Determination of Hg2þ Ions Using Electrodes Modified withDithia-Podands Having Different End Alkyl Chain Lengths

Mi-Sook Won,+,+++ Yun-Jung Bae,+ Shim-Sung Lee,++ and Yoon-Bo Shim*+

+ Department of Chemistry and Chemistry Institute of Functional Materials, Pusan National University, Pusan 609-735, S. Korea

e-mail: ybshim@hywon.pusan.ac.kr++ Department of Chemistry, Gyeongsang National University, Chinju 660-701, S. Korea+++ Present address: Korea Basic Science Institute, Pusan 609-735, S. Korea

Received: August 31, 2000

Final version: November 14, 2000

Abstract

A series of dithia-podands (1–3) having different chain lengths (C6–C12) of alkyl endgroups has been used to modify the glassy carbonelectrode by a spin coating method for the analysis of trace amounts of Hg. The analytical parameters that affect the electrode reactionprocess have been studied in terms of pH, preconcentration temperature, preconcentration time, influence of interference from other ions,and the effect of the length of the alkyl endgroup of ligands using linear sweep voltammetry (LSV) and differential pulse voltammetry(DPV). The calibration plot using 1 yields a linearity between 1.061076 M and 7.061078 M and the least-square treatment of these dataproduced an equation of I [mA] ¼ 128.78 [mA] þ 16.66 log C with r¼ 0.997 (n¼ 6). On the basis of signal to background characteristicsof the response (S=N¼ 3), the detection limit was about 3.061078 M of Hg2þ ion at 25 min preconcentration time. This method wasverified by applying for the analysis of Hg2þ ion in a human urine sample (SRM 2670, NIST).

Keywords: Modified electrode, Podands, Hg2þ ion analysis, Stripping voltammetry

1. Introduction

Acyclic analogs of crown ether [1] including podands,

monesine, nigericin, and emercide, wrap themselves around thecation in a very similar way to that of the cyclic species [2, 3]. Ofthese, podands are obtained simply and cheaply and are knownfor their efficiency and selectivity in binding ions. Several metal

complexes of podands with alkaline metal [3, 4], alkaline earthmetal [5], and transition metal ions [3] are known. Ion-selectiveelectrodes based on podands are described for the detection of

alkali and alkaline earth metal ions [6, 7]. In addition, a lot ofvoltammetric studies based on macrocyclic and acyclic mole-cules [8–12] have been carried out sofar, but limited studies have

focused on the development of a preconcentration method forstripping voltammetry using electrodes modified with macro-cyclic and acyclic molecules.

The aims for modifying electrodes are to improve theiranalytical performance by increasing their sensitivity and selec-tivity. A promising approach to increase selectivity is by modi-fying the host molecules, which selectively interact with specific

guest molecules. Of these, macrocyclic and acyclic compoundsas host molecules are now routinely used in many analyticalapplications for enhancing the selectivity. Especially, one of the

advantages using CMEs is to bypass interferences in the samplesolution which are not overcome using conventional strippingvoltammetry that preconcentrates metal ions with potential

application. CMEs based on podands are likely to have usefulanalytical characteristics, because they have the conformationallability and facility of structural variation. However, there are few

reports on the application of podands as modifiers to determineheavy metal ions in voltammetric techniques. One of them is the1,11-bis(8-quinolyloxy)-3,6,9-trioxaundecane (BQT) modifiedglassy carbon electrode (by spin coating) to analyze Hg2þ ions in

human urine [13]. Until now, there are some reports for strippingvoltammetric analyses of Hg2þ using modified carbon electrodescontaining organic ligands [14–16].

In the present study, we used a series of dithia-podands havingdifferent length of alkyl endgroups to determine mercury. Theeffects of analytical parameters (e.g., pH, preconcentration

temperature, preconcentration time, the effect of interference, theeffect of alkyl endgroup-length of ligands, etc.) which affect theelectrode reaction and analysis processes were studied using

linear sweep voltammetry (LSV) and differential pulse voltam-metry (DPV). This method using dithia-podands has been evalu-ated by analyzing Hg2þ ions in a human urine sample (SRM 2670,

NIST).

2. Experimental

2.1. Reagents and Apparatus

Dithia-podands [1,8-bis(dodecylthio)-3,6-dioxaoctane (DTDO,

1), 1,8-bis(nonylthio)-3,6-dioxaoctane (NTDO, 2), and 1,8-bis(hexylthio)-3,6-dioxaoctane (HTDO, 3)] as shown in Figure 1were prepared by the reaction of the corresponding thiols with

1,8-dichloro-3,4-dioxaoctane in the presence of KOH in ethanolas reported earlier [17–19]. The pure products were obtained ascolorless solid except with DTDO. 1H and 13C NMR spectra forthe compound used in this study were consistent with the desired

structures. A methanolic dithia solution was prepared and storedin a refrigerator with a concentration of 5.0 mM, and it wasimmediately used after dilution to 1.0 mM before every

measurement. A 0.1 M acetic acid/acetate buffer (pH 3–5) and a0.1 M KNO3 solution served as a supporting electrolyte in thisstudy. Metal salts of nitrate or chloride solutions were obtained

from Junsei Chemical Co. (Japan) and diluted as required beforeuse. Human urine sample was used as the reference material thatwas purchased from the National Institute of Standards and

Technology (SRM 2670, NIST; USA). All test aqueous solutionsused in this study were prepared with doubly distilled water puri-fied by using the Milli-Q water purification system (18 MO/cm).

1003

Electroanalysis 2001, 13, No. 12 # WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 1040-0397/01/1208–1003 $17.50þ.50=0

Linear sweep and differential pulse voltammograms were

recorded by employing Pine Instrument Model AFRED4 bipo-tentiostat/galvanostat and EG&G PAR Model 273 potentiostat/galvanostat. A typical H-type cell consisting of a glassy carbon

electrode (GCE) coated with dithia compounds as a workingelectrode, a Ag/AgCl electrode as reference electrode, and a Ptwire as counter electrode were used throughout the study.Modification of the glassy carbon electrode surface with dithia

compounds was performed employing a spin-coating method.A continuous stream of nitrogen gas was maintained over themeasuring solution while voltammetric measurements were

under progress. Two 50 mL Pyrex cells were separately used. Onecontained an electrolyte solution only and the other contained ametal solution. During the preconcentration of Hg2þ ion, the

sample solution was stirred without applying any potential to themeasuring cell.

2.2. Fabrication of Modified Electrodes

Prior to modifying the GCE surface with dithia compounds

using spin coating technique, bare GCEs were pretreated byvarious ways: The GCE was polished with alumina slurry(0.3mm) and washed with deionized water followed by sonica-

tion. Electrochemical pretreatment was performed by the poten-tial step method after mechanically polishing, which was appliedalternatively at þ0.8 V for 5 min and at ÿ1.4 V for 1 min in 0.1 MH2SO4. Then, modification of the surface of the GC electrodes

with dithia compounds was performed by spin coating employinga homemade rotator with a speed of 200 rpm. Two drops of 10 mLof 1.0610ÿ3 M dithia-podands dissolved in chloroform were

dropped on the spinning electrode and then dried in air.

2.3. Analytical Procedure

Hg2þ ions were preconcentrated in a solution containing testions with stirring at various experimental conditions that are

expected to affect the analysis. After the preconcentrationprocess, the CME was taken out from the solution, then washedwith distilled water thoroughly and transferred to a separate cell

containing a 0.1 M KNO3 or a buffer solution, which serves assupporting electrolytes. Linear sweep and differential pulseanodic voltammetry were employed to get an anodic stripping of

Hg0 to Hg2þ. The potential was scanned from ÿ0.3 V to 0.6 Vwith scan rates of 100 mV/s and 20 mV/s for LSV and DPV,respectively, during the stripping procedure. All solutions were

diluted to a required concentration from stock solutions beforeuse. A fresh test solution and a modifier-coated electrode wasused in every measurement.

3. Results and Discussion

3.1 Electrochemical Behavior of Dithia-Podands

Modified Electrodes

Figure 2 shows representative linear sweep stripping voltam-mograms of the Hg2þ ion using a) a GCE and b) a 1,8-

bis(dodecylthio)-3,6-dioxaoctane (DTDO) coated GCE in ablank electrolyte solution. Other podands modified with NTDO,HTDO showed the same voltammograms with different strippingpeak currents of Hg2þ ion (not shown). The voltammogram

recorded in a base electrolyte solution after preconcentration ofHg for 20 min in a 1.0610ÿ5 M Hg2þ/0.1 M KNO3 solution.Two anodic peaks appeared at þ0.7 and þ0.3 V (vs. Ag/AgCl

electrode) during scanning from ÿ0.3 V to þ0.9 V. The firstanodic peak corresponds to the oxidation of reduced Hg2þ atÿ0.3 V to Hg2þ

2 species and the second to Hg2þ ions. In the

present experiment, the first oxidation peak was reproducible andsensitive for low concentrations of Hg2þ ions. Thus, we used thisanodic peak as the analytical signal. The deposition and stripping

mechanism is shown in Scheme 1.To understand the effect of stripping media on the anodic

stripping peak current, the experiment was also done in 0.1 Macetate, phosphate, boric acid/KNO3, potassium hydrogen

phthalate, citrate, HCl/NaOH, HClO4/NaOH and KOH/HNO3

buffer solutions. The stripping currents were affected by theelectrolytes, although the anodic stripping peak appears in all test

electrolytes. Of these, a 0.1 M KOH/HNO3 medium showed themost intense anodic peaks. This is different when compared withthe previous result reported by Shim et al. [13] in that they used

the electrolyte and preconditioning solutions with chloride ions.Where, they used a different type of podand having oxygen andnitrogen atoms as donor atoms for the complexation with Hg2þ

ions. The Hg2þ ion was not preconcentrated on the modified

electrode with BQT in the preconcentration solution withoutchloride ions. The chloride ion produced an insoluble speciesreacted with Hg2þ

2 ions that were possible to deposit a sparingly

Fig. 1. Structures of the dithia-podands used.

Fig. 2. Linear sweep voltammograms for the DTDO modified GCE in a0.1 M KNO3 a) after dipping the CME in a blank solution, and b) afterdipping the CME in a 1.0610–5 M Hg2+ ion solution for 15 min.Temperature: 22�C, scan rate: 100 mV/s.

1004 M. Won et al.

Electroanalysis 2001, 13, No. 12

soluble Hg2þ2 salt on the modified electrode surface. It was tested

for various stripping media that the anodic stripping currentswere strongly dependent on an electrolyte. In this case, the

chloride ion played an important role in the preconcentration ofHg2þ ions. On the other hand, the Hg2þ ion in the present workwas complexed with dithia-podands and deposited on the CMEfollowed by stripping to Hg2þ

2 or Hg2þ ions. Thus, the

mechanism of the stripping voltammetry using dithia-podands asdescribed previously differs from one using BQT. We observedthat additionally two kinds of dithia-podands, NTDO, HTDO

showed the same voltammograms when compared to that ofDTDO, with an exception that the oxidation current of Hg toHg2þ

2 is different from each other depending on the chain length

of podands species. This will be discussed in the subsequentsection of pH dependence on the anodic stripping current.

Figure 3 shows the pH dependence of the anodic stripping

current of Hg2þ ions preconcentrated on the electrode surface viathe complex formation reactions between Hg2þ ions witha) DTDO, b) NTDO, and c) HTDO. These modifiers havedifferent alkyl chain length, same ligating atoms, and the same

structure of the headgroup. The pH of the media was controlledby using 0.1 M KOH/HNO3 and buffer solutions with adequatepHs, and the preconcentration time was 20 min. As shown in

Figure 3, the largest anodic current was observed at pH 2.53 forDTDO, 3.45 for NTDO, and 4.45 for HTDO, respectively. Thedifferent pH revealing the maximum current should be rising

from the different protonation ability of ligands that affects theformation constant of the complex between Hg2þ ions andligands in different pHs. The pH showing the maximum strippingcurrent was shifted to the higher value as the chain length of

dithia-podands become short. This can be explained by the factthat protonation of oxygen and sulfur atoms on the dithia-podands depends on the pHs of the media and affect the

complexation reaction between Hg2þ ions and the ligands.

However, there are no data of this due to the insolubility of thesespecies in an aqueous solution.

In addition, the anodic stripping currents recorded under the

same pH condition increased as the alkyl chain length increaseddue to the more hydrophobic property for the longer chainlength. Thus, the solubility of the dithia complexes decrease asthe alkyl chain length of the ligands increase. This result showed

that the deposition of Hg2þ ions is easier on the modified elec-trode with a ligand having a long alkyl chain length. From the pHdependence of the peak current of dithia compounds, DTDO was

chosen as a modifier to optimize the analysis condition, whichwas most sensitive to Hg2þ analysis.

3.2. Analytical Conditions for Hg2þ Ions

In order to determine the optimum condition for the analysisof Hg2þ ions using the DTDO modified electrode, we investi-gated various experimental parameters, such as preconcentration

time, temperature, pH of the media, etc. which affect the analy-tical sensitivity. As shown in Figure 3a, a maximum responsetowards deposition was obtained at pH 2.5. It may be explained

by the fact that the interaction between Hg2þ ions and DTDO isstronger at pH 2.5 when compared with one at other pHs, and thestability of the Hg2þ-DTDO complex should be the highest. As

the pHs of the media become higher, the anodic stripping currenttends to be smaller. Thus, all measurements for the determinationof Hg2þ ions were undertaken at pH 2.5. The complex formation

between Hg2þ and DTDO has take place during the preconcen-tration process, and the extent of the complex formation reactionshould affect the anodic stripping current as well as the stabilityof the complex.

The voltammograms were recorded in buffer media containing1.0610ÿ5 M Hg2þ ions, which were composed of such as aceticacid, acetate, potassium phosphate/phosphoric acid, potassium

biphthalate, NaOH, HClO4, and HNO3. On comparing these,0.1 M KOH/HNO3 media gave the highest anodic peak currentconcerned with the Hg2þ ion analysis. Thus, this medium was

selected to record anodic stripping current for test ions. Theelectrode modified with DTDO was immersed for 20 min in a0.1 M KOH/HNO3 (pH 2.54) solution containing 1.0610ÿ5 MHg2þ ion with stirring at 600 rpm. After this, CME was rinsed

thoroughly with deionized water and transferred into a blankelectrolyte solution and the anodic stripping voltammogram wasrecorded. The effect of the electrode spin rate during coating the

electrode with the modifier was investigated. The electrode spinrate was varied in order to get a uniform film of modifiers and tocontrol the thickness of the film. The stripping peak current

decreased when the rotation rate of the electrode became higherand a constant current was reached at 2500 rpm. However, theoxidation current using CME coated at 2500 rpm was reduced by

about 40% when compared with one obtained at 600 rpm. Thisresulted from the spin out of modifier due to the high rotationspeed of the electrode.

Fig. 3. Effect of pH on the stripping peak currents of Hg2þ ions usinga) DTDO, b) NTDO, and c) HTDO modified electrodes. Hg2+ ionconcentration: 1.0610–5 M, temperature: 22�C, preconcentration time:15 min, and scan rate: 100 mV/s.

Hg2þ þ dithia ðon the GCEÞ ! Hg2þÿ dithia complex : preconcentration step

Hg2þÿ dithia complexþ 2eÿ ! Hg ðon the GCEÞ þ dithia : reduction step

Hg ðon the GCEÞ ! Hg2þ2 þ 2eÿðþ0:3 VÞ;Hg! Hg2þ þ 2eÿðþ0:7 VÞ : oxidation steps

Scheme 1.

Hg2þ Determination 1005

Electroanalysis 2001, 13, No. 12

Variation in stripping current was obtained according to the

preconcentration time of Hg2þ ions that was tested with the CMEis as shown in Figure 4. The preconcentration times were 3, 5, 10,15, 20, 25, and 30 min in a 0.1 M KOH/HNO3 solution of pH

2.53 containing 1.0610ÿ5 M Hg2þ ions. The scan rate was100 mV/s for linear sweep stripping voltammetry. As thepreconcentration time increased, the anodic peak current gradu-ally increased; after 20 min the current finally decreased. A

limited current at the longer preconcentration time was observeddue to reaching the equilibrium state of the complexation reac-tion between DTDO and Hg2þ ion, followed by the saturation

at the active site of CMEs. Decreasing of the current after 20 minin the same concentration may arise from detaching the over-deposited test ion from the surface of CME. In addition, the

effect of concentration of DTDO on the preconcentration timewas also tested and showed that the shorter accumulationtime was required for the lower quantity of DTDO (1.0 mM,10mL-1 drop) to reach a constant value of peak current at about

15 min. However, the sensitivity for the Hg2þ ion analysisdecreased. Thus, we have chosen the quantity of modifier as 2drops of 10mL-1 mM DTDO.

The dependence of the anodic peak height on the temperatureof the preconcentration solution was observed as shown inFigure 5. The data in Figure 5 were obtained from LSV recorded

in the electrolyte solution of pH 2.53, followed by preconcen-tration in a 1.0610ÿ5 M Hg2þ ion solution at various tempera-tures. The test range of the deposition temperature was from

17�C to 32�C and a maximum stripping current at 22�C wasobserved and fixed as optimum for further studies. When thedeposition temperature increased from 22�C to 32�C, the strip-ping current gradually decreased. This indicates that the Hg2þ-

DTDO complex formed on the CME surface is most stable at22�C, and above this temperature the complex should be moresoluble or unstable in the adsorbed state. This makes a decrease

in the magnitude of anodic stripping current of Hg2þ ions athigher temperatures above 22�C. At lower temperatures less than22�C, the complex formation reaction should not be easier than

that of above 22�C. Thus, we have decided on 22�C as theoptimum preconcentration temperature for this study.

The interference effect of other metal ions which affect thedetermination of Hg2þ ions, possibly by forming complexes with

DTDO, were tested in the solution containing 1.0610ÿ5 M ofHg2þ(II) ions. The interfering ions were as follows: Mn2þ, Ca2þ,Ba2þ, Zn2þ, Ti3þ, Ni2þ, Co2þ, Fe3þ, Cr3þ, Al3þ, Mg2þ, Pb2þ,Agþ, and Hg2þ

2 ions that were expected to interfere by disturbing

the complex formation between Hg2þ and DTDO, and/or byoverlapping on the anodic peak potentials of the metal ionsdeposited competitively on the CME. Among these ions, the

presence of Hg2þ2 in the sample solution affected the anodic

stripping current of Hg2þ. The coexistence of the sameconcentration of Hg2þ

2 ions in the test solution made a decrease in

the stripping current of Hg2þ to about 50 % compared with that ifonly Hg2þ ions are present. This suggests that the Hg2þ

2 ions aswell as Hg2þ ions may be deposited on the CME modified

DTDO, and can give the same anodic stripping mechanism.Thus, we examined the method to get rid of the mutual inter-ference of Hg2þ

2 and Hg2þ ions with each other. In order tochange the two types of oxidation states of Hg into one, an excess

amount of hydroxylamine hydrochloride was added to themixture containing both Hg2þ and Hg2þ

2 ions thereby all theHg2þ ions were reduced to the Hg2þ

2 state. If the quantity of

the reducing agent was five times more when compared to theconcentration of Hg2þ ions, all the Hg2þ ions were completelyreduced to the Hg2þ

2 ions. As evidence on adding five times

excess of the reducing agent to the test solution containing thesame concentration of Hg2þ and Hg2þ

2 ions no interference fromHg2þ

2 was observed in the analysis of Hg2þ ions. On the otherhand, coexistence of Agþ ion in the test solution containing Hg2þ

ions showed a decrease by about 20% in the stripping current ofHg2þ ions when compared to its absence. However, the inter-ference by Agþ ion can be overcome by pretreating sample

solution with chloride ions and thereby we avoided the inter-ference from Agþ ion. On the basis of the results reported in the

Fig. 4. Effect of preconcentration time on stripping peak current ofDTDO modified electrode in a 0.1 M KOH/HNO3 solution (pH 2.5)containing 1.0610–5 M Hg2þ ion. Temperature: 22�C and scan rate:100 mV/s.

Fig. 5. Effect of preconcentration temperature on the stripping peakcurrent of Hg2+ ion with DTDO-modified electrode. Concentration ofHg2+ ion: 1.0610–5 M in 0.1 M KOH/KNO3 (pH: 2.53), pre-concentration time: 20 min, and scan rate: 100 mV/s.

Table 1. Analysis results from the reference urine material.

Sample(SRM 2670)

Concentration

No. ofdeterminations

StandarddeviationCertified Mean

Hg 84 ppb 90 ppb 6 2.34

1006 M. Won et al.

Electroanalysis 2001, 13, No. 12

previous section, the optimum condition for the Hg analysis is

tabulated in Table 1.Figure 6 shows the calibration plot obtained from a) LSV and

b) DPV with the DTDO-coated CME in a 0.1 M KOH/HNO3

solution (pH 2.53) at a preconcentration time of 25 min. Thecalibration plot from LSV yields a linearity between 1.0610ÿ5

and 5.0610ÿ7 and the one from DPV between 1.0610ÿ6 M and7.0610ÿ8 M and the least-square treatment of DPV data

produced an equation of I [mA] = 128.78 [mA] þ 16.66 log C

with r¼ 0.997 (n¼ 6). On the basis of signal to backgroundcharacteristics response (S/N=3), a detection limit of about

3.0610ÿ8 M of Hg2þ ions at the preconcentration time of 25 minwas obtained.

3.3. Analytical Application for Real Sample

To evaluate the validity of the analytical method using theDTDO modified electrode, we attempted to determine Hg in a

well-characterized real sample. The sample selected for thispurpose was a freeze-dried human urine sample that was obtainedfrom NIST-USA (SRM 2670). Preconcentration of Hg2þ ion on

the DTDO modified GCE was done for 25 min in a 0.1 M HNO3/KOH solution (pH 2.5). Prior to the preconcentration andvoltammetric measurements, the decomposition of the urine

sample was carried out in concentrated nitric acid. In this sample,the certified Hg2þ concentration was 105 ppb (diluted to 84 ppb)and the other metal ions present in the sample was within the

range of 2–370 ppb which includes Ag, Al, Pb, As, Be, Cd, Cr,Ca, Au, Mg, Mn, Ni, Pt, K, Se, Na, and V. The observed

concentration of the Hg2þ ion with this method was 90 ppb(SD: 2.34 ppb) from six differential voltammetric measurements.Table 1 shows the analysis results obtained from the referenceurine material and suggests that the observed value has good

agreement with the certified value of the urine sample withoutany interference from the diverse metal ions.

4. Conclusions

The sensitivities of the electrode modified with dithia-podandswith difference alkyl chain lengths for the determinations of

Hg2þ were compared. Among these, the longest alkyl chainlength having dithia-podand (DTDO) showed the most sensitiveresponse when compared to the shorter alkyl chain length having

dithia-podands. High selectivity for Hg2þ ion was achieved withopen-chain dithia-podands, such as DTDO, NTDO, and HTDO.The Hg2þ ion was spontaneously deposited on the CMEs and theresulting surface was characterized by anodic stripping voltam-

metry. The detection limit adopted from differential pulsevoltammetry was 3.0610–8 M for Hg2þ ion, and the standarddeviation was 2.34 ppb for 90 ppb Hg2þ ion. This method was

successfully applied for the determination of trace amounts ofHg2þ ion in a human urine sample without any interference fromother metal ions.

5. Acknowledgement

The Korean Science and Engineering Foundation to ‘‘TheCenter for Integrated Molecular Systems’’ supported this work.

6. References

[1] B. Tummer, G. Mass, E. Weber, W. Wehner, F. Vogtle, J. Am. Chem.Soc. 1977, 99, 4683.

[2] F. Vogtle, E. Weber, Angew. Chem. Int. Ed. Engl. 1979, 18, 753.[3] B. Tummler, G. Maass, E. Weber, W. Wehner, F. Vogtle, J. Am.

Chem. Soc. 1977, 99, 4683.[4] J. Grandjean, P. Laszlo, W. Offermann, P.L. Rinaldi, J. Am. Chem.

Soc. 1981, 103, 1380.[5] H. Sieger, F. Vogtle, Tetrahedron Lett. 1978, 30, 2709.[6] O.M. Petrukhin, E.V. Shipulo, S.A. Krylova, S.L. Rogatinskaya,

A.F. Zhukov, S. Wilke, H. Muller, E.N. Tsvetkov, V.E. Baulia,V.K. Syundyukova, N.A. Bondarenko, J. Anal. Chem. 1994, 49,1175.

[7] M. Bochenska, J.F. Biernat, J.S. Bradshaw, J. Choord. Chem. 1992,27, 129.

[8] J. Wang, M. Bonakdar, Talanta 1988, 35, 277.[9] I. Turyan, D. Mandler, Anal. Chem. 1993, 65, 2089.

[10] S. Dong, Y. Wang, Talanta 1988, 35, 819.[11] I. Turyan, D. Mandler, Electroanalysis 1994, 6, 838.[12] S. Lee, M.-K. Ahn, S. B. Park, Analyst 1998, 123, 383.[13] H.-J. Kim, D.-S. Park, M.H. Hyun, Y.-B. Shim, Electroanalysis

1998, 10, 303.[14] J. Wang, M. Bonakdar, Talanta 1998, 35, 277.[15] E.-D. Jeong, M.-S. Won, Y.-B. Shim, Electroanalysis 1994, 6, 887.[16] S.B. Khoo, Q. Cai, Electroanalysis 1996, 8, 549.[17] A.P. Paiva, Sep. Sci. Technol. 1993, 28, 947.[18] F. Dietze, K. Gloe, R. Jacobi, P. Muehl, J. Beger, M. Petrich,

L. Beyer, E. Hoyer, Solv. Extn. Ion Exch. 1989, 7, 223.[19] S. Chung, W. Kim, S.B. Park, D.Y. Kim, S.S. Lee, Talanta 1997,

44, 1291.

Fig. 6. Calibration plot for Hg2+ ions with a) linear sweep voltammetry(preconcentration time: 20 min, temperature: 22 �C, pH: 2.53, and scanrate: 100 mV/s), and b) differential pulse voltammetry (pulse width:0.05 s, pulse amplitude: 0.05 V, scan rate: 20 mV/s, preconcentrationtime: 20 min, and pH: 2.53).

Hg2þ Determination 1007

Electroanalysis 2001, 13, No. 12