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  • 1. METHODS FOR THE PHOTOMETRIC DETERMINATION OF REACTIVE BROMINE AND CHLORINE SPECIES WITH ABTS ULRICH PINKERNELL, BERND NOWACK*, HERVE 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. 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