1. METHODS FOR THE PHOTOMETRIC DETERMINATION
OF REACTIVE BROMINE AND CHLORINE SPECIES WITH
ABTS
ULRICH PINKERNELL, BERND NOWACK*, HERVEÂ GALLARD and URS VON
GUNTEN6
Swiss Federal Institute for Environmental Science and Technology (EAWAG), Ueberlandstr. 133,
CH-8600, DuÈ bendorf, Switzerland
(First received 15 October 1999; accepted 10 March 2000)
AbstractÐNew methods for the determination of reactive bromine and chlorine species are presented.
Hypobromous acid (HOBr) and all three bromamines species (NH2Br, NHBr2, NBr3) are analyzed as a
sum parameter and hypochlorous acid (HOCl), monochloramine (NH2Cl) and chlorine dioxide (ClO2)
can be determined selectively. However, no distinction is possible between HOCl and the active
bromine species. The bromine and chlorine species react with ABTS (2,2-azino-bis(3-
ethylbenzothiazoline)-6-sulfonic acid-diammonium salt) to a green colored product that is measured at
405 or 728 nm. Free chlorine and NH2Cl can be measured in the presence of ozone. The method is
therefore suitable if combinations of disinfectants are used, such as chlorine/chlorine dioxide or
chlorine/ozone. In natural waters, the method provides a detection limit for all chlorine/bromine species
of less than 0.1 mM. The colored reaction product is very stable and allows a ®xation of the chlorine/
bromine species in the ®eld and subsequent determination of the absorption in the laboratory. 7 2000
Elsevier Science Ltd. All rights reserved
Key wordsÐhypobromous acid, bromamines, chlorine, chlorine dioxide, monochloramine, drinking
water, analysis
INTRODUCTION
Bromine species: Reactive bromine species, such as
hypobromous acid (HOBr) and bromamines
(NH2Br, NHBr2, NBr3) may occur due to two main
reasons during water disinfection: (1) they are used
directly as disinfectants; and (2) they are disinfec-
tion by-products during ozonation and chlorination
of bromide-containing waters:
1. HOBr is used for disinfection of swimming pool
water and for control of biofouling (Fisher et al.,
1999). Due to the presence of ammonia in these
applications, bromamines are formed immedi-
ately, which have comparable disinfection prop-
erties as HOBr (Floyd et al., 1978). All these
active bromine species show a higher toxicity to
biofouling organisms as their corresponding
chlorine analogues (Fisher et al., 1999).
2. HOBr and bromamines are formed as disinfec-
tion by-products when ozone or chlorine is
applied to treat bromide-containing natural
waters. It has been shown in studies on bromate
formation during ozonation of bromide-contain-
ing natural waters that hypobromous acid is a
key intermediate (von Gunten and Hoigne ,
1994). In the presence of ammonia HOBr is sca-
venged in a fast reaction forming bromamine
(NH2Br) and dibromamine (NHBr2) (Inman and
Johnson, 1984), which are not oxidized to bro-
mate directly. Therefore, ammonia addition may
be one possible control option for bromate mini-
mization (Pinkernell and von Gunten, 1999). To
get an overall bromine mass balance during
water treatment, it is therefore necessary to
assess the fate of these reactive bromamines in
addition to HOBr.
HOBr and bromamines can be determined by a
DPD (diphenylendiamine)-based method. However,
the stability of the colored reaction product can be
expected to be low as described for the chlorine
analysis (Jandik and EichelsdoÈ rfer, 1980). In ad-
dition, this method requires internal calibration
using standard solutions of HOBr and bromamines.
Wat. Res. Vol. 34, No. 18, pp. 4343±4350, 2000
7 2000 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
0043-1354/00/$ - see front matter
4343
www.elsevier.com/locate/watres
PII: S0043-1354(00)00216-5
*
Present address: Institute of Terrestrial Ecology (ITOÈ ),
Swiss Federal Institute of Technology (ETH),
Grabenstr. 11a, CH-8952 Schlieren, Switzerland.
6Author to whom all correspondence should be addressed.
Tel.: +41-1-823-5270; fax: +41-1-823-5028; e-mail:
vongunten@eawag.ch

2. This is dicult to carry out due to the low stability
of these reactive compounds.
Chlorine species: Chlorine, consisting of hypo-
chlorous acid (HOCl) and of hypochlorite ion
(ClOÀ
) at neutral pH, and chlorine dioxide are used
worldwide in large quantities for disinfection.
Therefore, numerous analytical techniques are avail-
able for their determination (APHA, 1989). The
need for another analytical method to determine
one of the disinfectants alone is therefore not
urgent. However, chlorine and chlorine dioxide are
also increasingly used as a mixture in water treat-
ment to minimize the formation of by-products
(Katz et al., 1994). The exact analysis of these mix-
tures is troublesome. The standard procedure for
the distinction of the two oxidants consists in the
addition of glycine to the sample, which masks
chlorine by formation of chloroaminoacetic acid
(APHA, 1989). Chlorine dioxide can then be
measured selectively by a colorimetric method and
the chlorine concentration is calculated by di€er-
ence of the total oxidant concentration and chlorine
dioxide. Chlorite, which is always present when
chlorine dioxide is added to water, is also measured
and further complicates the analysis (APHA, 1989).
A method to distinguish between bromine or chlor-
ine and chlorine dioxide has also been described
(Emerson, 1994). However, this method has a detec-
tion limit of about 0.5 mg lÀ1
Cl2, which is not suf-
®ciently sensitive for the analysis of drinking water
samples. The method also uses the addition of gly-
cine to suppress free chlorine. It has been reported,
however, that glycine interferes with the ClO2
measurement in the DPD method (Jandik and
EichelsdoÈ rfer, 1980; Palin, 1975). Alternatively,
chlorine dioxide can be purged with nitrogen from
the water (Aieta et al., 1984). This method is very
time-consuming because every sample has to be
purged for more than 30 min.
Photometric methods for the selective determi-
nation of chlorine dioxide with no interference from
chlorine have been described (Hofmann et al., 1998;
Sweetin et al., 1996). It is, however, not possible to
also analyze chlorine using one of these methods.
The need for a simple method to distinguish
between chlorine and chlorine dioxide at low con-
centrations is therefore given. A method that uses
only one reagent produces a stable color and allows
the distinction between chlorine, chloramine and
chlorine dioxide without interference of chlorite,
should be found. The same need is given for reac-
tive bromine species. As there are no applications
where signi®cant concentrations of bromine and
chlorine species are present in the water treatment
®eld, a distinction between chlorine and bromine is
not necessary.
We have found that ABTS (2,2-azino-bis(3-ethyl-
benzothiazoline)-6-sulfonic acid-diammonium salt),
a color reagent, ful®lls these requirements. ABTS is
a well-known substrate for enzymatic peroxide tests
(Bergmeyer, 1986) and percarboxylic acid analysis
(Pinkernell et al., 1997). The colorless ABTS is oxi-
dized undergoing a one-electron transfer as depicted
in Scheme 1. The reaction product ABTSÁ+
is a
stable green colored radical. It has a broad absor-
bance spectrum containing several maxima with
high molar absorptivities at 415, 650, 728, and
815 nm (Scott et al., 1993).
It has been shown before that because the reac-
tion with ABTS is fast and has a known stoichi-
ometry, a direct calculation of the analyte using the
molar absorptivity of the colored oxidation product
is possible (Pinkernell et al., 1997).
EXPERIMENTAL SECTION
Materials
All water used was from a Barnstead B-Pure system. All
chemicals have been of p.a. quality (Fluka) if not stated
otherwise. ABTS (2,2-azino-bis(3-ethylbenzothiazoline)-6-
sulfonic acid-diammonium salt, Aldrich) stock solutions
(1 g/l) were used for all experiments and stored at 48C.
After a few weeks the ABTS solution had a slightly higher
blank absorption and was replaced. A 1 mM KI solution
(puriss p.a., Fluka, Buchs, Switzerland) was prepared
weekly. A phosphate bu€er (0.5 M) with a pH of 6.1 was
prepared from NaH2PO4 and NaOH. Dilution of this buf-
fer with the sample and the other reagents results in a pH
of 6.5 in the ®nal solution.
Stock solutions of chlorine free hypobromous acid
(HOBr) were prepared from a 0.8 mM solution of potass-
ium bromide (KBr) by addition of 1 mM ozone at pH 4
(10 mM phosphate bu€er). After 24 h, the residual ozone
was removed by purging the solution with nitrogen gas for
15 min. The HOBr solution was standardized by direct
photometric determination of the hypobromite (OBrÀ
) at
329 nm (E=332 MÀ1
cmÀ1
) (Troy and Margerum, 1991)
after adjusting the pH to 11 by sodium hydroxide. The
yield was typically 95% and the bromine solution was
stable for several days when stored at 48C.
All three bromamines, monobromamine (NH2Br),
dibromamine (NHBr2) and tribromamine (NBr3), were
prepared from HOBr and ammonia according to Inman
and Johnson (1984) and Galal-Gorchev and Morris
(1965). They were checked for their purity by direct
measurements of their characteristic UV-spectra. No pure
solutions of NHBr2 could be prepared. They always con-
Scheme 1. Chemical structure of 2,2-azino-bis(3-ethylben-
zothiazoline)-6-sulfonate (ABTS) and its oxidation product
(ABTSÁ+
).
Ulrich Pinkernell et al.4344

3. tained small quantities of NH2Br and NBr3 (approx.
10%).
A sodium hypochlorite solution (5% in water, Fluka)
was used to prepare stock solutions of aqueous chlorine,
which was standardized by UV-spectrometry via the for-
mation of triiodide (Bichsel and von Gunten, 1999).
Chlorine dioxide (ClO2) was prepared from K2S2O8 and
NaClO2 as described by Gates (1997). Solutions of ClO2
were standardized by direct UV-measurement at 359 nm
using a molar absorptivity of 1200 MÀ1
cmÀ1
(Hoigne and
Bader, 1994) and also by formation of triiodide (Bichsel
and von Gunten, 1999). The two methods for ClO2 gave
similar results (e.g. direct UV: 1.83 mM ClO2; via triio-
dide: 1.84 mM ClO2).
Monochloramine (NH2Cl) was prepared by adding
HOCl to NH4NO3 (pH 8) at a molar ratio of 1:5. The
reaction is complete within seconds. Such solutions were
prepared daily.
Apparatus
All photometric measurements were made on a spectro-
photometer (UVIKON 940, Kontron Instruments) in 1, 5,
or 10 cm quartz cells. Kinetic experiments were done in
the spectrophotometric cell and absorbance data were
automatically recorded every second.
Analytical conditions
A detection wavelength of 405 nm instead of the near
absorption maximum at 415 nm was chosen because the
absorbance spectrum of the ABTSÁ+
provides a broad
shoulder at 405 nm and optical ®lters for portable photo-
meters are obtainable for this wavelength.
Table 1 gives an overview of the conditions for the
analysis of the bromine and chlorine species as described
herein. The following procedures can be simply adapted to
di€erent analyte concentrations by changing the sample
volume, ABTS concentration and the pathlength of the
measuring cell.
Determination of HOBr with ABTS. The procedure is
described for a low concentration range of 1±20 mM
HOBr. In a 25 ml volumetric ¯ask, 10 ml of the HOBr
sample is added to 1 ml of the ABTS solution (1 g lÀ1
)
and 1 ml 0.05 M H2SO4 and diluted with H2O. The
absorption at 405 nm is measured in a 1 cm cell after
1 min. The HOBr concentration is calculated from the
molar absorptivity of the ABTSÁ+
E(405 nm)=31,600 MÀ1
cmÀ1
(Pinkernell et al., 1997).
Determination of bromamines with ABTS. Same as for
HOBr. The concentration ranges decrease for di- and tri-
bromamine due to their higher degree of bromination.
Determination of HOBr as Br3
À
. For stoichiometry
check the concentration range of 0.07±0.7 mM HOBr,
0.5 ml of the sample is added to 8 ml of 1 M NaBr and
1 ml of 0.05 M H2SO4. After dilution with water to 10 ml,
the absorbance of the Br3
À
is measured at 260 nm in a 1 cm
cell (E=37,200 MÀ1
cmÀ1
; Beckwith and Margerum,
1997).
All chlorine analyses. For low chlorine concentrations,
20 ml of sample are added to a 25 ml volumetric ¯ask.
The resulting total available chlorine should be less than
20 mM. The absorbance is recorded at 405 or 728 nm,
10 min after addition of ABTS because the reaction is
slower than the corresponding with HOBr.
Determination of total chlorine. One milliliter ABTS sol-
ution (1 g lÀ1
), 0.15 ml KI solution (1 mM) and 3 ml
phosphate bu€er (pH 6.1) are added to a 25 ml volumetric
¯ask. The water sample is added and the ¯ask is ®lled
with chlorine-free water to 25 ml. 0.05 ml NH4NO3
(0.1 M) can additionally be added to the ¯ask together
with ABTS to avoid consumption of free chlorine by natu-
ral organic matter (see below).
Determination of monochloramine. 0.05 ml NaNO2
(0.1 M) is added to a 25 ml volumetric ¯ask to quench
HOCl or ClO2 (Table 1). If the sample has a pH of less
than 7, bu€er has to be added to raise the pH to 8 for
nitrite addition. Then, the water sample is added. After
4 min, 1 ml ABTS (1 g lÀ1
), 3 ml phosphate bu€er
(pH 6.1) and 0.15 ml KI (1 mM) are added and the ¯ask
is ®lled with chlorine-free water to 25 ml.
Determination of chlorine dioxide (for explanation see
below). 0.2 ml glycine (50 g lÀ1
), 0.2 ml HgII
Cl2 (3 g lÀ1
),
3 ml phosphate bu€er (pH 6.1) and 1 ml ABTS (1 g lÀ1
)
are added to a 25 ml volumetric ¯ask. Then, the water
sample is added and the ¯ask is ®lled with chlorine-free
water to 25 ml. The absorbance is read after 2 min.
Determination of total chlorine in the presence of ozone.
0.05 ml NH4NO3 (0.1 M) is added to a 25 ml ¯ask before
addition of the water sample. After 1 min, 0.05 ml NaNO2
(0.1 M) is added to quench ozone. Again after 1 min, 1 ml
ABTS (1 g lÀ1
), 3 ml bu€er (pH 6.1) and 0.15 ml KI
(1 mM) are added and the ¯ask is ®lled with water to
25 ml.
The blanks (not chlorinated natural water) prepared as
described above have absorptions of <0.001 in a 5 cm cell
at 728 nm.
RESULTS AND DISCUSSION
HOBr and bromamines
HOBr, NH2Br, NHBr2 and NBr3 react with
ABTS as follows:
HOBr ‡ 2ABTS ‡ H‡
4BrÀ
‡ 2ABTSÁ‡
‡ H2O …1†
NH2Br ‡ 2ABTS ‡ 2H‡
4BrÀ
‡ 2ABTSÁ‡
‡ NH‡
4 …2†
NHBr2 ‡ 4ABTS ‡ 3H‡
42BrÀ
‡ 4ABTSÁ‡
‡ NH‡
4 …3†
NBr3 ‡ 6ABTS ‡ 4H‡
43BrÀ
‡ 6ABTSÁ‡
‡ NH‡
4 …4†
In reactions 1±4 bromine is in the oxidation state
Br(+I) and therefore oxidizes two molecules of
Table 1. Overview of the conditions for analysis of bromine and chlorine species with ABTS
Analyte Speci®c conditions and scavenger added Bu€er/acid pH Catalyst
HOBr, bromamines ± Sulfuric acid 2 ±
HOCl+NH2Cl+ClO2 ± Phosphate 6.5 Iodide
NH2Cl Nitrite for HOCl and ClO2 (pH 8) Phosphate 6.5 Iodide
HOCl+NH2Cl in the presence of O3 (1) NH4
+
for HOCl stabilization. (2) Nitrite for O3 Phosphate 6.5 Iodide
ClO2 Glycine for HOCl; HgII
for trace of iodide Phosphate 6.5 ±
Analysis of bromine and chlorine species 4345

4. ABTS. At pH 2, all reactions were found to be
quantitative within seconds.
To verify the stoichiometry of the oxidation of
ABTS according to equations (1)±(4), the analysis
of di€erent HOBr solutions using ABTS has been
compared with the results obtained by an additional
analysis method: in excess of bromide, HOBr oxi-
dizes bromide to bromine (Br2). Under strong acidic
conditions and in excess of bromide, bromine forms
tribromide …BrÀ
3 ). Both series of HOBr analysis in
the range of 0.07±0.7 mM HOBr showed results
with a 100% consistence, while the correlation coef-
®cients of both calibrations were 0.999 or higher.
A check of the degree of reaction for the three
bromamines is more dicult because a direct
photometric quanti®cation of the prepared broma-
mines is inaccurate due to overlapping bands.
Therefore, the concentration of the hypobromous
acid, used for the preparation of all three broma-
mines, was taken as a reference, because the rapid
and stoichiometric reaction of HOBr with ammonia
leads to the di€erent bromamines without loss of
active bromine (Br(+I)). Figure 1 shows the cali-
bration graphs for all three bromamines directly
after preparation at low concentrations from 1 to
15 mM active bromine (Br(+I)). The recoveries of
95% (NH2Br), 84% (NHBr2) and 86% (NBr3)
result from the instability of bromamines, in par-
ticular NHBr2 and NBr3 (Inman and Johnson,
1984).
Therefore, ABTS is a suitable reagent for the de-
termination of hypobromous acid and all three bro-
mamines in drinking and swimming pool waters.
However, due to their similar reactivity, it is not
possible to determine the di€erent species selec-
tively. In case of a mixture of the bromine com-
pounds, the obtained result must be taken as a sum
parameter for the total concentration of the active
bromine Br(+I). The quanti®cation of these bro-
mine species with ABTS in the presence of chlorine
is not possible due to their similar reactivity.
Total available chlorine
At pH 2, chlorine species (HOCl, NH2Cl, ClO2
and ClOÀ
2 † react rapidly with ABTS and no distinc-
tion between the di€erent species is possible:
HOCl ‡ 2ABTS ‡ H‡
4ClÀ
‡ 2ABTSÁ‡
‡ H2O …5†
NH2Cl ‡ 2ABTS ‡ 2H‡
4ClÀ
‡ 2ABTSÁ‡
‡ NH‡
4 …6†
ClO2 ‡ ABTS4ClOÀ
2 ‡ ABTSÁ‡
…7†
ClOÀ
2 ‡ 4ABTS ‡ 4H‡
4ClÀ
‡ 4ABTSÁ‡
‡ 2H2O …8†
However, at pH 6.5 the di€erent species can be dis-
tinguished: chlorite does not react with ABTS,
HOCl and NH2Cl slowly and ClO2 very rapidly. In
the presence of low iodide concentrations (6 mM)
the reaction with HOCl and NH2Cl is fast and
complete within a few minutes. Iodide is oxidized to
Fig. 1. Calibration for NH2Br (R, 0.71±14 mM), NHBr2
(W, 0.39±7.6 mM) and NBr3 (*, 0.26±5.1 mM) using
ABTS. Values are given for active bromine (Br(+I)).
Fig. 2. Kinetics of the reaction of the chlorine species
(12.2 mM HOCl, 12.2 mM NH2Cl, 8.4 mM ClO2, 10 mM
NaClO2) with ABTS (405 nm, 5 cm cell) under the di€er-
ent conditions speci®ed in the methods section. (a) In the
presence of iodide (6 mM). (b) In the presence of glycine
and HgII
.
Ulrich Pinkernell et al.4346

5. hypoiodous acid, which reacts quickly with ABTS
keeping the original stoichiometry. The sum of all
three chlorine species can therefore be determined
in the presence of iodide at pH 6.5. Figure 2(a)
shows the kinetics of the color formation at 208C
for 12.2 mM HOCl, 12.2 mM NH2Cl, 8.4 mM ClO2
and 10 mM ClO2
À
(pH 6.5, 6 mM iodide). The reac-
tion of ABTS with ClO2 is complete after 2 min,
whereas it takes 6 min for HOCl and NH2Cl. All
three reactions are not very sensitive to tempera-
ture. At 58C they proceed with almost the same
rate. A reaction time of 10 min is therefore su-
cient for all temperatures.
Ammonia can be added in excess for the analysis
to avoid a fast consumption of free chlorine by or-
ganic matter during the time of color development.
The free chlorine is then rapidly converted to
monochloramine at pH 8. Monochloramine reacts
much slower with NOM in the water than free
chlorine (Qualls and Johnson, 1983) and therefore
no consumption of chlorine takes place.
In the presence of bromide HOBr is formed from
chlorine. The determination of 10 mM of hypochlor-
ous acid in the presence of various concentrations
of bromide (0±20 mM) resulted in a constant
amount of oxidized ABTS although the concen-
tration of HOCl was decreased. Analysis of total
available chlorine then gives the sum of the concen-
tration of reactive bromine and chlorine species.
Determination of the NH2Cl fraction
To selectively determine NH2Cl, both HOCl and
ClO2 are destroyed by the addition of nitrite prior
to ABTS addition. Nitrite rapidly reacts with ClO2
and HOCl but only slowly with NH2Cl. Nitrite
does not react with ABTS at pH 6.5 and does not
interfere in the determination of NH2Cl fraction
(Fig. 3). Four minutes are sucient for a complete
destruction of ClO2 and HOCl. Afterwards, ABTS
and iodine can be added. The rate of the reaction
between NH2Cl and NO2
À
increases with decreasing
pH. Therefore, to avoid signi®cant losses for an
extended reaction time waters with pH <8 have to
be bu€ered to pH 8 before nitrite addition. The
NH2Cl fraction can be calculated directly from the
absorption at either 405 or 728 nm using the corre-
sponding molar absorptivities at pH 6.5 and stoichi-
ometry according to equation (6).
When the ratio Cl/N increases dichloramine
(NHCl2) is also formed. Dichloramine was analyzed
at less than 10% of its concentration in the mono-
chloramine fraction and no condition was found for
its selective analysis.
In natural and sewage waters, some free and
combined amino acids and some heterocyclic bases
react with chlorine to form N-chloroamino acids
and N-chloroheterocyclic compounds that oxidize
iodide to iodine (Maccrehan et al., 1998). Therefore,
these compounds are also analyzed in the combined
chlorine fraction. For example, chlorination of an
excess of glycine, arginine and alanine showed that
the three N-chloroamino acids are detected to
100% in the monochloramine fraction with the
ABTS method. The monochloramine fraction can
therefore be overestimated in waters containing
high concentrations of organic nitrogen.
Selective determination of ClO2
For a selective analysis of ClO2, glycine can be
added to scavenge any HOCl under formation of
chloroaminoacetic acid. In addition, HgII
is added
to mask traces of iodide by complexation (Jensen
and Johnson, 1990). Iodide present in the sample or
in the phosphate bu€er would otherwise catalyze
the reaction of chloroaminoacetic acid and NH2Cl
with ABTS. In the presence of HgII
, the reaction of
ABTS with chloroaminoacetic acid and NH2Cl is
extremely slow, whereas the reactivity of ClO2 is
not a€ected. Figure 2(b) depicts the kinetics of the
color formation at 208C for 12.2 mM HOCl,
12.2 mM NH2Cl, 8.4 mM ClO2 and 10 mM ClO2
À
(pH 6.5, 0.4 g lÀ1
glycine, 0.024 g lÀ1
HgII
). The
reaction with ClO2 is complete after 2 min, while
HOCl, NH2Cl, NHCl2 and ClO2
À
do not react.
Figure 3 also shows that iodide, glycine and HgII
have no e€ect on the ClO2 calibration. ClO2 can be
calculated directly from the absorption at either 405
or 728 nm using the corresponding molar absorptiv-
ities at pH 6.5 and according to the stoichiometry
of equation (7).
Determination of free chlorine
The concentration of free chlorine is calculated
by subtracting monochloramine and chlorine diox-
ide from the total available chlorine fraction. There-
fore, for small fraction of HOCl, the precision for
Fig. 3. Calibration for HOCl (R, 1.5±6.1 mM), NH2Cl
(W,1.5±6.7 mM) and ClO2 (T, 2.1±10.5 mM) using ABTS
(calibration curves are given for solution of the single
compound). The in¯uence of NOÀ
2 on NH2Cl (Q) and the
in¯uence of glycine/HgII
on ClO2 (*) are also shown.
Analysis of bromine and chlorine species 4347

6. its quanti®cation can be greatly a€ected and should
be determined for each condition.
Determination of free chlorine and monochloramine
fraction in ozonation processes
If waters containing a chlorine residual are ozo-
nated, both chlorine and ozone may be present in
the water at the same time. Ozone also reacts with
ABTS and therefore interferes with the analysis of
the chlorine residual. Free chlorine and mono-
chloramine in ozonation processes can be measured
by the following procedure:
1. Addition of ammonia to the sample transforms
free chlorine to NH2Cl. This compound only
reacts slowly with ozone.
2. Addition of nitrite, which rapidly reacts with
ozone, therefore, destroys a residual of this oxi-
dant. The free chlorine can then be measured as
NH2Cl without interference from ozone. NH2Cl
initially present in the sample is quanti®ed with-
out the addition of ammonia.
Molar absorptivity of ABTSÁ+
The molar absorptivity E of ABTSÁ+
has been
previously determined at pH 0 (15,000 MÀ1
cmÀ1
at
728 nm; Scott et al., 1993) and pH 2 (31,600 MÀ1
cmÀ1
at 405 nm; Pinkernell et al., 1997). For the
analysis of the bromine species, the value deter-
mined at pH 2 was used.
To mimic our experimental conditions, we have
determined the value of E at pH 6.5 with standar-
dized solutions of HOCl and ClO2. E was found to
be 28,5002950 MÀ1
cmÀ1
at 405 nm (n = 33) and
13,0002440 MÀ1
cmÀ1
at 728 nm (n = 28). No sig-
ni®cant di€erence was found between HOCl and
ClO2. These values of E are 14% (728 nm) and 10%
(405 nm) lower than at low pH.
Stability of ABTSÁ+
The colored ABTSÁ+
decays with a ®rst-order
kinetic. At room temperature and in pure water, its
half-life is approximately 47 h. Therefore, the decay
in the absorption is less than 1 and 5% after 30 min
and for 5 h of storage, respectively. At 48C, the
half-life increases to a value of 357 h. Under these
conditions, it can be stored for 24 h with less than
3% of decrease in the absorption.
Linear range, detection limits and precision
The linear ranges for these analytical methods
using ABTS depend on the analytical procedure.
Concentrations in the lower micromolar range have
been chosen for all analytes to describe their linear
ranges. These ranges can be shifted to higher con-
centrations by decreasing the sample volume and
increasing the pathlength of the measuring cell.
The linear range for the determination of HOBr
in a 1 cm cell was found to be 1±20 mM HOBr with
a correlation coecient of r 2
> 0.999 for 10
samples. Figure 1 shows the calibration graphs for
mono-, di- and tribromamine, depicted as recovery
of active bromine (Br(+I)) for a concentration
range of 1±15 mM Br(+I). A high linearity (r 2
>
0.999 each) was determined.
The detection limits for all bromine species were
determined as three times the variability of the
blank signal using a 1 cm cell and were found to
be: HOBr and NH2Br 0.05 mM, NHBr2 0.03 mM
and NBr3 0.02 mM. The precision for the analysis
of all bromine species was determined by the analy-
sis of 10 samples each and resulted in a standard
deviation of 1% for all bromine species.
Calibration graphs for HOCl, NH2Cl, and ClO2
are linear in the range 1±10 mM (0.07±0.7 mg lÀ1
chlorine) with correlation coecients of 0.998 or
better, using a 1 cm cell and a detection wavelength
of 405 nm (Fig. 3). Calibration curve for HOCl in
the low concentration range from 0.1 to 1.5 mM
(0.007±0.1 mg lÀ1
chlorine) measured at 728 nm in
a 5 cm cell also gives a good correlation coecient
(r2
> 0.999).
The detection limits for the di€erent chlorine
species were determined in river water as three
times the variability of the blank signal using a 5 cm
cell. At 728 nm it was found to be 0.04 mM for
HOCl and NH2Cl and 0.1 mM for ClO2. For all
natural water analyses, the absorption was
measured at 728 nm because the background
absorption of the water matrix is much lower than
at 405 nm. This results in a lower detection limit
despite the smaller value of E.
Standard deviation was found to be less than 4%
for all chlorine species in pure water (n = 33
samples). For mixtures of HClO, ClO2 and NH2Cl,
Fig. 4. Behavior of reactive chlorine species (HOCl,
NH2Cl, ClO2) during Limmat river water treatment
(Limmatwasserwerk ZuÈ rich, Switzerland). A mixture of
chlorine/chlorine dioxide is added to the river water. For
speci®c conditions see text.
Ulrich Pinkernell et al.4348

7. the detection limit and the precision for the deter-
mination of HOCl calculated as the di€erence
between the total active chlorine and the sum of
ClO2 and NH2Cl will depend on the concentrations
of each compound. In this case, precision can be
signi®cantly lower than for pure solution of HOCl.
Application to drinking water samples
The fate of reactive chlorine species at a drinking
water treatment plant in ZuÈ rich using chlorine/
chlorine dioxide as initial disinfectant has been
determined using the new method. A mixture of
chlorine/chlorine dioxide (4.3 mM (0.31 mg lÀ1
)/
1.3 mM (0.088 mg lÀ1
)) is added to the river water
(pH 8, 2 mM bicarbonate, 1.5 mg/l DOC, 15 mg lÀ1
ammonia (H1 mM), 0.2 mM BrÀ
), followed by sedi-
mentation, ozonation, and sand- and activated car-
bon ®ltration. Finally, a mixture of chlorine/
chlorine dioxide is added for disinfection again
(2.1 mM (0.149 mg lÀ1
)/0.7 mM (0.047 mg lÀ1
)).
Figure 4 shows a typical development of the con-
centrations of HOCl, ClO2 and NH2Cl through the
whole process. A rapid consumption of chlorine
and chlorine dioxide occurs between the addition
and the ®rst sampling point (t = 8 min) with the
concurrent formation of NH2Cl. After sedimen-
tation (t = 3 h) HOCl and ClO2 have almost
entirely reacted. At this point of the treatment,
NH2Cl is still present at a concentration of about
1 mM, which corresponds to the concentration of
ammonia in the river water. During ozonation
(HRT=30 min) and the subsequent activated car-
bon ®ltration, NH2Cl disappears completely. The
®nal disinfection results again in the formation of a
low concentration of NH2Cl.
Despite the small amount of added disinfectants
(0.4 mg as Cl2), their concentration can be easily
followed by the new method.
Compared to the added HOCl the BrÀ
concen-
tration in natural waters is low. Once HOBr is
formed from HOCl and BrÀ
, it is consumed much
more rapidly by natural organic matter compared
to chlorine. Therefore, the overestimation of free
chlorine by the cross-reactivity towards HOBr is
negligible.
CONCLUSIONS
The reaction of ABTS with BrI
and ClI
is strictly
stoichiometric, so the molar absorptivity E can be
used for the calculation of the concentration and a
calibration is not necessary.
The advantage of using ABTS over DPD consists
in the much higher stability of the color that is
formed. With ABTS, the di€erent oxidant species
can be ®xed in the ®eld and measured after transfer
to the laboratory.
The determination of hypobromous acid and all
three bromamines using ABTS was found to be re-
liable and accurate for concentrations down to
0.1 mM.
The selectivity of the reaction of ABTS towards
ClO2 and the other chlorine species allows a very
simple determination of ClO2. The detection limits
for the chlorine species are 0.07 mM for NH2Cl and
HOCl (0.005 mg lÀ1
Cl2) and 0.1 mM for ClO2
(0.007 mg lÀ1
). The main disadvantage of the
method is that free chlorine cannot be measured
independently. No conditions could be found where
HOCl reacts with ABTS but NH2Cl did not. There-
fore, free chlorine can only be determined by di€er-
ence. Moreover, no distinction is possible between
HOCl and HOBr.
AcknowledgementsÐU.P. thanks the Compagnie Ge ne rale
des Eaux, Paris, and B.N. and H.G. thank the
Waterworks ZuÈ rich for ®nancial support.
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