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Electrochemical detection of lead using overoxidized polypyrrole films
Journal of Electroanalytical Chemistry 537 (2002) 135 /143 www.elsevier.com/locate/jelechem
Electrochemical detection of lead using overoxidized polypyrrole films Adam Wanekaya, Omowunmi A. Sadik Department of Chemistry, State University of New York-Binghamton, P.O. Box 6016, Binghamton, NY 13902-6016, USA Received 11 June 2002; received in revised form 9 September 2002; accepted 11 October 2002
Abstract An electrochemical method for the determination of lead has been developed using overoxidized polypyrrole (OPPy) electrode doped with 2(2-pyridylazo)chromotropic acid anion (PACh2 ). The PACh2 acts both as a chelating agent and a counter anion within the polypyrrole matrix. In a typical assay, Pb2 is accumulated on a solid electrode via the formation of a lead /PACh complex at open circuit. The electrode containing the Pb2 PACh2 is then transferred to a 0.1 M acetate buffer where it is subjected to differential pulse anodic stripping voltammetry. The resulting stripping peak current was linearly related to the concentration of lead. The method has been optimized with respect to pH, concentration of chelating agent, accumulation time, reduction potential and time. The detection limit was found to be 10 ng ml 1 with a linear range of 0 /200 ng ml 1. The method has been validated for the determination of lead using spiked potable water at 25 ng ml 1. The average recovery was 93.4% with a relative standard deviation of 8.54%. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Accelerated solvent extraction; Pressure-assisted chelation; PACE; Digestion; Metals
1. Introduction Electrochemical methods enable the preparation of thin-films on electrodes that are subsequently used for the development of monitoring techniques for metal ions in the environment. In order to improve the selectivity of such techniques, the electrochemical measurements are carried out by first accumulating or preconcentrating the target metal from a dilute solution onto the electrode surface. This accumulation step can be performed with or without the use of applied potential. The latter approach called the open circuit enables the separation of the metal species under consideration thereby eliminating interferences [1] [2]. Electrically conducting polymers (ECPs) can act as a means of immobilizing metal/ligand complexes on electrodes. ECPs can also be used to mediate redox transport between metal centers. We have used these characteristics to develop a method for monitoring
Corresponding author. Fax: /1-607-777-4132 E-mail address: [email protected] (O.A. Sadik).
Pb2. Electrodes modified with electrically conducting polypyrrole (PPy) containing complexing ligands have already been employed for the determination of various transition metals [1 /9] with such ligand-modified electrodes; accumulation involves only a chemical complexation reaction without any applied potential. Once the metal has been chemically accumulated from the sample solution onto the electrode, it can be transferred to a second metal-free solution for subsequent stripping analysis. This mode of medium exchange allows many potential interferants to be removed from the sample solution and can greatly improve the selectivity of the method. The feasibility of using electrodes modified with functionalized polymer films for electroanalysis of metal ions in solution was demonstrated by Guadalupe and Abruna [6]. Numerous ligands have been used for electroanalysis of metals and the factors affecting their electrochemical behavior extensively studied [1]. This include the electropolymerization and redox properties of bipyridyl-polypyrrole and Cu(II) bipyridyl-polypyrrole film electrodes [7], the electrochemistry and electronic conductivities of conducting polymers containing
0022-0728/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 2 ) 0 1 2 6 1 - 5
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[Ru(2,2?-bipyridine)2(3-{pyrrol-1-ylmethyl)Cl] [8], the electropolymerization of PPy films functionalized with ruthenium polypyridyl [9] and nickel cyclam complexes [10] and the determination of silver and mercury with electrodes modified with both PPy and poly(pyrrole-N carbodithionate) [11,12]. In a recent report, PPy coated with mercury film electrodes have been used in the detection of lead and cadmium [13]. Polypyrrole films are produced typically via electrochemical oxidation with anionic counterions that are conveniently sequestered within the film in order to maintain electroneutrality [14]. Various anionic species have been incorporated into PPy film simply by oxidizing pyrrole monomers in the presence of the target anions. However, detection limits in these voltammetrybased methods have not been adequate [15]. Thus these modified electrodes have not been competitive relative to spectrophotometric techniques. A major limitation lies in the high background currents typically encountered at modified PPy films. These currents mask the relatively smaller metal /chelate binding thereby making metal detection difficult. One way to overcome this problem is to completely overoxidize the polymer matrix prior to metal accumulation and measurement [16]. In the present study, we report the development of an electrochemical technique for the detection of Pb2 using 2(2-pyridylazo)chromotropic acid anion (PACh2) sequestered within a overoxidized polypyrrole (OPPy) matrix.
If sufficient PACh2 is retained within the overoxidized polymer matrix, rapid and sensitive detection of lead can be achieved. PACh2 is a dark purple, water-soluble, amorphous powder and forms watersoluble complexes with many metal ions. The potential of PACh2 as a metal complexant has been known for a long time [17]. It was proposed that PACh2 could exist in two tautomeric forms as follows [17a].
ing agent after being immobilized within the PPy matrix. Lead was chosen as the analyte because of its clinical and environmental significance. Therefore, there is a need to develop an inexpensive and accurate method for its detection. We are not aware of previous reports where either mercury-free PPy or OPPy has been used for the determination of Pb2.
2. Experimental
2.1. Reagents and instrumentation Pyrrole was obtained from Aldrich (Milwaukee, WI) and was distilled before use. PACh2 disodium salt was obtained from Research Plus Inc. (Bayonne, NJ) and was used as received. All the other reagents were of analytical grade. Electrochemical measurements were performed by using an EG&G potentiostat/galvanostat Model 273, EG&G Instruments (Princeton, NJ) equipped with a glassy carbon working electrode, a Pt counter electrode, and a Ag/AgCl/KCl(sat) reference electrode (Bioanalytical Systems Inc., West Lafayette, IN). All potentials were quoted versus the Ag/AgCl reference. All data collection and analysis were done using Model 270/250 Research Electrochemistry Software 4.30. Glassy carbon voltammetric electrodes of 3.0 mm diameter, obtained from Bioanalytical Systems Inc. (West Lafayette, IN) were polished thoroughly with alumina and cleaned in an ultrasonicating bath before use. A stock solution of Pb (1000 ppm) was obtained from Perkin /Elmer Corp. (Norwalk, CT). Test solutions were diluted from this stock using deionized water obtained by purification through a NanoPure model D4741 (Barnstead/Thermolyne, Dubuque, IA). Ammonia buffer was made from NH3 solution and NH4Cl. Acetate buffer was made from AcONa and AcOH. pH measurements were made with an Accument Basic pH meter from Fisher Scientific (Pittsburgh, PA).
2.2. Film preparation
It has also been known that Pb2 reacts with various arylazo derivatives of chromotropic acid [18]. The reason why we chose PACh2 is because it not only reacts with metals to form metal complexes but it is also exists as a disodium salt. We believe that since PACh2 is an anion, it can act both as a dopant in the electrochemical synthesis of PPy and later as a complex-
PACh2 doped PPy films were prepared by potentiostatic electropolymerization at /800 mV for 1/2 s from aqueous solutions of Py (40 mM) and various concentrations of PACh2 disodium salt. Since PACh2 was the only anion present in the solution it was incorporated in the growing cationic PPy film as the chargebalancing counter anion. Overoxidation of the PPy films was carried out in aq. 0.1 M NaOH by cycling the potential between 0 and /1200 mV at 50 mV s 1 for two cycles.
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2.3. Voltammetric measurements Cyclic voltammograms (CV) were carried out after purging the solution with nitrogen for 5 min. The CVs were obtained by scanning from 700 to /1200 mV and back at a scan rate of 50 mV s1. For accumulation, the polymer film electrode was immersed in a stirred buffer solution containing a known concentration of a metal ion for a specific accumulation time. The electrode was removed after the accumulation, rinsed with pure water, dried and then transferred to a metal ion free electrolyte solution for differential pulse anodic stripping voltammetric (DPASV) in 0.1 M acetate buffer at pH 4.7. Differential pulse anodic stripping voltammograms were recorded at 40 mV s 1 scan rate, 50 mV pulse height, and 0.2 s pulse period. All measurements were carried out at room temperature. An average of at least three runs of the differential pulse anodic stripping voltammograms were recorded.
Fig. 1. CVs for PPy/PACh2 before overoxidation: (a) before; and (b) after accumulation in 0.37 M NH3 buffer pH 10 containing 2.5 ppm Pb for 5 min. Voltammetric conditions: 0.1 M acetate buffer pH 4.7, scan rate 50 mV s 1.
3. Results and discussion Overoxidation of PPy is a destructive process whereby the conducting PPy is oxidized to a higher oxidation state after which it becomes susceptible to nucleophilic attack [19]. This process usually results in the addition of a carbonyl functionality to the pyrrolic rings with loss of conjugation and hence the inherent electronic conductivity [19,20]. After undergoing oxidation a net electronegative character is imparted to the polymer film and it then undergoes cation permselective behavior resulting in overoxidized (OPPy) films [21,22] and derivative films [23]. It has further been shown that small doping anions can be ejected from OPPy [24]. Such studies were carried out using hexacyanoferrate (III/II) and much larger ions, namely 3-(2-pyridyl)-5,6diphenyl-4,4?-disulfonate-1,2,4-triazine (PDTDS2) and anthraquinone-2,6-disulfonate (AQDS2) [16]. It was shown that the relatively compact hexacyanoferrate (III/ II) could not be detected in the OPPy whereas the larger, more flexible PDTDS2 and AQDS2 were detected at lower amounts than those trapped in the PPy film. Since the PACh2 disodium (Scheme 1) salt was the only electrolyte present in the electropolymerized solution, it was incorporated into the growing cationic PPy film to maintain electroneutrality in the polymer matrix. Fig. 1 shows the CVs of PPy/PACh2 before and after accumulation in a solution containing lead ions. The voltammograms showed that apart from a slight change in the current magnitude, there is no noticeable potential shift before and after the accumulation steps. Also, the voltammograms did not indicate that any Pb2 ion has been complexed by the PACh2 into the PPy matrix. This is attributed to the inaccessibility of the complexing agent (PACh2) to the metal ions. The binding sites
have probably been buried within the polymer film. Also, the large background current resulting from the conducting and electroactive PPy is likely to mask the relatively smaller current that may result from the lead / chelate complex. Consequently, Fig. 2a shows the voltammograms recorded for the overoxidation process of PPy/PACh2. There is a prominent peak at approximately 500 mV during the first cycle. During the second cycle, however, a featureless voltammogram was recorded (Fig. 2b). This indicates that an irreversible electrochemical transformation of the polymer film has been achieved during the first cycle. As stated earlier, this transformation is accomplished by a loss of
Fig. 2. Repetitive CVs for the overoxidation process of PPy doped with PACh2 in 0.1 M NaOH. Rate: 50 mV s 1: (a) first cycle; (b) second cycle.
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conjugation and hence electronic conductivity [21,22]. Although not electrochemically active, the polymer can still allow complexation of lead if the chelator is immobilized in the overoxidized film. Fig. 3 shows the CVs recorded for OPPy/PACh2 before and after accumulation in a solution containing Pb2ions. The voltammogram of OPPy/PACh2 before accumulation is featureless (Fig. 3a) whereas that recorded after the accumulation of lead exhibited prominent anodic and cathodic peaks at approximately /500 and /650 mV, respectively. The anodic peak was more prominent and sharper than the cathodic peak. These peaks can be attributed only to the lead /chelate complex. Since the magnitude of the anodic peak was much larger than the cathodic peak, we recorded the differential pulse voltammetry experiments by sweeping in the positive direction as opposed to the negative direction for the purpose of optimization and analytical applications. The cathodic peak clearly corresponds to the reduction of Pb2 taken up chemically from the accumulation solution by complexation with the PACh2. The anodic peak, on the other hand, is due to the oxidation of the Pb0 to Pb2. The appearance of the peaks is also indicative of the role of the OPPy as a suitable matrix for the metal / chelate binding as the peaks are clearly visible probably because the overoxidation process eliminated the high background currents. The important observation from Figs. 1 and 3 is that the voltammogram of PPy/PACh2 (Fig. 1b) did not exhibit the oxidation and reduction peaks of lead species after accumulation while the voltammograms due to OPPy/PACh2 (Fig. 3b) clearly did. As stated earlier, the fact that the peaks were clearly
evident in the OPPy/PACh2 and not in the original PPy/PACh2 is a confirmation of the hypothesis that sufficient PACh2 was doped and was still present even after the polymer has been overoxidized and is still capable of complexation with Pb2; ii) elimination of the large background currents, which were present in PPy/PACh2 but are now absent in OPPy/PACh2. Thus the overoxidation is important in enhancing the metal signals.
i)
To prove further the importance of PACh2 both as a complexant and a dopant, a CV of OPPy/PACh2 and OPPy/Br after accumulation in lead solution were compared. Fig. 4b shows the CV of OPPy/Br whereas the CV of OPPy/PACh2 is shown in Fig. 4a. It is evident that the magnitude of the peaks in OPPy/ PACh2 is probably due to complexation of PACh2 with Pb2. No peaks were expected in the case of OPPy/ Br . However, the minor bump in Fig. 4b is attributed to residual lead adsorbed on the OPPy/Br polymer matrix. Fig. 5 shows the differential pulse anodic stripping voltammograms of the OPPy/PACh2 after exposure to Pb2 solution, reduction at /1000 mV and scanning from /1000 to /300 mV. As expected, there is a peak at approximately /580 mV, which is attributed to the oxidation of lead taken up chemically from the accumulation solution via complexation with PACh2. 3.1. Effect of accumulation time, pH and concentration of accumulating solution Accumulation time is very important as it represents the length of time allowed for the complexation reaction
Fig. 3. CV of OPPy/PACh2 : (a) before; and (b) after accumulation in 0.37 M NH3 buffer pH 10 containing 2.5 ppm Pb for 5 min. Voltammetric conditions as in Fig. 1.
Fig. 4. CV of: (a) OPPy/PACh2 ; and (b) OPPy/Br after accumulation in 0.37 M NH3 buffer pH 10 containing 2.5 ppm Pb for 5 min. Voltammetric conditions as in Fig. 1.
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Fig. 5. Differential pulse anodic stripping voltammogram of a PPy/ PACh2 electrode after accumulation time in 0.37 M NH3 buffer pH 10 containing 5 ppm Pb for 2 min. Voltammetric conditions: electrolyte 0.1 M acetate buffer pH 4.7, reduction time 120 s at / 1000 mV.
between Pb2 and PACh2 prior to metal detection. Fig. 6a shows the dependence of the differential pulse anodic voltammetric peak current on the accumulation time. The peak current increases with increasing accumulation time, indicating an enhancement of Pb2 uptake at the electrode surface. The peak current reaches a maximum at 150 s after which leveling occurs. This result indicates that the complexation reaction between PACh2 and Pb2 reaches equilibrium at this time and a steady state situation is achieved. Fig. 6a also shows that at 180 s, a complete equilibrium is possible. In order to ensure maximum accumulation of Pb2, subsequent experiments were carried out after an accumulation time of 180 s. Metal complexation reactions are pH-dependent. Therefore, the effect of pH on stripping peak current was studied in the range of 7.5 /11.4 as shown in Fig. 6b. It was found that the peak current increased sharply up to pH 8.5 and then decreased. This suggests that as with other 2-pyridylazo compounds, metal complexation using PACh2 is not selective [18]. However, with appropriate pH adjustment and applied potential, the selectivity can be tuned. In that case, hydrogen ion will compete with the metal ion during complexation. The higher the stability of the metal complex, the lower the pH at which it can exist. Also, the lower the pH, the fewer is the number of metals complexed. The control of pH alone, or in combination with the accumulation potential may be used for selective metal accumulation and complexation. PACh2 may form tridentate 1:1 and 1:2 complexes with the lead, through coordination at the pyridine nitrogen, the azo nitrogen further from
Fig. 6. (a) Effect of accumulation time on stripping voltammetric peak current. Accumulation in 1 M NH3 buffer (pH 10) containing 2.5 ppm Pb. Conditions: electrolyte 0.1 M acetate buffer, reduction time 120 s at /1000 mV. (b) Effect of accumulation pH on the differential pulse anodic stripping voltammetric peak current. One hundred and eighty seconds accumulation time in 1 M NH3 buffer (pH /7.5 / /11.4) containing 2.5 ppm Pb. Other conditions as in Fig. 6a. (c) Effect of NH3 buffer concentration on the differential pulse anodic stripping voltammetric peak current. One hundred and eighty seconds accumulation time in NH3 buffer pH 8.5 (0.5 /3 M) containing 2.5 ppm Pb. Other conditions as in Fig. 6b.
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the heterogeneous cyclic ring and the ortho hydroxyl group [18]. The bimodal distribution in the pH trend noticed in Fig. 6b may then be attributed to the formation of either tri- or monodentate chelate with the lead ions. Hence we selected pH 8.5 in all our subsequent experiments. Any formation of hydroxides of Pb2 was minimized by spiking the Pb2 solution into the accumulation cell containing the ammonia buffer just before dipping the OPPy/PACh2 electrode in that cell. This procedure has been used successfully in the determination of Pb2 in water using a carbon-paste electrode modified with N -p-chlorophenylcinnamo-hydroxamic acid [25]. The effect of the concentration of the ammonia buffer (accumulating solution) was also studied in the 0.5 /3.0 M range. The result showed that the maximum stripping peak current occurred at 1.5 M as is shown in Fig. 6c. We therefore used this concentration for subsequent studies. 3.2. Effect of reduction potential and time Using the conditions discussed earlier, we studied the optimum times and magnitudes of reduction potential necessary to plate Pb0 on the electrode by reducing the Pb2 within the complex. The OPPy/PACh2 electrode with accumulated Pb2 was transferred to 0.1 M acetate buffer solution (pH 4.7) and the potential varied from / 900 to /1700 mV (Fig. 7a). The stripping peak current increased with increasing negative potential reaching a steady state at /1200 mV. Hence /1200 mV was employed as the reduction potential in all subsequent studies. Next we optimized the reduction time. The stripping peak current was found to increase with increasing reduction time up to 120 s after which it became constant as shown in Fig. 7b. Hence a reduction time of 120 s was used for further studies. 3.3. Chelate concentration The effect of change in the concentration of the Na2PACh in the polymer growth solution was studied. The amount of PACh2 within the polymer matrix on the electrode surface is expected to have a significant effect on the amount of Pb2 accumulating and hence on the stripping peak current. We kept the concentration of the pyrrole monomer constant at 40 mM and varied the concentration of Na2PACh from 2 to 16 mM during the electrochemical synthesis of PPy/PACh2 before overoxidation to OPPy/PACh2. Fig. 8 shows that there is an increase in stripping current of OPPy/ PACh2 made from a concentration of up to 8 mM Na2PACh followed by a gradual decrease. The peak currents increased with the increasing concentration of Na2PACh used in the electrosynthesis of PPy/PACh2 because more PACh2 is available for complexation
Fig. 7. (a) Effect of reduction potential on the differential pulse anodic stripping voltammetric peak current. One hundred and eighty seconds accumulation time in 1.5 M NH3 buffer pH 8.5 containing 2.5 ppm Pb. Voltammetric conditions: electrolyte 0.1 M acetate buffer pH 4.7, reduction time 120 s with variation in reduction potential /900 // 1700 mV. (b) Effect of reduction time on the differential pulse anodic stripping voltammetric peak. One hundred and eighty seconds accumulation time in 1.5 M NH3 buffer pH 8.5 containing 2.5 ppm Pb. Voltammetric conditions: electrolyte 0.1 M acetate buffer pH 4.7, reduction potential /1200 mV with variation in reduction time from 2 to 180 s.
and also accumulation of Pb2. The decrease in peak current after 8 mM could be attributed to the presence of excess PACh2 resulting in a possible decrease in the conductivity of the modified electrode. The calibration curve was therefore generated by using an electrode containing OPPy/PACh2 synthesized from a solution containing 8 mM of Na2PACh and 40 mM of pyrrole, i.e. a ratio of 1:5. 3.4. Principle of detection Based on the above observations the proposed mechanism for the accumulation and differential pulse voltammetric procedure is outlined below:
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of other metals ions once appropriate chelators are identified. 3.5. Calibration, detection limit and interference
Fig. 8. Variation in differential pulse anodic stripping voltammetric peak currents with the change in [Na2PACh2 ] to pyrrole ratio used in the synthesis of PPy/PACh. One hundred and eighty seconds accumulation time in 1.5 M NH3 buffer pH 8.5 containing 2.5 ppm Pb. Voltammetric conditions: electrolyte 0.1 M acetate buffer pH 4.7, reduction time 120 s at /1200 mV.
1) The first step is the chelation of Pb2 from the accumulation solution on to the surface at the OPPy/PACh2 at open circuit (1.5 M NH3 buffer solution pH 8.5) 2 PACh2 Pb2 (aq)PACh2 surface 0 Pb surface
(1)
2) The second step is the reduction of the accumulated Pb2 ions to Pb0 at closed circuit by the application of a constant negative potential of /1200 mV (0.1 M acetate buffer pH 4.7) Pb2 PACh2 surface 2e
0 Pb0surface PACh2 surface
(2)
3) The final step is the stripping step where the Pb0 is electrochemically stripped back into the solution by scanning from /1200 to /100 mV. The resulting stripping peak obtained represents the analytical signal (0.1 M acetate buffer pH 4.7) Pb0surface 0 Pb2 (aq)2e
(3)
The resulting signal is proportional to concentration of Pb2. The usefulness of this approach lies in the fact that the sample preparation, accumulation, trapping and metal uptake can be achieved via electrochemical steps. The approach may be used for the determination
Standard solutions containing Pb2 in the concentration range 0/10 000 ng ml 1 were prepared in 1.5 M ammonia buffer at pH 8.5 and exposed to the OPPy/ PACh2 for 180 s. The ratio of the Na2PACh to pyrrole used for the electrochemical synthesis of OPPy/PACh2 was 1:5. The electrode was then subjected to the optimized differential pulse anodic stripping voltammetric procedures described above. These conditions were: 0.1 M acetate buffer pH 4.7 as the electrolyte, reduction time 120 s at /1200 mV. The voltammograms at different concentrations of Pb2 are shown in Fig. 9a and b. The peak potential of the voltammogram shifted to less negative potentials with increasing concentration of Pb2. A calibration curve of concentration versus peak current was plotted (Fig. 10). The calibration plot was linear between 0 and 200 ng ml 1 (y/0.03x/ 0.3232, R2 /0.988). At higher concentrations (/1000 ng ml 1) deviations from linearity occurred due to saturation of the electrode surface. The detection limit (based on three times the mean of blank measurements) was calculated by making replicate current measurements at /550 mV for blank solutions and it was found to be 10 ng ml 1. No peaks were observed when the OPPy/PACh2 was accumulated separately in Zn2, Fe3, Al3 or Mn2 at 1000 ng ml 1 and subjected to the differential pulse anodic stripping procedures described earlier. When similar solutions were spiked with Pb2 solutions to get 200 ng ml 1 of Pb2, the peak current magnitudes due to lead were the same as those obtained in electrolyte only. Cu2 exhibited some interference. However, the interference was less prominent than that experienced in the determination of Pb2 using a carbon-paste electrode modified with N -p -chlorophenylcinnamo-hydroxamic acid [25] where equimolar amounts of Cu2 depressed the peak current by as much as 36%. In the present study, equimolar amounts of Pb2 and Cu2 (200 ng ml1 each) produced no significant effects on the stripping peak current. However, when the amount of Cu2 was increased threefold to 600 ng ml1 the peak current was depressed by as much as 31%. Such effects have been reported [26] and are related to the formation of Pb /Cu solid solutions [27]. The phenomenon may result in either suppression or enhancement of the Pb2 stripping peak current [27]. Interference due to anions was also investigated. Accumulation of the modified electrode in 1000 ng ml 1 of Pb2 in excess of 50-fold SO2 and 4 ; 20-fold Cl 2 threefold CO3 followed by differential pulse anodic stripping voltammetry did not to have any measurable effect on the peak current due to lead.
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Fig. 10. Calibration curve from 0 to 200 ng ml 1. Conditions are as in Fig. 9.
method. Three determinations were made for each addition and three replicates were run. The average recovery of Pb2 was 93.4% with a relative standard deviation of 8.54%.
4. Conclusions Fig. 9. (a) Differential pulse anodic stripping voltammograms of Pb solutions (0 /80 ppb) accumulated on the OPPy/PACh2 electrode: (a) 0; (b) 20; (c) 40; (d) 80 ppb Pb. Conditions: PACh2 /Py ratio/1:5, accumulation time 180 min in 1.5 M NH3 buffer pH 8.5. Voltammetric conditions: 0.1 M acetate buffer pH 4.7, reduction potential /1200 mV for 120 s. (b) Differential pulse anodic stripping voltammograms of Pb solutions (200 /10 000 ppb) accumulated on the OPPy/PACh2 electrode: (a) 200; (b) 500; (c) 1000; (d) 5000; (e) 10 000 ppb Pb. Other conditions are as in Fig. 9a.
3.6. Environmental application In order to evaluate the performance of the modified electrode in practical analytical work, quantitation of lead in potable water was attempted in spiked and unspiked water samples using the standard addition method and the optimized conditions realized above. Since Pb2 was not detected in the potable water sample, a standard Pb2 solution was spiked into the water sample resulting in a total concentration of 25 ng ml 1. Analysis was done by the standard addition
A new differential pulse anodic stripping voltammetry method has been developed for the determination of trace levels of Pb2. The optimum conditions for metal determination depends on the uptake of lead from solution followed by its rapid complexation with an immobilized metal chelator at OPPy electrode. This step introduces a significant selectivity for the determination of lead and may be suitable for other metal ions. Open circuit accumulation of Pb2 was achieved in 1.5 M ammonia buffer (pH 8.5) for 180 s. The optimized conditions for differential pulse anodic voltammetric studies were identified as /1200 mV reduction potential for 120 s. Most metal ions studied with concentrations greater than the Pb2 did not interfere with the capability of the method. The detection limit and selectivity of this method are very impressive and are comparable to other mercury-free stripping voltammetric methods with low detection limits [28,29]. The linear range of up to 200 ng ml1 is much more than the linear ranges exhibited by most methods [25,29].
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[18] B. Budesinsky, in: H.A. Flaschka, A.J. Barnard, Jr. (Eds.), Chelates in Analytical Chemistry, vol. 2, Marcel Dekker, Inc, New York, 1969, p. 1. [19] F. Beck, P. Braun, M. Oberst, Ber. Bunsenges. Phys. Chem. 91 (1987) 967. [20] P.A. Christensen, A. Hammett, Electrochim. Acta 36 (1991) 1263. [21] (a) M. Freund, L. Bodalbhai, A. Brajter-Toth, Talanta 38 (1991) 95; (b) A. Witkowski, M.S. Freund, A. Brajter-Toth, Anal. Chem. 63 (1991) 622; (c) A. Witkowski, A. Brajter-Toth, Anal. Chem. 64 (1992) 635; (d) C.C. Hsueh, A. Brajter-Toth, Anal. Chem. 66 (1994) 2458. [22] (a) Z. Gao, M. Zi, B. Chen, J. Electroanal. Chem. 373 (1994) 141; (b) Z. Gao, B. Chen, M. Zi, Analyst 119 (1994) 459; (c) Z. Gao, M. Zi, B. Chen, Anal. Chim. Acta 286 (1994) 213; (d) Z. Gao, A. Ivaska, Anal. Chim. Acta 284 (1993) 393. [23] (a) D.W.M. Arrigan, Anal. Commun. 34 (1997) 241; (b) J. Wang, V.A.P. Prasad, G. Cepria, S. Basak, K. Rajeshwar, Analyst 122 (1997) 981. [24] F. Beck, P. Braun, F. Schloten, J. Electroanal. Chem. 267 (1989) 141. [25] T.H. Degefa, B.S. Chandravanshi, H. Alemu, Electroanalysis 11 (1999) 1305. [26] O.L. Kabanova, S.B. Beniaminova, J. Anal. Chem. USSR 26 (1970) 94. [27] (a) F. Vydra, K. Stulik, E. Julakova, Electrochemical Stripping Analysis, Wiley, Chichester, 1976, p. 83; (b) H. Gunasingham, R.R. Dalangin, Anal. Chim. Acta 246 (1991) 309. [28] K.C. Honeychurch, J.P. Hart, D.C. Cowell, Anal. Chim. Acta 431 (2001) 89. [29] K.C. Honeychurch, J.P. Hart, D.C. Cowell, D.W.M. Arrigan, Sens. Actuators B 77 (2001) 642.
Electrochemical detection of lead using overoxidized polypyrrole films Adam Wanekaya, Omowunmi A. Sadik Department of Chemistry, State University of New York-Binghamton, P.O. Box 6016, Binghamton, NY 13902-6016, USA Received 11 June 2002; received in revised form 9 September 2002; accepted 11 October 2002
Abstract An electrochemical method for the determination of lead has been developed using overoxidized polypyrrole (OPPy) electrode doped with 2(2-pyridylazo)chromotropic acid anion (PACh2 ). The PACh2 acts both as a chelating agent and a counter anion within the polypyrrole matrix. In a typical assay, Pb2 is accumulated on a solid electrode via the formation of a lead /PACh complex at open circuit. The electrode containing the Pb2 PACh2 is then transferred to a 0.1 M acetate buffer where it is subjected to differential pulse anodic stripping voltammetry. The resulting stripping peak current was linearly related to the concentration of lead. The method has been optimized with respect to pH, concentration of chelating agent, accumulation time, reduction potential and time. The detection limit was found to be 10 ng ml 1 with a linear range of 0 /200 ng ml 1. The method has been validated for the determination of lead using spiked potable water at 25 ng ml 1. The average recovery was 93.4% with a relative standard deviation of 8.54%. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Accelerated solvent extraction; Pressure-assisted chelation; PACE; Digestion; Metals
1. Introduction Electrochemical methods enable the preparation of thin-films on electrodes that are subsequently used for the development of monitoring techniques for metal ions in the environment. In order to improve the selectivity of such techniques, the electrochemical measurements are carried out by first accumulating or preconcentrating the target metal from a dilute solution onto the electrode surface. This accumulation step can be performed with or without the use of applied potential. The latter approach called the open circuit enables the separation of the metal species under consideration thereby eliminating interferences [1] [2]. Electrically conducting polymers (ECPs) can act as a means of immobilizing metal/ligand complexes on electrodes. ECPs can also be used to mediate redox transport between metal centers. We have used these characteristics to develop a method for monitoring
Corresponding author. Fax: /1-607-777-4132 E-mail address: [email protected] (O.A. Sadik).
Pb2. Electrodes modified with electrically conducting polypyrrole (PPy) containing complexing ligands have already been employed for the determination of various transition metals [1 /9] with such ligand-modified electrodes; accumulation involves only a chemical complexation reaction without any applied potential. Once the metal has been chemically accumulated from the sample solution onto the electrode, it can be transferred to a second metal-free solution for subsequent stripping analysis. This mode of medium exchange allows many potential interferants to be removed from the sample solution and can greatly improve the selectivity of the method. The feasibility of using electrodes modified with functionalized polymer films for electroanalysis of metal ions in solution was demonstrated by Guadalupe and Abruna [6]. Numerous ligands have been used for electroanalysis of metals and the factors affecting their electrochemical behavior extensively studied [1]. This include the electropolymerization and redox properties of bipyridyl-polypyrrole and Cu(II) bipyridyl-polypyrrole film electrodes [7], the electrochemistry and electronic conductivities of conducting polymers containing
0022-0728/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 2 ) 0 1 2 6 1 - 5
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[Ru(2,2?-bipyridine)2(3-{pyrrol-1-ylmethyl)Cl] [8], the electropolymerization of PPy films functionalized with ruthenium polypyridyl [9] and nickel cyclam complexes [10] and the determination of silver and mercury with electrodes modified with both PPy and poly(pyrrole-N carbodithionate) [11,12]. In a recent report, PPy coated with mercury film electrodes have been used in the detection of lead and cadmium [13]. Polypyrrole films are produced typically via electrochemical oxidation with anionic counterions that are conveniently sequestered within the film in order to maintain electroneutrality [14]. Various anionic species have been incorporated into PPy film simply by oxidizing pyrrole monomers in the presence of the target anions. However, detection limits in these voltammetrybased methods have not been adequate [15]. Thus these modified electrodes have not been competitive relative to spectrophotometric techniques. A major limitation lies in the high background currents typically encountered at modified PPy films. These currents mask the relatively smaller metal /chelate binding thereby making metal detection difficult. One way to overcome this problem is to completely overoxidize the polymer matrix prior to metal accumulation and measurement [16]. In the present study, we report the development of an electrochemical technique for the detection of Pb2 using 2(2-pyridylazo)chromotropic acid anion (PACh2) sequestered within a overoxidized polypyrrole (OPPy) matrix.
If sufficient PACh2 is retained within the overoxidized polymer matrix, rapid and sensitive detection of lead can be achieved. PACh2 is a dark purple, water-soluble, amorphous powder and forms watersoluble complexes with many metal ions. The potential of PACh2 as a metal complexant has been known for a long time [17]. It was proposed that PACh2 could exist in two tautomeric forms as follows [17a].
ing agent after being immobilized within the PPy matrix. Lead was chosen as the analyte because of its clinical and environmental significance. Therefore, there is a need to develop an inexpensive and accurate method for its detection. We are not aware of previous reports where either mercury-free PPy or OPPy has been used for the determination of Pb2.
2. Experimental
2.1. Reagents and instrumentation Pyrrole was obtained from Aldrich (Milwaukee, WI) and was distilled before use. PACh2 disodium salt was obtained from Research Plus Inc. (Bayonne, NJ) and was used as received. All the other reagents were of analytical grade. Electrochemical measurements were performed by using an EG&G potentiostat/galvanostat Model 273, EG&G Instruments (Princeton, NJ) equipped with a glassy carbon working electrode, a Pt counter electrode, and a Ag/AgCl/KCl(sat) reference electrode (Bioanalytical Systems Inc., West Lafayette, IN). All potentials were quoted versus the Ag/AgCl reference. All data collection and analysis were done using Model 270/250 Research Electrochemistry Software 4.30. Glassy carbon voltammetric electrodes of 3.0 mm diameter, obtained from Bioanalytical Systems Inc. (West Lafayette, IN) were polished thoroughly with alumina and cleaned in an ultrasonicating bath before use. A stock solution of Pb (1000 ppm) was obtained from Perkin /Elmer Corp. (Norwalk, CT). Test solutions were diluted from this stock using deionized water obtained by purification through a NanoPure model D4741 (Barnstead/Thermolyne, Dubuque, IA). Ammonia buffer was made from NH3 solution and NH4Cl. Acetate buffer was made from AcONa and AcOH. pH measurements were made with an Accument Basic pH meter from Fisher Scientific (Pittsburgh, PA).
2.2. Film preparation
It has also been known that Pb2 reacts with various arylazo derivatives of chromotropic acid [18]. The reason why we chose PACh2 is because it not only reacts with metals to form metal complexes but it is also exists as a disodium salt. We believe that since PACh2 is an anion, it can act both as a dopant in the electrochemical synthesis of PPy and later as a complex-
PACh2 doped PPy films were prepared by potentiostatic electropolymerization at /800 mV for 1/2 s from aqueous solutions of Py (40 mM) and various concentrations of PACh2 disodium salt. Since PACh2 was the only anion present in the solution it was incorporated in the growing cationic PPy film as the chargebalancing counter anion. Overoxidation of the PPy films was carried out in aq. 0.1 M NaOH by cycling the potential between 0 and /1200 mV at 50 mV s 1 for two cycles.
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2.3. Voltammetric measurements Cyclic voltammograms (CV) were carried out after purging the solution with nitrogen for 5 min. The CVs were obtained by scanning from 700 to /1200 mV and back at a scan rate of 50 mV s1. For accumulation, the polymer film electrode was immersed in a stirred buffer solution containing a known concentration of a metal ion for a specific accumulation time. The electrode was removed after the accumulation, rinsed with pure water, dried and then transferred to a metal ion free electrolyte solution for differential pulse anodic stripping voltammetric (DPASV) in 0.1 M acetate buffer at pH 4.7. Differential pulse anodic stripping voltammograms were recorded at 40 mV s 1 scan rate, 50 mV pulse height, and 0.2 s pulse period. All measurements were carried out at room temperature. An average of at least three runs of the differential pulse anodic stripping voltammograms were recorded.
Fig. 1. CVs for PPy/PACh2 before overoxidation: (a) before; and (b) after accumulation in 0.37 M NH3 buffer pH 10 containing 2.5 ppm Pb for 5 min. Voltammetric conditions: 0.1 M acetate buffer pH 4.7, scan rate 50 mV s 1.
3. Results and discussion Overoxidation of PPy is a destructive process whereby the conducting PPy is oxidized to a higher oxidation state after which it becomes susceptible to nucleophilic attack [19]. This process usually results in the addition of a carbonyl functionality to the pyrrolic rings with loss of conjugation and hence the inherent electronic conductivity [19,20]. After undergoing oxidation a net electronegative character is imparted to the polymer film and it then undergoes cation permselective behavior resulting in overoxidized (OPPy) films [21,22] and derivative films [23]. It has further been shown that small doping anions can be ejected from OPPy [24]. Such studies were carried out using hexacyanoferrate (III/II) and much larger ions, namely 3-(2-pyridyl)-5,6diphenyl-4,4?-disulfonate-1,2,4-triazine (PDTDS2) and anthraquinone-2,6-disulfonate (AQDS2) [16]. It was shown that the relatively compact hexacyanoferrate (III/ II) could not be detected in the OPPy whereas the larger, more flexible PDTDS2 and AQDS2 were detected at lower amounts than those trapped in the PPy film. Since the PACh2 disodium (Scheme 1) salt was the only electrolyte present in the electropolymerized solution, it was incorporated into the growing cationic PPy film to maintain electroneutrality in the polymer matrix. Fig. 1 shows the CVs of PPy/PACh2 before and after accumulation in a solution containing lead ions. The voltammograms showed that apart from a slight change in the current magnitude, there is no noticeable potential shift before and after the accumulation steps. Also, the voltammograms did not indicate that any Pb2 ion has been complexed by the PACh2 into the PPy matrix. This is attributed to the inaccessibility of the complexing agent (PACh2) to the metal ions. The binding sites
have probably been buried within the polymer film. Also, the large background current resulting from the conducting and electroactive PPy is likely to mask the relatively smaller current that may result from the lead / chelate complex. Consequently, Fig. 2a shows the voltammograms recorded for the overoxidation process of PPy/PACh2. There is a prominent peak at approximately 500 mV during the first cycle. During the second cycle, however, a featureless voltammogram was recorded (Fig. 2b). This indicates that an irreversible electrochemical transformation of the polymer film has been achieved during the first cycle. As stated earlier, this transformation is accomplished by a loss of
Fig. 2. Repetitive CVs for the overoxidation process of PPy doped with PACh2 in 0.1 M NaOH. Rate: 50 mV s 1: (a) first cycle; (b) second cycle.
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conjugation and hence electronic conductivity [21,22]. Although not electrochemically active, the polymer can still allow complexation of lead if the chelator is immobilized in the overoxidized film. Fig. 3 shows the CVs recorded for OPPy/PACh2 before and after accumulation in a solution containing Pb2ions. The voltammogram of OPPy/PACh2 before accumulation is featureless (Fig. 3a) whereas that recorded after the accumulation of lead exhibited prominent anodic and cathodic peaks at approximately /500 and /650 mV, respectively. The anodic peak was more prominent and sharper than the cathodic peak. These peaks can be attributed only to the lead /chelate complex. Since the magnitude of the anodic peak was much larger than the cathodic peak, we recorded the differential pulse voltammetry experiments by sweeping in the positive direction as opposed to the negative direction for the purpose of optimization and analytical applications. The cathodic peak clearly corresponds to the reduction of Pb2 taken up chemically from the accumulation solution by complexation with the PACh2. The anodic peak, on the other hand, is due to the oxidation of the Pb0 to Pb2. The appearance of the peaks is also indicative of the role of the OPPy as a suitable matrix for the metal / chelate binding as the peaks are clearly visible probably because the overoxidation process eliminated the high background currents. The important observation from Figs. 1 and 3 is that the voltammogram of PPy/PACh2 (Fig. 1b) did not exhibit the oxidation and reduction peaks of lead species after accumulation while the voltammograms due to OPPy/PACh2 (Fig. 3b) clearly did. As stated earlier, the fact that the peaks were clearly
evident in the OPPy/PACh2 and not in the original PPy/PACh2 is a confirmation of the hypothesis that sufficient PACh2 was doped and was still present even after the polymer has been overoxidized and is still capable of complexation with Pb2; ii) elimination of the large background currents, which were present in PPy/PACh2 but are now absent in OPPy/PACh2. Thus the overoxidation is important in enhancing the metal signals.
i)
To prove further the importance of PACh2 both as a complexant and a dopant, a CV of OPPy/PACh2 and OPPy/Br after accumulation in lead solution were compared. Fig. 4b shows the CV of OPPy/Br whereas the CV of OPPy/PACh2 is shown in Fig. 4a. It is evident that the magnitude of the peaks in OPPy/ PACh2 is probably due to complexation of PACh2 with Pb2. No peaks were expected in the case of OPPy/ Br . However, the minor bump in Fig. 4b is attributed to residual lead adsorbed on the OPPy/Br polymer matrix. Fig. 5 shows the differential pulse anodic stripping voltammograms of the OPPy/PACh2 after exposure to Pb2 solution, reduction at /1000 mV and scanning from /1000 to /300 mV. As expected, there is a peak at approximately /580 mV, which is attributed to the oxidation of lead taken up chemically from the accumulation solution via complexation with PACh2. 3.1. Effect of accumulation time, pH and concentration of accumulating solution Accumulation time is very important as it represents the length of time allowed for the complexation reaction
Fig. 3. CV of OPPy/PACh2 : (a) before; and (b) after accumulation in 0.37 M NH3 buffer pH 10 containing 2.5 ppm Pb for 5 min. Voltammetric conditions as in Fig. 1.
Fig. 4. CV of: (a) OPPy/PACh2 ; and (b) OPPy/Br after accumulation in 0.37 M NH3 buffer pH 10 containing 2.5 ppm Pb for 5 min. Voltammetric conditions as in Fig. 1.
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Fig. 5. Differential pulse anodic stripping voltammogram of a PPy/ PACh2 electrode after accumulation time in 0.37 M NH3 buffer pH 10 containing 5 ppm Pb for 2 min. Voltammetric conditions: electrolyte 0.1 M acetate buffer pH 4.7, reduction time 120 s at / 1000 mV.
between Pb2 and PACh2 prior to metal detection. Fig. 6a shows the dependence of the differential pulse anodic voltammetric peak current on the accumulation time. The peak current increases with increasing accumulation time, indicating an enhancement of Pb2 uptake at the electrode surface. The peak current reaches a maximum at 150 s after which leveling occurs. This result indicates that the complexation reaction between PACh2 and Pb2 reaches equilibrium at this time and a steady state situation is achieved. Fig. 6a also shows that at 180 s, a complete equilibrium is possible. In order to ensure maximum accumulation of Pb2, subsequent experiments were carried out after an accumulation time of 180 s. Metal complexation reactions are pH-dependent. Therefore, the effect of pH on stripping peak current was studied in the range of 7.5 /11.4 as shown in Fig. 6b. It was found that the peak current increased sharply up to pH 8.5 and then decreased. This suggests that as with other 2-pyridylazo compounds, metal complexation using PACh2 is not selective [18]. However, with appropriate pH adjustment and applied potential, the selectivity can be tuned. In that case, hydrogen ion will compete with the metal ion during complexation. The higher the stability of the metal complex, the lower the pH at which it can exist. Also, the lower the pH, the fewer is the number of metals complexed. The control of pH alone, or in combination with the accumulation potential may be used for selective metal accumulation and complexation. PACh2 may form tridentate 1:1 and 1:2 complexes with the lead, through coordination at the pyridine nitrogen, the azo nitrogen further from
Fig. 6. (a) Effect of accumulation time on stripping voltammetric peak current. Accumulation in 1 M NH3 buffer (pH 10) containing 2.5 ppm Pb. Conditions: electrolyte 0.1 M acetate buffer, reduction time 120 s at /1000 mV. (b) Effect of accumulation pH on the differential pulse anodic stripping voltammetric peak current. One hundred and eighty seconds accumulation time in 1 M NH3 buffer (pH /7.5 / /11.4) containing 2.5 ppm Pb. Other conditions as in Fig. 6a. (c) Effect of NH3 buffer concentration on the differential pulse anodic stripping voltammetric peak current. One hundred and eighty seconds accumulation time in NH3 buffer pH 8.5 (0.5 /3 M) containing 2.5 ppm Pb. Other conditions as in Fig. 6b.
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the heterogeneous cyclic ring and the ortho hydroxyl group [18]. The bimodal distribution in the pH trend noticed in Fig. 6b may then be attributed to the formation of either tri- or monodentate chelate with the lead ions. Hence we selected pH 8.5 in all our subsequent experiments. Any formation of hydroxides of Pb2 was minimized by spiking the Pb2 solution into the accumulation cell containing the ammonia buffer just before dipping the OPPy/PACh2 electrode in that cell. This procedure has been used successfully in the determination of Pb2 in water using a carbon-paste electrode modified with N -p-chlorophenylcinnamo-hydroxamic acid [25]. The effect of the concentration of the ammonia buffer (accumulating solution) was also studied in the 0.5 /3.0 M range. The result showed that the maximum stripping peak current occurred at 1.5 M as is shown in Fig. 6c. We therefore used this concentration for subsequent studies. 3.2. Effect of reduction potential and time Using the conditions discussed earlier, we studied the optimum times and magnitudes of reduction potential necessary to plate Pb0 on the electrode by reducing the Pb2 within the complex. The OPPy/PACh2 electrode with accumulated Pb2 was transferred to 0.1 M acetate buffer solution (pH 4.7) and the potential varied from / 900 to /1700 mV (Fig. 7a). The stripping peak current increased with increasing negative potential reaching a steady state at /1200 mV. Hence /1200 mV was employed as the reduction potential in all subsequent studies. Next we optimized the reduction time. The stripping peak current was found to increase with increasing reduction time up to 120 s after which it became constant as shown in Fig. 7b. Hence a reduction time of 120 s was used for further studies. 3.3. Chelate concentration The effect of change in the concentration of the Na2PACh in the polymer growth solution was studied. The amount of PACh2 within the polymer matrix on the electrode surface is expected to have a significant effect on the amount of Pb2 accumulating and hence on the stripping peak current. We kept the concentration of the pyrrole monomer constant at 40 mM and varied the concentration of Na2PACh from 2 to 16 mM during the electrochemical synthesis of PPy/PACh2 before overoxidation to OPPy/PACh2. Fig. 8 shows that there is an increase in stripping current of OPPy/ PACh2 made from a concentration of up to 8 mM Na2PACh followed by a gradual decrease. The peak currents increased with the increasing concentration of Na2PACh used in the electrosynthesis of PPy/PACh2 because more PACh2 is available for complexation
Fig. 7. (a) Effect of reduction potential on the differential pulse anodic stripping voltammetric peak current. One hundred and eighty seconds accumulation time in 1.5 M NH3 buffer pH 8.5 containing 2.5 ppm Pb. Voltammetric conditions: electrolyte 0.1 M acetate buffer pH 4.7, reduction time 120 s with variation in reduction potential /900 // 1700 mV. (b) Effect of reduction time on the differential pulse anodic stripping voltammetric peak. One hundred and eighty seconds accumulation time in 1.5 M NH3 buffer pH 8.5 containing 2.5 ppm Pb. Voltammetric conditions: electrolyte 0.1 M acetate buffer pH 4.7, reduction potential /1200 mV with variation in reduction time from 2 to 180 s.
and also accumulation of Pb2. The decrease in peak current after 8 mM could be attributed to the presence of excess PACh2 resulting in a possible decrease in the conductivity of the modified electrode. The calibration curve was therefore generated by using an electrode containing OPPy/PACh2 synthesized from a solution containing 8 mM of Na2PACh and 40 mM of pyrrole, i.e. a ratio of 1:5. 3.4. Principle of detection Based on the above observations the proposed mechanism for the accumulation and differential pulse voltammetric procedure is outlined below:
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of other metals ions once appropriate chelators are identified. 3.5. Calibration, detection limit and interference
Fig. 8. Variation in differential pulse anodic stripping voltammetric peak currents with the change in [Na2PACh2 ] to pyrrole ratio used in the synthesis of PPy/PACh. One hundred and eighty seconds accumulation time in 1.5 M NH3 buffer pH 8.5 containing 2.5 ppm Pb. Voltammetric conditions: electrolyte 0.1 M acetate buffer pH 4.7, reduction time 120 s at /1200 mV.
1) The first step is the chelation of Pb2 from the accumulation solution on to the surface at the OPPy/PACh2 at open circuit (1.5 M NH3 buffer solution pH 8.5) 2 PACh2 Pb2 (aq)PACh2 surface 0 Pb surface
(1)
2) The second step is the reduction of the accumulated Pb2 ions to Pb0 at closed circuit by the application of a constant negative potential of /1200 mV (0.1 M acetate buffer pH 4.7) Pb2 PACh2 surface 2e
0 Pb0surface PACh2 surface
(2)
3) The final step is the stripping step where the Pb0 is electrochemically stripped back into the solution by scanning from /1200 to /100 mV. The resulting stripping peak obtained represents the analytical signal (0.1 M acetate buffer pH 4.7) Pb0surface 0 Pb2 (aq)2e
(3)
The resulting signal is proportional to concentration of Pb2. The usefulness of this approach lies in the fact that the sample preparation, accumulation, trapping and metal uptake can be achieved via electrochemical steps. The approach may be used for the determination
Standard solutions containing Pb2 in the concentration range 0/10 000 ng ml 1 were prepared in 1.5 M ammonia buffer at pH 8.5 and exposed to the OPPy/ PACh2 for 180 s. The ratio of the Na2PACh to pyrrole used for the electrochemical synthesis of OPPy/PACh2 was 1:5. The electrode was then subjected to the optimized differential pulse anodic stripping voltammetric procedures described above. These conditions were: 0.1 M acetate buffer pH 4.7 as the electrolyte, reduction time 120 s at /1200 mV. The voltammograms at different concentrations of Pb2 are shown in Fig. 9a and b. The peak potential of the voltammogram shifted to less negative potentials with increasing concentration of Pb2. A calibration curve of concentration versus peak current was plotted (Fig. 10). The calibration plot was linear between 0 and 200 ng ml 1 (y/0.03x/ 0.3232, R2 /0.988). At higher concentrations (/1000 ng ml 1) deviations from linearity occurred due to saturation of the electrode surface. The detection limit (based on three times the mean of blank measurements) was calculated by making replicate current measurements at /550 mV for blank solutions and it was found to be 10 ng ml 1. No peaks were observed when the OPPy/PACh2 was accumulated separately in Zn2, Fe3, Al3 or Mn2 at 1000 ng ml 1 and subjected to the differential pulse anodic stripping procedures described earlier. When similar solutions were spiked with Pb2 solutions to get 200 ng ml 1 of Pb2, the peak current magnitudes due to lead were the same as those obtained in electrolyte only. Cu2 exhibited some interference. However, the interference was less prominent than that experienced in the determination of Pb2 using a carbon-paste electrode modified with N -p -chlorophenylcinnamo-hydroxamic acid [25] where equimolar amounts of Cu2 depressed the peak current by as much as 36%. In the present study, equimolar amounts of Pb2 and Cu2 (200 ng ml1 each) produced no significant effects on the stripping peak current. However, when the amount of Cu2 was increased threefold to 600 ng ml1 the peak current was depressed by as much as 31%. Such effects have been reported [26] and are related to the formation of Pb /Cu solid solutions [27]. The phenomenon may result in either suppression or enhancement of the Pb2 stripping peak current [27]. Interference due to anions was also investigated. Accumulation of the modified electrode in 1000 ng ml 1 of Pb2 in excess of 50-fold SO2 and 4 ; 20-fold Cl 2 threefold CO3 followed by differential pulse anodic stripping voltammetry did not to have any measurable effect on the peak current due to lead.
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Fig. 10. Calibration curve from 0 to 200 ng ml 1. Conditions are as in Fig. 9.
method. Three determinations were made for each addition and three replicates were run. The average recovery of Pb2 was 93.4% with a relative standard deviation of 8.54%.
4. Conclusions Fig. 9. (a) Differential pulse anodic stripping voltammograms of Pb solutions (0 /80 ppb) accumulated on the OPPy/PACh2 electrode: (a) 0; (b) 20; (c) 40; (d) 80 ppb Pb. Conditions: PACh2 /Py ratio/1:5, accumulation time 180 min in 1.5 M NH3 buffer pH 8.5. Voltammetric conditions: 0.1 M acetate buffer pH 4.7, reduction potential /1200 mV for 120 s. (b) Differential pulse anodic stripping voltammograms of Pb solutions (200 /10 000 ppb) accumulated on the OPPy/PACh2 electrode: (a) 200; (b) 500; (c) 1000; (d) 5000; (e) 10 000 ppb Pb. Other conditions are as in Fig. 9a.
3.6. Environmental application In order to evaluate the performance of the modified electrode in practical analytical work, quantitation of lead in potable water was attempted in spiked and unspiked water samples using the standard addition method and the optimized conditions realized above. Since Pb2 was not detected in the potable water sample, a standard Pb2 solution was spiked into the water sample resulting in a total concentration of 25 ng ml 1. Analysis was done by the standard addition
A new differential pulse anodic stripping voltammetry method has been developed for the determination of trace levels of Pb2. The optimum conditions for metal determination depends on the uptake of lead from solution followed by its rapid complexation with an immobilized metal chelator at OPPy electrode. This step introduces a significant selectivity for the determination of lead and may be suitable for other metal ions. Open circuit accumulation of Pb2 was achieved in 1.5 M ammonia buffer (pH 8.5) for 180 s. The optimized conditions for differential pulse anodic voltammetric studies were identified as /1200 mV reduction potential for 120 s. Most metal ions studied with concentrations greater than the Pb2 did not interfere with the capability of the method. The detection limit and selectivity of this method are very impressive and are comparable to other mercury-free stripping voltammetric methods with low detection limits [28,29]. The linear range of up to 200 ng ml1 is much more than the linear ranges exhibited by most methods [25,29].
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