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Electrochemical study and flow-injection amperometric detection of trace NO2− at CuPtCl6 chemically modified electrode
Talanta 51 (2000) 1107 – 1115 www.elsevier.com/locate/talanta
Electrochemical study and flow-injection amperometric detection of trace NO− 2 at CuPtCl6 chemically modified electrode Jianhong Pei 1, Xiao-yuan Li * Department of Chemistry, The Hong Kong Uni6ersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Received 26 April 1999; received in revised form 26 November 1999; accepted 10 December 1999
Abstract A thin film of mixed-valent CuPtCl6 is deposited on a glassy carbon electrode by continuous cyclic scanning in a solution containing 3 ×10 − 3 M CuCl2 +3×10 − 3 M K2PtCl6 + 1 M KCl in the potential range from 700 to −800 mV. The cyclic voltammetry is used to study the electrochemical behaviors of nitrite on CuPtCl6/GC modified electrode and the electrode displays a good catalytic activity toward the oxidation of nitrite. The effects of the film thickness, pH, the electrode stability and precision have been evaluated. Experiments in flow-injection analysis are performed to characterize the electrode as an amperometric sensor for the detection of nitrite. The modified electrode shows a wide dynamic range, quite a low detection limit and short response time. The linear relationship between the flow-injection peak currents and the concentrations of nitrite is at a range of 1 ×10 − 7 –2× 10 − 3 M with a detection limit of 5 ×10 − 8 M. © 2000 Elsevier Science B.V. All rights reserved. Keywords: CME; CuPtCl6; Nitrite and FIA
1. Introduction The chemical transformation in the nitrogen cycle has been the subject of considerable interest [1], nitrite is one of active intermediate in nitrogen cycle, resulting from incompletely oxidation of ammonia or from reduction of nitrate. Also the * Corresponding author. E-mail addresses: [email protected] (J. Pei), [email protected] (X.-y. Li) 1 Present address: Department of Inorganic, Analytical and Applied Chemistry, Science II, The University of Geneva, 30 Quai, E.-Ansermet, 1211 Geneva 4, Switzerland.
occurring of nitrite salts in environment and their use as food preservation is widespread. However, now there has been increasing concern about the role of nitrite ion in the formation of N-nitrosamines, many of which have been shown to be carcinogenic [2–4]. It is, therefore, an important challenge for the medical and environmental applications that a sensitive analytical method be available for the determination of nitrite ion at lower concentration and in a fast manner. Many of analytical methods have been developed for the determination of nitrite. But not all of them are suitable for the routine trace detec-
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tion. Spectrophotometric detection techniques have widely used in the determination of nitrite, and most of them are based on the different modification of Griss reaction [5 – 8], in which the absorbance of the products generated in the reaction of nitrite with an aromatic amine is proportional to the concentration of nitrite in the sample. But the two-step introduction of reagents, pH and temperature regulation during the reaction is inconvenient, especially for the flow-injection analysis. Also these methods suffer the interference from the powerful oxidants and other colored substances. Other spectrophotometric methods are based on the interaction of nitrite in acid solution with active aromatic substances [9,10]. But the detection limits are not good for real applications. Some polarographic [11] and voltammetric methods for the detection of nitrite have been reported. They involve the catalytic reduction of NO− 2 in the presence of some polyvalent metal ions, such as molybdate [12,13], ytterbium(III) [14] and chromium [15]. All of these methods suffer from poor sensitivity and the detection limit is only 10 − 5 – 10 − 6 M. Furthermore, these methods are subject to the interference from nitrate. Glassy carbon (GC) electrode is widely used [16,17]. It is characterized by several interesting features, such as good electrical conductivity, large anodic window, very low surface porosity and finally, low and constant residual current. Its major drawback is due to a rather large overpotential for the redox of some compounds, which affects both the selectivity and detection limit. NO− 2 has a high overpotential at the surface of bare GC electrode and makes it difficult to detect directly on bare GC electrode. Oxidative NO− 2 amperometry has been reported to detect of nitrite at the bare GC electrode[18,19]. Since the oxidation involves large overpotential, the usefulness of this approach is limited. Chemically modified electrodes (CME) have been developed to decrease the overpotential for nitrite oxidation. Attempts included the use of platinum electrode coated with chemisorbed iodine with further coating of quaternized poly(4-vinylpyridine) (PVP) [20]. Some redox polymers containing the electrocatalysts such as Ru(bipy)2Cl2 [21], Os(bipy)2Cl2
[22] coordinated to PVP were also reported as electrode modifiers. The detection limit of these methods is about 10 − 6 –10 − 7 M. We have successfully developed CuPtCl6 CME on various substrates [23]. Here, we will describe the catalytic effect of this electrode toward the oxidation of nitrite. And the electrode is used as amperometric sensor for NO− 2 in flow-injection analysis (FIA) regarding the quantitative detection of the nitrite.
2. Experimental section
2.1. Chemicals Analytic grade K2PtCl6 (Aldrich, USA) and CuCl2·2H2O (Riedel-de Hae¨n, Germany) were used to prepare the modifier film on the surface of glassy carbon electrode. A stock solution of nitrite (0.100 M) was prepared by directly dissolving an appropriate amount of sodium nitrite (Nacalai Tesque, Japan) in ‘triple-distilled’ water (deionized water followed by double-distillation) and stored in the dark. The working solutions were prepared by freshly diluting the stock solution. The stock solution is stable for a week. Other chemicals employed were all of analytical grade or higher purity and used as received. All the experimental water was ‘triple-distilled’ water and the experiments were carried out at room temperature (229 1°C).
2.2. Instruments Modification of the electrode surface and cyclic voltammetric measurements were carried out on a BAS 100 B/W electrochemical workstation (Bioanalytical System, USA) controlled by the BAS 100B/W software from a personal computer. A 10-ml BAS model VC-2 glass vial as electrochemical cell was fitted with a Teflon cap. The conventional three-electrode system was employed, which include a bare GC or CuPtCl6/GC electrode (area: 0.071 cm2, BAS) as the working electrode, Platinum wire as a counter electrode and an Ag/AgCl (3 M NaCl) as a reference electrode. The whole cell is placed in a C-2 BAS
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faraday cage to avoid external interference. Prior to the measurements, the solution was deaerated by high purity nitrogen gas and a nitrogen gas atmosphere was maintained over the solution during measurement. The surface of the GC electrode was carefully polished with 0.05 mm alumina powder, and rinsed in 1 M HNO3 solution and water in sequence and then sonicated for 5 min in alcohol and 10 min in water, respectively. Amperometric measurements in flowing stream were performed on a BAS LC-4C electrochemical amperometric detector, and a flow through thin layer electrochemical cell consisting of CuPtCl6/ GC working electrode, an Ag/AgCl (3 M NaCl) reference electrode and a stainless steel auxiliary electrode. The output signal was recorded by a BAS dual-pen strip-chart Y-t recorder (MF-9045, BAS). The flow injection experiments were carried out with a PM-80 BAS solvent delivery system equipped with a rheodyne (Berkeley, CA) model 9125 injector using a 5-ml sample loop. An in-line LC-26 BAS Vacuum degasser was used to remove oxygen from the mobile phase. The mobile phase used in the experiments was 0.01 M KCl (pH= 1.5 adjusted by 1 M HCl).
Fig. 1. Cyclic voltammograms for the electrodeposition of the film. (a) the first scan. (b) continuous scans in 3× 10 − 3 M CuCl2 + 3 ×10 − 3 M K2PtCl6 + 1 M KCl, scan rate: 100 mV s − 1, Substrate: f 3 mm glassy carbon disk electrode.
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2.3. Electrode preparation The CuPtCl6 modifier film was formed onto the electrode surface by electropolymerization in the modification solution by continuous cyclic scanning (at the scan rate of 100 mV/s) the potential from 700 to −800 mV. The typical components of the modification solution are 3×10 − 3 M CuCl2 + 3× 10 − 3 M K2PtCl6 + 1 M KCl. The film thickness can be controlled by the concentration of the modification solution and the continuous scan time as well as the scan rate. There are no special electrochemical pretreatment needed before the deposition. But a clear, polished electrode surface is very important for the modification.
3. Results and discussion
3.1. Formation of CuPtCl6 film on the electrode surface The deposition of CuPtCl6 film on a GC electrode has been discussed in detail in our previous report [23]. Fig. 1 shows the typical results of modification. Fig. 1(a) (the insert) is the first cyclic voltammogram (CV). Three redox couples are observed, which correspond to Cu2 + /Cu+ (Ic/Ia), Cu+/Cu0 (IIc/IIa), and Pt4 + /Pt2 + (IIIc/ IIIa) in the complex, respectively. Fig. 1(b) is the result of continuous potential scan in the solution. They display many changes with respect to the first scan. First of all, the second redox couple in Fig. 1(a) (Cu+/Cu0 couple), disappears after three continuous scans in Fig. 1(b). Secondly, the first redox couple (Ic/Ia) in the Fig. 1(a) (Cu2 + /Cu+ couple) shifts for about 200 mV toward positive direction in Fig. 1(b) in later scans. The Ic/Ia pair disappeared or merged into the new peak at the more positive potential, which increases with the number of scans. A shoulder on the negative side of the new peaks was observable in the anodic process. Thirdly, the similar observations as that for Ic/Ia were also made for the redox peaks of Pt4 + /Pt2 + pair (IIIc/IIIa) in the complex. The reduction peak of Pt4 + shifted to a more positive position. It appeared at the potential near −400
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IIIc/IIIa, increase markedly with the number of scans, especially for the Cu2 + /Cu+ redox couple in the film. Fig. 1(b) clearly shows the gradual increase of peak currents associated with the elec-
Fig. 2. Cyclic voltammograms of (1) bare GC, (2) CuPtCl6/GC electrodes in 4 × 10 − 4 M NaNO2 + 1 M KCl (pH 1.5). Scan rate: 50 mV/s.
Fig. 4. The effect of pH on the response current of 3× 10 − 4 M nitrite in 1 M KCl at CuPtCl6/GC. (1) The effect on the peak potential. (2) The effect on the peak current.
Fig. 3. Cyclic voltammograms of nitrite on CuPtCl6/GC modified electrode: The concentrations of nitrite: (1) 0, (2) 4 × 10 − 4 M, (3) 8× 10 − 4 M, (4) 1.2× 10 − 3 M, (5) 1.6× 10 − 3 M. Supporting electrolyte: 1 M KCl (pH 1.5). Scan rate: 50 mV/s.
mV (vs SCE) and also bears a shoulder at the negative side in the continuous scans. Fourthly, the currents of the two redox couples, Ic/Ia and
Fig. 5. The relationship between the peak current and the flow rate on CuPtCl6/GC. Applied potential: 850 mV (vs Ag/AgCl). Carrier electrolyte: 0.01 M KCl/HCl, pH 1.5. Concentration of nitrite injection: 5 ×10 − 5 M nitrite. Loop: 5 ml.
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Fig. 6. Hydrodynamic voltammograms of 5 × 10 − 5 M nitrite in FIA at (1) bare GC. (2) CuPtCl6/GC. Flow rate: 0.2 ml/min. Others are same as Fig. 5 except potential.
tropolymerization/deposition of CuPtCl6. It reflects the accumulation of the modifier at the electrode surface. In our previous report [23], the mechanism of the film formation on the electrode surface have been discussed in detailed. It proposes that the film is a mixed-valent, one-dimensional stacked, inorganic conductive polymer. The component of the film is comprised of alternating octahedral d6 Pt4 + complexes and square planar d8 Pt2 + complexes. Cu2 + as copolymer is also deposited in the film. This kind of one-dimensional crystal had been reported to be prepared by the evaporation of the saturated solution [24,25].
3.2. Electrochemically catalytic oxidation of nitrite The cyclic voltammetry was used to investigate the electrocatalytic activity of CuPtCl6/GC electrode toward the oxidation of nitrite. The strategy used in these investigations was essentially straightforward and involved scanning the potential at the working electrode from 400 to 900 mV in a solution only containing supporting electrolyte of 1 M KCl (pH =1.5 adjusted by diluted
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HCl) with a bare GC electrode and repeating with a CuPtCl6/GC electrode. The same procedure was then carried out in the solution, in addition to the supporting electrolyte, also containing 4×10 − 4 M nitrite in order to ensure the electrochemical reaction observed was that of nitrite oxidation at the modified electrode rather than some reaction occurring in the film itself or occurring on the unmodified electrode. The results show that the bare GC electrode does not display any peak current in either presence or absence of nitrite in 1 M KCl solution within the studied potential range. Neither CuPtCl6/GC electrode itself displays any peak current in the absence of nitrite. Nevertheless, CuPtCl6/GC electrode displays a greatly enhancement to the oxidation current of nitrite. Fig. 2 shows the typical CVs of the solution containing 4×10 − 4 M nitrite+ 1 M KCl (pH= 1.5) on the bare GC (curve 1) and CuPtCl6/GC (curve 2) electrodes, respectively. The oxidation of nitrite does not show a clear peak at the bare GC electrode at the scanned potential range (curve 1), and only a small shoulder is observed at potentials where large background anodic currents commence. It means that nitrite is not effectively oxidized in the studied potential range at bare GC electrode. In contrast to the response in the bare GC electrode, a greatly enhanced oxidation current of nitrite in the same solution appears on the surface of a CuPtCl6/GC electrode (curve 2 in Fig. 2). It indicates that the catalytic oxidation of nitrite occurs at the CuPtCl6/GC electrode. Fig. 3 shows the CVs recorded at different concentrations of nitrite on the surface of a CuPtCl6/GC electrode. Curve 1 in Fig. 3 shows the CV response of a CuPtCl6/GC electrode in a solution only containing supporting electrolyte (1 M KCl, pH= 1.5 adjusted by HCl). It was found that no obvious peak current was observed except the residual current at the scanned potential range. However, a strongly enhanced oxidation current was appeared and increased with the concentrations of nitrite (curves 2–5 in Fig. 3). The reproducibility of the electrode was tested in 1 × 10 − 4 M nitrite. After each measurement, the electrode was taken out of the solution and rinsed with water and then re-inserted into the
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solution for the next measurement. The results were shown that the relative standard deviation of eight measurements of 1× 10 − 4 M nitrite is 3.1%. The modified electrode has good stability and it can be stored in the air without any special protection step. It can maintain at least 1 month stable response (longer time has not been tested) without observable current change. The electrode can be carried out more than 100 measurements without noticeable current decrease.
3.3. The effect of film thickness It is found that the film thickness has much influence on the oxidation current of nitrite. The modifier on the surface of the GC electrode is not completely uniform and the surface is very rough and some particles also appear on the electrode surface [23]. The more scans, the rougher of the electrode surface and the more particles appear at
the electrode surface, which will result in increasing the active area. However, if the film is too thick, the film will hinder the electron propagation in the film and spoil the current response, which will result in decreasing the current. In order to find out a suitable thickness of the modifier film for the detection of nitrite, the effect of the film thickness on the response of nitrite was carried out on the CuPtCl6/GC electrode. It was found that the response currents increased with the scanning time. But the shape of the peaks broadened when the scanning time was more than 20 min. Too long scanning time was not of benefit to the lower detection limit either since the background current also increased. If the scanning time is too short and the modifier film is too thin, the poor stability and short using life will be observed. With the compromise, usually a cyclic scanning time of 15 min is taken for the electrode preparation in this work.
Fig. 7. FIA-EC detection of nitrite. Flow rate: 0.1 ml/min; Applied potential: 850 mV. Other conditions are same as Fig. 5.
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3.4. The effect of pH The pH has much effect on both the peak current and the peak potential of the oxidation of nitrite at CuPtCl6/GC electrode. The influence of pH was studied at pH from 0.5 to 4 and the results are depicted in Fig. 4. The maximal peak current response was appeared at pH 1.5, and the response current decreased when pH greater than or less than this value. So pH 1.5 was adopted for the detection of nitrite in all of the following measurements unless otherwise stated. The pH effect on the peak potential is very complicated, which displays two different response slope in the pH range of 0.5 – 1.5, and 1.5 – 4, respectively. When pH is less than 1.5, the pH effect on the peak potential is not obvious compared with that of pH greater than 1.5. When pH is at a range of 1.5–4, the peak potential shifts to a more negative direction at a rate of 32 mV/pH with the increase of pH value. It indicates that protons were involved in the electrochemical reaction process. If we think that the electrochemical reaction is the oxidation of nitrite to nitrate, and the equation of electrode reaction can be described as following: NO2− + H2O-2e − NO3− +2H+ In this equation, the number of protons involved in the electrochemical reaction is equal to the number of electrons involved in the electrochemical reaction. According to the Nernst equation, the peak potential should be shifted at a rate of 60 mV/pH instead of 32 mV/pH. The reason may be the electrochemical oxidation of nitrite at CuPtCl6/GC is not a simply one-step reaction as described as the above equation. Some preceding reactions and multi-step reactions must be involved in the electrochemical reaction processes and make the peak potential shift at a rate of 32 mV/pH instead of 60 mV/pH. The pH effect on the peak potential is different at different pH ranges, such as at a lower pH ( B1.5), the shiftiness of the peak potential is much slower than that of pH\ 1.5, indicating that the electrochemical reaction mechanism is different at different pH media. As we know, the redox of nitrite is very complicated, most of them involving proton participation and nitrite exist mainly as NO+ in the
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acidic media, the self-catalytic reaction will happen to produce NO and NO− 3 . The exact reaction mechanism of catalytic oxidation of nitrite at CuPtCl6/GC is still under study.
3.5. The interference experiment The common ions were tested for the possible interference for the detection of nitrite. The criterion of the interference is based on the current response. If the addition of given molecule or ion causes the current change of 10% or more, we consider it to interfere. In the experiments of testing the possible interference, a 1× 10 − 4 M nitrite was used. It is found that 500-folds or above Na+, K+, Ca2 + , Mg2 + , Ba2 + , PO34 − , 2+ , SO24 − ; 200-folds Ni2 + , CO23 − , Cl−, NO− 3 , Zn 2+ 2+ + 3− Co , Cu , Fe(CN)6 , NH+ 4 , Li ; 100-folds − 2+ 2+ 4− Br , Fe(CN)6 , Pb , Cd , SO23 − , S2O23 − , C2O24 − ; Although ascorbic acid, H2O2 can produce oxidation current at CuPtCl6/GC electrode surface, the peak potential is much negative compared with that of nitrite, 50-folds ascorbic acid, H2O2 did not interfere with the determination of nitrite. But the same concentration of I− can interfere with the determination of nitrite, which causes the current response to decrease. It is because I− can react with nitrite rapidly and cause the concentration of nitrite to decrease.
3.6. FIA and amperometrical determination of nitrite The long-term stability of the attached moieties is one of the key factors for the performance of CMEs in the flowing system [26]. The rigorous hydrodynamic conditions of the flowing systems, which can dissolve the modifier layer, can have an adverse effect on the stability of CMEs. And some modifiers easily decomposed on the electrode surface make them unsuitable for using in the flowing system and this has limited the applications of CME in FIA. However, the experiment results show that the CuPtCl6/GC modified electrode is very stable even in the flowing stream system. There are no obvious observable current decrease even at a fast flow rate and the potential applying to the electrode. The lifetime of this modified
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electrode is at least 1 week in the flowing system. It displays that CuPtCl6 film can be strongly attached on the surface of glassy carbon.
3.6.1. The effect of carrier flow-rate Fig. 5 shows the effect of the carrier flow rate on the amperometric response at a CuPtCl6/GC electrode to 5×10 − 5 M nitrite in FIA, which the operating potential is at 850 mV. The flow injection peak current is strongly dependent upon the flow rate, which displays an exponential decrease in current response on increasing the flow rate from 0.05 to 0.3 ml/min. But according to the theoretical expectations based on laminar flow assumptions, the dispersion and hydrodynamic effects would predict the opposite pattern, i.e. increasing the flow-rate would increase the electrode response current due to the increase of the hydrodynamics and reduction of the diffusion layer thickness [26,27]. However, this kind of contrary behavior was also found on other systems using CME in the flowing stream system [28,29]. Some explanations [28] attributed the reason causing the difference between the theoretical expectation and the experimental results to the slow adsorption of the reactant or slow desorption of the product. But in this work, there is no adsorption involved in the electrode process. Apparently, the reason for the decreased response on higher flow-rate is due to the slow catalytic reaction between the modifier film and analyte. The decreased residence time of the analyte on the electrode surface on increasing the flow rate and the slow kinetic process make it produce smaller current response at the surface of CuPtCl6/GC electrode. At a slower flow-rate, the analyte has a longer residence time and can be full electrochemically oxidized on the surface of CuPtCl6/GC electrode and got a higher response. But if the flow rate is too slow ( B0.05 ml/min), a significant broadening peak was observed. This is due to the dispersion of the sample plug. Also if the flow rate is too slow, the analytical time will be prolonged. So a flow rate of 0.1 ml/min was recommended. 3.6.2. Hydrodynamic 6oltammograms of nitrite The optimum applied potential was also deter-
mined by hydrodynamic voltammetry (HDV), in which the response current was measured against the applied potential, point by point every 50 mV in the range from 600 to 950 mV on both the bare GC (curve 1) and CuPtCl6/GC electrodes (curve 2). The results are shown in Fig. 6. At the bare GC electrode, only a very small current was observed at the potential of more than 750 mV, indicative of poor sensitivity for the detection of nitrite. However, a greatly enhanced current versus applied potential was obtained at a CuPtCl6/GC. Every point in the figure was taken from the average values of three measurements. Although the higher potential is of benefit to the current enhancement, we choose the applied potential of 850 mV in this work. It can get high sensitive response and satisfy usually analytic purpose. Another reason is that choosing relatively lower applied potential is good both to minimize the interference and to maintain a relatively lower background signal. Lower applied potential is also good for the stability and the using life of the electrochemical sensor.
3.6.3. FIA-amperometric determination of nitrite From the results discussed above, the CuPtCl6/ GC electrode in a thin-layer electrochemical cell can maintain its catalytic stability under constantpotential operation. With all these experiments in mind, the responses in the FIA can be obtained for the oxidation of nitrite under the optimized conditions: The carrier solution of 0.01 M KCl+ HCl, pH 1.5, the applied potential of 850 mV and the flow rate of the carrier solution of 0.1 ml/min are used in the measurements. Fig. 7 illustrates the flow-injection current-time profile for different concentrations of nitrite at a CuPtCl6/GC. Good linear relationship between the peak currents and the concentrations of nitrite was obtained over the range of 1× 10 − 7 –1× 10 − 3 M. The detection limit is 5 ×10 − 8 M, which was calculated from the signal to noise level of 3. This CME can stand for more than 100 injections without noticeable current decrease. It is also easy to refresh the electrode surface just by keeping the potential cyclic scanning in the modification solution.
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4. Conclusions
References
CuPtCl6/GC modified electrode was prepared by cyclic scanning in a solution containing 3× 10 − 3 M CuCl2 +3 × 10 − 3 M K2PtCl6 + 1 M KCl and the applied potential is from 700 to −800 mV at a scan rate of 100 mV/s. This electrode was used for the catalytic oxidation of nitrite. At the bare GC electrode, nitrite was not effectively oxidized prior to the discharge of the supporting electrolyte. But CuPtCl6/GC electrode shows a greatly catalytic effect on the oxidation of nitrite. Cyclic voltammetry was used to investigate the electrochemical behavior of nitrite at the CuPtCl6/GC electrode. The effects of the film thickness, pH and coexisting ions on the response current have been evaluated. The electrode was used as an amperometric sensor in FIA for the detection of nitrite. Several experimental parameters (flow rate, applied potential) and the electrode stability and precision have been evaluated. This electrode displays a very wide dynamic range, quite low detection limits and short response times and easy preparation. The linear relationship between the flow-injection peak currents and the concentrations of nitrite is at a range of 1×10 − 7 – 2 ×10 − 3 M, with a detection limit of 5×10 − 8 M.
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Acknowledgements We acknowledge the support of this project by Research Grant Council of Hong Kong and by The Hong Kong University of Science and Technology. It is also partially supported by NNSF of China.
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Electrochemical study and flow-injection amperometric detection of trace NO− 2 at CuPtCl6 chemically modified electrode Jianhong Pei 1, Xiao-yuan Li * Department of Chemistry, The Hong Kong Uni6ersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Received 26 April 1999; received in revised form 26 November 1999; accepted 10 December 1999
Abstract A thin film of mixed-valent CuPtCl6 is deposited on a glassy carbon electrode by continuous cyclic scanning in a solution containing 3 ×10 − 3 M CuCl2 +3×10 − 3 M K2PtCl6 + 1 M KCl in the potential range from 700 to −800 mV. The cyclic voltammetry is used to study the electrochemical behaviors of nitrite on CuPtCl6/GC modified electrode and the electrode displays a good catalytic activity toward the oxidation of nitrite. The effects of the film thickness, pH, the electrode stability and precision have been evaluated. Experiments in flow-injection analysis are performed to characterize the electrode as an amperometric sensor for the detection of nitrite. The modified electrode shows a wide dynamic range, quite a low detection limit and short response time. The linear relationship between the flow-injection peak currents and the concentrations of nitrite is at a range of 1 ×10 − 7 –2× 10 − 3 M with a detection limit of 5 ×10 − 8 M. © 2000 Elsevier Science B.V. All rights reserved. Keywords: CME; CuPtCl6; Nitrite and FIA
1. Introduction The chemical transformation in the nitrogen cycle has been the subject of considerable interest [1], nitrite is one of active intermediate in nitrogen cycle, resulting from incompletely oxidation of ammonia or from reduction of nitrate. Also the * Corresponding author. E-mail addresses: [email protected] (J. Pei), [email protected] (X.-y. Li) 1 Present address: Department of Inorganic, Analytical and Applied Chemistry, Science II, The University of Geneva, 30 Quai, E.-Ansermet, 1211 Geneva 4, Switzerland.
occurring of nitrite salts in environment and their use as food preservation is widespread. However, now there has been increasing concern about the role of nitrite ion in the formation of N-nitrosamines, many of which have been shown to be carcinogenic [2–4]. It is, therefore, an important challenge for the medical and environmental applications that a sensitive analytical method be available for the determination of nitrite ion at lower concentration and in a fast manner. Many of analytical methods have been developed for the determination of nitrite. But not all of them are suitable for the routine trace detec-
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tion. Spectrophotometric detection techniques have widely used in the determination of nitrite, and most of them are based on the different modification of Griss reaction [5 – 8], in which the absorbance of the products generated in the reaction of nitrite with an aromatic amine is proportional to the concentration of nitrite in the sample. But the two-step introduction of reagents, pH and temperature regulation during the reaction is inconvenient, especially for the flow-injection analysis. Also these methods suffer the interference from the powerful oxidants and other colored substances. Other spectrophotometric methods are based on the interaction of nitrite in acid solution with active aromatic substances [9,10]. But the detection limits are not good for real applications. Some polarographic [11] and voltammetric methods for the detection of nitrite have been reported. They involve the catalytic reduction of NO− 2 in the presence of some polyvalent metal ions, such as molybdate [12,13], ytterbium(III) [14] and chromium [15]. All of these methods suffer from poor sensitivity and the detection limit is only 10 − 5 – 10 − 6 M. Furthermore, these methods are subject to the interference from nitrate. Glassy carbon (GC) electrode is widely used [16,17]. It is characterized by several interesting features, such as good electrical conductivity, large anodic window, very low surface porosity and finally, low and constant residual current. Its major drawback is due to a rather large overpotential for the redox of some compounds, which affects both the selectivity and detection limit. NO− 2 has a high overpotential at the surface of bare GC electrode and makes it difficult to detect directly on bare GC electrode. Oxidative NO− 2 amperometry has been reported to detect of nitrite at the bare GC electrode[18,19]. Since the oxidation involves large overpotential, the usefulness of this approach is limited. Chemically modified electrodes (CME) have been developed to decrease the overpotential for nitrite oxidation. Attempts included the use of platinum electrode coated with chemisorbed iodine with further coating of quaternized poly(4-vinylpyridine) (PVP) [20]. Some redox polymers containing the electrocatalysts such as Ru(bipy)2Cl2 [21], Os(bipy)2Cl2
[22] coordinated to PVP were also reported as electrode modifiers. The detection limit of these methods is about 10 − 6 –10 − 7 M. We have successfully developed CuPtCl6 CME on various substrates [23]. Here, we will describe the catalytic effect of this electrode toward the oxidation of nitrite. And the electrode is used as amperometric sensor for NO− 2 in flow-injection analysis (FIA) regarding the quantitative detection of the nitrite.
2. Experimental section
2.1. Chemicals Analytic grade K2PtCl6 (Aldrich, USA) and CuCl2·2H2O (Riedel-de Hae¨n, Germany) were used to prepare the modifier film on the surface of glassy carbon electrode. A stock solution of nitrite (0.100 M) was prepared by directly dissolving an appropriate amount of sodium nitrite (Nacalai Tesque, Japan) in ‘triple-distilled’ water (deionized water followed by double-distillation) and stored in the dark. The working solutions were prepared by freshly diluting the stock solution. The stock solution is stable for a week. Other chemicals employed were all of analytical grade or higher purity and used as received. All the experimental water was ‘triple-distilled’ water and the experiments were carried out at room temperature (229 1°C).
2.2. Instruments Modification of the electrode surface and cyclic voltammetric measurements were carried out on a BAS 100 B/W electrochemical workstation (Bioanalytical System, USA) controlled by the BAS 100B/W software from a personal computer. A 10-ml BAS model VC-2 glass vial as electrochemical cell was fitted with a Teflon cap. The conventional three-electrode system was employed, which include a bare GC or CuPtCl6/GC electrode (area: 0.071 cm2, BAS) as the working electrode, Platinum wire as a counter electrode and an Ag/AgCl (3 M NaCl) as a reference electrode. The whole cell is placed in a C-2 BAS
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faraday cage to avoid external interference. Prior to the measurements, the solution was deaerated by high purity nitrogen gas and a nitrogen gas atmosphere was maintained over the solution during measurement. The surface of the GC electrode was carefully polished with 0.05 mm alumina powder, and rinsed in 1 M HNO3 solution and water in sequence and then sonicated for 5 min in alcohol and 10 min in water, respectively. Amperometric measurements in flowing stream were performed on a BAS LC-4C electrochemical amperometric detector, and a flow through thin layer electrochemical cell consisting of CuPtCl6/ GC working electrode, an Ag/AgCl (3 M NaCl) reference electrode and a stainless steel auxiliary electrode. The output signal was recorded by a BAS dual-pen strip-chart Y-t recorder (MF-9045, BAS). The flow injection experiments were carried out with a PM-80 BAS solvent delivery system equipped with a rheodyne (Berkeley, CA) model 9125 injector using a 5-ml sample loop. An in-line LC-26 BAS Vacuum degasser was used to remove oxygen from the mobile phase. The mobile phase used in the experiments was 0.01 M KCl (pH= 1.5 adjusted by 1 M HCl).
Fig. 1. Cyclic voltammograms for the electrodeposition of the film. (a) the first scan. (b) continuous scans in 3× 10 − 3 M CuCl2 + 3 ×10 − 3 M K2PtCl6 + 1 M KCl, scan rate: 100 mV s − 1, Substrate: f 3 mm glassy carbon disk electrode.
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2.3. Electrode preparation The CuPtCl6 modifier film was formed onto the electrode surface by electropolymerization in the modification solution by continuous cyclic scanning (at the scan rate of 100 mV/s) the potential from 700 to −800 mV. The typical components of the modification solution are 3×10 − 3 M CuCl2 + 3× 10 − 3 M K2PtCl6 + 1 M KCl. The film thickness can be controlled by the concentration of the modification solution and the continuous scan time as well as the scan rate. There are no special electrochemical pretreatment needed before the deposition. But a clear, polished electrode surface is very important for the modification.
3. Results and discussion
3.1. Formation of CuPtCl6 film on the electrode surface The deposition of CuPtCl6 film on a GC electrode has been discussed in detail in our previous report [23]. Fig. 1 shows the typical results of modification. Fig. 1(a) (the insert) is the first cyclic voltammogram (CV). Three redox couples are observed, which correspond to Cu2 + /Cu+ (Ic/Ia), Cu+/Cu0 (IIc/IIa), and Pt4 + /Pt2 + (IIIc/ IIIa) in the complex, respectively. Fig. 1(b) is the result of continuous potential scan in the solution. They display many changes with respect to the first scan. First of all, the second redox couple in Fig. 1(a) (Cu+/Cu0 couple), disappears after three continuous scans in Fig. 1(b). Secondly, the first redox couple (Ic/Ia) in the Fig. 1(a) (Cu2 + /Cu+ couple) shifts for about 200 mV toward positive direction in Fig. 1(b) in later scans. The Ic/Ia pair disappeared or merged into the new peak at the more positive potential, which increases with the number of scans. A shoulder on the negative side of the new peaks was observable in the anodic process. Thirdly, the similar observations as that for Ic/Ia were also made for the redox peaks of Pt4 + /Pt2 + pair (IIIc/IIIa) in the complex. The reduction peak of Pt4 + shifted to a more positive position. It appeared at the potential near −400
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IIIc/IIIa, increase markedly with the number of scans, especially for the Cu2 + /Cu+ redox couple in the film. Fig. 1(b) clearly shows the gradual increase of peak currents associated with the elec-
Fig. 2. Cyclic voltammograms of (1) bare GC, (2) CuPtCl6/GC electrodes in 4 × 10 − 4 M NaNO2 + 1 M KCl (pH 1.5). Scan rate: 50 mV/s.
Fig. 4. The effect of pH on the response current of 3× 10 − 4 M nitrite in 1 M KCl at CuPtCl6/GC. (1) The effect on the peak potential. (2) The effect on the peak current.
Fig. 3. Cyclic voltammograms of nitrite on CuPtCl6/GC modified electrode: The concentrations of nitrite: (1) 0, (2) 4 × 10 − 4 M, (3) 8× 10 − 4 M, (4) 1.2× 10 − 3 M, (5) 1.6× 10 − 3 M. Supporting electrolyte: 1 M KCl (pH 1.5). Scan rate: 50 mV/s.
mV (vs SCE) and also bears a shoulder at the negative side in the continuous scans. Fourthly, the currents of the two redox couples, Ic/Ia and
Fig. 5. The relationship between the peak current and the flow rate on CuPtCl6/GC. Applied potential: 850 mV (vs Ag/AgCl). Carrier electrolyte: 0.01 M KCl/HCl, pH 1.5. Concentration of nitrite injection: 5 ×10 − 5 M nitrite. Loop: 5 ml.
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Fig. 6. Hydrodynamic voltammograms of 5 × 10 − 5 M nitrite in FIA at (1) bare GC. (2) CuPtCl6/GC. Flow rate: 0.2 ml/min. Others are same as Fig. 5 except potential.
tropolymerization/deposition of CuPtCl6. It reflects the accumulation of the modifier at the electrode surface. In our previous report [23], the mechanism of the film formation on the electrode surface have been discussed in detailed. It proposes that the film is a mixed-valent, one-dimensional stacked, inorganic conductive polymer. The component of the film is comprised of alternating octahedral d6 Pt4 + complexes and square planar d8 Pt2 + complexes. Cu2 + as copolymer is also deposited in the film. This kind of one-dimensional crystal had been reported to be prepared by the evaporation of the saturated solution [24,25].
3.2. Electrochemically catalytic oxidation of nitrite The cyclic voltammetry was used to investigate the electrocatalytic activity of CuPtCl6/GC electrode toward the oxidation of nitrite. The strategy used in these investigations was essentially straightforward and involved scanning the potential at the working electrode from 400 to 900 mV in a solution only containing supporting electrolyte of 1 M KCl (pH =1.5 adjusted by diluted
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HCl) with a bare GC electrode and repeating with a CuPtCl6/GC electrode. The same procedure was then carried out in the solution, in addition to the supporting electrolyte, also containing 4×10 − 4 M nitrite in order to ensure the electrochemical reaction observed was that of nitrite oxidation at the modified electrode rather than some reaction occurring in the film itself or occurring on the unmodified electrode. The results show that the bare GC electrode does not display any peak current in either presence or absence of nitrite in 1 M KCl solution within the studied potential range. Neither CuPtCl6/GC electrode itself displays any peak current in the absence of nitrite. Nevertheless, CuPtCl6/GC electrode displays a greatly enhancement to the oxidation current of nitrite. Fig. 2 shows the typical CVs of the solution containing 4×10 − 4 M nitrite+ 1 M KCl (pH= 1.5) on the bare GC (curve 1) and CuPtCl6/GC (curve 2) electrodes, respectively. The oxidation of nitrite does not show a clear peak at the bare GC electrode at the scanned potential range (curve 1), and only a small shoulder is observed at potentials where large background anodic currents commence. It means that nitrite is not effectively oxidized in the studied potential range at bare GC electrode. In contrast to the response in the bare GC electrode, a greatly enhanced oxidation current of nitrite in the same solution appears on the surface of a CuPtCl6/GC electrode (curve 2 in Fig. 2). It indicates that the catalytic oxidation of nitrite occurs at the CuPtCl6/GC electrode. Fig. 3 shows the CVs recorded at different concentrations of nitrite on the surface of a CuPtCl6/GC electrode. Curve 1 in Fig. 3 shows the CV response of a CuPtCl6/GC electrode in a solution only containing supporting electrolyte (1 M KCl, pH= 1.5 adjusted by HCl). It was found that no obvious peak current was observed except the residual current at the scanned potential range. However, a strongly enhanced oxidation current was appeared and increased with the concentrations of nitrite (curves 2–5 in Fig. 3). The reproducibility of the electrode was tested in 1 × 10 − 4 M nitrite. After each measurement, the electrode was taken out of the solution and rinsed with water and then re-inserted into the
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solution for the next measurement. The results were shown that the relative standard deviation of eight measurements of 1× 10 − 4 M nitrite is 3.1%. The modified electrode has good stability and it can be stored in the air without any special protection step. It can maintain at least 1 month stable response (longer time has not been tested) without observable current change. The electrode can be carried out more than 100 measurements without noticeable current decrease.
3.3. The effect of film thickness It is found that the film thickness has much influence on the oxidation current of nitrite. The modifier on the surface of the GC electrode is not completely uniform and the surface is very rough and some particles also appear on the electrode surface [23]. The more scans, the rougher of the electrode surface and the more particles appear at
the electrode surface, which will result in increasing the active area. However, if the film is too thick, the film will hinder the electron propagation in the film and spoil the current response, which will result in decreasing the current. In order to find out a suitable thickness of the modifier film for the detection of nitrite, the effect of the film thickness on the response of nitrite was carried out on the CuPtCl6/GC electrode. It was found that the response currents increased with the scanning time. But the shape of the peaks broadened when the scanning time was more than 20 min. Too long scanning time was not of benefit to the lower detection limit either since the background current also increased. If the scanning time is too short and the modifier film is too thin, the poor stability and short using life will be observed. With the compromise, usually a cyclic scanning time of 15 min is taken for the electrode preparation in this work.
Fig. 7. FIA-EC detection of nitrite. Flow rate: 0.1 ml/min; Applied potential: 850 mV. Other conditions are same as Fig. 5.
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3.4. The effect of pH The pH has much effect on both the peak current and the peak potential of the oxidation of nitrite at CuPtCl6/GC electrode. The influence of pH was studied at pH from 0.5 to 4 and the results are depicted in Fig. 4. The maximal peak current response was appeared at pH 1.5, and the response current decreased when pH greater than or less than this value. So pH 1.5 was adopted for the detection of nitrite in all of the following measurements unless otherwise stated. The pH effect on the peak potential is very complicated, which displays two different response slope in the pH range of 0.5 – 1.5, and 1.5 – 4, respectively. When pH is less than 1.5, the pH effect on the peak potential is not obvious compared with that of pH greater than 1.5. When pH is at a range of 1.5–4, the peak potential shifts to a more negative direction at a rate of 32 mV/pH with the increase of pH value. It indicates that protons were involved in the electrochemical reaction process. If we think that the electrochemical reaction is the oxidation of nitrite to nitrate, and the equation of electrode reaction can be described as following: NO2− + H2O-2e − NO3− +2H+ In this equation, the number of protons involved in the electrochemical reaction is equal to the number of electrons involved in the electrochemical reaction. According to the Nernst equation, the peak potential should be shifted at a rate of 60 mV/pH instead of 32 mV/pH. The reason may be the electrochemical oxidation of nitrite at CuPtCl6/GC is not a simply one-step reaction as described as the above equation. Some preceding reactions and multi-step reactions must be involved in the electrochemical reaction processes and make the peak potential shift at a rate of 32 mV/pH instead of 60 mV/pH. The pH effect on the peak potential is different at different pH ranges, such as at a lower pH ( B1.5), the shiftiness of the peak potential is much slower than that of pH\ 1.5, indicating that the electrochemical reaction mechanism is different at different pH media. As we know, the redox of nitrite is very complicated, most of them involving proton participation and nitrite exist mainly as NO+ in the
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acidic media, the self-catalytic reaction will happen to produce NO and NO− 3 . The exact reaction mechanism of catalytic oxidation of nitrite at CuPtCl6/GC is still under study.
3.5. The interference experiment The common ions were tested for the possible interference for the detection of nitrite. The criterion of the interference is based on the current response. If the addition of given molecule or ion causes the current change of 10% or more, we consider it to interfere. In the experiments of testing the possible interference, a 1× 10 − 4 M nitrite was used. It is found that 500-folds or above Na+, K+, Ca2 + , Mg2 + , Ba2 + , PO34 − , 2+ , SO24 − ; 200-folds Ni2 + , CO23 − , Cl−, NO− 3 , Zn 2+ 2+ + 3− Co , Cu , Fe(CN)6 , NH+ 4 , Li ; 100-folds − 2+ 2+ 4− Br , Fe(CN)6 , Pb , Cd , SO23 − , S2O23 − , C2O24 − ; Although ascorbic acid, H2O2 can produce oxidation current at CuPtCl6/GC electrode surface, the peak potential is much negative compared with that of nitrite, 50-folds ascorbic acid, H2O2 did not interfere with the determination of nitrite. But the same concentration of I− can interfere with the determination of nitrite, which causes the current response to decrease. It is because I− can react with nitrite rapidly and cause the concentration of nitrite to decrease.
3.6. FIA and amperometrical determination of nitrite The long-term stability of the attached moieties is one of the key factors for the performance of CMEs in the flowing system [26]. The rigorous hydrodynamic conditions of the flowing systems, which can dissolve the modifier layer, can have an adverse effect on the stability of CMEs. And some modifiers easily decomposed on the electrode surface make them unsuitable for using in the flowing system and this has limited the applications of CME in FIA. However, the experiment results show that the CuPtCl6/GC modified electrode is very stable even in the flowing stream system. There are no obvious observable current decrease even at a fast flow rate and the potential applying to the electrode. The lifetime of this modified
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electrode is at least 1 week in the flowing system. It displays that CuPtCl6 film can be strongly attached on the surface of glassy carbon.
3.6.1. The effect of carrier flow-rate Fig. 5 shows the effect of the carrier flow rate on the amperometric response at a CuPtCl6/GC electrode to 5×10 − 5 M nitrite in FIA, which the operating potential is at 850 mV. The flow injection peak current is strongly dependent upon the flow rate, which displays an exponential decrease in current response on increasing the flow rate from 0.05 to 0.3 ml/min. But according to the theoretical expectations based on laminar flow assumptions, the dispersion and hydrodynamic effects would predict the opposite pattern, i.e. increasing the flow-rate would increase the electrode response current due to the increase of the hydrodynamics and reduction of the diffusion layer thickness [26,27]. However, this kind of contrary behavior was also found on other systems using CME in the flowing stream system [28,29]. Some explanations [28] attributed the reason causing the difference between the theoretical expectation and the experimental results to the slow adsorption of the reactant or slow desorption of the product. But in this work, there is no adsorption involved in the electrode process. Apparently, the reason for the decreased response on higher flow-rate is due to the slow catalytic reaction between the modifier film and analyte. The decreased residence time of the analyte on the electrode surface on increasing the flow rate and the slow kinetic process make it produce smaller current response at the surface of CuPtCl6/GC electrode. At a slower flow-rate, the analyte has a longer residence time and can be full electrochemically oxidized on the surface of CuPtCl6/GC electrode and got a higher response. But if the flow rate is too slow ( B0.05 ml/min), a significant broadening peak was observed. This is due to the dispersion of the sample plug. Also if the flow rate is too slow, the analytical time will be prolonged. So a flow rate of 0.1 ml/min was recommended. 3.6.2. Hydrodynamic 6oltammograms of nitrite The optimum applied potential was also deter-
mined by hydrodynamic voltammetry (HDV), in which the response current was measured against the applied potential, point by point every 50 mV in the range from 600 to 950 mV on both the bare GC (curve 1) and CuPtCl6/GC electrodes (curve 2). The results are shown in Fig. 6. At the bare GC electrode, only a very small current was observed at the potential of more than 750 mV, indicative of poor sensitivity for the detection of nitrite. However, a greatly enhanced current versus applied potential was obtained at a CuPtCl6/GC. Every point in the figure was taken from the average values of three measurements. Although the higher potential is of benefit to the current enhancement, we choose the applied potential of 850 mV in this work. It can get high sensitive response and satisfy usually analytic purpose. Another reason is that choosing relatively lower applied potential is good both to minimize the interference and to maintain a relatively lower background signal. Lower applied potential is also good for the stability and the using life of the electrochemical sensor.
3.6.3. FIA-amperometric determination of nitrite From the results discussed above, the CuPtCl6/ GC electrode in a thin-layer electrochemical cell can maintain its catalytic stability under constantpotential operation. With all these experiments in mind, the responses in the FIA can be obtained for the oxidation of nitrite under the optimized conditions: The carrier solution of 0.01 M KCl+ HCl, pH 1.5, the applied potential of 850 mV and the flow rate of the carrier solution of 0.1 ml/min are used in the measurements. Fig. 7 illustrates the flow-injection current-time profile for different concentrations of nitrite at a CuPtCl6/GC. Good linear relationship between the peak currents and the concentrations of nitrite was obtained over the range of 1× 10 − 7 –1× 10 − 3 M. The detection limit is 5 ×10 − 8 M, which was calculated from the signal to noise level of 3. This CME can stand for more than 100 injections without noticeable current decrease. It is also easy to refresh the electrode surface just by keeping the potential cyclic scanning in the modification solution.
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4. Conclusions
References
CuPtCl6/GC modified electrode was prepared by cyclic scanning in a solution containing 3× 10 − 3 M CuCl2 +3 × 10 − 3 M K2PtCl6 + 1 M KCl and the applied potential is from 700 to −800 mV at a scan rate of 100 mV/s. This electrode was used for the catalytic oxidation of nitrite. At the bare GC electrode, nitrite was not effectively oxidized prior to the discharge of the supporting electrolyte. But CuPtCl6/GC electrode shows a greatly catalytic effect on the oxidation of nitrite. Cyclic voltammetry was used to investigate the electrochemical behavior of nitrite at the CuPtCl6/GC electrode. The effects of the film thickness, pH and coexisting ions on the response current have been evaluated. The electrode was used as an amperometric sensor in FIA for the detection of nitrite. Several experimental parameters (flow rate, applied potential) and the electrode stability and precision have been evaluated. This electrode displays a very wide dynamic range, quite low detection limits and short response times and easy preparation. The linear relationship between the flow-injection peak currents and the concentrations of nitrite is at a range of 1×10 − 7 – 2 ×10 − 3 M, with a detection limit of 5×10 − 8 M.
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Acknowledgements We acknowledge the support of this project by Research Grant Council of Hong Kong and by The Hong Kong University of Science and Technology. It is also partially supported by NNSF of China.
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