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Spectrophotometric determination of nitrite based on its catalytic effect on the oxidation of carminic acid by bromate
Talanta 46 (1998) 1379 – 1386
Spectrophotometric determination of nitrite based on its catalytic effect on the oxidation of carminic acid by bromate Jamshid L. Manzoori *, Mohammad H. Sorouraddin, Ali M. Haji-Shabani Department of Analytical Chemistry, Faculty of Chemistry, Uni6ersity of Tabriz, Tabriz, Iran Received 7 July 1997; received in revised form 8 December 1997; accepted 8 December 1997
Abstract A highly sensitive and selective method is described for the determination of trace amounts of nitrite based on its effect on the oxidation of carminic acid with bromate. The reaction was monitored spectrophotometrically by measuring the decrease in absorbance of carminic acid at 490 nm after 3 min of mixing the reagents. The optimum reaction conditions were 1.8 × 10 − 1 mol l − 1 H2SO4, 3.8×10 − 3 mol l − 1 KBrO3, and 1.2× 10 − 4 mol l − 1 carminic acid at 30°C. By using the recommended procedure, the calibration graph was linear from 0.2 to 14 ng ml − 1 of nitrite; the detection limit was 0.04 ng ml − 1; the R.S.D. for six replicate determinations of 6 ng ml − 1 was 1.7%. The method is mostly free from interference, especially from large amounts of nitrate and ammonium ions. The proposed method was applied to the determination of nitrite in rain and river water. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Nitrite determination; Catalytic spectrophotometric method; Carminic acid – bromate redox reaction; Natural water
1. Introduction Nitrite is an active intermediate in the nitrogen cycle, resulting from incomplete oxidation of ammonia or from reduction of nitrates. This ion is an important precursor of nitrosamines, which are potential carcinogens. Nitrite is one of the pollutants found in the atmosphere and natural water [1,2]. Nitrite salts is sometimes used as a preserva* Corresponding author. Tel.: +98 41 355998; fax: + 98 41 340191.
tive in the food industry and a corrosion inhibitor in industrial process water. Thus, its determination at ng ml − 1 levels is important in environmental studies. Many papers have been published on the determination of nitrite in different samples, but not all are suitable for routine trace determination. Among these methods spectrophotometry based on diazatization of an aromatic amine and subsequent coupling to form an azo dye are most widely used. These methods are characterized by high sensitivity but often have drawbacks of inter-
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ference by oxidizing and reducing agents, as well as relatively long reaction times and large sample volumes [3– 7]. Other methods such as fluorometry [8,9], chemiluminescence [10,11], chromatography [12,13], pulse polarography [14,15] and flow injection [16 – 18] are also used but suffer from more or less time consuming procedures or complicated instrumentation. A number of kinetic methods have been reported for nitrite determination [19 – 28]. Most of these procedures are based on its catalytic effect on bromate oxidation of organic compounds such as thionine [19], pyrogallol red [20], prochlorperazine [21], brilliant cresyl blue [22], methyl orange [23], pyridine-2-aldehyde 2-pyridylhydrazone [24], chlorophosphonazo-pN [25], and nile blue A [26]. However, many of these method are time consuming or have high limit of detection or need rigid control of acidity, temperature or reagents. The hydrogen peroxide oxidation of carminic acid has been used for the kinetic determination of cobalt [29] and iron [30], but its oxidation by bromate has not been used for nitrite determination so far. The aim of this work was to study the catalytic oxidation of carminic acid with bromate in the presence of nitrite and to use the results obtained to develop a catalytic method for nitrite determination. The reaction was monitored spectrophotometrically and the method was successfully applied to the determination of nitrite in rain and river water.
2. Experimental
2.1. Reagents Analytical-reagent grade chemicals and triply distilled water were used throughout. A stock standard nitrite solution, 1000 mg ml−1, was prepared by dissolving 0.150 g sodium nitrite, pre-dried at 110°C for 4 h, in water. A small amount of sodium hydroxide was added to this solution to prevent its decomposition and a few drops of chloroform were also added to prevent bacterial growth. The resulting solution was made up to the mark in a 100 ml calibrated flask and was kept in a refrigerator and used within 2 weeks of preparation.
Potassium bromate solution (3.8× 10 − 2 mol l ) was prepared by dissolving 1.5866 g KBrO3 in water in a 250 ml volumetric flask. Carminic acid (Merck) (structure shown in Fig. 1) was used without further purification. A stock solution (1.2× 10 − 3 mol l − 1) was prepared by dissolving 0.0591 g of carminic acid in water in a 100 ml standard flask and kept in a refrigerator. Sulphuric acid (1.8 mol l − 1) was prepared from concentrated H2SO4 (98%, Merck). Stock solutions (1000 mg ml − 1) of interfering ions were prepared by dissolving suitable salts in water, hydrochloric acid or sodium hydroxide solution. −1
2.2. Apparatus A UV-265 FW spectrophotometer (Shimadzu) was used for measurements of absorption spectra. A model UV-120-02 spectrophotometer (Shimadzu) with 1.0 cm glass cuvettes was used to measure the absorbance at 490 nm. A thermostat (Tokyo Rikakika, LTD UA-1) was used to keep the reaction temperature at 30°C. A stop-watch was used for recording the reaction time.
2.3. Recommended procedure 1 ml 1.8 mol l − 1 Sulphuric acid and 1 ml 1.2× 10 − 3 mol l − 1 carminic acid were added to each sample or standard solution containing 2– 140 ng of nitrite in a 10 ml volumetric flask. Each solution was diluted to ca. 8 ml with water and was kept in a thermostated water-bath at 30°C for 10 min. 1 ml 3.8×10 − 2 mol l − 1 Bromate previously brought to 30°C, was added and the solution was diluted to the mark with water. Time was measured from just after the addition of the
Fig. 1. The structure of carminic acid.
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Fig. 2. Absorption spectra of the carminic acid– bromate system in the presence of nitrite, conditions: carminic acid, 1.2 ×10 − 4 mol l − 1; sulphuric acid, 1.8 × 10 − 1 mol l − 1; bromate, 3.8 ×10 − 3 mol l − 1; nitrite, 5.0 ng ml − 1; temperature, 30°C; after: a, 0.5; b, 1.5; c, 2.5; d, 3.5; e, 4.5; and f, 5.5 min from initiation of the reaction.
bromate solution. The blank solution was prepared by the same procedure. After 3.0 min, 0.1 g solid urea was added to the solutions to stop the catalysed reaction. The solutions were transferred into 1.0 cm glass cuvettes and after 3.5 min the absorbences of the solutions were measured
against water at 490 nm. The absorbences of the sample and blank were labeled A and A0 respectively. Then the amount of nitrite was determined from a calibration graph which was constructed by plotting the log (A0/A) versus the nitrite concentration.
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Fig. 3. Effect of sulphuric acid concentration on the reaction rate, conditions: carminic acid, 1.0 × 10 − 4 mol l − 1; bromate, 4.0 × 10 − 3 mol l − 1; nitrite, 10.0 ng ml − 1; temperature, 30°C; reaction time, 3.0 min.
Fig. 4. Effect of carminic acid concentration on the reaction rate, conditions: sulphuric acid, 1.8 × 10 − 1 mol l − 1; bromate, 4.0 × 10 − 3 mol l − 1; nitrite, 10.0 ng ml − 1; temperature, 30°C; reaction time, 3.0 min.
3. Results and discussion Some oxidants such as bromate could oxidize carminic acid irreversibly in acidic media at a
slow rate. This oxidation is increased in the presence of ultra-trace amounts of nitrite. This process was monitored by measuring the decrease in absorbance at 490 nm. Fig. 2 shows absorption
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Fig. 5. Effect of bromate concentration on the reaction rate, conditions: sulphuric acid, 1.8 ×10 − 1 mol l − 1; carminic acid, 1.2 × 10 − 4 mol l − 1; nitrite, 10.0 ng ml − 1; temperature, 30°C; reaction time, 3.0 min.
spectra of carminic acid at different times. As shown in this figure, the change in absorbance at 490 nm with time can be used for the study. Several acidic media including sulphuric, hydrochloric, nitric and phosphoric acids were investigated for the carminic acid – bromate system. The results indicated that among these four acids, sulphuric acid is the best due to its acidity strength relative to other acids at equimolar concentrations.
3.1. Effect of 6ariables The effect of sulphuric acid concentration on the rate of the reaction with and without nitrite was studied. The optimum concentration of sulphuric acid is 1.8× 10 − 1 mol l − 1 as shown in Fig. 3. Fig. 4 shows the effect of carminic acid concentration on the catalysed and uncatalysed reaction. The results show that the sensitivity increased up to 1.2 ×10 − 4 mol 1 − 1 carminic acid, whereas a higher concentration of the reagent caused a decrease in the reaction rate. This effect may be due to the change of mechanism of the reac-
tion or deviation from Beer’s law. Thus 1.2 × 10 − 4 mol l − 1 carminic acid was used for the study. The effect of bromate concentration on obtaining maximum sensitivity was investigated. Fig. 5 shows that sensitivity increases up to 3.8× 10 − 3 mol l − 1 BrO3− . At higher concentrations, the sensitivity decreases, owing to the decrease of A0. Thus 3.8× 10 − 3 mol l − 1 bromate concentration was selected for use. The effect of the reaction temperature was studied in the range 5–40°C at optimum conditions. Fig. 6 shows that by increasing the temperature to 30°C, A0 − A increases with temperature, while at higher temperatures it decreases. This effect is due to the fact that at higher temperatures, the rate of the uncatalysed reaction increases with temperature to a greater extent than the catalysed reaction and the difference between the rates of the catalysed and uncatalysed reactions diminishes. A temperature of 30°C, which gives high sensitivity, was selected. A chemical inhibitor or rapid cooling is sometimes used to stop the catalytic reaction in practical applications of the fixed time method. It was
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Fig. 6. Effect of temperature on the reaction rate, conditions: sulphuric acid, 1.8 ×10 − 1 mol l − 1; carminic acid, 1.2 ×10 − 4 mol l − 1; bromate, 3.8 × 10 − 3 mol l − 1; nitrite, 10.0 ng ml − 1; temperature, 30°C; reaction time, 3.0 min.
found that urea is a good inhibitor which decomposes nitrite rapidly. In this study urea was used to stop the catalytic reaction. 0.1 g Solid urea suffices for 10 ml of solution and simultaneously quenches the catalysed reaction. The absorbance of the solution remains constant for a few minutes in the presence of urea. The effect of ionic strength on the rate of reaction was investigated by using NaNO3 (2 mol l − 1). The reaction rate increases very slightly up to 0.5 mol l − 1 NaNO3. The effect of measuring time on the rate of the reaction was studied at the optimum concentraTable 1 Tolerance limits of foreign ions in the determination of 10 ng ml-1 of nitrite following the recommended procedure Tolerance limit (mg ml-1)
Foreign ion
100
23NO-3, F-, SO24 , CO3 , ClO3, PO4 , Cl , ac+ + 2+ etate, tartarate, NH+ , Na , K , Mg , 4 2+ 2+ 2+ 2+ 3+ Ca , Cu , Zn , Ni , La , U (VI) Citrate, Cd2+, Co2+, Mn2+, Cr3+, Al3+ CN-, EDTA, Ag+, Pb2+ 2+ C2O2, Ba2+, Mo(VI), W(VI) 4 , Hg 3+ 2IO3, CrO4 , S2O2, Fe3+ 3 , Bi
50 10 1 0.2
tions of the reagents at 30°C. The results show that 3.0 min yields the best sensitivity. Thus, a fixed reaction time of 3 min was chosen for this study.
3.2. Calibration, precision and detection limit Under the conditions chosen, in the concentration range 0.2–14 ng ml − 1 of nitrite the following regression equation was obtained: log(A0/A)= 1.6× 10 − 3 + 0.011C
(r=0.9995)
−1
where C is the ng ml of nitrite. The R.S.D. for six replicate determinations is 1.7% for 6.0 ng ml − 1 nitrite. The experimental limit of detection is 0.04 ng ml − 1, which was calculated as three times the S.D. of the blank (3s criterion).
3.3. Interference The effects of possible interfering species, which commonly accompany nitrite in natural waters, were studied in the determination of 10 ng ml − 1 of nitrite following the recommended procedure. A foreign ion was considered to interfere seriously when it gave a determination error of more than 5%. The tolerance limit of foreign ions are sum-
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Table 2 Analytical results and recoveries for rain and river water samples Samplea
River
Rain
Nitrite (ng)
Recovery (%)
Added
In reaction solutionb
0.0 50.0 80.0
33.6 82.1 111.4
97.0 97.2
0.0 30.0 50.0
55.2 84.8 104.2
98.7 98.0
Content of nitrite in sample (ng ml-1)
R.S.D. (%)
This method
Standard method
16.8
16.9
2.6 1.4 0.8
27.6
26.4
1.6 1.0 1.2
a
Collected at Iran; the river water was collected from the Talkheroud river on 28 April 1997; the rain water was collected on the roof of the Chemistry building of Tabriz University on 29 April 1997. b A sample of 2 ml was used for one measurement. Each result is the average of five determinations.
marized in Table 1. Large amounts of nitrate and ammonium ions have little effect. Thus, the determination of trace amounts of nitrite in nitrate or ammonium salt is possible.
tion of nitrate to nitrite by using a copper cadmium reduction column [17] and measuring the sum. Nitrite is determined directly by the proposed method and nitrate can be determined by the difference.
3.4. Determination of nitrite in rain and ri6er water A freshly collected water sample was filtered through a membrane filter of 0.45 mm pore size into a poly(propylene) bottle, kept in a refrigerator at about 4°C and analysed by recommended procedure within 12 h of collection. Since the concentration of common pollutants in natural water is generally far below the tolerance levels shown in Table 1, the method was applied directly to the determination of nitrite in rain and river water. As shown in Table 2, the results obtained with the proposed method agree well with those obtained by the standard method [31]. The reliability of the method to analyse real samples was also checked by recovery experiments. The results show that the method gives good recoveries of added nitrite. The simplicity, high sensitivity and its freedom from interference effects are significant advantages of the proposed method. The method could be applied to drinking water after removing the chlorine. Furthermore, with the proposed method it is also possible to combine the nitrate and nitrite analysis. This could be feasible by reduc-
References [1] I.A. Wolff, A.E. Wasserman, Science 177 (1972) 15. [2] K.K. Chio, K.W. Fung, Analyst 105 (1980) 241. [3] M. Nakamura, T. Mazuka, M. Yamashita, Anal. Chem. 56 (1984) 2242. [4] E. Szekely, Talanta 15 (1968) 795. [5] G. Norwitz, P.N. Keliher, Analyst 110 (1985) 689. [6] P.K. Tarafder, D.P.S. Rathore, Analyst 113 (1988) 1073. [7] A. Chaube, A.K. Baveja, V.K. Gupta, Talanta 31 (1984) 391. [8] S. Motomizu, M. Hiroshi, Talanta 33 (1986) 792. [9] P. Damiani, G. Burini, Talanta 33 (1986) 649. [10] R.D. Cox, Anal. Chem. 52 (1980) 332. [11] R.S. Braman, S.A. Hendrix, Anal. Chem. 61 (1989) 2715. [12] Z. Iskandarani, D.J. Pietrzyk, Anal. Chem. 54 (1982) 2601. [13] S.H. Lee, L.R. Field, Anal. Chem. 56 (1984) 2647. [14] Z. Gao, G. Wang, Z. Zhao, Anal. Chim. Acta 230 (1990) 105. [15] S. Sabharwal, Analyst 115 (1990) 1305. [16] M.J. Ahmed, C.D. Stalikas, S.M. Tzouwara-Karayanni, M.I. Karayannis, Talanta 43 (1996) 1009. [17] J.F. Van Staden, A.E. Joubert, H.R. Van Vliet, Fresenius Z. Anal. Chem. 325 (1986) 150. [18] I.M.P.L.V.O. Ferreira, J.L.F.C. Lima, M.C.B.S.M. Montenegro, R. Perez Olmos, A. Rios, Analyst 121 (1996) 1393.
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[19] M. Jiang, F. Jiang, J. Duan, X. Tang, Z. Zhao, Anal. Chim. Acta 234 (1990) 403. [20] A.A. Ensafi, M. Samimifar, Talanta 40 (1993) 1375. [21] A.A. Mohamed, M.F. El-shahat, T. Fukasawa, M. Iwatsuki, Analyst 121 (1996) 89. [22] A.A. Ensafi, B. Rezaii, Microchem. J. 50 (1994) 169. [23] J. Zhi-Liang, Q. Hai-Cuo, W. Da-Qiang, Talanta 39 (1992) 1239. [24] R. Montes, J.J. Laserna, Talanta 34 (1987) 1021. [25] C. Xingguo, W. Ketai, H. Zhide, Z. Zhengfeng, Anal. Lett. 29 (1996) 2015.
[26] A.A. Ensafi, M.S. Kolagar, Anal. Lett. 28 (1995) 1245. [27] C. Sanchez-Pedreno, M.T. Sierra, M.I. Sierra, A. Sanz, Analyst 112 (1987) 837. [28] B. Liang, M. Iwatsuki, T. Fukasawa, Analyst 119 (1994) 2113. [29] Z. Zhang, G. Zhang, Fenxi Huaxue 18 (1990) 929. [30] T.G. Pecev, S.S. Mitic, J. Serb. Chem. Soc. 59 (1994) 195. [31] F.J. Welcher (ed.), Standard Methods of Chemical Analysis, vol. 2, part B, Van Nostrand, New York, 1963, pp. 2448.
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.
Spectrophotometric determination of nitrite based on its catalytic effect on the oxidation of carminic acid by bromate Jamshid L. Manzoori *, Mohammad H. Sorouraddin, Ali M. Haji-Shabani Department of Analytical Chemistry, Faculty of Chemistry, Uni6ersity of Tabriz, Tabriz, Iran Received 7 July 1997; received in revised form 8 December 1997; accepted 8 December 1997
Abstract A highly sensitive and selective method is described for the determination of trace amounts of nitrite based on its effect on the oxidation of carminic acid with bromate. The reaction was monitored spectrophotometrically by measuring the decrease in absorbance of carminic acid at 490 nm after 3 min of mixing the reagents. The optimum reaction conditions were 1.8 × 10 − 1 mol l − 1 H2SO4, 3.8×10 − 3 mol l − 1 KBrO3, and 1.2× 10 − 4 mol l − 1 carminic acid at 30°C. By using the recommended procedure, the calibration graph was linear from 0.2 to 14 ng ml − 1 of nitrite; the detection limit was 0.04 ng ml − 1; the R.S.D. for six replicate determinations of 6 ng ml − 1 was 1.7%. The method is mostly free from interference, especially from large amounts of nitrate and ammonium ions. The proposed method was applied to the determination of nitrite in rain and river water. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Nitrite determination; Catalytic spectrophotometric method; Carminic acid – bromate redox reaction; Natural water
1. Introduction Nitrite is an active intermediate in the nitrogen cycle, resulting from incomplete oxidation of ammonia or from reduction of nitrates. This ion is an important precursor of nitrosamines, which are potential carcinogens. Nitrite is one of the pollutants found in the atmosphere and natural water [1,2]. Nitrite salts is sometimes used as a preserva* Corresponding author. Tel.: +98 41 355998; fax: + 98 41 340191.
tive in the food industry and a corrosion inhibitor in industrial process water. Thus, its determination at ng ml − 1 levels is important in environmental studies. Many papers have been published on the determination of nitrite in different samples, but not all are suitable for routine trace determination. Among these methods spectrophotometry based on diazatization of an aromatic amine and subsequent coupling to form an azo dye are most widely used. These methods are characterized by high sensitivity but often have drawbacks of inter-
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ference by oxidizing and reducing agents, as well as relatively long reaction times and large sample volumes [3– 7]. Other methods such as fluorometry [8,9], chemiluminescence [10,11], chromatography [12,13], pulse polarography [14,15] and flow injection [16 – 18] are also used but suffer from more or less time consuming procedures or complicated instrumentation. A number of kinetic methods have been reported for nitrite determination [19 – 28]. Most of these procedures are based on its catalytic effect on bromate oxidation of organic compounds such as thionine [19], pyrogallol red [20], prochlorperazine [21], brilliant cresyl blue [22], methyl orange [23], pyridine-2-aldehyde 2-pyridylhydrazone [24], chlorophosphonazo-pN [25], and nile blue A [26]. However, many of these method are time consuming or have high limit of detection or need rigid control of acidity, temperature or reagents. The hydrogen peroxide oxidation of carminic acid has been used for the kinetic determination of cobalt [29] and iron [30], but its oxidation by bromate has not been used for nitrite determination so far. The aim of this work was to study the catalytic oxidation of carminic acid with bromate in the presence of nitrite and to use the results obtained to develop a catalytic method for nitrite determination. The reaction was monitored spectrophotometrically and the method was successfully applied to the determination of nitrite in rain and river water.
2. Experimental
2.1. Reagents Analytical-reagent grade chemicals and triply distilled water were used throughout. A stock standard nitrite solution, 1000 mg ml−1, was prepared by dissolving 0.150 g sodium nitrite, pre-dried at 110°C for 4 h, in water. A small amount of sodium hydroxide was added to this solution to prevent its decomposition and a few drops of chloroform were also added to prevent bacterial growth. The resulting solution was made up to the mark in a 100 ml calibrated flask and was kept in a refrigerator and used within 2 weeks of preparation.
Potassium bromate solution (3.8× 10 − 2 mol l ) was prepared by dissolving 1.5866 g KBrO3 in water in a 250 ml volumetric flask. Carminic acid (Merck) (structure shown in Fig. 1) was used without further purification. A stock solution (1.2× 10 − 3 mol l − 1) was prepared by dissolving 0.0591 g of carminic acid in water in a 100 ml standard flask and kept in a refrigerator. Sulphuric acid (1.8 mol l − 1) was prepared from concentrated H2SO4 (98%, Merck). Stock solutions (1000 mg ml − 1) of interfering ions were prepared by dissolving suitable salts in water, hydrochloric acid or sodium hydroxide solution. −1
2.2. Apparatus A UV-265 FW spectrophotometer (Shimadzu) was used for measurements of absorption spectra. A model UV-120-02 spectrophotometer (Shimadzu) with 1.0 cm glass cuvettes was used to measure the absorbance at 490 nm. A thermostat (Tokyo Rikakika, LTD UA-1) was used to keep the reaction temperature at 30°C. A stop-watch was used for recording the reaction time.
2.3. Recommended procedure 1 ml 1.8 mol l − 1 Sulphuric acid and 1 ml 1.2× 10 − 3 mol l − 1 carminic acid were added to each sample or standard solution containing 2– 140 ng of nitrite in a 10 ml volumetric flask. Each solution was diluted to ca. 8 ml with water and was kept in a thermostated water-bath at 30°C for 10 min. 1 ml 3.8×10 − 2 mol l − 1 Bromate previously brought to 30°C, was added and the solution was diluted to the mark with water. Time was measured from just after the addition of the
Fig. 1. The structure of carminic acid.
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Fig. 2. Absorption spectra of the carminic acid– bromate system in the presence of nitrite, conditions: carminic acid, 1.2 ×10 − 4 mol l − 1; sulphuric acid, 1.8 × 10 − 1 mol l − 1; bromate, 3.8 ×10 − 3 mol l − 1; nitrite, 5.0 ng ml − 1; temperature, 30°C; after: a, 0.5; b, 1.5; c, 2.5; d, 3.5; e, 4.5; and f, 5.5 min from initiation of the reaction.
bromate solution. The blank solution was prepared by the same procedure. After 3.0 min, 0.1 g solid urea was added to the solutions to stop the catalysed reaction. The solutions were transferred into 1.0 cm glass cuvettes and after 3.5 min the absorbences of the solutions were measured
against water at 490 nm. The absorbences of the sample and blank were labeled A and A0 respectively. Then the amount of nitrite was determined from a calibration graph which was constructed by plotting the log (A0/A) versus the nitrite concentration.
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Fig. 3. Effect of sulphuric acid concentration on the reaction rate, conditions: carminic acid, 1.0 × 10 − 4 mol l − 1; bromate, 4.0 × 10 − 3 mol l − 1; nitrite, 10.0 ng ml − 1; temperature, 30°C; reaction time, 3.0 min.
Fig. 4. Effect of carminic acid concentration on the reaction rate, conditions: sulphuric acid, 1.8 × 10 − 1 mol l − 1; bromate, 4.0 × 10 − 3 mol l − 1; nitrite, 10.0 ng ml − 1; temperature, 30°C; reaction time, 3.0 min.
3. Results and discussion Some oxidants such as bromate could oxidize carminic acid irreversibly in acidic media at a
slow rate. This oxidation is increased in the presence of ultra-trace amounts of nitrite. This process was monitored by measuring the decrease in absorbance at 490 nm. Fig. 2 shows absorption
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Fig. 5. Effect of bromate concentration on the reaction rate, conditions: sulphuric acid, 1.8 ×10 − 1 mol l − 1; carminic acid, 1.2 × 10 − 4 mol l − 1; nitrite, 10.0 ng ml − 1; temperature, 30°C; reaction time, 3.0 min.
spectra of carminic acid at different times. As shown in this figure, the change in absorbance at 490 nm with time can be used for the study. Several acidic media including sulphuric, hydrochloric, nitric and phosphoric acids were investigated for the carminic acid – bromate system. The results indicated that among these four acids, sulphuric acid is the best due to its acidity strength relative to other acids at equimolar concentrations.
3.1. Effect of 6ariables The effect of sulphuric acid concentration on the rate of the reaction with and without nitrite was studied. The optimum concentration of sulphuric acid is 1.8× 10 − 1 mol l − 1 as shown in Fig. 3. Fig. 4 shows the effect of carminic acid concentration on the catalysed and uncatalysed reaction. The results show that the sensitivity increased up to 1.2 ×10 − 4 mol 1 − 1 carminic acid, whereas a higher concentration of the reagent caused a decrease in the reaction rate. This effect may be due to the change of mechanism of the reac-
tion or deviation from Beer’s law. Thus 1.2 × 10 − 4 mol l − 1 carminic acid was used for the study. The effect of bromate concentration on obtaining maximum sensitivity was investigated. Fig. 5 shows that sensitivity increases up to 3.8× 10 − 3 mol l − 1 BrO3− . At higher concentrations, the sensitivity decreases, owing to the decrease of A0. Thus 3.8× 10 − 3 mol l − 1 bromate concentration was selected for use. The effect of the reaction temperature was studied in the range 5–40°C at optimum conditions. Fig. 6 shows that by increasing the temperature to 30°C, A0 − A increases with temperature, while at higher temperatures it decreases. This effect is due to the fact that at higher temperatures, the rate of the uncatalysed reaction increases with temperature to a greater extent than the catalysed reaction and the difference between the rates of the catalysed and uncatalysed reactions diminishes. A temperature of 30°C, which gives high sensitivity, was selected. A chemical inhibitor or rapid cooling is sometimes used to stop the catalytic reaction in practical applications of the fixed time method. It was
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Fig. 6. Effect of temperature on the reaction rate, conditions: sulphuric acid, 1.8 ×10 − 1 mol l − 1; carminic acid, 1.2 ×10 − 4 mol l − 1; bromate, 3.8 × 10 − 3 mol l − 1; nitrite, 10.0 ng ml − 1; temperature, 30°C; reaction time, 3.0 min.
found that urea is a good inhibitor which decomposes nitrite rapidly. In this study urea was used to stop the catalytic reaction. 0.1 g Solid urea suffices for 10 ml of solution and simultaneously quenches the catalysed reaction. The absorbance of the solution remains constant for a few minutes in the presence of urea. The effect of ionic strength on the rate of reaction was investigated by using NaNO3 (2 mol l − 1). The reaction rate increases very slightly up to 0.5 mol l − 1 NaNO3. The effect of measuring time on the rate of the reaction was studied at the optimum concentraTable 1 Tolerance limits of foreign ions in the determination of 10 ng ml-1 of nitrite following the recommended procedure Tolerance limit (mg ml-1)
Foreign ion
100
23NO-3, F-, SO24 , CO3 , ClO3, PO4 , Cl , ac+ + 2+ etate, tartarate, NH+ , Na , K , Mg , 4 2+ 2+ 2+ 2+ 3+ Ca , Cu , Zn , Ni , La , U (VI) Citrate, Cd2+, Co2+, Mn2+, Cr3+, Al3+ CN-, EDTA, Ag+, Pb2+ 2+ C2O2, Ba2+, Mo(VI), W(VI) 4 , Hg 3+ 2IO3, CrO4 , S2O2, Fe3+ 3 , Bi
50 10 1 0.2
tions of the reagents at 30°C. The results show that 3.0 min yields the best sensitivity. Thus, a fixed reaction time of 3 min was chosen for this study.
3.2. Calibration, precision and detection limit Under the conditions chosen, in the concentration range 0.2–14 ng ml − 1 of nitrite the following regression equation was obtained: log(A0/A)= 1.6× 10 − 3 + 0.011C
(r=0.9995)
−1
where C is the ng ml of nitrite. The R.S.D. for six replicate determinations is 1.7% for 6.0 ng ml − 1 nitrite. The experimental limit of detection is 0.04 ng ml − 1, which was calculated as three times the S.D. of the blank (3s criterion).
3.3. Interference The effects of possible interfering species, which commonly accompany nitrite in natural waters, were studied in the determination of 10 ng ml − 1 of nitrite following the recommended procedure. A foreign ion was considered to interfere seriously when it gave a determination error of more than 5%. The tolerance limit of foreign ions are sum-
J.L. Manzoori et al. / Talanta 46 (1998) 1379–1386
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Table 2 Analytical results and recoveries for rain and river water samples Samplea
River
Rain
Nitrite (ng)
Recovery (%)
Added
In reaction solutionb
0.0 50.0 80.0
33.6 82.1 111.4
97.0 97.2
0.0 30.0 50.0
55.2 84.8 104.2
98.7 98.0
Content of nitrite in sample (ng ml-1)
R.S.D. (%)
This method
Standard method
16.8
16.9
2.6 1.4 0.8
27.6
26.4
1.6 1.0 1.2
a
Collected at Iran; the river water was collected from the Talkheroud river on 28 April 1997; the rain water was collected on the roof of the Chemistry building of Tabriz University on 29 April 1997. b A sample of 2 ml was used for one measurement. Each result is the average of five determinations.
marized in Table 1. Large amounts of nitrate and ammonium ions have little effect. Thus, the determination of trace amounts of nitrite in nitrate or ammonium salt is possible.
tion of nitrate to nitrite by using a copper cadmium reduction column [17] and measuring the sum. Nitrite is determined directly by the proposed method and nitrate can be determined by the difference.
3.4. Determination of nitrite in rain and ri6er water A freshly collected water sample was filtered through a membrane filter of 0.45 mm pore size into a poly(propylene) bottle, kept in a refrigerator at about 4°C and analysed by recommended procedure within 12 h of collection. Since the concentration of common pollutants in natural water is generally far below the tolerance levels shown in Table 1, the method was applied directly to the determination of nitrite in rain and river water. As shown in Table 2, the results obtained with the proposed method agree well with those obtained by the standard method [31]. The reliability of the method to analyse real samples was also checked by recovery experiments. The results show that the method gives good recoveries of added nitrite. The simplicity, high sensitivity and its freedom from interference effects are significant advantages of the proposed method. The method could be applied to drinking water after removing the chlorine. Furthermore, with the proposed method it is also possible to combine the nitrate and nitrite analysis. This could be feasible by reduc-
References [1] I.A. Wolff, A.E. Wasserman, Science 177 (1972) 15. [2] K.K. Chio, K.W. Fung, Analyst 105 (1980) 241. [3] M. Nakamura, T. Mazuka, M. Yamashita, Anal. Chem. 56 (1984) 2242. [4] E. Szekely, Talanta 15 (1968) 795. [5] G. Norwitz, P.N. Keliher, Analyst 110 (1985) 689. [6] P.K. Tarafder, D.P.S. Rathore, Analyst 113 (1988) 1073. [7] A. Chaube, A.K. Baveja, V.K. Gupta, Talanta 31 (1984) 391. [8] S. Motomizu, M. Hiroshi, Talanta 33 (1986) 792. [9] P. Damiani, G. Burini, Talanta 33 (1986) 649. [10] R.D. Cox, Anal. Chem. 52 (1980) 332. [11] R.S. Braman, S.A. Hendrix, Anal. Chem. 61 (1989) 2715. [12] Z. Iskandarani, D.J. Pietrzyk, Anal. Chem. 54 (1982) 2601. [13] S.H. Lee, L.R. Field, Anal. Chem. 56 (1984) 2647. [14] Z. Gao, G. Wang, Z. Zhao, Anal. Chim. Acta 230 (1990) 105. [15] S. Sabharwal, Analyst 115 (1990) 1305. [16] M.J. Ahmed, C.D. Stalikas, S.M. Tzouwara-Karayanni, M.I. Karayannis, Talanta 43 (1996) 1009. [17] J.F. Van Staden, A.E. Joubert, H.R. Van Vliet, Fresenius Z. Anal. Chem. 325 (1986) 150. [18] I.M.P.L.V.O. Ferreira, J.L.F.C. Lima, M.C.B.S.M. Montenegro, R. Perez Olmos, A. Rios, Analyst 121 (1996) 1393.
1386
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[19] M. Jiang, F. Jiang, J. Duan, X. Tang, Z. Zhao, Anal. Chim. Acta 234 (1990) 403. [20] A.A. Ensafi, M. Samimifar, Talanta 40 (1993) 1375. [21] A.A. Mohamed, M.F. El-shahat, T. Fukasawa, M. Iwatsuki, Analyst 121 (1996) 89. [22] A.A. Ensafi, B. Rezaii, Microchem. J. 50 (1994) 169. [23] J. Zhi-Liang, Q. Hai-Cuo, W. Da-Qiang, Talanta 39 (1992) 1239. [24] R. Montes, J.J. Laserna, Talanta 34 (1987) 1021. [25] C. Xingguo, W. Ketai, H. Zhide, Z. Zhengfeng, Anal. Lett. 29 (1996) 2015.
[26] A.A. Ensafi, M.S. Kolagar, Anal. Lett. 28 (1995) 1245. [27] C. Sanchez-Pedreno, M.T. Sierra, M.I. Sierra, A. Sanz, Analyst 112 (1987) 837. [28] B. Liang, M. Iwatsuki, T. Fukasawa, Analyst 119 (1994) 2113. [29] Z. Zhang, G. Zhang, Fenxi Huaxue 18 (1990) 929. [30] T.G. Pecev, S.S. Mitic, J. Serb. Chem. Soc. 59 (1994) 195. [31] F.J. Welcher (ed.), Standard Methods of Chemical Analysis, vol. 2, part B, Van Nostrand, New York, 1963, pp. 2448.
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