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Measurement of nitrate and chlorate in swimming pool water by capillary zone electrophoresis
Talanta 45 (1998) 657 – 661
Measurement of nitrate and chlorate in swimming pool water by capillary zone electrophoresis P. Wang a, S.F.Y. Li b, H.K. Lee a,* b
a Department of Chemistry, National Uni6ersity of Singapore, Kent Ridge, Singapore 119260, Singapore Department of Chemistry and Institute of Materials Research and Engineering, National Uni6ersity of Singapore, Kent Ridge, Singapore 119260, Singapore
Received 5 February 1997; accepted 31 March 1997
Abstract Capillary zone electrophoresis (CZE) of nitrate and chlorate in swimming pool water are described. Nitrate and chlorate were determined simultaneously with an indirect detection method in an electrolyte containing 10 mM chromate and 0.1 mM cetyltrimethylammonium bromide (CTAB). Where chloride concentration was so high that nitrate could not be determined satisfactorily because of interference, a direct detection technology was developed in which 10 mM sulfate and 0.1 mM CTAB were used as the buffer. The wavelength for indirect detection was 254 nm and 214 nm for direct detection. Relative standard deviations of the quantification of nitrate and chlorate in real samples were below 6%. The detection limits were 7 mg ml − 1 for chlorate, and 4 mg ml − 1 (indirect detection) and 0.4 mg ml − 1 (direct detection) for nitrate. © 1998 Elsevier Science B.V. Keywords: Nitrate; Chlorate; Capillary zone electrophoresis; Water analysis
1. Introduction Water analysis has an important place in the chemical analysis of environmental samples. The development of, and improvement in, methods for the analysis of water is a major task of analytical chemists. This is especially so since each type of water has its own specific constituents and any one procedure may not be completely amenable to different water-types.
* Corresponding author. Tel.: +65 7756666; fax: +65 7791691.
Swimming pool water should basically have the quality of drinking water. Swimming pool water generally has a higher concentration of chloride because chlorine or hypochlorous acid is the principal sterilizing agent used in such water. Because of the nature of its usage, it is desirable that swimming pool water be analyzed regularly to monitor its quality. According to the US National Interim Primary Drinking Water Regulations [1], the nitrate nitrogen level should be below 10 mg ml − 1. That is to say, the nitrate concentration should be below 45 mg ml − 1. No standard has been published for chlorate in drinking water.
0039-9140/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 9 - 9 1 4 0 ( 9 7 ) 0 0 2 9 6 - 8
658
P. Wang et al. / Talanta 45 (1998) 657–661
The principal methods for the analysis of nitrate in swimming pool water include colorimetry [2,3], titrimetry [2], ion-specific electrode electroanalysis [2] and ion chromatography (IC) [4]. Chlorate can be determined by iodometry [3] or by IC [4]. However, the high chloride content and free chlorine usually interferes with the determination of nitrate in the above chemical analysis methods; the former must be precipitated before determination [2]. These operations are time-consuming and are not suitable for automated analysis of large numbers of samples. IC is a powerful method for the analysis of anions in water [5]. However, nitrate and chlorate cannot be easily separated on IC columns because their interaction with ion exchangers are very similar. Nitrate can be determined by UV detection at 215 nm at which chlorate has little absorbance, thus minimize the interference posed by the later [4]. Mobile phase IC (MPIC) has been used to determine both ions [4]. In this method, 10% acetonitrile must be added to the eluent. In this communication, a new method for the simultaneous determination of nitrate and chlorate in swimming pool water by capillary zone electrophoresis (CZE) is described. Another more sensitive method for the analysis of trace nitrate by CZE is also described. This method is useful where chloride levels are so high as to cause interference with nitrate analysis. In general, the methods reported are simple, fast, reproducible and require no volatile organic solvent.
volume (pH =9.4). The buffer used in the direct CZE was prepared by diluting 1 ml of 100 mM sodium sulfate and 0.1 ml of 10 mM CTAB to 10 ml total volume (pH= 5.2). These working electrolytes were replaced daily. The standard nitrate and chlorate storage solutions (each 1000 mg ml − 1) were prepared by dissolving the appropriate amounts of the corresponding sodium salts in deionized water. All the above solutions were filtered through 0.45 mm membrane filters before use.
2.2. Apparatus A Lauerlabs CE system (Emmen, The Netherlands) equipped with a Bischoff (Leonberg, Germany) model 1000 UV detector and a Shimadzu
2. Experimental
2.1. Reagents Sodium nitrate, sodium chlorate, sodium chromate and anhydrous sodium sulfate were obtained from Fluka (Switzerland). Cetyltrimethylammonium bromide (CTAB) was purchased from Aldrich Chemical (Milwaukee, WI). All solutions were prepared using water purified by a Barnstead NANOpure system (Dubuque, IA). The buffer used in the indirect CZE was prepared by diluting 1 ml 100 mM sodium chromate and 0.1 ml of 10 mM CTAB to 10 ml total
Fig. 1. Electropherograms of (a) standard anions by indirect detection. Peaks: 1 =nitrate; 2 = chlorate. (b) A real swimming pool water sample by indirect detection. Peaks: 1 = nitrate; 2 = chlorate; 3=chloride; 4= sulfate. Separation conditions: 10 mM chromate and 0.1 mM CTAB (pH =9.4), −20 kV, 65 cm ×50 mm fused silica (52 cm to detector), 254 nm, 0.1 min injection time under 50 mbar, ambient temperature.
P. Wang et al. / Talanta 45 (1998) 657–661
659
set at 20 kV (negative mode). The detector was set at 254 and 214 nm for the indirect and direct modes, respectively. Hydrostatic injection was realized in 0.1 min under a pressure of 50 mbar. All experiments were carried out at ambient temperature (22–25°C).
3. Results and discussion
3.1. Analytical conditions
Fig. 2. Electropherograms of (a) standard anions by direct detection. Peak: 1 =nitrate. (b) A real sample by direct detection. Peaks: 1 =nitrate; 5 =not identified. Separation conditions: 10 mM sulfate and 0.1 mM CTAB (pH =5.2), −20 kV, 65 cm× 50 Bm fused silica (52 cm to detector), 214 nm, 0.1 min injection time under 50 mbar, ambient temperature.
(Tokyo, Japan) C-R6A integrator were used in this study. A 65 cm× 50 mm i.d. uncoated fused-silica capillary with the detection window placed 52 cm from the injection end was used. The voltage was
Because of the high ratio of the concentration of chloride to nitrate and chlorate, analytical conditions must be chosen carefully in order to determine nitrate and chlorate simultaneously by indirect detection. A series of solutions which contained 10 mM chromate and 0.1 mM CTAB with different pH values, adjusted by 0.1 M NaOH or 0.1 M H2SO4, were used as buffer solutions. A buffer at pH=9 gave the best results. Fig. 1a shows the electropherogram of a standard solution of nitrate and chlorate, while Fig. 1b is an electropherogram of a real swimming pool water sample. As shown in Fig. 1b, the nitrate peak is much smaller than that of chloride. Greater sensitivity and accuracy for nitrate analysis could be achieved by direct detection, in which nitrate was monitored at 214 nm. Direct detection is easy to perform, because chloride, sulfate and chlorate have little response at 214 nm whereas nitrate has a strong absorbance signal. Except for the change of buffer from chromate to sulfate and wavelength from 254 to 214 nm, the other conditions for direct detection method were the same as those for the indirect method. The time needed to re-equilibrate the column was very short. The column was rinsed for 10 min by water under
Table 1 Quantitative results for nitrate and chlorate in water from three open-air swimming pools Sample
n
Chlorate (mg ml−1)
Nitrate (by indirect detection) (mg ml−1)
Nitrate (by direct detection) (mg ml−1)
1 2 3
3 3 3
14.59 0.40 22.39 0.58 12.69 0.33
8.789 0.31 18.29 0.44 24.19 0.68
9.98 90.12 17.390.21 23.3 90.23
P. Wang et al. / Talanta 45 (1998) 657–661
660
Table 2 Recoveries of nitrate and chlorate from a real sample Analyte
n
Added (mg ml−1)
Found (mg ml−1)
Recovery (%)
Chlorate
3
5.0 15.0 25.0
5.53 9 0.17 15.5 90.32 25.7 90.66
111 104 103
Nitrate (by indirect detection)
3
5.0 15.0 25.0
4.36 9 0.16 15.1 90.45 25.7 9 0.92
87.2 101 103
Nitrate (by direct detection)
3
2.0 6.0 10.0
2.06 9 0.03 5.69 9 0.08 9.98 90.09
103 94.8 99.8
y = 47.02x + 465.9
R 2 = 0.9917
pressure (2000 mbar), and then for another 10 min with sulfate buffer. Fig. 2a shows an electropherogram of standard nitrate and Fig. 2b a typical electropherogram of swimming pool water, both obtained by the direct detection method.
3.2. Quantification As shown in Fig. 2, the peak tailing in the standard nitrate solution was more significant than that in the sample. This is probably due to the difference in conductivity between the standard and the real samples. In order to eliminate this matrix effect, the quantitative method of standard additions was used in the chlorate and nitrate analyses. The linear equations for one of the real samples, which were constructed by adding standard solutions with different concentrations of chlorate or nitrate to the sample, are as follows: chlorate (added range: 0 – 25 mg ml − 1) y= 21.32x + 314.0
R 2 =0.9939
nitrate (indirect method, added range: 0–25 mg ml − 1)
nitrate (direct method, added range: 0–10 mg ml − 1) y= 370.9x +1043
R 2 = 0.9943
where y is the peak height, and x is the concentration (mg ml − 1). The negative intercepts of the above regression lines represent the amount of the corresponding analytes in the sample. Samples from three open-air swimming pools were analyzed using the above quantification method. The results are listed in Table 1.
3.3. Reco6ery, precision and detection limits The results of recovery tests are shown in Table 2. The mean recoveries of nitrate and chlorate added to swimming pool water were 87.2–103 and 103–111%, respectively. The intra- and inter-assay precisions were evaluated as coefficients of variation (C.V.) which are listed in Table 3. The values were all less than 6%. The detection limits were 7 mg ml − 1 for chlorate, and 4 mg ml − 1 for nitrate by the indirect
Table 3 The precision of the methods Analyte
n
Intra-day C.V. (%)
Inter-day C.V. (%)
Chlorate Nitrate (by indirect detection) Nitrate (by direct detection)
5 5 5
3.1 4.2 1.6
5.1 5.5 2.2
P. Wang et al. / Talanta 45 (1998) 657–661
method and 0.4 mg ml − 1 by the direct method, when the signal-to-noise ratio was 3:1.
661
Acknowledgements The authors thank the National University of Singapore for financial support.
4. Conclusion This paper demonstrates the use of CZEbased methods for the analysis of nitrate and chlorate in swimming pool water. The two anions of interest may be separated and determined simultaneously even if the chloride content in the real sample is much higher than each of the two ions. Trace amounts of nitrate can be analyzed by a direct detection method without any interference. The CZE methods are convenient, sensitive, reproducible and require no pernicious volatile organic solvent. CZE can be used in the routine monitoring of these pollutants in swimming pool water.
.
References [1] National Interim Primary Drinking Water Regulation, US EPA Publication No. 570/9-76-003, 1976. [2] K. Holl, Water — Examination Assessment Conditioning Chemistry Bacteriology Biology, Walter de Gruyter, Berlin, Germany, 1972, 389 pp. [3] J.A. Beech, R. Diaz, C. Ordaz, B. Palomeque, Am. J. Public Health 70 (1980) 79. [4] E.L. Johnson, K.K. Haak, Liquid Chromatography in Environmental Analysis, Humana, Clifton, NJ, 1983, 291 pp. [5] O.A. Shpigun, Y.A. Zolotov, Ion Chromatography in Water Analysis, Ellis Horwood, Chichester, UK, 1988, 188 pp.
Measurement of nitrate and chlorate in swimming pool water by capillary zone electrophoresis P. Wang a, S.F.Y. Li b, H.K. Lee a,* b
a Department of Chemistry, National Uni6ersity of Singapore, Kent Ridge, Singapore 119260, Singapore Department of Chemistry and Institute of Materials Research and Engineering, National Uni6ersity of Singapore, Kent Ridge, Singapore 119260, Singapore
Received 5 February 1997; accepted 31 March 1997
Abstract Capillary zone electrophoresis (CZE) of nitrate and chlorate in swimming pool water are described. Nitrate and chlorate were determined simultaneously with an indirect detection method in an electrolyte containing 10 mM chromate and 0.1 mM cetyltrimethylammonium bromide (CTAB). Where chloride concentration was so high that nitrate could not be determined satisfactorily because of interference, a direct detection technology was developed in which 10 mM sulfate and 0.1 mM CTAB were used as the buffer. The wavelength for indirect detection was 254 nm and 214 nm for direct detection. Relative standard deviations of the quantification of nitrate and chlorate in real samples were below 6%. The detection limits were 7 mg ml − 1 for chlorate, and 4 mg ml − 1 (indirect detection) and 0.4 mg ml − 1 (direct detection) for nitrate. © 1998 Elsevier Science B.V. Keywords: Nitrate; Chlorate; Capillary zone electrophoresis; Water analysis
1. Introduction Water analysis has an important place in the chemical analysis of environmental samples. The development of, and improvement in, methods for the analysis of water is a major task of analytical chemists. This is especially so since each type of water has its own specific constituents and any one procedure may not be completely amenable to different water-types.
* Corresponding author. Tel.: +65 7756666; fax: +65 7791691.
Swimming pool water should basically have the quality of drinking water. Swimming pool water generally has a higher concentration of chloride because chlorine or hypochlorous acid is the principal sterilizing agent used in such water. Because of the nature of its usage, it is desirable that swimming pool water be analyzed regularly to monitor its quality. According to the US National Interim Primary Drinking Water Regulations [1], the nitrate nitrogen level should be below 10 mg ml − 1. That is to say, the nitrate concentration should be below 45 mg ml − 1. No standard has been published for chlorate in drinking water.
0039-9140/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 9 - 9 1 4 0 ( 9 7 ) 0 0 2 9 6 - 8
658
P. Wang et al. / Talanta 45 (1998) 657–661
The principal methods for the analysis of nitrate in swimming pool water include colorimetry [2,3], titrimetry [2], ion-specific electrode electroanalysis [2] and ion chromatography (IC) [4]. Chlorate can be determined by iodometry [3] or by IC [4]. However, the high chloride content and free chlorine usually interferes with the determination of nitrate in the above chemical analysis methods; the former must be precipitated before determination [2]. These operations are time-consuming and are not suitable for automated analysis of large numbers of samples. IC is a powerful method for the analysis of anions in water [5]. However, nitrate and chlorate cannot be easily separated on IC columns because their interaction with ion exchangers are very similar. Nitrate can be determined by UV detection at 215 nm at which chlorate has little absorbance, thus minimize the interference posed by the later [4]. Mobile phase IC (MPIC) has been used to determine both ions [4]. In this method, 10% acetonitrile must be added to the eluent. In this communication, a new method for the simultaneous determination of nitrate and chlorate in swimming pool water by capillary zone electrophoresis (CZE) is described. Another more sensitive method for the analysis of trace nitrate by CZE is also described. This method is useful where chloride levels are so high as to cause interference with nitrate analysis. In general, the methods reported are simple, fast, reproducible and require no volatile organic solvent.
volume (pH =9.4). The buffer used in the direct CZE was prepared by diluting 1 ml of 100 mM sodium sulfate and 0.1 ml of 10 mM CTAB to 10 ml total volume (pH= 5.2). These working electrolytes were replaced daily. The standard nitrate and chlorate storage solutions (each 1000 mg ml − 1) were prepared by dissolving the appropriate amounts of the corresponding sodium salts in deionized water. All the above solutions were filtered through 0.45 mm membrane filters before use.
2.2. Apparatus A Lauerlabs CE system (Emmen, The Netherlands) equipped with a Bischoff (Leonberg, Germany) model 1000 UV detector and a Shimadzu
2. Experimental
2.1. Reagents Sodium nitrate, sodium chlorate, sodium chromate and anhydrous sodium sulfate were obtained from Fluka (Switzerland). Cetyltrimethylammonium bromide (CTAB) was purchased from Aldrich Chemical (Milwaukee, WI). All solutions were prepared using water purified by a Barnstead NANOpure system (Dubuque, IA). The buffer used in the indirect CZE was prepared by diluting 1 ml 100 mM sodium chromate and 0.1 ml of 10 mM CTAB to 10 ml total
Fig. 1. Electropherograms of (a) standard anions by indirect detection. Peaks: 1 =nitrate; 2 = chlorate. (b) A real swimming pool water sample by indirect detection. Peaks: 1 = nitrate; 2 = chlorate; 3=chloride; 4= sulfate. Separation conditions: 10 mM chromate and 0.1 mM CTAB (pH =9.4), −20 kV, 65 cm ×50 mm fused silica (52 cm to detector), 254 nm, 0.1 min injection time under 50 mbar, ambient temperature.
P. Wang et al. / Talanta 45 (1998) 657–661
659
set at 20 kV (negative mode). The detector was set at 254 and 214 nm for the indirect and direct modes, respectively. Hydrostatic injection was realized in 0.1 min under a pressure of 50 mbar. All experiments were carried out at ambient temperature (22–25°C).
3. Results and discussion
3.1. Analytical conditions
Fig. 2. Electropherograms of (a) standard anions by direct detection. Peak: 1 =nitrate. (b) A real sample by direct detection. Peaks: 1 =nitrate; 5 =not identified. Separation conditions: 10 mM sulfate and 0.1 mM CTAB (pH =5.2), −20 kV, 65 cm× 50 Bm fused silica (52 cm to detector), 214 nm, 0.1 min injection time under 50 mbar, ambient temperature.
(Tokyo, Japan) C-R6A integrator were used in this study. A 65 cm× 50 mm i.d. uncoated fused-silica capillary with the detection window placed 52 cm from the injection end was used. The voltage was
Because of the high ratio of the concentration of chloride to nitrate and chlorate, analytical conditions must be chosen carefully in order to determine nitrate and chlorate simultaneously by indirect detection. A series of solutions which contained 10 mM chromate and 0.1 mM CTAB with different pH values, adjusted by 0.1 M NaOH or 0.1 M H2SO4, were used as buffer solutions. A buffer at pH=9 gave the best results. Fig. 1a shows the electropherogram of a standard solution of nitrate and chlorate, while Fig. 1b is an electropherogram of a real swimming pool water sample. As shown in Fig. 1b, the nitrate peak is much smaller than that of chloride. Greater sensitivity and accuracy for nitrate analysis could be achieved by direct detection, in which nitrate was monitored at 214 nm. Direct detection is easy to perform, because chloride, sulfate and chlorate have little response at 214 nm whereas nitrate has a strong absorbance signal. Except for the change of buffer from chromate to sulfate and wavelength from 254 to 214 nm, the other conditions for direct detection method were the same as those for the indirect method. The time needed to re-equilibrate the column was very short. The column was rinsed for 10 min by water under
Table 1 Quantitative results for nitrate and chlorate in water from three open-air swimming pools Sample
n
Chlorate (mg ml−1)
Nitrate (by indirect detection) (mg ml−1)
Nitrate (by direct detection) (mg ml−1)
1 2 3
3 3 3
14.59 0.40 22.39 0.58 12.69 0.33
8.789 0.31 18.29 0.44 24.19 0.68
9.98 90.12 17.390.21 23.3 90.23
P. Wang et al. / Talanta 45 (1998) 657–661
660
Table 2 Recoveries of nitrate and chlorate from a real sample Analyte
n
Added (mg ml−1)
Found (mg ml−1)
Recovery (%)
Chlorate
3
5.0 15.0 25.0
5.53 9 0.17 15.5 90.32 25.7 90.66
111 104 103
Nitrate (by indirect detection)
3
5.0 15.0 25.0
4.36 9 0.16 15.1 90.45 25.7 9 0.92
87.2 101 103
Nitrate (by direct detection)
3
2.0 6.0 10.0
2.06 9 0.03 5.69 9 0.08 9.98 90.09
103 94.8 99.8
y = 47.02x + 465.9
R 2 = 0.9917
pressure (2000 mbar), and then for another 10 min with sulfate buffer. Fig. 2a shows an electropherogram of standard nitrate and Fig. 2b a typical electropherogram of swimming pool water, both obtained by the direct detection method.
3.2. Quantification As shown in Fig. 2, the peak tailing in the standard nitrate solution was more significant than that in the sample. This is probably due to the difference in conductivity between the standard and the real samples. In order to eliminate this matrix effect, the quantitative method of standard additions was used in the chlorate and nitrate analyses. The linear equations for one of the real samples, which were constructed by adding standard solutions with different concentrations of chlorate or nitrate to the sample, are as follows: chlorate (added range: 0 – 25 mg ml − 1) y= 21.32x + 314.0
R 2 =0.9939
nitrate (indirect method, added range: 0–25 mg ml − 1)
nitrate (direct method, added range: 0–10 mg ml − 1) y= 370.9x +1043
R 2 = 0.9943
where y is the peak height, and x is the concentration (mg ml − 1). The negative intercepts of the above regression lines represent the amount of the corresponding analytes in the sample. Samples from three open-air swimming pools were analyzed using the above quantification method. The results are listed in Table 1.
3.3. Reco6ery, precision and detection limits The results of recovery tests are shown in Table 2. The mean recoveries of nitrate and chlorate added to swimming pool water were 87.2–103 and 103–111%, respectively. The intra- and inter-assay precisions were evaluated as coefficients of variation (C.V.) which are listed in Table 3. The values were all less than 6%. The detection limits were 7 mg ml − 1 for chlorate, and 4 mg ml − 1 for nitrate by the indirect
Table 3 The precision of the methods Analyte
n
Intra-day C.V. (%)
Inter-day C.V. (%)
Chlorate Nitrate (by indirect detection) Nitrate (by direct detection)
5 5 5
3.1 4.2 1.6
5.1 5.5 2.2
P. Wang et al. / Talanta 45 (1998) 657–661
method and 0.4 mg ml − 1 by the direct method, when the signal-to-noise ratio was 3:1.
661
Acknowledgements The authors thank the National University of Singapore for financial support.
4. Conclusion This paper demonstrates the use of CZEbased methods for the analysis of nitrate and chlorate in swimming pool water. The two anions of interest may be separated and determined simultaneously even if the chloride content in the real sample is much higher than each of the two ions. Trace amounts of nitrate can be analyzed by a direct detection method without any interference. The CZE methods are convenient, sensitive, reproducible and require no pernicious volatile organic solvent. CZE can be used in the routine monitoring of these pollutants in swimming pool water.
.
References [1] National Interim Primary Drinking Water Regulation, US EPA Publication No. 570/9-76-003, 1976. [2] K. Holl, Water — Examination Assessment Conditioning Chemistry Bacteriology Biology, Walter de Gruyter, Berlin, Germany, 1972, 389 pp. [3] J.A. Beech, R. Diaz, C. Ordaz, B. Palomeque, Am. J. Public Health 70 (1980) 79. [4] E.L. Johnson, K.K. Haak, Liquid Chromatography in Environmental Analysis, Humana, Clifton, NJ, 1983, 291 pp. [5] O.A. Shpigun, Y.A. Zolotov, Ion Chromatography in Water Analysis, Ellis Horwood, Chichester, UK, 1988, 188 pp.