determination of copper(i) ion with a chemically modified carbon paste electrode based on...

Ebctroanalvsis. 5( 199’3) 421-426

Determination of Copper(1) Ion with a Chemically Mc dified Carbon Paste Electrode Based on Di(2- Im no-Cyclopentylidine Mercaptomethyl) Disulfide

Mi-Sook Won, Jin-hyo Park, and Yoon-Bo Shim’ Department of Chemistry, Pusan National UnivwsiQ, Pusan 609-735, Korea Recezved March 2 5, 1992

ABSTRACT A copper-sensitive chemically modified electrode (CME) has been constructed by incorporating di(2- imino-cyclopentylidine mercaptomethyl) disulfide (didd) into the carbon paste composed of graphite powder and Nujol oil. Copper(1) ion was chemically deposited on the CME by immersing it in the Cu(1) solution. The resulting surfaces were characterized by cyclic and differential pulse voltammetry. The CME’s surface could be regenerated by applying more positive potential than the stripping potential of the Cu(1) ion and then used for another deposition. After seven deposition/measurement/ regeneration cycles, the peak current of voltammograms of the analyte decreases slightly. The differential pulse technique was also applied to the above system. In this case, the detection limit for Cu(1) ion was 5 X lo-’’ M for 30 minutes of deposition time. After reduction of Cu(I1) ion to Cu(1) in the sample solution with hydroxylamine, satisfactory results were obtained for the determination of copper in certified standard urine reference material SRM’s 2670 (trace elements in urine).

KEY WORDS: Chemically modified electrode, Determination of Cu( I), Di(2-imino-cyclopentylidine mercaptomethyl) disulfide.

INTRODUCTION

Until now, stripping voltammetry has been recognized as one of the most sensitive electrochemical methods for the determination of trace metal ions. This technique has been used for the determination of trace amounts o f both metals and organic species. In general, this method is based on the preconcentration and stripping of an analyte in the sample solution. There are various techniques for the preconcentration and stripping of the an- alytes [1,2]. A number of investigators have recently reported new preconcentration procedures for trace metal analysis. The new method adopts chemically modified electrodes (CMEs), on which various ligands are incor- porated [3-51.

Baldwin et al. [6] have reported CMEs coated with dimethylglyoxime for the trace analysis of Ni(I1) ion in a variety of complicated samples. They have also reported copper-sensitive CME containing 2,9-dimethyl-1 ,lo- phenanthroline [7]. O’Riordan and Wallace reported a poly-(pyrrole-N-carbodithiate) modified electrode [8 ] . McCracken et al. [9] demonstrated the utility of electrodes modified with Mordant Violet for determination of Ni(I1). This method was based on the coordination of Ni(I1) from the sample solution by an immobilized layer (via ion exchange) of the dye. Wang et al. [ 101 have even

employed a CME based on an ion exchange resin for the determination of Cu(I1) ion. Cheek and Nelson reported determination of Ag(1) employing a CME containing amino silanes [ 111.

There are several advantages for analytical applications of the CMEs. First, they have a wider adjustable preconcentration range than the conventional stripping voltammetric techniques, in which one has to apply a constant potential to preconcentrate test ions on the electrode. Second, this method has higher selectivity due to the ability of the modifier to form a complex with specific metal ions. If experimental conditions are optimized, one has very little interference from other metal ions presented in the sample solution. Furthermore, by replacing the sample solution with a clean electrolysis medium prior to performing the stripping analysis step, one may effectively bypass a host of electroactive sub- stances that easily interfere in anodic stripping voltammetry. In using CMEs, it is not necessary to apply a potential for the deposition of analyte during preconcentration procedures. Of these CMEs, the carbon paste electrode (CPE) especially provides another ad- vantage, since it can be readily prepared and its elec-

‘To whom correspondence should be addressed

0 1993 VCH Publishers, Inc. 1040-0397/93/%5.00 + .25 42 1

422 Won et al.

trode surface can be easily regenerated. We reported preliminary work [ 121 on the electrochemical behavior of di(2-imino-cyclopenthylidene mercaptomethyl) disulfide (didd) in an acetone and DMSO solvent employing a glass): carbon electrode. Besides, stripping analyses of 2-amino-1-cyclopentene-1-dithio-carboxylate (acdc) itself [12] and of Ag(1) ion with a CME containing protonated acdc they have been studied previously [ 131. It was possible to determine the quantity of acdc by stripping voltammetry because its dimerized product (didd) was strongly adsorbed on the glassy carbon electrodes.

In this work, the CMEs have been prepared by mak- ing carbon paste mixtures containing an appropriate amount of didd coated onto graphite particles treated under nitrogen atmosphere. These modified electrodes were used to analyze trace metal ions via complexation followed by stripping voltammetry. The technique in- volves a process of complex formation of the metal ion with didd on the modified electrode surface. The complex formed, as a sparingly soluble precipitate should be electrochemically reduced on an electrode. Various experimental parameters effecting the response were care- fully optimized, and the CME performance was characterized.

EXPERTMN7" Reagents Di(2-imino-cyclopentylidine mercaptomethyl) disulfide was prepared by following a modified procedure of Tdk- eshima et al. [14]. A solution of 0.01 M I, was added dropwise with stirring to a solution of 3 g acdc in 0.01 M sodium hydroxide. The yellow product was collected by filtration, washed with water, and dried in vacuum at room temperature for 1 day. The recrystallization solvent was ethanol. The result of elemental analysis: Cals.: 45.53% C, 5.05% H, 8.85% N ; Found: 43.56% C, 4.92% H, 8.62% N. Metal ion solutions were prepared from either nitrate or chloride salts. Addition of a twofold excess of hydroxylarnine sulfate led to the generation of Cu(1) ion from Cu(1l) ion having the desired concentration. The experiments were performed in 0.1 M acetate buffer solution adjusted 'to pH 5.0. All chemicals used in this experiment are reagent grade or better.

Electrodes An unmodified carbon paste was prepared by handmix- ing 5 g of reagent-grade graphite powder (Fluka Chem- ical Co.) previously rinsed with 96% ethanol (Fluka Chemical Co.) four times-with 3 ml of Nujol oil (Sigma Co.) in a mortar. A modified paste was prepared in a similar fashion, except that the graphite powder was coated with the desired weight of didd. Both unmodified and modified pastes were packed into 1-ml polyethylene syringes ( 5 mm diameter) the tips of which had been cut off. Electrical contact to the paste was established via a thin copper wire. The surface of a fresh CME was pre- conditioned by exposure to a 1.0 X lo-' M Cu(1) ion solution for 3 minutes. Then, the electrode was rinsed

with deionized water and placed in 0.1 M KC1 solution at 0.1 V (vs. SCE) for 5 minutes with stirring. This conditioning cycle was repeated three times for each new CME surface. A fresh electrode surface was obtained by squeezing more out. The surplus of paste was cut out with a glass rod and the exposed end polished on a pa- per until the surface showed a shiny appearance.

Apparatus Two 25-ml Pyrex cells were used, one containing the electrolytic blank solution and the other the sample solution. The preconcentration vessel was placed on a magnetic stirrer (600 rpm) with a 1-cm stirring bar. The deposit solutions containing Cu(1) ion were thermostat- ted at 25 * 0.1"C. A three-electrode system was used, which was connected to an EG&G PAR Model 273 Po- tentiostat/Galvanostat. The output was displayed on a Kipp & Zonen Model BD 90 X-Y recorder. The reference electrode was a SCE, and the auxiliary electrode was a platinum wire. The usual scan rate of cyclic voltammetry was 100 mV/s in this experiment. A continuous stream of nitrogen gas was passed through the solution while mea- surements were being taken.

Ana lytica l Procedure

Chemical accumulation of Cu(1) was carried out in a buffered preconcentration solution containing Cu(1) ion. The CME depositing the Cu(1) ion via complex formation was removed from the preconcentration solution, washed with distilled water thoroughly, and transferred to the separate measuring cell. After this, the stripping voltammograms were obtained in one having only 0.1 M KCI solution.

RESULTS AND mscussroN Cu(I,l Ion Deposition and Electrochemical Behauior

of the CMEs. In Figure 1, cyclic voltammograms for two plain carbon paste elecrrodes (a and b) and for two didd- containing carbon paste electrodes (c and d) are shown, respectively. Where (a) and (c) are CVs from a blank solution without dipping an electrode in the solution having a test ion. (b) and (d) are CVs obtained after dipping an electrode in Cu(1) ion solution. There were no significant differences in the CVs between (a) and (c) over the potential range of +0.8 to -0.8 V (vs. SCE). The only difference was that the background current of the CMEs (Figure lc) was higher than that of the unmodified plain carbon paste electrode (Figure la). However, voltammograms subsequently recorded exhibit well-defined redox peaks in a 0.1 M K CI blank solution. The CME was then immersed for 5 minutes in a pH 5 buffer solution having 1 X M Cu(1) ion and rinsed thoroughly with distilled water (Figure Id).

A cyclic voltammogram of CME deposited in Cu(1) ion solution shows two large anodic peaks at -0.05 and +0.4 V, respectively (Figure Id), and two cathodic ones at -0.03 and -0.6 V (vs. SCE). The anodic peak at -0.05 V resulted from the exposure of the CME into a Cu(1)

Determination of Copper(1) Ion with a Chemically Modified Carbon Paste Electrode 423

c, c Q)

3 0

L L

2

1

I I I I I

0.8 0.0 -0.8 Volt vs.SCE

FIGURE 1. CVs of the carbon paste electrodes: (a and b) the plain carbon paste electrodes and (c and d) the CMEs. Electrodes were exDosed for 5 min to (a and c) 0.1 M KCI blank solution and (b and d) 1 x ion one. (Scan rate: 100 rnV/s.)

M Cu(l)

ion solution, but not from a Cu(I1) ion solution or the Cu(I) ion on an unmodified electrode surface. De- creases in height of the anodic and cathodic peaks corresponding to redox of Cu(1) ion were shown on the repeating cycles. These peaks could be eliminated completely by holding the potential sufficiently positive for a prolonged period in a blank solution. Hydroxylamine sulfate was used to generate Cu(1) ion from Cu(I1) ion solutions which gave no electrochemical response on the electrode itself over the potential range used in this work. The height of these redox peaks gradually increased as the concentration of Cu(1) ion in a deposition solution

I I I I

05 OD -0.5 -10

E (V vs AglAgCl)

FIGURE 2. CVs for the copper metal-coated platinum electrode at various pH the peaks correspond to the oxidation process of Cu(0) to Cu(l) in (a) pH 7.0 (b) pH 8.3, and (c) pH 10.0, respectively. (Scan rate: 100 mV/s; 0.1 M sodium borate buffer solution)

increased and the deposition time increased at any concentration. The results indicate that the peaks are due to the presence of Cu(1) ion. We obtained CVs, for the anodic oxidation of copper film coated on a platinum electrode in aqueous solution, to assign the anodic peak ap- pearing at -0.05 V (vs. Ag/AgCl).

In Figure 2 , the anodic peak corresponding to the oxidation of Cu to Cu(1) (the first anodic peak) shifts to a more negative potential as the pH of the solution increases. However, as shown in the CV, it is not possible to confirm the anodic peak corresponding to the process of Cu(0) to Cu(1) in the case of pH 7. From these results and the assignment of the redox peaks of copper in KOH solution, which had been extensively studied by Pyun and Park [ 151, it has been confirmed that the anodic peak at -0.05 V corresponds to the oxidation reaction of Cu(0) to Cu(1). Therefore, the electrode reaction for the analysis of Cu(1) with the CME should be as follows: Cu(1) + didd (on the CME)

-+ Cu(1)didd; accumulation step Cu(1)didd + e-

+ Cu(0) + didd; reduction step (-0.6 V)

Cu(0) + Cu(I) + e-; measurement step (-0.05 V)

Several cycles of deposition and the oxidative re- moval of copper on the electrode show that fresh CME surfaces are somewhat less efficient for Cu(1) ion uptake from the solution than the surfaces which had been previously exposed to Cu(1). The initial deposition on a CME surface always gave relatively small oxidation waves for the analyte. However, after three or four depositions/ stripping cycles were carried out for a given CME surface, CVs and differential pulse voltammograms (DPVs) of Cu(1) became highly reproducible and sensitive. For

424 Won et al.

5

- Y a -. * a. E ?! L 3 4 - 3 - /' /q I 1 I I I I 2

7 i

1 ; : /q 3 1 I I

FIGURE 3. Effect of peak height versus electrode composition (didd/graphite w/w%). The concentration of Cu(l) ion in deposition solution was 1 x rate: 100 niV/s; deposition time: 5 min; temperature: 25°C; and pH 5.)

FIGURE 4. Effect of peak height versus deposition temperature of Cu(l) ion solution. The concentration of Cu(l) ion in deposition solution was 1 x M. (Scan rate: 100 mV/s; deposition time: 5 min; and pH 5.)

M. (Scan

runs 7 or 8 times, the relative standard deviation o f the peak height for each scan was about 5%. The surface o f these CMEs was quite stable unless it was exposed t o extreme potentials or extreme pH conditions. The up- d u n g of Cu(1I) ion by the didd-modified electrode shows a similar conditioning sequence to that of Ni(I1) ion uptake by dimethylglyoxime [5]. The reasons for this behavior of the CMEs are not been completely understand yet. However, it might be assumed that a ligand used in this study forms a complex having a specific geometry. One of the functions of the predeposition stages might be to assist that the ligand molecules will be initially ori- ented on the electrode surface, which facilitates complexation with the specific metal ions.

AnalyiicuL Conditions. The analytical conditions for copper were studied by anodic stripping voltammetry. The CMEs composed of various ratios of didd and graphite (10-50% w,/w) were immersed for 2 minutes in 1 x lo-' M Cu(1) ion solution at room temperature, then rinsed thoroughly with distilled water several times, and transferred to a 0.1 M KCI blank solution to obtain the anodic stripping voltammograms (Figure 3). Copper(1) ion responded on the CMEs in all composition ratios. However, for the 50% didd-contained electrode, the peak heights of the voltammograms were low and not reproducible. This might presumably be due to the high re- sistance of the CMEs when the ratio of didd to graphite is large. The other electrodes, except the 50% -CME, ex- hibited well-defined peaks. The signal-to-noise ratio of the CMEs decreased as the content of didd increases. The optimum contents ratio of didd and graphite was 40% in this study.

The changes in the anodic peak height were studied as a function of the deposition temperature (Figure 4). The CME (40% w/w) was immersed for 5 minutes in a distilled water spiked with 1 X lod5 M Cu(1) ion at each

temperature. At the temperature range above 50"C, the peak height for Cu(I) ion became lower. This was due to the roughness of CME surface as a result of a slow dissolution of the complex into the bulk solution. How- ever, when the temperature was increased gradually from 25 to 45°C in the preconcentration cell, the sensitivity for copper( I) ion was better. The maximum voltammetric peak height was obtained at 45"C, but it was not reproducible. So the deposition temperature was 40°C in all subsequent experiments.

The size of the anodic stripping peak was observed according to the change of pH (for values 3.4, 4.6, 7.0, 8.3, and 9.0). In this case, chemical deposition was done under identical conditions [l X M Cu(1) ion solution for 5 minutes at 40"Cl. Over the region of pH <3 and pH >9.0, the oxidation signal was not observed due to the increase of background current. At pH 5.0, the signal was very sensitive to the copper(1) ion. Therefore, in all subsequent work, the buffer solution of pH 5.0 was used. Figure 5 shows that the deposition time for Cu(1) ion, required by a variation of the stripping current nearly becoming constant, was different for each of these three concentration conditions. The higher concentrations of Cu(1) ion have larger anodic currents and slightly shorter deposition times to attain constant current. At these three concentrations, the deposition process was relatively slow. As shown in Figure 5, the peak current increased lin- early, by any point, as a function of deposition time at each concentration. The peak height was nearly constant when the deposition time was about 20 minutes.

The interference effect of other metal ions [Mn(II), Ni(II), Cd(lI), V(IV), Co(II), Pb(II), Hg(II), and Ag(I)] on the copper determination was also studied. The CMEs were first immersed in a buffer solution of pH 5.0 containing several metal ions for 5 minutes at 40°C. The copper oxidation signal was not influenced by the presence of these metal ions at optimum conditions. However, another experiment for Cu(1) ion determination using

Determination of Copper(1) Ion with a Chemically Modified Carbon Paste Electrode 425

25 1

20 -

= 10 -

0 5 10 15 20 25 30 35

Time (min.)

FIGURE 5. Effect of peak height versus deposition time for Cu(l) at different bulk concentrations: 1 x M (*), 5 x M (A). (Scan rate: 100 mV/sec; pH 5; and temperature: 25°C.)

M (O), and 1 x

neocuproine-containing CME was seriously affected by Ag(1) ion [16]. While this experiment was not affected by the presence of excess Ag(1) ion in the deposition solution of about pH 5.0, is more accurate quantitation might be achieved by a didd-containing CME.

Figure 6A shows differential pulse stripping voltammograms for the analysis of Cu(1) ion with a didd-containing CME in acetate buffer solution (pH 5). In this case, the lower the concentration of analyte is, the more negative the potential shifts. A calibration plot obtained from data of the differential pulse stripping voltammetry yields a linear plot (Figure 6B). Least-squares treatment of these data yielded the equation of I (PA) = (0.658 + 0.038) C (log M ) + 6.736 p A and Y = 0.997.

At higher concentrations than about 1 X lo-' M, a deviation from linearity is expected. In such cases, shorter preconcentration periods would be expected to allow

2.0

extension of the linear range. On the basis of the signal- to-background characteristics of the response, a limit of detection was about 5 x lo-" M copper ion in standard solution by using 30 minutes preconcentration. Better detectability might be attained by using longer preconcentration periods.

Analytical Applications. Because of the high selectivity of the didd-containing CME, it is of interest to at- tempt the determination of copper in a well-characterized but realistic sample matrix such as that afforded by various standard reference materials available for metal ions. To demonstrate the suitability of the method for analyses of a real sample, an experiment for the concentration dependence and a precision test was performed with urine samples. The sample selected in this study was the SRM 2670 consisting freeze-dried urine. The certified copper content of this material was 130 ppb (low level) and 370 ppb (elevated level); other com- ponents present in the 2-300 ppb range include Ag, Al, As, Be, Cd, C1, Ca, Au, Pb, Mg, Mn, Hg, Ni, Pt, K, Se, Na, and V. A sample treatment step with nitric acid was used prior to the preconcentration and undertaking of the differential pulse voltammetric procedure described above. Since possible adsorption of nonelectroactive urine con- stituents may affect the surface active sites for the analyte adsorption, the reference urine sample and the standard copper solution were heated in concentrated nitric acid, separately and then the pH of the solution was adjusted to 5 by addition of acetate buffer. Two quantitative methods were used to compare the results of a calibration plot method and a standard addition one. As shown in Table 1, the standard addition method was more precise than the calibration plot method. The negative error on both results might be from the incomplete degradation of the reference urine sample and/or loss in processing.

The didd-containing CME can be effectively used for the selective detection of copper in a physiological urine sample.

O*O' 10 9 8 7 -0.1 -0.2 -0.3 -L0gtM1 Volt vs. SCE

FIGURE 6. (A) DPVs of each Cu(l) concentration; a 1 x

(c) 1 x lo-' M and (d) 1 x lo-'' M. (B) A calibration curve for Cu(l). (The DPV measuring conditions were as follows: scan rate: 5 mv/s; pulse height: 0.05 V; pulse width: 0.05 s; conditioning temperature: 40°C deposition time: 20 min; and pH 5.)

M, (b) 1 x 10- L ' M, and

426 Won et al.

TABLE 1 Reference Material

Comparative Data for the Standard Addition Method and the Calibration Plot Derived from the Urine

Quantitation Method Sample

Cu Found (ppb)

Mean Ranae SD*

Calibration Curve method Standard Addition method

*Standard deviation

Low level (130 ppb) Elevated level (370 ppb) Low level Elevated level

~ ~~~~~~~ ~

150 140-1 50 4.1 350 340-360 5.0 135 130-1 40 4.1 380 370-380 4.1

ACMVO WLEDGMENT

We thank the Ni w-Directed Research Fun4 Korea Research Foundation (19901, its $nuncia1 support

REFEWCES

1.

2.

3. 4.

5.

6.

F. Vydrd, K. Stulik, and E. Julacova, Electrochemical Strip- ping AnaZysZ,sis, Ellis Horwood Publishing Co., USA, 1976. J. Wang, Stripping Ana&sis; Princ@le, Instrumentation and Application, Verlag Chemie, Deerfield Beach, EL, USA, 1985. H. D. Abruna, Coord. Chmz. Rev. 86 (1988) 135. R. W. Murray, Electrocanalytical Chemistry, A. J. Bard, ed., Marcel Dekker, New York, 1984, vol. 13, pp. 191-368. M. J. Gehron ,and Anna Brajter-Toth, Anal. Chem. 58 (1986) 1488. R. P. Baldwin. J. K. Christensen, and L. Kryger, Anal. Chem. 58 (1986) 1790.

7. S. V. Prabhu and R. P. Baldwin, Anal. Chem. 59 (1987) 1078. 8. D. M. T. O’IZlordan and G. G. Wallace, Anal. Chem. 58 (1986)

9. L. L. McCracken, L. M. Wier, and H. D. Abruna, Anal. Lett.

10. G. Wang, B. Greene, and C. Morgan, Anal. Chem. Acta. 158

11. G. T. Cheek and R. F. Nelson, Anal. Lett. 11 (1978) 393. 12. Y.-B. Shim, D. S. Park, S. N. Choi, and M . 3 . Won,J. Korean

13. J. S . Yeom, M.-S. Won, S. N. Choi, and Y.-B. Shim, Bull.

14. T. Takeshima, M. Yokoydma, T. Iamaoto, M. Akano, and €1.

15. C.-H. Pyun and S.-M. Park, J , Electrochem. Soc. 13.3 (1986)

16. S. V. Prabhu and R P. Baldwin, Anal. Chem. 59 (1987) 1074.

1766.

20(10) (1987) 1521.

(1984) 15.

Chma. Soc. 32 (1988) 37.

Korean Chem. SOC. 11 (1990) 200.

Asaka, J. Org. Chem. 34 (1969) 730.

2024.

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