ORIGINAL PAPER A novel kinetic-spectrophotometric

Chemical Papers 63 (4) 385–390 (2009) DOI: 10.2478/s11696-009-0038-2

ORIGINAL PAPER

A novel kinetic-spectrophotometric method for determination of nitrites in water Zenovia Moldovan* University of Bucharest, Faculty of Chemistry, Department of Analytical Chemistry, 90-92 Panduri Road, 050663-Bucharest, Romania Received 27 October 2008; Revised 7 December 2008; Accepted 9 December 2008

A simple, selective and sensitive kinetic method for the determination of nitrite in water was developed. The method is based on the catalytic effect of nitrite on the oxidation of methylene blue (MB) with bromate in a sulfuric acid medium. During the oxidation process, absorbance of the reaction mixture decreases with the increasing time, inversely proportional to the nitrite concentration. The reaction rate was monitored spectrophotometrically at λ = 666 nm within 30 s of mixing. Linear calibration graph was obtained in the range of 0.005–0.5 µg mL−1 with a relative standard deviation of 2.09 % for six measurements at 0.5 µg mL−1 . The detection limit was found to be 0.0015 µg mL−1 . The effect of different factors such as acidity, time, bromate concentration, MB concentration, ionic strength, and order of reactants additions is reported. Interference of the most common foreign ions was also investigated. The optimum experimental conditions were: 0.38 mol L−1 H2 SO4 , 5 × 10−4 mol L−1 KBrO3 , 1.25 × 10−5 mol L−1 MB, 0.3 mol L−1 sodium nitrate, and 25 ◦C. The proposed method was conveniently applied for the determination of nitrite in spiked drinking water samples. c 2008 Institute of Chemistry, Slovak Academy of Sciences Keywords: nitrite ion, catalytic, methylene blue, spectrophotometry, water sample

Introduction Nitrite is a naturally occurring ion that is a part of the nitrogen cycle. Nitrite can be formed chemically in distribution pipes by Nitrosomonas bacteria during stagnation of nitrate-containing and oxygen-poor drinking-water in galvanized steel pipes or if chloramination is used to provide a residual disinfectant and the process is not sufficiently well controlled. In soil, fertilizers containing inorganic nitrogen and wastes containing organic nitrogen are first decomposed to give ammonia which is then oxidized to nitrite and nitrate. Nitrite is also present in air. Air-borne nitrogen oxides are converted into nitrite ions which are removed by wet and dry deposition. With respect to its influence on human metabolism, nitrite in the bloodstream is involved in the oxidation of haemoglobin to methaemglobin (Fe2+ present in the haem group is oxidized to its Fe3+ form) which does not allow oxy-

gen transport owing to the strong binding of oxygen. Therefore, methaemglobinaemia can lead to cyanosis. Nitrite may react in stomach with nitrosable compounds (e.g. secondary and tertiary amines or amides in food) to form N-nitroso compounds which are well known potential carcinogens (Patty, 1963; Lijinsky & Epstein, 1970). Maximum permissible limit of nitrite concentration as fixed by the U.S. Public Health Association is 0.06 µg mL−1 in potable water. Taking into account its toxicity, many analytical methods for nitrite determination have been developed and proposed for environmental monitoring. Among them, visible spectrophotometric methods, convenient, sensitive, relatively inexpensive, are preferred for nitrite determination (Afkhami et al., 2005; Al-Okab & Syed, 2007; Mansour et al., 2007; Melchert et al., 2007). These methods employ different routes of nitrite determination. The most known procedure is based on the Griess

*Corresponding author, e-mail: z [email protected]

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Z. Moldovan/Chemical Papers 63 (4) 385–390 (2009)

diazo-coupling reaction (Nagaraja et al., 1998). In this method, nitrite is first treated with a diazotizing reagent, namely sulfanilamide, in acidic media to form a transient diazonium salt. This intermediate is then allowed to react with a coupling reagent, namely naphtyl-1-amine, to form a deep red-colored azo dye. On basis of this reaction, a series of methods proposed different reagents to obtain strong azo dyes in the presence of nitrite ions (Amin, 1986; Kaveeshwar et al., 1991; Rathore et al., 1991; Kumar et al., 1993; Horita et al., 1997; Helaleh & Korenaga, 2001; Manzoori & Soflaee, 2001; Revanasiddappa et al., 2001; Revanasiddappa & Kumar, 2001; Nagaraja et al., 2002; Ivanov, 2004; Dayananda & Revanasiddappa, 2007). This class of methods often presents some disadvantages, e.g. the diazo coupling reaction is time consuming, careful control of acidity is needed and some of the used reagents are toxic (Gabbay et al., 1977; Greenberget et al., 2000). Another series of spectrophotometric methods for nitrite determination is based on its catalytic action in the oxidation of some organic reagents with suitable oxidizing agents (Pettas et al., 1998; Pouretedal & Nazari, 2004; Ghasemi et al., 2004; Mubarak et al., 2007). Some kinetic spectrophotometric methods are based on the fading of colored species upon their reaction with nitrite ions (Ensafi & Keyvanfard, 1994; Barzegar et al., 2000). The current paper describes another kinetic spectrophotometric method for the determination of nitrite based on the methylene blue-bromate redox reaction.

Experimental All chemicals were of analytical reagent grade and were purchased from Merck (Darmstadt, Germany). Deionized-distilled water was used throughout the experiment. A stock standard nitrite solution (1000 µg mL−1 ) was prepared by dissolving 150 mg of pre-dried sodium nitrite in water containing a few milligrams of NaOH to prevent its decomposition. The resulting solution was diluted in a 100 mL volumetric flask after adding a few drops of chloroform as a stabilizer, to prevent bacterial growth. This solution was stored in a brown bottle and kept at 4 ◦C; it was used within two weeks of preparation. A stock solution of 5 × 10−3 mol L−1 MB was prepared by dissolving MB and its dilution to the mark with 0.32 mol L−1 sulfuric acid in a 100 mL volumetric flask wrapped in aluminum foil and keeping it at 4 ◦C when not in use. The solution was stable for at least two months. A 4 mol L−1 solution of sulfuric acid was prepared by diluting concentrated sulfuric acid with water. Potassium bromate and sodium nitrate stock solutions were prepared in deionized-distilled water. The required working standard solutions were prepared by diluting the corresponding stock solutions.

Absorbance measurements were performed on a UV-VIS spectrophotometer (V-530 Jasco-Japan) equipped with a cell holder thermostated by an external circulating water bath. Quartz cells of 1 cm path length were used. The temperature was kept constant at (25.0 ± 0.1) ◦C using a thermostated water bath, GFL 1003 type (Burgwedal, Germany), with an accuracy of ± 1 ◦C. Eppendorf vary-pipettes (10–100 µL, 100–1000 µL, and 500–2500 µL) were used to deliver accurate volumes. Working solutions, sample solutions and pure water were kept at 25 ◦C in the thermostated water bath for at least 15 min to reach the equilibrium temperature. A 1 mL sample solution containing 0.001–0.7 µg mL−1 NO− 2 was transferred into a quartz cell. Then, 0.5 mL of 0.5 × 10−5 –2.0 × 10−5 mol L−1 MB solution, 0.3 mL of 1.33–3.33 mol L−1 H2 SO4 , and 0.2 mL of 2–4 mol L−1 NaNO3 solution were added sequentially. Taking into account that the MB solution was prepared in 0.32 mol L−1 H2 SO4 , the final concentration of sulfuric acid in the prepared samples (after adding aliquots of 0.3 mL of 1.33–3.33 mol L−1 H2 SO4 ) was between 0.28 mol L−1 and 0.58 mol L−1 . The reaction was initiated by an injection of 20 µL of 10−3 –0.45 mol L−1 bromate solution. The prepared sample was mixed thoroughly. Redox reaction, therefore the reagent mixture discoloration, was monitored by recording the absorbance (A) and time (t ) graph against water as reference at λ = 666 nm allowing a lag time of 5 s. The rate was calculated from the slope of the initial linear part of the A–t graph, within 30 s of the reagents mixing. Nitrite content of the synthetic and real samples was determined from the calibration graph; the mentioned samples were prepared according to the general procedure.

Results and discussion Reaction between methylene blue ((7-dimethylaminophenothiazin-3-ylidene)-dimethyl-azanium chloride) and bromate in acid medium takes place very slowly which was confirmed by a slow decrease in absorbance. When the nitrite ion catalyzes this reaction, the oxidation of MB is much faster resulting in a considerable discoloration of the mixture (Fig. 1). The wavelength of maximum absorbance attributed to MB was found to be λ = 666 nm. Preliminary experiments showed that the position of the MB characteristic band does not change with the varying acidity and reagent concentrations. Main parameters influencing the performance of the proposed method were studied to determine the optimum working configuration. The rate values were calculated as a difference between the rates of catalyzed and non-catalyzed reactions performed under the same experimental conditions. The effect of H2 SO4 on the reaction of MB oxidation with bromate was studied in the concentration


387


0.7

0.40 0.35

A

k/min

−1

0.6

0.30

0.5

0.25 0.4

0

10

t/s

20

0.20

30

0.28 0.38 0.48 −1 c(H2SO4)/(mol L )

Fig. 1. Variation of MB absorbance at λ = 666 nm with time in various mixtures: NO− 2 –MB–H2 SO4 –NaNO3 ( );

− MB–H2 SO4 –NaNO3 –BrO− 3 (); NO2 –MB–H2 SO4 –

0.58

Fig. 3. Rate dependence of redox reaction as a function of sulfuric acid concentration.

•

− −1 ; c(MB) NaNO3 –BrO− 3 ( ); c(NO2 ) = 0.5 µg mL −5 −1 = 1.25 × 10 mol L ; c(NaNO3 ) = 0.3 mol L−1 ; −4 mol L−1 . c(BrO− 3 ) = 5 × 10

0.5

k/min

−1

0.4

0.7

0.3 0.2

0.6

A

0.1

0.5

0.0 2.5

5.0

0.4 0.3

0

5

10

15

20

25

30

7.5 10.0 12.5 15.0 17.5 -1 c(MB)/(µg mL )

Fig. 4. Rate dependence of redox reaction as a function of MB concentration.

t/s 1.0

Fig. 2. Variation of MB absorbance at λ = 666 nm with time as a function of sulfuric acid concentration: 0.28 mol L−1 (); 0.38 mol L−1 ( ); 0.48 mol L−1 (); 0.58 mol L−1 ().

0.8

A

0.6

range of 0.28–0.58 mol L−1 . As can be seen in Fig. 2, the initial rate, in the presence of 0.5 µg mL−1 NO− 2, increased with H2 SO4 concentration up to 0.48 mol L−1 ; then, a considerable decrease was observed. At the concentration of ≥ 0.48 mol L−1 H2 SO4 , linearity of the A–t graphs became poor and linear parts of the A–t graphs were shortened to values below 10 s resulting in poor precision (Fig. 3). The chosen concentration of H2 SO4 was 0.38 mol L−1 . The effect of MB concentration was studied in the concentration range of 0.5 × 10−5 –2.0 × 10−5 mol L−1 MB. The initial rates increased with MB concentration up to 1.25 × 10−5 mol L−1 , then they remained almost constant (Fig. 4). Therefore, 1.25 × 10−5 mol L−1 MB was used in the recommended procedure. The effect of bromate concentration was studied in the concentration range of 1 × 10−5 –4.5 × 10−3 mol L−1 . As shown in Fig. 5, the initial rates increased with bromate concentration up to 5 × 10−4 mol L−1 ; over this value, a decrease of linearity in

0.4 0.2 0.0

0

10

20

30

t/s Fig. 5. Variation of MB absorbance at λ = 666 nm with time as a function of bromate concentration: 10−5 mol L−1 (); 10−4 mol L−1 (); 5 × 10−4 mol L−1 ( ); 10−3 mol L−1 ( ); 4.5 × 10−3 mol L−1 (∗).

•

the A–t graph was observed. Moreover, the linear part of these graphs was considerable shortened to values reaching 5 s when bromate concentration of 4.5 × 10−3 mol L−1 was used. Thus, 5 × 10−4 mol L−1 was chosen to be the optimum value of bromate concentration and it was adopted for further experiments. The effect of ionic strength on the catalyzed reaction was studied in the presence of NaNO3 . This


388


0.40

Table 1. Precision and accuracy of the proposed method Nitrite content/(µg mL−1 )

0.39

a Found

k/min

−1

Taken 0.100 0.300 0.500

0.38

0.37

0.0 0.1 0.2 −1 0.3 c(NaNO3)/(mol L )

0.4

Fig. 6. Rate dependence of redox reaction as a function of sodium nitrate concentration.

−1

0.099 ± 0.002 0.299 ± 0.009 0.500 ± 0.011

RSD/%

R/%

2.34 2.76 2.09

99.25 99.67 100.05

a) Mean ± 95 % confidence limit for N = 6; t = 2.57, t – distribution for confidence level of 95 % with N − 1 degrees of freedom. RSD – relative standard deviation; R – percent recovery.

0.4

on samples of the following composition and respecting the order of reagents addition: 1 mL sample solution containing a known amount of nitrite + 0.5 mL of 5 × 10−5 mol L−1 MB + 0.3 mL of 2 mol L−1 H2 SO4 + 0.2 mL of 3 mol L−1 NaNO3 + 20 µL of 5 × 10−2 mol L−1 bromate.

0.3

Analytical figures of merit

0.2

Once the optimum working conditions were established, the proposed spectrophotometric method was evaluated with respect to linearity, LOD, LOQ, accuracy, and precision. The calibration graph was plotted using absorbance values obtained from five replicate samples of the same nitrite content. Parameters of the calibration graph were as follows: the linear regression equation: k = 0.6991c(NO− 2 ) + 0.009, where c(NO− ) is the nitrite concentration expressed in µg 2 mL−1 ; the squared correlation coefficient r2 = 0.9991; LOD (calculated as three times the standard deviation of the blank) is 0.0015 µg mL−1 and LOQ is 0.005 µg mL−1 . LOQ was considered as the lowest concentration of the calibration standard solution which was approximately equal to the calculated 10 times the standard deviation of the results for the series of replicates used to determine the limit of detection. In order to estimate the accuracy and precision of the proposed method, standard solutions of 0.10 µg mL−1 , 0.30 µg mL−1 , and 0.50 µg mL−1 nitrite were analyzed according to the recommended procedure. For this purpose, six replicate determinations of each concentration were prepared. As can be seen in Table 1, relative standard deviations ranged from 2.09 % to 2.76 % and the percent recovery from 99.25 % to 100.05 %. The results in Table 1 were obtained by performing the experiments on 1 mL sample solutions containing the nitrite ion at different concentration level + 0.5 mL of 5 × 10−5 mol L−1 MB + 0.3 mL of 2 mol L−1 H2 SO4 + 0.2 mL of 3 mol L−1 NaNO3 + 20 µL of 5 × 10−2 mol L−1 bromate.

0.5

k/min

± tSN −0.5

0.1 0.0

1

2

3

4

5

Sequence/No. Fig. 7. Rate variation with reactants addition order MB– − − H2 SO4 –NaNO3 –NO− 2 –BrO3 (1); BrO3 –MB–H2 SO4 – − − NaNO3 –NO− 2 (2); BrO3 –NO2 –MB–H2 SO4 –NaNO3

− − (3); BrO− 3 –H2 SO4 –NO2 –MB–NaNO3 (4); NO2 –MB–

H2 SO4 –NaNO3 –BrO− 3 (5).

reagent was also introduced to increase the oxidizing − potential of the NO− 3 /NO2 system and so to reduce the oxidation of nitrite. As shown in Fig. 6, the reaction rate was independent of the ionic strength up to 0.3 mol L−1 NaNO3 . This value was chosen to maintain the ionic strength at a constant value. It was observed that the sequence of reactants addition can influence the rate of the oxidation process. Thus, sequences MB–H2 SO4 –NaNO3 –NO− 2– − − BrO− (1); NO –MB–H SO –NaNO –BrO (5), and 2 4 3 3 2 3 − even BrO− 3 –MB–H2 SO4 –NaNO3 –NO2 (2) gave higher values of the oxidation rate. While sequences BrO− 3– − NO− –MB–H SO –NaNO (3), and BrO –H SO 2 4 3 2 4– 2 3 NO− –MB–NaNO (4) gave lower values of the oxi3 2 dation rate. These results can be explained: in case of sequence (3), adding bromate before MB, partial oxidation of nitrite takes place (Mubarak et al., 2007). Moreover, this process is favored by the addition of nitrite to the acidified bromate solution (sequence 4). On basis of the oxidation rates illustrated in Fig. 7, sequence 5 was chosen to be applied in the proposed method. Performance of the proposed method was verified

Effect of diverse ions As in case of other organic reagents, oxidation of MB with bromate can be catalyzed, under different


389


Table 2. Influence of foreign ions on the determination of nitrite (0.5 µg mL−1 ) Tolerance limit/(µg mL−1 )

Foreign ions

+ + 2+ , Mg2+ , Zn2+ NH+ 4 , Na , K , Ca

300

2− 3− 2− − NO− 3 , SO4 , PO4 , CO3 , CH3 COO − − − F , Cl , Br Mn2+ , Co2+ , Ni Fe3+ , Al3+ Cu2+ Fe2+ , Cr3+ , Ti4+ , V4+ , Zr4+ , Sn4+ , Se4+ Hg2+ , Ag1+ , I− SCN−

250 150 120 20 15 5 0.4 0.02

have low tolerance limits. The majority of foreign ions mentioned in Table 2 are present in drinking water at much lower concentration limits. Hence, the proposed method could be applied for the determination of nitrite in water samples (quality control). Method application Analytical potential of the presented method was tested by its application in the determination of nitrite in spiked drinking water samples. Samples of potable water were collected from packaged water bottles. As can be seen in Table 3, very good recoveries of nitrite were obtained considering the presence of common constituents normally encountered in drinking water.

working conditions, by not only nitrite but also by other species such as Br− (Uraisin et al., 2006) or SCN− (Shishehbore et al., 2005). To evaluate the selectivity of the proposed method, the effect of foreign ions on the determination of nitrite was studied by adding known quantities of each ion to a solution containing 1 µg of nitrite (V = 2 mL) and determining the nitrite by the proposed method. The tolerance limits of foreign ions, taken as the concentrations (µg mL−1 ) which cause errors lower than 3 %, are given in Table 2. These results clearly show that most ions normally associated with nitrite in water samples do not interfere. However, Hg2+ , Ag+ , I− , and SCN−

Conclusions Oxidation of MB by bromate in a sulfuric acid medium and in the presence of nitrite ion is an analytical reaction that can be applied in the kineticspectrophotometric determination of nitrite-containing water samples. The proposed method is inexpensive, fairly rapid and sensitive. Its analytical parameters, especially sensitivity, recommend the proposed method as an alternative to other reported kineticspectrophotometric methods (Table 4) and as an instrument for the quality control of drinking water.

Table 3. Recovery data for drinking water samples spiked with nitrite Nitrite content/(µg mL−1 ) R/%

Mineral water sample, No. Added 1 2 3 4

a Found

0.300 0.500 0.300 0.500 0.300 0.500 0.300 0.500

0.295 0.492 0.303 0.508 0.292 0.491 0.305 0.504

±

tSN −0.5

± ± ± ± ± ± ± ±

0.015 0.016 0.010 0.009 0.010 0.014 0.013 0.011

98.33 98.40 101.00 101.60 97.33 98.20 101.66 100.80

a) Mean ± 95 % confidence limit, for N = 4; t = 3.18, t – distribution for confidence level of 95 % with N − 1 degrees of freedom. Table 4. Comparison of dynamic ranges and detection limits of the present analysis method with previously reported methods

Reaction system

Perphenasine–BrO− 3 Methyl Red–BrO− 3

Methylene Blue–BrO− 3

Methylthymol Blue–BrO− 3 Thymol Blue–BrO− 3

Dynamic range

Detection limit

t

ng mL−1

ng mL−1

s

up to 4.5

0.07

30

Mubarak et al., 2007

50–1200

45

50

Ghasemi et al., 2004

Reference

5–500

1.5

30

2–100

0.6

240

5–80

4.5

–


This work Pouretedal et al., 2004 Pettas et al., 1998

390


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