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Analytica Chimica Acta 577 (2006) 178–182

Square wave anodic stripping voltammetric determination of Pb2+ usingacetylene black paste electrode based on the inducing

adsorption ability of I−

Gang Li, Zhiming Ji, Kangbing Wu ∗Department of Chemistry, Huazhong University of Science and Technology, Wuhan 430074, PR China

Received 22 May 2006; received in revised form 22 June 2006; accepted 24 June 2006Available online 30 June 2006

bstract

Herein, a sensitive and simplified electrochemical method was proposed for the determination of trace levels of Pb2+ by anodic strippingoltammetry (ASV) based on the inducing adsorption ability of I− toward Pb2+. In the presence of low concentration of I−, Pb2+ was induced toccumulate onto the acetylene black (AB) paste electrode surface, and then reduced at −0.90 V. During the following square wave sweep from0.90 to −0.30 V, the reduced Pb was oxidized, resulting in a sensitive and well-shaped stripping peak at −0.56 V. Further studies indicate that

ow concentration of I− significantly enhances the sensitivity of determination of Pb2+. After all the experimental parameters were optimized,2+ −8

novel and sensitive method was developed for the electrochemical determination of Pb . The linear range is found to be from 2.0 × 10

o 4.0 × 10−6 mol L−1, and the lowest detectable concentration is estimated to be 6.0 × 10−9 mol L−1. This newly proposed method was finallyemonstrated with water samples. Otherwise, the anodic stripping responses of Pb2+ on AB paste electrode and graphite paste electrode wereompared.

2006 Elsevier B.V. All rights reserved.

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eywords: Lead; Electrochemical determination; Anodic stripping voltammetr

. Introduction

The pollution caused by heavy metal ions is becoming morend more severe all over the world, promoting extensive atten-ion into developing sensitive, reliable and rapid analytical

ethod. Lead is toxic and linked to various adverse health effects1,2]. Pb2+ easily accumulates in the environment and producesoxic effects on plants and animals, even at low concentrations3–5] since Pb2+ is not biodegradable. Therefore, it is very valu-ble and welcome to develop rapid and sensitive method for theetermination of Pb2+.

The commonly used method for Pb2+ analysis is atomicpectrometry, including atomic absorption spectrometry (AAS)nd atomic emission spectrometry (AES). These methods have

igh sensitivity and excellent selectivity, however, they have thentrinsic drawbacks: requirement of complicated and expensivenstruments, high cost, not for in situ measurement, et al. Dislike

∗ Corresponding author. Fax: +86 27 8754 3632.E-mail address: kbwu@mail.hust.edu.cn (K. Wu).

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003-2670/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.aca.2006.06.061

etylene black; Inducing adsorption

his, electrochemical method is attractive for in situ determin-ng Pb2+ since it exhibits high sensitivity, good selectivity, rapidesponse, easy data read-out and low cost. What is more, thenstruments employed in electrochemical method are relativelyimple, and conveniently miniaturized for in situ and automatedetection.

Anodic stripping voltammetry (ASV) is an ideal electro-hemical technique for the determination of trace levels ofetal ions because it possesses very high sensitivity. In the past

ears, mercury electrodes, consisting of dropping mercury elec-rode (DME), hanging drop mercury electrode (HDME) andhin mercury film electrode (MFE), were dominantly used inSV. This is due to the predominant properties of mercury elec-

rode: easy formation of amalgam with reduced metal, excellenteproducibility, easy surface renewal. However, mercury itself islso toxic, so numerous efforts have been made to develop var-ous mercury-free solid electrodes. Among these mercury-free

olid electrodes, paste electrode (also called carbon paste elec-rode) has obtained increasing attention and extensively usedn electroanalysis or electrochemistry since it possesses follow-ng advantages: easy preparation, porous surface, wide potential

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Fig. 1 shows the anodic stripping voltammetric responsesof Pb2+ in different conditions. After 4-min accumulationat −0.90 V in 0.1 mol L−1 HClO4 solution, an ill-shaped

Fig. 1. Square wave anodic stripping voltammograms of Pb2+ in 0.1 mol L−1

G. Li et al. / Analytica Chi

ange (from −1.40 to +1.30 V), low residual current, low costnd convenient surface renewal. To date, all kinds of mercury-ree solid electrodes or chemically modified solid electrodesCMEs) have been reported for the determination of Pb2+ [6–11].owever, to the best of our knowledge, determination of trace

evels of Pb2+ using acetylene carbon black (AB) paste electrodeased on the inducing adsorption ability of I− is firstly reportedere.

The main objective of the current work is to develop a sensi-ive and convenient electrochemical method for the determina-ion of Pb2+ utilizing the excellent properties of AB as well ashe inducing adsorption ability of I− toward Pb2+.

AB, a special type of carbon black, is made by the con-rolled combustion of acetylene in air under pressure. Dueo its excellent electric conductivity, large specific surfacerea and strong adsorptive ability, AB has been widely usedn electrochemistry and electroanalytical chemistry. Other-ise, previous reports [12,13] prove that I− can induce someetal ions to adsorb at electrode surface, and proper mech-

nism has been given. To achieve the goal, low concentra-ion of KI was added into supporting electrolyte to induceb2+ to accumulate onto AB paste electrode, significantlynhancing the surface amount of Pb2+ as well as its strippingeak current. Therefore, the sensitivity of Pb2+ analysis wasemarkably improved in the presence of I−. This new sens-ng and determining system possesses following advantages:xtreme simplicity, low detection limit, free of mercury and lowost.

. Experimental section

.1. Reagents

Stock solution of 1.00 × 10−2 mol L−1 Pb2+ was prepared byissolving Pb(NO3)2 (Shanghai Reagent Corporation, China)nto redistilled water, and then diluted to working solution atesired concentration with redistilled water. Other chemicalssed are analytical reagents and water used is re-distilled.

Acetylene carbon black (purity > 99.99%, particle size =50–200 nm) was purchased from STREM Chemicals (USA).raphite powder (spectral reagent) and paraffin oil were pur-

hased from Shanghai Reagent Corporation, China.

.2. Instruments

Electrochemical experiments were carried out using a CHnstruments 650B electrochemical workstation (CH Instrument,ustin, TX). A conventional three-electrode system, consistingf an AB paste working electrode, a saturated calomel referencelectrode (SCE) and a platinum wire auxiliary electrode, wasmployed. The body of working electrode was a polytetraflu-roethylene (PTFE) cylinder that was tightly packed with ABaste. A copper wire inserted into the paste providing electrical

ontact.

Atomic absorption spectrometric measurements were con-ucted with AA 6300 Atomic Absorption spectrophotometerSHIMADZU, Japan).

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Acta 577 (2006) 178–182 179

.3. Preparation of AB paste electrode

The AB paste electrode was prepared by mixing 100.0 mgB and 100.0 �L paraffin oil in a small mortar to form a homo-eneous AB paste. Then the paste was pressed into the end cavity3-mm in diameter, 1-mm in depth) of working electrode body.he electrode surface was smoothed against weighing paper.

t is important to note that the amount of paraffin oil must bearefully controlled because excessive paraffin oil will lower theonductivity of AB paste, while insufficient paraffin oil is noteneficial to obtain uniform AB paste. The graphite paste elec-rode was also prepared as above-mentioned procedure but theatio of graphite powder and paraffin oil is 100.0 mg–40.0 �L.

.4. Analytical procedure

Unless otherwise stated, 0.1 mol L−1 HClO4 solution con-aining 7.00 × 10−3 mol L−1 KI was used as supporting elec-rolyte for Pb2+ determination. The accumulation step was pro-eeded at −0.90 V for a desired time with stirring solution, thenhe square wave stripping voltammograms were recorded from

0.90 to −0.30 V after 15 s quiescence. The peak current waseasured at −0.56 V for Pb2+. After each measurement, the used

aste was carefully removed from the end cavity and a new ABaste was pressed into. That is to say, each AB paste electrodeas used one time to achieve better reproducibility.

. Results and discussion

.1. Electrochemical response of Pb2+ at AB pastelectrode

ClO4 at AB paste electrode: (a) blank voltammograms; (b) 1.0 × 10−7 mol L−1

b2+; (c–e) 5.0 × 10−8, 1.0 × 10−7, 2.0 × 10−7 mol L−1 Pb2+ in presence of.0 × 10−3 mol L−1 KI. Accumulation potential: −0.90 V, accumulation time:min, pulse amplitude = 25 mV, frequency = 25 Hz, potential increment = 4 mV.rrow means potential sweep direction.

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nodic stripping peak appears at AB paste electrode for.0 × 10−7 mol L−1 Pb2+ (Fig. 1b) during the square waveweep from −0.90 to −0.30 V with following parame-ers: amplitude = 25 mV, frequency = 25 Hz, potential incre-

ent = 4 mV. The stripping peak locates at −0.54 V and theeak current is very low (about 0.17 �A). When adding.0 × 10−3 mol L−1 KI into solution, the stripping peak cur-ent increases significantly (about 10 times), and stripping peakotential shifts negatively to −0.56 V (Fig. 1d). From the com-arison of curves (b) and (d), it is very clear that low concentra-ion of I− can remarkably enhance the stripping peak current ofb2+. According to previous reports [12,13], I− can induce someetal ions to adsorb at electrode surface (such as mercury) and

wo different proposals have been given. In the current system,b2+ is much easily accumulated at AB paste electrode surfacender the strong inducing adsorption ability of I−. Therefore, theurface amount of Pb2+ increases greatly and then the strippingeak current increases obviously in the presence of I−.

Otherwise, as improving the concentration of Pb2+ to.0 × 10−7 mol L−1, the anodic stripping peak current almostncreases by two times (Fig. 1e). If we lower the concentrationf Pb2+ to 5.0 × 10−8 mol L−1, the stripping peak current alsoecreases by 50% (Fig. 1c). When the concentration of Pb2+

urther decreases to zero (no Pb2+ in 0.1 mol L−1 HClO4 solu-ion), the anodic stripping peak current will vanish (Fig. 1a).hese phenomena reveal that the stripping peak at −0.56 V cor-

esponds to Pb2+ and what is more, the anodic stripping peakurrent exhibits good linearity with concentration of Pb2+, whichan be used as Pb2+ analytical signal.

.2. Electrochemical response of Pb2+ at graphite pastelectrode

In order to show the unique properties of AB, the anodic

tripping behaviors of Pb2+ at graphite paste electrode was inves-igated for comparison. The results are shown in Fig. 2. Curvesa) and (b) depict the square wave anodic stripping voltammo-rams of 1.0 × 10−7 mol L−1 Pb2+ after 4-min accumulation

ig. 2. Square wave anodic stripping voltammograms of 1.0 × 10−7 mol L−1

b2+ in 0.1 mol L−1 HClO4 at graphite paste (a and c) and AB (b and) paste electrodes; (a and b) in absence of KI; (c and d) in presence of.0 × 10−3 mol L−1 KI. Other conditions are the same as in Fig. 1.

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Acta 577 (2006) 178–182

nder −0.90 V at graphite paste and AB paste electrode, respec-ively. As seen, two stripping peaks are poor-shaped and notensitive. However, the stripping peak current at AB paste elec-rode (curve b) is relatively higher than that at graphite pastelectrode (curve a). After addition of 7.0 × 10−3 mol L−1 KI,he stripping peak currents increase both at graphite paste elec-rode (Fig. 2c) and AB paste electrode (Fig. 2d). The peak currentnhancement is attributed to the inducing adsorption ability of−, explained as above. Comparing curves (c) and (d), it is foundhat the stripping peak current at AB paste electrode is muchigher (about three times) than that at graphite paste electrode.hat is more, the stripping peak is well-defined at AB paste

lectrode. This is caused by the fact that AB possesses higherurface area and stronger adsorptive ability, resulting in higherccumulation efficiency to Pb2+.

.3. Choice of supporting electrolyte

In electrochemical determination, choice of suitable support-ng electrolyte (sometimes called determining medium) is verymportant since the electrochemical responses of Pb2+ showreat difference in different supporting electrolytes. In this work,he anodic stripping responses of Pb2+ in a variety of determining

ediums, such as HCl, HClO4, KCl, pH 3.5 ∼ 5.6 HAc-NaAcuffer (each 0.1 mol L−1) containing different concentration ofI from 0 to 1.0 × 102 were investigated in details. It is found

hat the stripping peak current is highest in 0.1 mol L−1 HClO4olution containing 7.0 × 10−3 mol L−1 KI, synchronously thetripping peak shape is best-defined. Therefore, 0.1 mol L−1

ClO4 solution containing 7.0 × 10−3 mol L−1 KI is selecteds best determining medium for Pb2+ analysis.

.4. Optimization of I− concentration

−

Fig. 1 tells that low concentration of I can improve the sensi-ivity of determining Pb2+. The influence of I− concentration onhe stripping peak current was investigated, and the results shownn Fig. 3. The stripping peak current firstly increases linearly with

ig. 3. Influence of KI concentration on stripping peak current of.0 × 10−7 mol/L Pb2+ at AB paste electrode. Other conditions are the sames in Fig. 1.

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− concentration over the range from 0 to 5.0 × 10−3 mol L−1,hen remains unchangeable over the range from 5.0 × 10−3 to.015 mol L−1. Finally, the peak current begins to decline when− concentration exceeds 0.015 mol L−1. In the determination,oncentration of I− was chosen as 7.0 × 10−3 mol L−1.

.5. Accumulation potential and time

Accumulation potential and time are two important param-ters in the anodic stripping voltammetric determination ofb2+, directly determining the sensitivity and selectivity to somextent. Thus, the anodic stripping peak currents of Pb2+ underifferent accumulation potential for 4 min were compared inig. 4. The anodic stripping peak current gradually increasess accumulation potential shifting from −0.60 to −0.90 V. Atore negative potential, Pb2+ adsorbed at AB paste electrode

s reduced more completely. Therefore, the following strippingeak current shows increase. However, the anodic stripping peakurrent of Pb2+ improves very slightly and the background cur-ent increases greatly when the accumulation potential is moreegative than −0.90 V. What is more, more negative accumula-ion potential will cause other metal ions or H+ to be reduced,ndoubtedly causing interference for the determination of Pb2+.hus, the optimized accumulation potential is −0.90 V in thisystem.

Under a fixed accumulation potential of −0.90 V, the strip-ing peak currents will improve as extending accumulationime. As expected in Fig. 5, the stripping peak current of.0 × 10−7 mol L−1 Pb2+ obviously increases as improvingccumulation time from 0 to 4 min. Longer accumulation timeauses much more Pb2+ to be adsorbed onto AB paste electrodeurface, and finally leads to peak current enhancement. Althoughhe striping peak current also gradually increases as furtherxtending accumulation time, the increment degree becomesmaller. From Fig. 5, it is very obvious that the sensitivity of

etermination of Pb2+ will increase when extending accumula-ion time. In order to shorten analysis time and achieve higherensitivity, the accumulation time is selected as 4 min in thisork.

ig. 4. Effect of accumulation potential on stripping peak current of 1.0 × 10−7

b2+. Other conditions are the same as in Fig. 1.

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ig. 5. Influence of accumulation time on anodic stripping peak current of.0 × 10−7 Pb2+. Other conditions are the same as in Fig. 1.

.6. Responses to Pb2+

The relationship between anodic stripping peak currentnd concentration of Pb2+ was studied. The peak current isirectly proportional to the concentration of Pb2+ in the rangerom 2.0 × 10−8 to 4.0 × 10−6 mol L−1 (R = 0.997). The low-st detectable concentration of Pb2+ after 4-min accumula-ion is 6.0 × 10−9 mol L−1. As above-mentioned, the limit ofetection will lower when increasing accumulation time. If theccumulation time extends to 10-min, this method can detect.0 × 10−9 mol L−1 Pb2+.

After each measurement, the AB paste was carefully removedrom the cavity and another new AB paste electrode was remades above-mentioned procedure. The reproducibility betweenultiple electrode preparations was estimated by comparing the

nodic stripping peak current of 1.0 × 10−7 mol L−1 Pb2+. Theelative standard deviation (R.S.D.) is 6.1% for 10 AB paste elec-rodes, revealing that this method possesses good reproducibilitynd potential applications.

.7. Interferences

The possible interferences of some inorganic speciesere also evaluated. Table 1 reveals that a great number

2+ 2+ 2+ 2+

f cations and anions, such as Ca , Zn , Mn , Co ,e3+, Al3+ (each 1.0 × 10−4 mol L−1), Cd2+ (5.0 × 10−6),u2+ (1.0 × 10−6 mol L−1), SCN−, Cl−, F−, Br−, SO4

2−,O3

−, PO43− (each 1.0 × 10−4 mol L−1) have no influences

able 1nterferences of some inorganic species on stripping peak current of.0 × 10−7 mol L−1 Pb2+

oreign species Tolerance level (mol L−1)a

a2+, Zn2+, Mn2+, Co2+, Fe3+, Al3+ 1.0 × 10−4

d2+ 5.0 × 10−6

u2+ 1.0 × 10−6

CN−, Cl−, F−, Br−, SO42−, NO3

−, PO43− 1.0 × 10−4

g2+ 1.0 × 10−5

a For 6% error.

182 G. Li et al. / Analytica Chimica

Table 2Determination of Pb2+ in water samples

Samples Detected byAAS (mol L−1)

Detected by thismethod (mol L−1)

R.S.D.(%)

Recovery(%)

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n the signals of 1.0 × 10−7 mol L−1 Pb2+, with peak cur-ent change below 6%. Only high concentration of Hg2+

>1.0 × 10−5 mol L−1) was found to interfere with the deter-ination of Pb2+. At −0.90 V, Hg2+ can be reduced and formthin film at AB paste electrode surface, causing Pb2+ to be

educed more easily since forming amalgam. Finally, the anodictripping peak current of Pb2+ increases in the presence of highoncentration of Hg2+.

.8. Determination of Pb2+ in water samples

In order to demonstrate its application in practical analysis,he new sensing and determining system was employed to detectb2+ in some water samples. Five milliliters of water sampleas added into 5.00 mL 0.2 mol L−1 HClO4 solution containing.014 mol L−1 KI, and then accumulated at −0.90 V for 4-minith stirring. After that, the anodic stripping peak current waseasured for Pb2+ as analytical procedure. The concentration ofb2+ was obtained by standard addition method, and the resultsollected in Table 2. The relative standard deviation (R.S.D.)f each sample for five times parallel detections was less than%. Additionally, the recovery was investigated and the values between 98.6% and 102.2%, suggesting this Pb2+ analysis

ethod is effective and feasible. Finally, in order to testify theeasibility and accuracy of this method, the typical and common

sed method: atomic absorption spectrometry (AAS) was usedo detect Pb2+ in water samples. From the comparisons listedn Table 2, conclusion can be made that this method has greatotential for practical sample analysis.

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Acta 577 (2006) 178–182

. Conclusions

The highlight of current work is that a sensitive and simplelectrochemical method was developed for the determination ofb2+ combining the excellent properties of AB (large surfacerea, strong adsorptive ability and excellent electric conductiv-ty) with inducing adsorption ability of I− to Pb2+. In the pres-nce of low concentration of I−, Pb2+ was effectively inducedo adsorb at AB paste electrode surface. As a result, the surfacemount of Pb2+ was remarkably improved, and then the corre-ponding stripping peak current greatly increases. In brief, it ishe inducing adsorption ability of I− that remarkably enhanceshe determining sensitivity of Pb2+.

cknowledgement

The authors are grateful to the financial support from theational Natural Science Foundation of China (No. 20507006).

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