Simultaneous Voltammetric Determination of Nitrate and

ANALYTICAL SCIENCES NOVEMBER 2010, VOL. 26 1173

Introduction

The determination and analysis of trace elements in environmental water samples have been a key factor for the development of analytical and environmental chemistry. Nitrate and nitrite ions are essential trace elements in environmental water, and their high concentrations above the recommended levels can be hazardous to both life and Earth. The environmental and biological importance of these ions has led to their analyses and the development of new determination methods in analytical and environmental chemistry. The main attention of this work has been paid to their simultaneous determination.

Several determination methods for nitrate and nitrite ions, such as spectrophotometric, fluorescence, chemiluminescence, and electrochemical detections, have been reviewed.1 High-performance liquid chromatography and ion chromatography have been widely used for the determination of nitrate and nitrite ions in natural water samples.2–5 Capillary zone electrophoresis methods have also been applied to the determination.6,7 A flow-injection analysis using membrane separation and chemiluminescence detection was proposed.8 A flow system coupled with on-line solid phase extraction and liquid waveguide capillary cell spectrophotometric detection was applied to the determination at nanomolar levels of nitrate and nitrite ions in surface seawater samples.9 Methods using Cu-Cd column reduction have been widely used as a part for the simultaneous determination.10,11 However, most of the above-mentioned methods reflect complexity in terms of the determination steps and the pretreatment of samples.

Voltammetric techniques have also been developed for the determination of nitrate and nitrite ions.12–14 In particular,

available electrode materials for the electroreduction of nitrate and nitrite ions have been sought and examined for many years. Among many electrode materials, copper is expected to be the most effective material in terms of electroreduction of nitrate and nitrite ions.15–21 Indeed, many studies have shown that pretreated and/or modified copper electrodes are effective for the determination of nitrate and nitrite ions.13,14,22–26 However, the pretreatments and/or modifications of copper electrodes in the studies seem to be complicated, time consuming and insufficiently sensitive for the determination of nitrate and nitrite ions in natural water samples. For example, some researchers have shown how to modify a copper electrode in one solution, and then place the electrode in a test solution.13 Some have determined nitrite first by anodic oxidation, and then pretreated the electrode in a complex and time-consuming manner for the determination of nitrate.12 Some lack information about the optimal conditions.14 In addition, most of the voltammetric studies are for an individual determination of nitrate or nitrite ions, and provide little information on the simultaneous determination of nitrate and nitrite ions, especially in natural water samples.

So far, we have developed voltammetric analyses of nutrient components in natural water.27 In this work, while taking into account that a Cu-Cd column is effective for the reduction of nitrate ion in spectrophotometry (the Griess method), we attempted to use a Cu electrode for the reduction of nitrate and nitrite ions. This paper describes a simple method for the simultaneous determinations of nitrate and nitrite ions in natural water samples by differential pulse voltammetry at an in situ pretreated copper electrode.

2010 © The Japan Society for Analytical Chemistry

† To whom correspondence should be addressed.E-mail: [email protected]

Simultaneous Voltammetric Determination of Nitrate and Nitrite Ions Using a Copper Electrode Pretreated by Dissolution/Redeposition

Shovon Mohammad SHARIAR† and Teruo HINOUE

Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto 390–8621, Japan

Nitrate and nitrite ions were found to be successfully electroreduced at an in situ electrochemically pretreated copper electrode in acidic media, and their reduction peaks of a cyclic voltammogram were found at two distinct electrode potentials. Cyclic voltammetric experiments revealed a highly sensitive behavior of the pretreated copper electrode upon the electroreduction of nitrate and nitrite ions, and showed that a simultaneous voltammetric determination of the ions was achievable. Differential pulse voltammetry (DPV) was applied to the simultaneous determination. As a result, the detection limits were 0.26 and 0.17 μmol dm–3 for nitrate and nitrite ions, respectively. Simultaneous determinations for real samples (river water) were carried out by DPV at the pretreated copper electrode and spectrophotometry (the Griess method). The determination values obtained by both methods were in a good agreement with each other.

(Received August 25, 2010; Accepted October 17, 2010; Published November 10, 2010)

1174 ANALYTICAL SCIENCES NOVEMBER 2010, VOL. 26

Experimental

Reagents and materialsSulfuric acid, sodium sulfate, lithium sulfate, potassium nitrate,

sodium nitrite, hydrochloric acid, N-(1-naphthyl)ethylenediamaine dihydrochloride, copper(II) sulfate, potassium chloride, sodium perchlorate, perchloric acid (60%), disodium dihydrogen ethylenediamine tetraacetate dehydrate, sodium silicate, ammonium chloride, sodium chloride, and ammonia solution (28%) were purchased from Nacalai Tesque. Sulfanilamide was purchased from Kanto Chemical Co. Inc. Cadmium granular small fragments (500 – 2000 μm), phosphoric acid and potassium sulfate were purchased from Wako Pure Chemical Industries, Ltd. A copper wire and a Pt wire of 0.1 mm in diameter, serving as working electrodes, were purchased from Nilaco Corporation. A copper plate for scanning electron microscopic observations was also purchased from Nilaco Corporation. A graphite-reinforced carbon (GRC) rod 0.1 mm in diameter was supplied from Tsukuba Materials Information Laboratory. All aqueous solutions were prepared with deionized water. All chemicals used were of analytical grade, and were used without further purification.

Instrumentation and procedureCyclic voltammetry (CV) was performed with a potentiostat

(Huso, HECS 314) and a potential sweeper (Huso, HECS 980). Differential pulse voltammetry (DPV) was performed with a lab-made apparatus. A conventional three-electrode system was used in all voltammetric experiments. A copper wire (f = 0.1 mm) embedded in a glass tube with epoxy cement was used as a working electrode, a saturated silver/silver chloride (Ag/AgCl) electrode as a reference electrode, and a platinum wire as a counter electrode. A GRC rod and platinum wire of 0.1 mm in diameter were also used as working electrodes in a comparative study. All solutions tested in this work were prepared by dissolving reagents with ion-exchanged water processed through a water-purification system (Yamato Autostill, WG 25). The pH values of the solutions were measured with a bench top pH meter (F-52, Horiba). Unless otherwise noted, each measurement was performed at least three times, and then the resultant values were averaged. Surface images of copper plate surfaces were captured with a scanning electron microscope (JSM-5200 LVP, JEOL).

Pretreatment of a copper electrodeA copper electrode was finely polished with 0.3 μm-alumina

slurry, sonicated in an ultrasonic bath and thoroughly washed with ion-exchanged water before a pretreatment. The copper electrode was placed in a sample solution containing 0.1 mol dm–3 Na2SO4, which was adjusted to pH 2.0 with H2SO4. The sample solution was deaerated with nitrogen gas for ca. 15 min. Two potentials were successively added to the copper electrode for electrochemically pretreating the electrode, prior to each voltammetric measurement. At the first potential, +0.5 V vs. Ag/AgCl, copper atoms of a copper electrode were anodically dissolved to copper ions, and at the second potential, –0.1 V, the copper ions were cathodically redeposited on the copper electrode. The first and second potentials were held for 60 and 10 s, respectively. The potential was manually stepped from +0.5 to –0.1 V.

Spectrophotometric analysisFor a comparison with the voltammetric determination,

nitrate and nitrite ions were determined by spectrophotometry.

The UV-Vis absorption spectra were measured with a spectrophotometer (UV-2500, Shimadzu), using a 1-cm quartz optical cuvette. We used the Griess protocol for this purpose.13,28 Nitrate is reduced to nitrite by passing through a copperized cadmium column. Nitrite reacts with sulfanilamide to form a diazonium compound, which then reacts with the N-(1-naphthyl)-ethylenediamine dihydrochloride to produce a pink-colored azo-compound with the maximum absorption at 540 nm.

Results and Discussion

Effect  of  a  pretreatment  of  a  Cu  electrode  on  the  reduction  of nitrate and nitrite ions

Cyclic voltammetry (CV) was used to examine the effect of the pretreatment. The pretreatment involved a dissolution/redeposition process, as described later in detail. Figure 1 shows cyclic voltammograms (CVs) of 1.0 mmol dm–3 nitrate and 1.0 mmol dm–3 nitrite ions in 0.1 mol dm–3 Na2SO4 (pH 2.0), at an unpretreated (only polished) and pretreated copper electrode. The dotted lines correspond to the blank solution, which is 0.1 mol dm–3 Na2SO4 (pH 2.0), and the solid lines correspond to the sample solution. As can be seen in Figs. 1(a) and 1(c), the unpretreated copper electrode produced ill-defined nitrate and nitrite reduction waves. However, the pretreated copper electrode produced well-defined reduction peaks for nitrate and nitrite ions at –0.56 and –0.28 V, respectively, as shown in Figs. 1(b) and 1(d). For a comparative study, we measured CVs of the two ions, using a GRC and platinum electrode. However, we could find no reduction waves at the GRC and platinum electrode. Some literature has reported that nitrate and nitrite ions are electroreduced to ammonium ions, i.e., NO3

– + 10H+ + 8e– → NH4+ + 3H2O and NO2

– + 8H+ + 6e– → NH4

+ + 2H2O in acidic sulfate media.29–31 While we assume that these electrode reactions are responsible for the

Fig. 1　 Cyclic voltammograms (CVs) of 1.0 mmol dm–3 nitrate ion at an unpretreated copper (a) and pretreated copper (b) electrode. CVs of 1.0 mmol dm–3 nitrite ion at an unpretreated copper (c) and pretreated copper (d). Supporting electrolyte, 0.1 mol dm–3 Na2SO4 (pH 2.0); scan rate, 100 mV s–1.


reduction peaks of the ions in Figs. 1(b) and 1(d), the electrode reactions have not yet been clarified. Anyway, it is obvious that the well-defined reduction waves of nitrate and nitrite ions could be observed at the pretreated copper electrode, though the ions were electroreduced at the unpretreated and pretreated copper electrodes. Consequently, the pretreatment of a copper electrode is indispensable in this work.

Effect of supporting electrolyteWe performed CV experiments to choose an optimal

supporting electrolyte. The experiments were carried out in various supporting electrolyte solutions, using the pretreated copper electrode. The pH values of supporting electrolytes were kept constant at 2.0. Table 1 indicates the peak currents obtained from cyclic voltammograms of nitrate and nitrite ions in different supporting electrolytes. As can be seen in Table 1, 0.1 mol dm–3 Na2SO4 (pH 2.0) was the most favorable supporting electrolyte to obtain a larger reduction peak current, compared with the other supporting electrolytes. It is assumed that SO4

2– and/or HSO4– possess some significant role for

pretreating a copper electrode. Consequently, 0.1 mol dm–3 Na2SO4 (pH 2.0) was thoroughly used as a supporting electrolyte in this work.

pH effectFigure 2 shows the pH effect on the peak currents and the

peak potentials of nitrate and nitrite ions. The concentrations used were 1.0 and 3.0 mmol dm–3 for nitrate and nitrite ions, respectively. As can be seen in Fig. 2(a), the peak currents for the ions linearly increased with the pH, and decreased after attaining a maximum value at ca. pH 2.0. No reduction waves were found beyond pH 3.6. In addition, reduction waves for the

ions gradually became ill-defined and difficult to measure, as the pH changed from 2.0 to lower and higher values. Since a similar pH effect was observed for both of nitrate and nitrite ions, it is reasonable that the pH influenced the pretreatment of copper electrode, rather than the electrode reaction. As can be seen in Fig. 2(b), the reduction potentials shifted to the positive direction with increasing pH. These shifts are also probably due to the pretreatment of the copper electrode. In addition, we observed the morphology of a copper plate surface pretreated at different pHs of the supporting electrolyte (0.1 mol dm–3 Na2SO4). Figure 3 shows scanning electron microscopic (SEM) images of the copper plate surfaces pretreated at pH 1.0, 2.0, 3.0 and 5.7. As can be seen in the SEM images, fine particles of deposited copper (white parts in this figure) were found all over the copper surface at pH 2.0, whereas the particles decreased at pH 1.0 and 3.0, and almost diminished at pH 5.7. It is therefore suggested that pH 2.0 was very favorable for the electrodeposition of copper on the electrode surface. The vertical lines in the SEM image of the unpretreated copper plate are probably due to polishing stretches. On the other hand, Fig. 4 shows an SEM image of the pretreated copper at pH 2.0 at higher magnification level. Based on several reports describing electrodeposited copper and copper crystals, the white parts in this figure are assumed to be copper crystals.32–34 Consequently, we think that copper crystals freshly deposited are favorable for the electroreduction of nitrate and nitrite ions.

The concentration dependence of the supporting electrolyte (Na2SO4, pH 2.0), on the peak currents of nitrate and nitrite ions was also examined. However, no significant dependence was found.

Optimal conditions for the pretreatmentOptimizing the dissolution/redeposition time is important for

the pretreatment of a copper electrode. We examined the dependence of the dissolution/redeposition time on the electroreduction of nitrate ion. Figure 5 shows a plot of the peak currents against the dissolution/redeposition time. This figure shows the peak currents obtained at a fixed redeposition time by changing the dissolution time and the peak currents obtained at a fixed dissolution time by changing the redeposition time. The dissolution and redeposition were carried out at +0.5 and –0.1 V, respectively. From this figure, it can be concluded that a dissolution time of 60 s and a redeposition time of 10 s was the best dissolution/redeposition time couple.

Cyclic  voltammograms  of  nitrate  and  nitrite  ions  under  the optimal conditions

We could successfully obtain cyclic voltammograms with two distinct reduction peaks for nitrate and nitrite ions in solutions containing both of the ions, using the pretreated copper electrode. Consequently, the simultaneous determination of

Table 1　Reduction peak currents (Ipcs) of nitrate and nitrite ions obtained from cyclic voltammograms in different supporting electrolytes

Supporting electrolyte pH Ipc/μAa Ipc/μAb

0.1 M Na2SO4 (pH adjusted by H2SO4)0.1 M K2SO4 (pH adjusted by H2SO4)0.1 M Li2SO4 (pH adjusted by H2SO4)0.1 M NaClO4 (pH adjusted by HClO4)0.3 M NaClO4 (pH adjusted by HClO4)0.1 M NaCl (pH adjusted by HCl)0.3 M NaCl (pH adjusted by HCl)

2.02.02.02.02.02.02.0

0.38 ± 0.010.37 ± 0.020.36 ± 0.030.28 ± 0.010.31 ± 0.020.33 ± 0.010.31 ± 0.01

0.24 ± 0.020.23 ± 0.020.22 ± 0.010.11 ± 0.010.12 ± 0.010.14 ± 0.020.10 ± 0.03

a. Corresponds to the peak currents obtained for 1.0 mmol dm–3 NO3–. b. Corresponds to the peak current obtained for 1.0 mmol dm–3 NO2

–.

Fig. 2　Influence of the pH on the reduction peak currents (a) and the peak potentials (b) of nitrate (▲) and nitrite ions (●). The concentrations used for nitrate and nitrite ion were 1.0 and 3.0 mmol dm–3, respectively. Scan rate used in CV, 100 mV s–1.


nitrate and nitrite ions is possible. Figure 6(a) shows the CVs for different concentrations of nitrite ion at a fixed concentration of nitrate ion (0.8 mmol dm–3), and Fig. 6(b) shows the relationship between the peak current of the ions and the concentration. The peak currents indicated a good proportionality (R2 = 0.999) to the nitrite ion concentration in the range to 0.7 mmol dm–3, whereas the peak currents of the nitrate ion were almost constant, regardless of the concentration of the nitrite ion. In a similar way, Fig. 6(c) shows CVs for different concentrations of the nitrate ion at a fixed concentration of

nitrite ion (0.7 mmol dm–3) and Fig. 6(d) shows a relationship between the peak current of the ions and the concentration. The peak currents also indicated a good linearity (R2 = 0.998) to the nitrate ion concentration in the range to 0.8 mmol dm–3, whereas the peak currents of nitrite ion were almost constant, regardless

Fig. 3　SEM images of the pretreated copper at different pH and an unpretreated copper surface.

Fig. 4　SEM image of the pretreated copper at pH 2.0.

Fig. 5　Plot of the peak currents of nitrate ion against the dissolution/redeposition time. Peak currents at a fixed redeposition time (10 s) at various dissolution times (●) and at a fixed dissolution time (60 s) at various redeposition times (▲), are shown. Concentration used for nitrate ion, 1.0 mmol dm–3. Scan rate used in CV, 100 mV s–1.


of the concentration of nitrate ion. Shifts in the peak potentials for both ions are likely to be caused by a lack of reproducibility of the dissolution/redeposition time due to manual controlling the potential. Thus, we have established a simple way to achieve the simultaneous voltammetric determination of nitrate and nitrite ions.

Application of differential pulse voltammetry for the simultaneous determination of nitrate and nitrite ions

In order to improve the sensitivity and resolution for simultaneous determinations of nitrate and nitrite ions, differential pulse voltammetry (DPV) was applied. DPV experiments were carried out under the optimal conditions established by CV experiments. As can be seen in DPVs of Fig. 7(a), nitrate and nitrite ions produced well-defined reduction peaks at –0.52 and –0.23 V, respectively. The peak currents of DPVs were measured as indicated in Fig. 7(a). In addition, in order to reconfirm that there is no direct interference between nitrate and nitrite ions under our experimental conditions with DPV, we carried out following experiments, in which a concentration of nitrate (nitrite) ion was maintained constant, and that of nitrite (nitrate) was changed. Figure 7(b) shows DPVs at various concentrations of nitrate ion and a constant concentration of nitrite ion, i.e., 10 μmol dm–3. As can be seen in the inset of Fig. 7(b), a good linearity (R2 = 0.998) was observed in the relationship between the peak current and the concentration of nitrate ion up to 40 μmol dm–3. On the other hand, Fig. 7(c) shows DPVs at various concentrations of nitrite ion and a constant concentration of nitrate ion, i.e., 20 μmol dm–3. In a similar manner to the inset of Fig. 7(b), a good linearity (R2 = 0.995) was observed between the peak current and the concentration of nitrite ion in the range up to 15 μmol dm–3. These results suggest no direct interference between the nitrate

and nitrite ions in the simultaneous determinations. Figure 7(d) shows DPVs at concentrations of nitrate and nitrite ions up to 50 μmol dm–3, and the inset indicates calibration curves of the ions in the concentration range to 50 μmol dm–3. Good linearity was found for nitrate (R2 = 0.998) and nitrite ions (R2 = 0.995). The detection limits of nitrate and nitrite ions were 0.26 and 0.17 μmol dm–3, based on a signal having three-times the standard deviation of the background signal. Further, for seven repetitive determinations of nitrate and nitrite ions, the relative standard deviations (RSDs) were decided to be 1.36 and 2.2% for 10 μmol dm–3 of nitrate and nitrite ions, respectively. It is worth mentioning that the concentrations of nitrate and nitrite ions in natural water samples were a few μmol dm–3. Consequently, this method has proved to be applicable for practical applications.

Simultaneous determination of nitrate and nitrite ions in natural water

River-water samples were sampled at seven points along Metoba River in Matsumoto, Japan. The water samples were filtered through 0.45 μm membrane filters, and analyzed within ca. 24 h. The nitrate and nitrite ion levels were determined by DPV at a copper electrode pretreated by the dissolution/redeposition procedure mentioned above, and by spectrophotometry using the Griess protocol. The concentrations determined are summarized in Table 2. As can be seen in Table 2, the determination values by the two methods are in good agreement with each other. An unpaired t-test confirmed that there was no significant difference between the concentrations obtained by the two methods at a 95% confidence level. It should be mentioned that the determination values given in Table 2 were obtained from one measurement, although we took three sample readings and averaged the values.

Fig. 6　(a) CVs for different concentrations of nitrite ion at a fixed concentration of nitrate ion (0.8 mmol dm–3); (b) shows a relationship between the peak current of nitrate and nitrite ion and the concentration; (c) CVs for different concentrations of nitrate ion at a fixed concentration of nitrite ion (0.7 mmol dm–3); (d) shows a relationship between the peak current of the ions and the concentration; scan rate, 100 mV s–1.


The RSDs were found to be less than 2% for three sample measurements for nitrate and higher than 7% for nitrite ions. The higher RSD for nitrite ion may be due to low concentration and baseline variation of the DPVs. Further, the correlation between the two methods was calculated to be 0.951 for nitrate and 0.869 nitrite ions. It was confirmed that phosphate, silicate and chloride ions did not interfere up to 100, 200, and 100 μmol dm–3 when the concentrations of nitrate and nitrite ions were 1 μmol dm–3. It can be said that the proposed

voltammetric method is applicable for the simultaneous voltammetric determination of nitrate and nitrite ions in natural water samples.

Conclusions

In this work, we demonstrated that a copper electrode pretreated by the dissolution/redeposition procedure was very favorable for the electroreduction of nitrate and nitrite ions. Fortunately, since the reduction peaks of nitrate and nitrite ions were observed at distinct potentials, we could establish a method for the simultaneous determinations of nitrate and nitrite ions. Using differential pulse voltammetry at the pretreated copper electrode, we could successfully determine nitrate and nitrite ions in river-water samples. A good proportionality of the reduction peak current to the concentration of nitrate and nitrite ions was found to be in the concentration range up to 100 μmol dm–3. The detection limits were 0.26 μmol dm–1 for nitrate ion and 0.17 μmol dm–3 for nitrite ion. The proposed method is simple and sensitive for the simultaneous determinations of nitrate and nitrite ions. More reproducible data can be achieved if the potential control steps in the pretreatment procedure can be done by computer programming.

Fig. 7　(a) Differential pulse voltammogram of 5.0 μmol dm–3 of nitrate and nitrite ions in 0.1 mol dm–3 Na2SO4 (pH 2.0); (b) DPVs at various concentrations of nitrate ion and a constant concentration of nitrite ion, 10 μmol dm–3 and in the inset, a good linearity (R2 = 0.998) was observed in a relationship between the peak current and the concentration of nitrate ion up to 40 μmol dm–3; (c) DPVs at various concentrations of nitrite ion and a constant concentration of nitrate ion, 20 μmol dm–3 and in the inset, a good linearity (R2 = 0.995) was observed between the peak current and the concentration of nitrite ion in the range up to 15 μmol dm–3; (d) DPVs and calibration curves (inset) of nitrate and nitrite ions in the concentration range to 50 μmol dm–3. ● corresponds to peak currents of nitrate and ▲ corresponds to peak currents of nitrite ions; pulse amplitude, 50 mV; sampling time, 50 ms; scan rate, 18 mV s–1.

Table 2　Concentrations of nitrate and nitrite ion determined in river-water samples by DPV and spectrophotometric methods

Sample

Spectrophotometric method

DPV method

Nitrate/ μmol dm–3

Nitrite/ μmol dm–3

Nitrate/ μmol dm–3

Nitrite/ μmol dm–3

1234567

11.211.912.115.413.513.813.7

0.550.580.771.331.130.870.88

11.011.812.115.514.815.014.5

0.830.971.101.431.270.900.94


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Simultaneous Voltammetric Determination of Nitrate and