mercury film modified electrode

Talanta 49 (1999) 59 – 68

Square-wave anodic stripping voltammetric determination of thallium(I) at a Nafion/mercury film modified electrode Tsai-Hwa Lu, Hao-Yun Yang, I. Wen Sun * Department of Chemistry, National Cheng Kung Uni6ersity, Tainan, 70101, Taiwan ROC Received 15 July 1998; received in revised form 29 October 1998; accepted 9 November 1998

Abstract A Nafion/mercury film electrode (NMFE) was used for the determination of trace thallium(I) in aqueous solutions. Thallium(I) was preconcentrated onto the NMFE from the sample solution containing 0.01 M ethylenediaminetetraacetate (EDTA), and determined by square-wave anodic stripping voltammetry (SWASV). Various factors influencing the determination of thallium(I) were thoroughly investigated. This modified electrode exhibits good resistance to interferences from surface-active compounds. The presence of EDTA effectively eliminated the interferences from metal ions, such as lead(II) and cadmium(II), which are generally considered as the major interferents in the determination of thallium at a mercury electrode. With 2-min preconcentration, linear calibration graphs were obtained over the range 0.05–100 ppb of thallium(I). An even lower detection limit, 0.01 ppb, were achieved with 5-min accumulation. The electrode is easy to prepare and can be readily renewed after each stripping experiment. Applicability of this procedure to various water samples is illustrated. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Mercury film electrode; Nafion; Square-wave voltammetry; Stripping analysis; Thallium

1. Introduction The high toxicity of thallium and its compounds has made the determination of traces of thallium in various samples important. While spectrometric methods including atomic absorption spectrometry [1], spectrophotometry [2], emission spectrophotometry [3] have been em* Corresponding author. Fax: +886-6-2740552. E-mail address: [email protected] (I.W. Sun)

ployed in such determination, anodic stripping voltammetry (ASV) is also very suitable for trace measurement of thallium due to its remarkable sensitivity. However, conventional voltammetric stripping methods for thallium determination are usually complicated by the interference from neighboring peaks associated with the stripping of coexisting metal ions [4,5]. Extensive efforts have thus been devoted to overcome such interference through the addition of certain adsorptive agents, or the use of chemically modified electrode [6–10].

0039-9140/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 9 8 ) 0 0 3 6 0 - 9

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These methods, however, could be interfered by ions which are also capable of forming complexes with the adsorptive agents. Furthermore, these methods may suffer from interferences due to organic surfactants adsorption on the electrode. This article describes a square-wave stripping voltammetric (SWSV) procedure for the determination of traces of cationic thallium(I) by using a Nafion/mercury film coated glassy carbon disk electrode (NMFE) with the presence of ethylenediaminetetraacetate (EDTA) in the sample solution. In this way, significant advantages come into effect: the cation-exchange and preconcentration feature of Nafion [11], the remarkable accumulation effect of the mercury film for thallium [12], and the high sensitivity of the SWSV [13]. In addition, the extraordinary chelating agent EDTA chelates to interfering metal ions forming bulky complexes that are excluded from the Nafion film. This simple procedure yields good sensitivity for thallium(I) and displays good resistance to the interference from common ions such as cadmium and lead as well as the interference from surfaceactive compounds such as sodium dodecyl sulfate (SDS) and Triton X-100.

2. Experimental

2.1. Apparatus All electrochemical experiments were performed with a Bioanalytical Systems BAS CV-50W electrochemical analyzer in conjunction with a BAS model C-2 electrochemical cell. The three-electrode system consisted of a glassy carbon disk working electrode (BAS, 3 mm diameter) coated with Nafion and mercury, an Ag/AgCl reference electrode, and a platinum spiral auxiliary electrode. All glassware was cleaned with 8 N nitric acid and rinsed with deionized water.

2.2. Chemicals and reagents Nafion perfluorinated ion-exchange powder, 5% m/6 solution in a mixture of lower aliphatic alcohols and 10% water, was obtained from Aldrich. Sodium perchlorate, sodium nitrate and nitric

acid were of analytical grade from Riedel de Haen (RDH). Standard metal solutions (1000 ppm) of Mg(II), Ni(II), Fe(III), Zn(II), Cr(VI), Cu(II), and Cd(II) were from Fisher. Standard solutions (1000 ppm) of Mo(VI), Se(IV), and Au(III) were from Mallinckrodt. Hg(II) standard solution (1000 ppm) was from Merck. Anionic surfactant sodium dodecyl sulphate (SDS) was received from RDH and nonionic surfactant Triton X-100 was received from Lancaster. Thallium(I) stock solutions (1000 ppm) were prepared by dissolving thallium(I) chloride (99.999%, Stream) in nitric acid. All the preparation and dilution of solutions were made with deionized water.

2.3. Preparation of Nafion mercury film electrode (NMFE) and sample solution After polished with a polishing cloth to a shiny surface, the glassy carbon electrode (GCE) was rinsed with deionized water and then cleaned ultrasonically in 1+1 nitric acid and deionized water. Then, 4 ml of Nafion coating solution was spin-coated onto the GCE at a spin rate of 3000 rpm. A uniform thin film was formed by evaporating the solvent at room temperature after about 3 min of spinning. Mercury was electrodeposited on the Nafion coated GCE from 5 ml of 10 ppm mercury(II) solution containing 0.1 M sodium perchlorate at an applied potential of − 0.8 V vs Ag/AgCl for 4 min with stirring. The sample solution medium contained thallium(I), 0.01 M EDTA to reduce the concentration of interfering metal ions and proper amount of 0.1 M nitric acid to adjust the solution pH, and 0.1 M sodium nitrate as the supporting electrolyte for all electrochemical experiments.

2.4. Procedure The freshly prepared NMFE was dipped into the stirred analyte solution containing Tl(I) and kept at −0.9 V vs Ag/AgCl for the time required for preconcentration (the optimized time was 2 min). Quantitative determinations were then performed in the SWSV mode. The potential was scanned in the anodic direction from − 1.0 to

T.-H. Lu et al. / Talanta 49 (1999) 59–68

− 0.4 V vs Ag/AgCl. A medium containing 0.01 M EDTA, 0.1 M sodium nitrate (pH = 4.5) was used in the electrochemical experiments. Solutions and samples were analyzed with 3 min deoxygenation. After recording the voltammogram, the electrode was regenerated by immersing the electrode in 1 M nitric acid at − 0.4 V vs Ag/AgCl for 30 s. The renewed electrode was then checked in the supporting electrolyte before the next measurement to ensure that it did not show any peak within the potential range. Tap water, ground water, and rain water were collected and stored in precleaned polyethylene bottles after filtration. The standard addition method was used to evaluate the content of Tl(I) in the water samples.

3. Results and discussion

3.1. Electrochemical beha6ior of Tl(I) at the 6arious electrodes The cyclic voltammograms for a solution containing 10 ppm thallium(I), 0.1 M NaNO3, and 0.01 M EDTA at a bare GCE and a NMFE are shown in Fig. 1. At the bare GCE (Fig. 1a), a cathodic peak which can be assigned to the cathodic deposition of thallium(I) to elemental thallium, and an anodic stripping peak corresponding to the reoxidation of thallium to thallium(I) are observed. The peak potential separation of these two peaks is fairly wide, indicative of a quasi-reversible electrochemical reaction. On the other hand, the cathodic deposition peak and the anodic stripping peak observed for the thallium(I)/ (0) redox couple at the NMFE (Fig. 1b) are fairly symmetric in shape. This behavior is typical for a electrochemical reaction occurring in a film at the electrode surface. Furthermore, the peak potential separation observed for thallium(I)/(0) redox reaction on the NMFE is much smaller than that observed on the bare GCE, indicating that the NMFE is more suitable than the bare GCE for the determination of thallium(I) by SWSV [14,15]. The advantage of using the NMFE in the determination of thallium(I) is further demonstrated in Fig. 2. This figure shows the responses of 10 ppb thallium(I) on different electrodes in the same

61

supporting electrolyte. As can be seen in Fig. 2a, no voltammetric response at all was detected on the bare GCE at this low thallium(I) concentration while a distinguishable oxidation peak of thallium at −0.70 and −0.61 V was observed on the Nafion film coated GCE (NFE)(Fig. 2b) and the mercury film electrode (MFE)(Fig. 2c), respectively. The results indicate that the preconcentration efficiency of the electrode could be enhanced by the ion-exchange property of Nafion or the good solubility of thallium in mercury. When the NMFE was used, a well-defined oxidation peak of thallium(I) at about − 0.66 V was observed (Fig. 2d). As can be seen, the peak height for NMFE is much higher than that of the NFE and MFE under the same experimental conditions. This confirms that remarkable improvement on the voltammetric detection of thallium(I) is achieved by combining the effect of Nafion film and mercury film.

Fig. 1. Cyclic voltammograms for a 10 ppm thallium(I) solution containing 0.1 M NaNO3 and 0.01 M EDTA: (a) at a glassy carbon electrode (GCE); (b) at the NMFE. Potential was scanned from about −0.2 to about − 1.2 V and reversed to − 0.1 V vs Ag/AgCl. Scan rate 100 mV s − 1.

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increases as the preconcentration potential becomes more negative. This behavior is not unexpected because thallium(I) bears a positive charge, and as a result the accumulation of thallium(I) is favored at more negative potentials. In addition, more negative potentials result in more complete reduction of the accumulated Tl(I), which may not be completely converted at less negative potentials. However, the peak current drops as the potential becomes more negative than − 0.9 V. A preconcentration potential of − 0.9V was therefore chosen in all subsequent work.

3.4. Effect of preconcentration time

Fig. 2. SWSV for 10 ppb thallium(I) in the 0.1 M NaNO3 + 0.01 M EDTA (pH = 4.5) solution: (a) on the bare GCE, (b) on the Nafion film coated GCE, (c) on the mercury film coated GCE and (d) on the Nafion/mercury film coated GCE. Preconcentration potential, Ep, was − 0.9V vs Ag/AgCl, and preconcentration time, tp, was 2 min. The SWSV was scanned from −1.0 to − 0.4 V. SWSV parameter: modulation pulse height 50 mV, modulation frequency 200 Hz, effective scan rate 800 mV s − 1.

3.2. Effect of pH For the NMFE, the optimum condition for thallium(I) accumulation was observed when the sample solution was slightly acidic (pH 4.5), as shown in Fig. 3. In the more acidic solutions, the SO3− sites of Nafion attract more proton ions and hold them inside the polymer matrix, and this would reduce its preconcentration ability for the thallium(I) cations. In the more basic solutions, however, the response of thallium(I) declines. Therefore, solutions of pH 4.5 were used in subsequent experiments.

The dependence of the thallium stripping peak current on the preconcentration time was studied with solutions of various thallium(I) concentration and the results are shown in Fig. 5. For higher concentration of thallium(I), (100 ppb), the peak current increase as the preconcentration time increases and starts to level off around 3 min. For a lower concentration of thallium(I) (10 ppb), it takes about 5 min for the current to level off. At even lower concentrations of thallium(I), (1 ppb), it takes more than 6 min for the peak current to level off. Apparently, the rate of thallium(I) uptake is dependent on concentration; higher concentration gradient enhances the diffusion of thallium(I) toward the NMFE during the accumulation and, thus, reaches the equilibrium state faster. The relation between the peak current and the thallium(I) concentration with preconcentration time of 1 and 2 min, respectively, was also studied. For both cases, good linear relationship between the peak current and the thallium(I) concentration are obtained between 1 and 100 ppb of thallium(I). Since a longer preconcentration time give a better sensitivity, the preconcentration time of 2 min was chosen in all the subsequent work.

3.3. Effect of preconcentration potential 3.5. Effect of square-wa6e parameters The effect of preconcentration potential on the SWSV response for thallium(I) on the NMFE is shown in Fig. 4. This figure shows that between −0.6 and −0.9 V vs Ag/AgCl, the peak current

The square-wave parameters that were investigated were the frequency and the pulse height. These parameters together affect the peak shape

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Fig. 3. Dependence of square-wave stripping peak height of 10 ppb thallium(I) (in 0.1 M NaNO3 supporting electrolyte) on pH for Tl(I) determination obtained on NMFE. Ep = − 0.9 V, tp =1 min. SWSV parameter: modulation pulse height 75 mV, modulation frequency 200 Hz, effective scan rate 800 mV s − 1.

Fig. 4. Relation between the preconcentration potential (Ep) and the square-wave stripping peak height of 10 ppb thallium(I) recorded on the NMFE. Supporting electrolyte: 0.1 M NaNO3. The pH of the solution was 4.5 and tp =1 min. SWSV parameters as in Fig. 3.

and peak current of the thallium response. It is found that the response for thallium increases with increasing SWSV frequency up to 200 Hz,

which is the limit of the instrument. An increase in the pulse height up to 50 mV causes an increase in the thallium peak. When the pulse height is

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higher than 65 mV the peak starts to broaden and finally peak split occurs when the pulse height reaches 100 mV. Overall, the best signal-to-background current characteristic can be obtained with the following instrumental settings: pulse height, 50 mV; frequency, 200 Hz, and effective scan rate 800 mV s − 1.

3.6. Effect of EDTA The function of EDTA in this procedure is to reduce the interference from metal ions by taking advantage of it’s reaction with most interfering metal ions to form complexes that can not diffuse

onto the electrode surface. Fig. 6A shows the square wave stripping voltammograms for a solution containing 10 ppb thallium(I) and 1 ppm lead(II) before and after the addition of 0.01 M EDTA. Without EDTA, lead(II) produces a stripping peak at ca. − 0.54 V which partially overlaps with the thallium(I) stripping peak. When EDTA is added to this solution, the lead(II) stripping peak is clearly removed. The efficacy of EDTA on reducing the interference from metal ion is further demonstrated in Fig. 6B with a 10 ppb thallium(I) solution containing 1 ppm of cadmium(II). This figure clearly shows that the thallium peak is completely buried in the cadmium peak when EDTA is not present in the solution. However, with the presence of EDTA, interference from the cadmium(II) is effectively suppressed while the thallium peak remains unaffected. Although EDTA is an effective reagent for masking interfering metal ions, too much EDTA may cause the formation of EDTA–thallium chelate, and this would reduce the effective concentration of thallium(I) that can be detected by the NMFE. Thus, the effect of EDTA concentration on the response for thallium(I) was studied. The results showed that for a 10 ppb thallium(I) solution, EDTA does not cause any significant depression in the thallium peak current unless the EDTA concentration is higher than 0.01 M. At EDTA concentration higher than this value the thallium stripping peak current drops rapidly. Consequently, 0.01 M EDTA was used in this procedure.

3.7. Calibration

Fig. 5. Effect of preconcentration time on the square-wave stripping peak height of: (a)100, (b) 10, (c) 1 ppb thallium(I) at the NMFE. The pH of the solution was 4.5 and Ep = −0.9 V. Supporting electrolyte: 0.1 M NaNO3. SWSV parameters as in Fig. 2.

Calibration data were obtained under optimum experimental conditions mentioned above. Fig. 7 presents some of the typical SWSV voltammograms for the NMFE after have been in contact with thallium(I) for 2 min preconcentration time at concentrations of 0.05, 0.2, 0.5, 2, 5, and 10 ppb, respectively. In all cases a stripping response was observed at a potential in the vicinity of − 0.75 V vs Ag/AgCl. A calibration graph was then constructed from the observed peak currents. The graph shows a very linear behavior with a slope (sensitivity) of 2.75 mA ppb − 1, and a corre-

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Fig. 6. Square-wave stripping voltammograms obtained at the NMFE for a 10 ppb thallium(I) + 0.1 M NaNO3 solution containing (A) 1 ppm lead(II), and (B) 1 ppm cadmium(II). (- - -) without EDTA, ( — ) with 0.01 M EDTA. The pH of the solution was 4.5, Ep = − 0.9 V, and tp = 2 min, and SWSV parameters as in Fig. 2.

lation coefficient of 0.995. The linear range is between 0.05 and 100 ppb thallium(I). Peak current due to thallium(I) with a concentration as low as 0.05 ppb can be actually distinguished from the background current. This detection limit is comparable to or better than the detection limits offered by previous reports on voltammetric determination of thallium [6 – 10]. An even lower detection limit could be achieved for thallium(I) provided that the preconcentration time is longer than 2 min. For example, 0.01 ppb (5× 10 − 11 M) thallium(I) can be detected with a preconcentration time of 5-min.

3.8. Interferences Various ions were examined regarding their interference in the determination of thallium(I). For a bare MFE, any metal ions that can be reduced on the MFE and subsequently stripped at a potential close to that of the thallium(I) ion are likely interferents. However, the use of a NMFE

with the presence of EDTA in the sample solution successively minimizes such interferences. For 10 ppb thallium(I), the results showed that over 1000-fold excess concentration of germanium(IV), molybdenum(VI), magnesium(II), nickel(II), iron(III), zinc(II), selenium(IV), chromium(VI) and over 100-fold excess concentration of gold(III), copper(II), lead(II), and cadmium(II) can be tolerated (Table 1). Note that lead(II) and cadmium(II) are generally considered as the major interference in the determination of thallium on a bare mercury electrode. However, this problem can actually be greatly improved by the proposed procedure as indicated above. The interference from surface-active compounds in stripping analysis using bare mercury electrode are well recognized. These compounds may adsorb on the electrode surface and reduces the analytical response of the analyte. Such interferences can be overcome by coating the electrode with Nafion. One of the function of the Nafion membrane coating on the mercury film electrode

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is to prevent the organic interferences from reaching the interface at which the deposition and stripping takes place. In this study, the nonionic surfactant Triton X-100 and anionic surfactant SDS were used to simulate the effect of typical surfactants. Fig. 8 shows how the 10 ppb thallium(I) stripping peak current is affected by different concentration of Triton X-100 and SDS. As can be seen, the detection can tolerate the presence of both surfactants for at least up to 10 ppm with the NMFE (solid lines). Compared to the same experiments performed with a bare mercury electrode (dashed lines), the tolerance was greatly improved.

3.9. Electrode renewal After each stripping analysis, the NMFE was renewed by immersing the electrode in 1 M nitric acid and regenerated at a potential of − 0.4 V vs

Table 1 Infuence of other ions on the response of Tl(I)a Ions

Concentration excess over Tl(I)

Contribution (%) [iTl(I) =100%]

Ge(IV) Mo(VI) Mg(II) Ni(II) Fe(III) Zn(II) Se(IV) Cr(VI) Au(III) Cu(II) Cd(II) Pb(II) SDS Triton X-100

1000× 1000× 1000× 1000× 1000× 1000× 1000× 1000× 500× 100× 100× 100× 1000× 1000×

0.76 −4.15 2.03 −8.59 −2.30 −1.12 3.43 0.16 −2.66 −4.38 −3.33 1.39 −14.52 −3.82

a

[Tl(I)]= 10 ppb

(Ag/AgCl) for about 30 s with stirring. This method showed an excellent reproducibility of the measurements, usually around 2.7% in terms of percent relative standard deviation for eight repetitive preconcentration/stripping/renewal experiments. Apparently, during this cleaning procedure, the residual thallium(I) from preceding deposition is removed from the NMFE by ion-exchange with the excess amount of proton.

3.10. Determination of thallium in real water samples

Fig. 7. Square-wave stripping voltammograms for increasing thallium(I) concentration of (a) 0, (b) 0.05, (c) 0.2, (d) 0.5, (e) 2, (f) 5 and (g) 10 ppb, respectively. The thallium(I) was preconcentrated at − 0.9 V for 2 min in pH = 4.5 solutions. Supporting electrolyte: 0.1 M NaNO3 and 0.01 M EDTA. SWSV parameters as in Fig. 2.

The analytical utility of the NMFE for the determination of thallium(I) was assessed by applying it to the determination of thallium(I) in tap water, ground water and rain water samples. No thallium was detected in all the three original water samples so they were spiked with appropriate amounts of thallium(I). The results collected in Table 2 are those for the original and spiked water samples. As can be seen, the recovery of the spiked thallium(I) is very good for all the three water samples, indicating that the proposed procedure is feasible for the determination of thallium(I) in different water samples. Note that the amount of thallium(I) in natural water is typically very low, and this is indeed the case in this study.

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Fig. 8. Effect of the surfactants Triton X-100 (") and SDS () at different concentrations on the SWSV response for 10 ppb thallium(I) with the NMFE (—) and MFE (- - -). The thallium(I) was preconcentrated at −0.9 V for 2 min in pH= 4.5 solutions. Supporting electrolyte contains 0.1 M NaNO3 and 0.01 M EDTA. SWSV parameters as in Fig. 2. Peak currents are given relative to those obtained with no added surfactants. Table 2 Determination of Tl(I) in tap water, rain water and ground water samplesa

Detected value original Spiked (ppb) Detected value after spike(ppb) Recovery (%) a

Tap water

Rain water

Ground water

ND 0.3 0.29 90.02 97

ND 1.0 0.98 9 0.01 98

ND 1.0 0.99 90.04 99

Number of samples assayed = 4.

Consequently, the amount of thallium(I) in the original water samples assayed in this study can not be detected by the proposed procedure with 2 min preconcentration time. Nevertheless, it was found that the spiked 0.05 ppb thallium stripping peak could be actually seen for these samples with 5 min of preconcentration time, and the amount of thallium(I) in these original water samples was therefore believed to be well below 0.05 ppb. Note that Nafion has unusually large selectivity coefficients for hydrophobic cations [16]. In real samples, such as biological materials, this may be a problem if these cations are not too bulky and will therefore be preferentially exchanged over Tl(I). Thus, the effect of such cations on the

determination of Tl(I) with the NMFE will be further studied.

4. Conclusions The results show that the anodic SWSV determination of trace thallium(I) based on the preconcentration and subsequent stripping of thallium on a NMFE in the presence of EDTA is simple and effective. The proposed procedure not only yields good sensitivity with a short preconcentration time but also offers improved resistance to metal ions and organic surfactants interferences than a bare MFE. These advantages result mainly

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from the ion-exchange property of Nafion, the good solubility of thallium in mercury, the good sensitivity of the square-wave voltammetry, and the chelating property of EDTA. The modified electrode is easily prepared and can be readily regenerated.

Acknowledgements The authors gratefully acknowledge the financial support of the National Science Council of the Republic of China (Taiwan) under Grants NSC87-2815-C-006-073M.

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