Electrochemical Method for Quantitative Determination of Trace

Anal. Chem. 2008, 80, 2042-2049

Electrochemical Method for Quantitative Determination of Trace Amounts of Lead Lasantha T. Viyannalage, Stoyan Bliznakov, and Nikolay Dimitrov*

Department of Chemistry, State University of New York at Binghamton, P.O. Box 6000, Binghamton, New York 13902-6000

An ultrasensitive chronoamperometric method for quantitative determination of trace amounts of lead (down to 20 ppb) in acidic solutions is proposed in this paper. The method is based on observations that a complete underpotentially deposited (UPD) lead layer inhibits the electroreduction of nitrate on a bare Cu(111) electrode. To asses the limits of the method, both the electroreduction of nitrate and UPD of lead monolayer on copper single (111) and polycrystalline electrodes in perchloric acidic solution are studied by means of cyclic voltammetry, chronoamperometry, and rotating disk electrode (RDE) experiments. It is found that an inexpensive polycrystalline copper electrode is sensitive enough for analytical detection of lead traces in electrolytes down to 1 × 10-8 M. Analytical results obtained by the proposed method in 2 orders of magnitude concentration range are compared to atomic absorption spectroscopy measurements to evaluate and assess the sensitivity of the employed experimental protocol. The excellent match between both analytical approaches validates the applicability of the proposed method. Lead has been mined, smelted, refined, and used in products (e.g., as an additive in paint, gasoline, leaded pipes, solder, crystal, and ceramics) for hundreds of years. All these activities resulted eventually in substantially higher lead levels in the environment, especially near mining and smelting sites, near some types of industrial and municipal facilities, and adjacent to highways. Lead poisoning can cause a number of adverse human health effects but is particularly detrimental to the neurological development of children. In children, lead poisoning can cause brain damage and/ or mental retardation, behavioral problems, anemia, liver and kidney damage, hearing loss, hyperactivity, developmental delays, and in extreme cases, death.1,2 Therefore, the controlling of lead contamination in the environment has always been of major importance. Key steps in this direction are represented by frequent testing and precise monitoring of Pb2+ ion content in the soil, air, and water. There are three groups of methods for detection of trace lead concentrations. Among those, flame atomic absorption * Corresponding author. Phone: +1 607-777-4271. Fax: +1 607-777-4478. E-mail: [email protected]. (1) National Safety Council Web site. http://www.nsc.org/library/facts/lead.htm (accessed Sept 24, 2007). (2) Environmental Protection Agency Web site. http://www.epa.gov/lead/ (accessed Sept 24, 2007).

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spectrometry,3 electrothermal atomic absorption spectrometry,4 isotope dilution inductively coupled plasma mass spectrometry,5 hydride generation atomic fluorescence spectrometry,6 etc., are known as spectroscopy methods. The second group that is spectrophotometric in nature is represented by a variety of UV methods.3 The third group that encompasses electroanalytical methods includes anodic stripping voltammetry7 and differential pulse polarography.8 The sensitivity, selectivity, duration, and costeffectiveness are the main criteria for assessment of the methods for lead detection. The spectroscopic methods are selective and highly sensitive (down to 1 ppb of Pb) but require a timeconsuming sample preparation and involve instruments that are originally expensive and demand additional investment to cover their high day-to-day maintenance cost. Spectrophotometric methods are simple and economical but are not sensitive and selective enough.3 The standard electroanalytical methods are fast and economical, but their selectivity and the sensitivity (down to 1 × 10-7 M of Pb2+ in solution) are relatively low. Stripping voltammetry9 and differential pulse anodic stripping voltammetry10 are rapid, low-cost electrochemical methods for lead detection with good selectivity and high sensitivity. Although a sensitivity of the order of 1 × 10-10 M of Pb2+ ions is reported in these papers,9,10 it should be noted that most sensitive measurements would heavily depend upon the preconcentrated amount of lead on the working electrode, that in turn requires time-consuming and complicated preparation procedures.10 In addition to this, most of the electroanalytical methods are best suited to work with mercury that is highly toxic and currently not recommended in most analytical practices. That is why we propose herein an innovative method for quantitative determination of lead traces that is fast, accurate, ultrasensitive, selective, and inexpensive. This method is based on the interference of two well-examined and documented electrochemical processes such as the electrochemical reduction of nitrate ions on copper electrodes11-19 and the underpotential (3) Taher, M. Croat. Chem. Acta 2003, 76 (3), 273-277. (4) Acar, O. Anal. Chim. Acta 2005, 542, 280-286. (5) Vassileva, E.; Quetel, C. Anal. Chim. Acta 2004, 519, 79-85. (6) Wan, Z.; Xu, Z.; Wang, J. Analyst 2006, 131, 141-147. (7) Application Note S-6 (Reprinted from American Laboratory, November, 1981). EG&G Princeton Applied Research Corporation Web site. http:// www.princetonappliedresearch.com/products/appnotes.cfm (accessed Sept 29, 2007). (8) Application Note P-2 (1980). EG&G Princeton Applied Research Corporation Web site. http://www.princetonappliedresearch.com/products/appnotes.cfm (accessed Sept 29, 2007). (9) Bartlett, N.; Denuanlt, G.; Sousa, F. Analyst 2000, 125, 1135-1138. (10) Li, W.; Kong, J. Anal. Lett. 2007, 40, 2161-2170. (11) Carpenter, N.; Pletcher, D. Anal. Chim. Acta 1995, 317, 287-293. 10.1021/ac702028h CCC: $40.75

© 2008 American Chemical Society Published on Web 02/15/2008

deposition (UPD) of lead on copper poly- and single-crystalline surfaces.20-24 The reduction of nitrate and nitrite has gained growing interest in recent years in view of its relevance to pollution control. Therefore, an extensive research on the electrochemical reduction of nitrate and nitrite ions in alkaline and acid solutions on noble and other metal electrodes such as Cu, Pt, Pd, Rh, Ru, Ir, Ag, and Au has been done in the past decade.11-19 The chemical and electrochemical reduction of nitrate are complex reactions with an outcome that depends strongly upon the experimental conditions. Thus, nitrite, nitrogen, and ammonia, involving the transfer of 2e-, 5e-, and 8e-, respectively, are the most common products. Vooys et al.12 have investigated the reduction of NO3on palladium/copper electrodes in acidic and alkaline solutions. In their work they reported on the critical role of copper in enhancement of the NO3- to NO2- reduction on Pd/Cu catalysts impacting both the initial adsorption of NO3- and the chargetransfer step, according to the adopted mechanism. The reaction is very dependent on the electrolyte composition. Equally important here are both the electrolyte pH and anion content. On pure Cu surface the nitrate ions are found to undergo reduction to NO in acidified solution.13-15 The electroreduction of nitrate and nitrite at a bare, freshly polished copper electrode and at the copperdeposited modified electrode in acidic solution is studied by Davis et al.15 These authors apply linear sweep voltammetry for analytical determination of NO3- and NO2- in lettuce and sewage treatment discharge water. A comparative study to determine the reactivity of nitrate ions on different polycrystalline electrodes in acidic solution is performed from Dima et al.13 This work reports on a strong dependence of the nitrate reduction current density upon the nature of the working electrode. According to the authors of ref 13 copper has the highest catalytic activity for the electroreduction of nitrate when compared with Au and Ag. The structure and properties of the Pb UPD monolayer on gold, silver, platinum, and copper are also extensively studied and well-understood.20-24 The Pb monolayer forms an unrotated yet incommensurate hexagonal structure on the Cu(111) surface.20 Later on it is shown for the first time by our group that the voltammetry of this system is highly sensitive to the presence of NO3- ions so that a moderate concentration of nitrate could cause complete disappearance of the Pb stripping peak.20 The proposed method in this paper is based on observations in that same work suggesting complete inhibition of the nitrate electroreduction process when a Pb UPD (12) Vooys, A.; Santen, R.; Veen, J. J. Mol. Catal. A: Chem. 2000, 154, 203215. (13) Dima, G.; Vooys, A.; Koper, M. J. Electroanal. Chem. 2003, 554-555, 1523. (14) Cacella, I.; Gatta, M. J. Electroanal. Chem. 2004, 568, 183-188. (15) Davis, J.; Moorcroft, M.; Wilkins, S.; Compton, R.; Cardosi, M. Analyst 2000, 125, 737-742. (16) Groot, M.; Koper, M. J. Electroanal. Chem. 2004, 562, 81-94. (17) Keita, B.; Abdeljalil, E.; Nadjo, L.; Contant, R.; Belgiche, R. Electrochem. Commun. 2001, 3, 56-62. (18) Gootzen, J.; Lefferts, L.; Veen, V. Appl. Catal., A 1999, 188, 127-136. (19) Beltramo, G.; Koper, M. Langmuir 2003, 19, 8907-8915. (20) Vasilic, R.; Vasiljevic, N.; Dimitrov, N. J. Electroanal. Chem. 2005, 580, 203212. (21) Brisard, G.; Zenati, E.; Gasteiger, H.; Markovic, N.; Ross, P. Langmuir 1995, 11, 2221-2230. (22) Chu, Y.; Robinson, I.; Gewirth, A. J. Chem. Phys. 1999, 110 (12), 5952. (23) Vasilic, R.; N.; Dimitrov, N.; Sieradzki, K. J. Electroanal. Chem. 2006, 595, 60-70. (24) Dickertman, D.; Koppitz, F.; Shultze, J. Electrochim. Acta 1976, 21, 967971.

Figure 1. Schematic of the proposed method: (I) nitrate electroreduction on the surface of a Cu electrode; (II) formation of the Pb UPD layer; (III) inhibition of the electroreduction of nitrate ions.

layer is formed on the Cu(111) substrate.20 The method is schematically outlined in Figure 1. Three distinct ranges are shown by a model chronoamperometric current transient: (I) the current is constant in the first range owing to a steady nitrate electroreduction on a bare Cu electrode, (II) the current decreases in the second range, where the Pb UPD layer blocks progressively the nitrate electroreduction, and (III) the current drops to zero as the completed Pb UPD monolayer “extinguishes” the electroreduction of nitrate in the third region. Apparently, the integral time for bringing the nitrate reduction on the electrode surface to a complete halt is directly proportional to the concentration of Pb2+ ions into the solution. EXPERIMENTAL SECTION Working Electrode. A Cu(111) single-crystalline (Monocrystals Co.) and copper (99.999%, Alfa Aesar) polycrystalline cylinders, both 3 mm thick and 10 mm in diameter, were used as working electrodes in our electrochemical experiments. The crystal was first mechanically polished with water-based alumina slurry (suspension of Buehler Micropolish II deagglomerated alumina polishing powder) down to 0.05 µm. Following the mechanical polishing, the crystal was rinsed with 2 L of Barnstead Nanopure water (>18.3 MΩ). It was then electrochemically polished in 5:3:2 ratio mixture of concentrated H3PO4/ethylene glycol/water at anodic dc current (0.1-0.15 A cm-2) for 10-15 s. After the electrochemical polishing, the crystal was thoroughly rinsed with 0.2 L of ethyl alcohol and 2 L of Barnstead Nanopure (>18.3 MΩ) water. Then, the crystal was terminated by a droplet of perchloric acid solution (pH 2) in order to deter possible surface oxidation and transferred to the electrochemical cell. The electrochemical measurements were made in a hanging meniscus configuration by employing a modification of the so-called dipping technique.25 This type of contact has been considered particularly suitable for investigation of electrode processes on single-crystal faces25 and for protocols where efficient deoxygenation is mandatory.20 In a set of rotating disk electrode (RDE) experiments a Cu polycrystalline cylinder 2 mm thick and 6 mm in diameter was used and prepared according to the procedure described above. After terminating the surface with a droplet of pH 2 perchloric acid the electrode was mounted on rotating disk holder (Pine Instruments) following a sequence of steps described in detail elsewhere.26 (25) Herrero, E.; Clavilier, J.; Feliu, J.; Aldaz, A. J. Electroanal. Chem. 1996, 410, 125-127. (26) Markovic, N.; Gasteiger, H.; Ross, P., Jr. J. Phys. Chem. 1995, 99, 3411.

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Figure 2. Cyclic voltammetry of single- and polycrystalline Cu electrodes in 2 × 10-4 M Pb(ClO4)2 + 1 × 10-2 M HClO4 solution at a sweep rate of 10 mV‚s-1.

Figure 3. Cyclic voltammograms of a copper polycrystalline electrode in 0.01 M HClO4 solution (a) and in 0.01 M HClO4 + 1 × 10-4 M HNO3 solution (b) at a scan rate of 20 mV‚s-1.

Cell and Electrolytes. The working electrolyte for investigation of the kinetics of Pb UPD in the absence of nitrate ions was a 1 × 10-2 M solution of HClO4 (double-distilled, GFS Chemicals) and X M Pb(ClO4)2 (99.999% metal basis, Sigma-Aldrich), where 1 × 10-3 > X > 1 × 10-7. The kinetics of NO3- electroreduction was investigated in a 1 × 10-2 M solution of HClO4 and X M HNO3 (double-distilled, GFS Chemicals), where 1 × 10-3 > X > 1 × 10-6. The analytical solution for method development consisted of 1 × 10-2 M HClO4 and 1 × 10-4 M HNO3 and X M Pb(ClO4)2 where X was varied. All solutions were made with Barnstead Nanopure water (>18.3 MΩ). The three-electrode glass cell used in the experiments was cleaned successively in concentrated HNO3 and concentrated H2SO4 both heated to 70 °C. Then, the cell was rinsed with 2 L of deionized water and finally terminated by Barnstead Nanopure water. A mercury sulfate electrode (MSE) was used as a reference electrode. However, for the sake of clarity the potentials in the text are reported versus lead pseudoreference electrode. A platinum wire served as a counter electrode. Prior to each experiment the electrolyte was deoxygenated for at least 2 h using ultrahigh purity nitrogen gas with less than 1 ppb oxygen, CO, CO2, and moisture content. Electrochemical and Atomic Absorption Measurements. Cyclic voltammetry and chronoamperometry experiments were performed with a Princeton Applied Research potentiostat, PAR 273, coupled with a digital storage oscilloscope Nicolet 310 to acquire experimental data. RDE experiments were performed by using a Pine Instrument bipotentiostat (model AFCBP1) operated by a PC with PinChem 2.7 software that also served for data acquisition purposes. The rotating rate was controlled by a Pine Instrument MSRX speed controller. A flame atomic absorption spectrophotometer (Perkin-Elmer, model A Analyst 300) equipped with a lead Lumina hollow cathode lamp was used for atomic absorption spectroscopy measurements. A standard procedure for calibration curve measurements in solutions with known Pb concentration was also applied at a wavelength of 283.3 nm.

potentials (340 and 176 mV vs Pb pseudoreference electrode (pre) for the anodic and cathodic peaks, respectively) on the copper electrode are observed in both CV curves. The cathodic and anodic peaks observed in CV voltammograms at about 0 V correspond to the bulk lead deposition and striping accordingly. It is seen that the peaks for Pb UPD on the polycrystalline Cu electrode are broader and are also shifted by about 20 mV toward lower potentials compared to the peaks of the Cu(111) electrode. The Pb peak intensity on the Cu(111) electrode is higher compared to the one measured on the polycrystalline electrode. The comparison of the charges, calculated from the cathodic peak area, 310 µC‚cm-2 for Cu(111) versus 232 µC‚cm-2 for Cu (poly), respectively, suggests presence of more UPD Pb atoms on Cu(111). This is expected as the Pb UPD layer on the Cu(111) surface is closer packed compared to the one on the polycrystalline electrode. In general, despite the above details the comparison of the CV for both Cu electrodes demonstrates no qualitative difference. The high catalytic activity of Cu to electroreduction of nitrate ions is clearly demonstrated in Figure 3, where the CV behavior of the Cu polycrystalline electrode in 1 × 10-2 M HClO4 solution (Figure 3a) and in 1 × 10-2 M HClO4 + 1 × 10-4 M HNO3 solution (Figure 3b) is presented. Apparently, in agreement with results published elsewhere22,27 no peaks are observed in perchloric acid solution (absence of NO3- ions) in the potential range of interest (Figure 3a). The electroreduction of nitrate that takes place on the Cu surface in NO3--containing solution is hinted at by the CV in curve (Figure 3b). Here, in a partially irreversible reduction process the rate of the 3e- transition from NO3- to NO is high enough to drive the current negatively even when potential is swept in the anodic direction. Figure 4, parts a and b, depicts the voltammetric behavior of a polycrystalline copper electrode immersed in perchloric solution containing only Pb2+ ions (Figure 4a, solid curve) and in the presence of both Pb2+ and NO3- ions, where the concentration of the nitrate ions is varied from 1 × 10-3 to 1 × 10-5 M. It could be seen that the anodic peak characteristic for UPD of Pb on Cu is registered only in the nitrate-free electrolyte. The dominant role of the nitrate electroreduction process on the bare copper

RESULTS AND DISCUSSION Current-Voltage Behavior of Nitrate Electroreduction and Pb UPD on Single- and Polycrystalline Copper Electrodes. Cyclic voltammograms for Cu(111) and Cu polycrystalline polished electrodes in 2 × 10-4 M Pb(ClO4)2 + 1 × 10-2 M HClO4 solution are presented in Figure 2. Characteristic peaks at Pb UPD 2044 Analytical Chemistry, Vol. 80, No. 6, March 15, 2008

(27) Vasiljevic, N.; Viyannalage, L.; Dimitrov, N.; Missert, N.; Copeland, R. J. Electrochem. Soc. 2007, 54 (4), C202-C208.

Figure 4. Cyclic voltammograms of a copper polycrystalline electrode in 0.01 M HClO4 + 1 × 10-3 M Pb(ClO4)2 + X M HNO3 (X ) 1 × 10-3 to 1 × 10-6) solution at a sweep rate of 1 mV‚s-1.

Figure 5. Levich plots of a Cu polycrystalline electrode in (a) 1 × 10-2 M HClO4, 1 × 10-4 M Pb(ClO4)2 solution at a potential of -100 mV (solid line) and at the corresponding potential in deoxygenated lead-free solution (dotted line) and (b) 1 × 10-2 M HClO4, 1 × 10-4 M HNO3 solution.

electrode is clearly demonstrated by the CV curves registered in presence of nitrate ions. Apparently, the increase in the NO3- ion concentration leads to their enhanced electroreduction on the bare copper surface resulting eventually in reduction currents that exceed the oxidation current associated with Pb UPD stripping. Thus, the rate of reduction is high enough not only to compensate the oxidation process of Pb to Pb2+ but also to drive the net current negatively.20 Figure 4 also shows that the cathodic current decays nearly to zero at potentials lower than 200 mV independently on NO3- concentration thereby indicating no nitrate electroreduction in this potential range. This suggests that the monolayer of Pb deposited underpotentially on the electrode surface inhibits completely the nitrate electroreduction activity. This result becomes clearer taking into account the reoccurrence of nitrate electroreduction upon stripping of the UPD Pb layer at a potential of about 160 mV during the anodic potential sweep. Kinetic Study of Competing Pb UPD and Nitrate Electroreduction Reactions on Copper in Perchlorate Solutions. In addition to CV, RDE experiments are used for detailed investigation of the reaction kinetics for both Pb UPD and nitrate electroreduction in the potential range of interest. Levich plots obtained from steady-state current measurements at a potential of -100 mV in the presence of 1 × 10-4 M Pb2+ ions and at potentials of 20, 40, and 60 mV in the presence of NO3- ions are

presented in Figure 5, parts a and b, respectively. The linear dependence of the current density with the square root of the electrode rotation speed (in revolutions per minute, rpm), Figure 5a, solid line, confirms the diffusion (mass transport) limitations that govern the deposition (respectively, UPD) of Pb on the Cu polycrystalline electrode in solutions at CPb2+ e 1 ×10-4 M. It should be noted that the intercept in Figure 5a is manifestation of a background current originating from the oxygen reduction reaction (ORR). In that potential range the ORR is mostly kinetically controlled as depicted by the steady-state current measurements performed at the corresponding potential in deoxygenated lead-free solution (Figure 5a, dotted line). This mass transport controlled Pb deposition suggests also that the increase of the rotation speed leads to substantially higher current density and in turn to enhanced sensitivity of this particular measurement to the presence of lead ions. This finding is certainly beneficial for adopting RDE measurements in the Pb2+ detection protocol. According to the Levich plots of nitrate electroreduction data, shown in Figure 5b, this reaction is a mixed-controlled process (certain diffusion and predominating kinetic limitations) that does not feature substantial current change with increase of the electrode rotation speed. On the basis of the above, when Pb2+ and NO3- ions are present in the electrolyte, both reactions (UPD of Pb and nitrate reduction) will concomitantly occur (despite their Analytical Chemistry, Vol. 80, No. 6, March 15, 2008

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Figure 6. Chronoamperometric transients on a Cu polycrystalline electrode, measured at 0.04 V in stagnant (a) 1 × 10-2 M HClO4, 1 × 10-4 M HNO3 solution and (b) 1 × 10-2 M HClO4, 1 × 10-4 M HNO3, 1 × 10-3 M Pb(ClO4)2 solution.

Figure 7. Chronoamperometric transients on a Cu polycrystalline electrode in stagnant 1 × 10-2 M HClO4 + 1 × 10-4 M HNO3 solution and various lead concentrations: (a) 1 × 10-4 M Pb(ClO4)2, (b) 1 × 10-5 M Pb(ClO4)2, and (c) 1 × 10-6 M Pb(ClO4)2.

different kinetics), at a rate depending upon the applied potential and electrode rotation speed. To systematically reveal the nature of Pb UPD/nitrate reduction competition, chromoamperometric measurements on a polycrystalline Cu electrode in stagnant electrolyte containing either only nitrate ions or both NO3- and Pb2+ ions are presented in Figure 6, parts a and b, respectively. It is clearly seen that at a potential of 0.04 V in the UPD range, the nitrate electroreduction current reaches its limited value of about 300 µA and levels off in the absence of Pb2+ ions. The same cathodic potential applied in solution containing both ions results in 2 times higher current because both reactions (Pb UPD and nitrate electroreduction) take place simultaneously. Right after that, however, the current undergoes decay with time. This decay could be rationalized as a steady passivation of the nitrate prone copper surface by growing clusters of Pb UPD atoms. The total passivation time associated with blockage of the entire electrode surface and the current decay to its minimum value, respectively, depend strictly upon the bulk concentration of Pb2+ ions as discussed later on in this work. This straightforward effect is indeed the rational emphasized by the proposed method. Inhibition of Nitrate Electroreduction on Copper by Pb UPD: Nature and Mechanism. A better insight on the nature and mechanism of this competitive reduction could be obtained in Figure 7a-c where chronoamperometric results on a Cu polycrystalline electrode are presented. The experiments here are performed at various concentrations of Pb2+ ions in the presence of constant concentration of NO3- ions in stagnant electrolyte. 2046 Analytical Chemistry, Vol. 80, No. 6, March 15, 2008

Figure 8. Model pictures illustrating the suggested mechanism: (I) nucleation of the Pb UPD layer; (II) establishment of steady-state diffusion of lead and growth of Pb clusters; (III) growth and completion of the UPD layer.

The extrapolation of the linear part of range III curves that intersects the abscissa axis accounts for the integral time for bringing the nitrate reduction on the electrode surface to a complete halt. This time is directly proportional to the concentration of Pb2+ ions into the solution as confirmed by the shorter integral time measured at higher lead concentrations in Figure 7. Ideally, this dependence should be linear as far as the Pb UPD process is purely mass transport controlled. In real life, however, at concentrations lower than 1 × 10-4 M Pb2+, the experimental transients change their shape. This is most likely due to the complex nature of competition between diffusion-limited UPD of Pb and mixed-controlled NO3- electroreduction that manifests itself by three distinct segments in the current transients. These segments correspond to different steps of the mechanism suggested in the following discussion for UPD of Pb on a Cu electrode and concomitant electroreduction of nitrate ions. A model that emphasizes these steps is presented in Figure 8. According to this model the first step (I) corresponds to the nucleation of the Pb UPD layer. The nucleation time is proportional to the CPb2+ as it exhausts the lead ions in the near-electrode vicinity right upon the process initiation. The second segment (II) appears at

Figure 9. Chronoamperometric curves, measured at 600 rpm in 1 × 10-2 M HClO4 + 1 × 10-4 M HNO3 solution with various lead concentrations in two sets: (a) two of solutions from the first set of electrolytes with concentrations of 1 × 10-5 and 1 × 10-6 M Pb(ClO4)2; (b) all solutions from the second set of electrolytes, presented in Table 1. Table 1. Prescribed Concentration, Integral Time of Current Decay, and Their Logarithm Values for the Investigated Sets of Electrolytes concn of Pb2+, M, (ppm), and [ppb]

time, s

log time

log CPb2+

set no. I

set no. II

set no. I

set no. II

set no. I

set no. II

set no. I

set no. II

1 × 10-7 [20.72] 1 × 10-6 [207.2] 1 × 10-5 (2.072) 1 × 10-4 (20.72) 1 × 10-3 (207.2)

2.4 × 10-6 (0.5) 4.8 × 10-6 (1.0) 1.2 × 10-5 (2.5) 2.4 × 10-5 (5.0) 4.8 × 10-5 (10.0)

3800 346 40 4.3 0.47

141 78 31 15 7.2

-7 -6 -5 -4 -3

-5.6 -5.3 -4.9 -4.6 -4.3

3.6 2.5 1.6 0.6 -0.3

2.15 1.89 1.49 1.18 0.86

concentrations lower than 1 × 10-4 M of Pb2+ ions and is associated with the establishment of a steady-state diffusion of Pb2+ ions from the bulk of solution to the electrode surface. According to the Fick’s first law the kinetics of this process is also concentration-dependent. The third step (III) is connected with the completion of the UPD layer, and its timing is undoubtedly proportional to CPb2+ as well. Thereby (albeit not in the most straightforward way), the entire curve should also be proportional to the bulk concentration of Pb2+ ions and in turn may be considered as a gauge for trace lead quantification. As it was mentioned above, the integral time determined by extrapolating the linear part of the chronoamperometric curves to the abscissa axis is a parameter proportional and/or associated to the bulk Pb2+ ions concentration. The relationship between the time of Pb UPD layer formation and lead ion concentration can be expressed as follows:

t)

Q 1 Q )k ≈ I CPb2+ CPb2+

(1)

where k is a coefficient of proportionality. After taking a logarithm of eq 1 we obtain eq 2, respectively,

log t ) log k - log CPb2+

(2)

In these equations t is the integral time for Pb UPD layer formation, Q is the charge, and CPb2+ is the concentration of lead ions in the solution. As seen from eq 2, the logarithm of time is directly proportional to the logarithm of the lead concentration.

This functional dependence allows us to determine trace amounts of Pb by simply measuring the time of lead UPD layer formation. To realize this method quantitatively and at higher sensitivity we switched to RDE experiments where the mass transport is controlled by convection and thereby the current is enhanced by the hydrodynamic flux. In RDE configuration we registered chronoamperometric curves of a Cu polycrystalline electrode in solutions with known concentrations of lead ions at different rotation speeds. We found that the speed of 600 rpm is optimal for lead detection down to the part-per-billion range of concentration. In order to prove the reproducibility of the experiments, we prepared two sets of electrolytes, as summarized in Table 1, and ran current transients in all of them. Chronoamperometric curves, registered at 600 rpm in solutions of the first and the second set, are presented in Figure 9, parts a and b, respectively. It is seen that the initial current is about 1-2 orders of magnitude higher than the one measured in stagnant electrolyte at corresponding concentrations, respectively. The comparison also suggests that the time for Pb UPD layer formation on the electrode surface is significantly reduced. It should be mentioned that the shape of the curves, albeit somewhat smoother, is similar to the one measured in stagnant solutions thereby suggesting that the process in RDE configuration follows an identical mechanism. A more detailed look indicates that the time for each step is reduced, because of a steadier supply of lead ions to the vicinity of the electrode as a result of being facilitated by convection mass transport. The integral times for the Pb UPD layer formation, the logarithms of the concentration, and the time values are also summarized in Table 1. Analytical Chemistry, Vol. 80, No. 6, March 15, 2008

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Table 2. Prescribed Concentration of the Investigated Electrolytes, Integral Time of Current Decay Determined by Chronoamperograms, and Concentrations of Pb2+ Determined by the Proposed Method and AAS

Figure 10. Comparison between the calibration curves for both sets of electrolytes with various concentrations.

Validation of the Proposed Chronoamperometric Method for Trace Amounts of Lead Detection. Final details of the proposed method validation can be found in Figure 10 where the calibration straight lines for the investigated sets of electrolytes are presented. As in the stagnant electrolyte case, the calibration curves here were plotted by extrapolating the linear part of the transients in Figure 9. A good match is observed for both curves. The linear relationship obtained in the investigated range of concentrations ascertains the validity of the proposed analytical concept. The results in Figure 10 also show that the limit of quantitation is well in the part-per-billion range and thereby singledigits part-per-billion traces of lead can be readily detected. It is now obvious that the integral time of the chronoamperometric curves measured on a Cu electrode in a solution of 1 × 10-2 M HClO4 + 1 × 10-4 M HNO3 with an unknown concentration of lead would allow for determining the Pb2+ concentration from the calibration curve in Figure 10. In order to validate the accuracy of the method we compared the concentration values determined by the suggested method with the values obtained from atomic absorption spectroscopy (AAS) for three different electrolytes with known lead concentrations. The actual concentration of the investigated electrolytes, integral times determined from chronoamperograms, and the concentrations determined by both methods are summarized in Table 2. The calibration curves and the match with measurements for solutions with prescribed concentration of Pb2+ ions, analyzed by

prescribed Pb2+ concn, ppm (M)

integral time, s

Pb2+ concn by chronoamperometry, ppm

Pb2+ concn by AAS, ppm

0.7 (3.4 × 10-6) 1.2 (5.6 × 10-6) 3.6 (17.4 × 10-6)

107.0 57.2 21.0

0.67 1.26 3.48

0.81 1.29 3.56

the new method and AAS, are also graphically presented in Figure 11, parts a and b, respectively. Linear dependences, with high correlation coefficients, in the investigated range of concentrations are observed for both calibration curves. The analysis of results presented in Table 2 shows that the concentration values measured by the method proposed herein are at least equally accurate to the actual ones with those registered by AAS. This comparison undoubtedly validates the proposed method and defines it as very accurate and highly sensitive in the detection of trace amounts of Pb in acidic solutions. CONCLUSIONS An innovative chronoamperometric method for analytical detection of trace amounts of lead in perchloric acidic solutions is proposed in the present paper. The method is based on the measurement of the steadily decaying rate of nitrate electroreduction on copper electrodes passivated by growing a Pb UPD layer in acidic solutions. Cyclic voltammetry, chronoamperometry, and RDE experiments in electrolytes containing Pb2+ and/or NO3ions are used to investigate the kinetics of participating reactions. It is shown that there are no qualitative differences in the mechanism of the competing processes on single- or polycrystalline Cu electrodes. This finding enables the application of an inexpensive polycrystalline Cu electrode without sensitivity loss in the proposed method. Enhanced sensitivity and reduced time for Pb detection are achieved by employing the RDE configuration for the working electrode. A qualitative model for a three-step mechanism for UPD layer formation in competition with a steadystate NO3- electroreduction is also proposed. This model suggests an overall proportionality between the integral time for UPD layer formation and the concentration of Pb2+ ions in the electrolyte. A

Figure 11. Experimental calibration curves for the investigated electrolytes, measured by (a) the proposed method and by (b) AAS. 2048 Analytical Chemistry, Vol. 80, No. 6, March 15, 2008

validation of the proposed method is provided by comparison of accordingly measured and obtained by AAS values for solutions with prescribed Pb2+ concentration. Nearly identical results ascertain the applicability of the concept employed in this study. In an attempt to shorten up by orders of magnitude the time for analysis we will emphasize in a future work quantitative aspects of the concentration dependence in the very beginning of each chronoamperometric measurement.

ACKNOWLEDGMENT This work is supported in part by the National Science Foundation, Division of Materials Research (DMR-0603019). Received for review September 28, 2007. Accepted January 9, 2008. AC702028H

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