acid, which has to be generated by protonation of hypochlorite, and then reacts
in a nucleophilic substitution of the hydroxo against the amino group. The reaction occurs
quickest at around pH 8. At higher pH values the concentration of hypochloric acid is lower,
at lower pH values ammonia is protonated to form ammonium ions NH4+, 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: 2 NH3(g) + Cl2(g) ⟺ NH2Cl(g) + NH4Cl(s)
Pure chloroamine can be prepared by passing fluoroamine through calcium chloride:
2 NH2F + CaCl2 ⟶ 2 NH2Cl + CaF2 Decomposition
The covalent N-Cl bonds of chloroamines are readily hydrolyzed with release of hypochlorous
acid: RR'NCl + H2O ⟺ RR'NH + HOCl
The quantitative hydrolysis constant 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 10-4 – 10-10:
K = / cRR'NCl In aqueous solution, chloroamine slowly decomposes
in a neutral or mildly alkaline medium: 3 NH2Cl ⟶ N2 + NH4Cl + 2 HCl
However, only a few percent of a 0.1 M chloroamine solution in water decompose according to the
formula in several weeks. At pH values above 11, the following reaction slowly occurs:
3 NH2Cl + 3 OH- ⟶ NH3 + N2 + 3 Cl- + 3 H2O In an acidic medium at pH values of around
4, chloroamine disproportionates to form dichloroamine, which in turn disproportionates at pH values
below 3 to form nitrogen trichloride: 2 NH2Cl + H+ ⟺ NHCl2 + NH4+
3 NHCl2 + H+ ⟺ 2 NCl3 + NH4+ At low pH values, nitrogen trichloride dominates
and between pH 3-5 dichloroamine dominates. These equilibria are disturbed by the irreversible
decomposition of both compounds: NHCl2 + NCl3 + 2 H2O ⟶ N2 + 3 HCl + 2 HOCl
Reactions In water chloroamine is pH neutral. It is
an oxidizing agent: NH2Cl + 2 H+ + 2 e- ⟶ NH4+ + Cl-
Reactions of chloroamine include radical, nucleophilic, and electrophilic substitution
of chlorine, electrophilic substitution of hydrogen, and oxidative additions.
Chloroamine can, like HOCl, donate positively charged chlorine in reactions with nucleophiles:
Nu- + NH3Cl+ ⟶ Nu–Cl + 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:
Nu- + NH2Cl ⟶ Nu–NH2 + Cl- The amination of ammonia with chloroamine
to form hydrazine is an example of this mechanism: NH2Cl + NH3 + NaOH ⟶ N2H4 + NaCl + H2O
Chloramine electrophilically aminates itself in neutral and alkaline media to start its
decomposition: 2 NH2Cl ⟶ N2H3Cl + HCl
The chlorohydrazine formed during self-decomposition is unstable and decomposes itself, which leads
to the net decomposition reaction: 3 NH2Cl ⟶ N2 + NH4Cl + 2 HCl
Uses and chemical reactions NH2Cl is a key intermediate in the traditional
synthesis of hydrazine. Monochloramine oxidizes sulfhydryls and disulfides
in the same manner as HClO, but only possesses 0.4% of the biocidal effect of HClO.
Uses in 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 is being displaced by chloramine—to be specific monochloramine—which is much
more stable and does not dissipate as rapidly as free chlorine. NH2Cl also has a very 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. 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. Pool test kits designed
for use by homeowners are sensitive to both free chlorine and chloramines, which can be
misleading. 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.
Removing chloramines 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 and fruit production
of plants. When a chemical or biological process that
changes the chemistry of chloramines is used, it falls under reductive dechlorination. Other
techniques use physical—not chemical—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 UV for chlorine or chloramine removal
is an established technology that has been widely accepted in pharmaceutical, beverage,
and dialysis applications. Ultraviolet light is also used at aquatic facilities.
Superchlorination Chloramine can be removed from tap water by
treatment with superchlorination while maintaining a pH of about 7. 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 Ascorbic acid and sodium ascorbate completely
neutralizes both chlorine and chloramines but degrades in a day or two, which make it
usable only for short-term applications; SFPUC determined that 1000 mg of Vitamin C remove
chloramine completely in a medium-size bathtub without significantly depressing pH.
Activated carbon Activated carbon have 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 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 during
circumstances. Campden tablets
Home brewers use reducing agents such as sodium metabisulfite or potassium metabisulfite 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 tap water for aquariums or treat effluent
from waste water treatments prior to release into rivers. The reduction reaction is analogous
to the iodine reduction reaction. Treatment of tap water requires between 0.1 grams and
0.3 grams of pentahydrated sodium thiosulfate per 10 liters of water. Many animals are sensitive
to chloramine, and it must be removed from water given to many animals in zoos.
Sodium thiosulfate has been used as treatment of calciphylaxis in hemodialysis patients
with end-stage renal disease. 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 from 10 gallons 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-chloromorpholine2O), N-chloropiperidine,
and N-chloroquinuclidinium chloride. Reduction of organic chloramines
Chloramines are often an unwanted side-product of oxidation reactions of organic compounds
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. Safety
US EPA drinking water quality standards limit chloramine concentration for public water
systems to 4 parts per million 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 by-products and N-nitrosodimethylamine. Both iodinated
disinfection by-products and N-nitrosodimethylamine have been shown to be genotoxic.
Chloramine has been implicated as a mutagen and as a toxic agent for aquatic life, hence
the US EPA proposes to prohibit its use in drinking water.
See also Disinfection
Disinfection by-products Water treatment
Pathogen References
External links "Chlorinated drinking water", IARC Monograph
EPA Maximum Contaminant Levels WebBook page for NH2Cl
Chlorine and chloramines in the freshwater aquarium