The silver amalgam film electrode in catalytic adsorptive stripping voltammetric determination of cobalt and nickel

Available online at www.sciencedirect.com Journal of

Electroanalytical Chemistry Journal of Electroanalytical Chemistry 617 (2008) 1–6 www.elsevier.com/locate/jelechem

The silver amalgam film electrode in catalytic adsorptive stripping voltammetric determination of cobalt and nickel Paweł Kapturski, Andrzej Bobrowski * Department of Building Materials Technology, Faculty of Materials Science and Ceramics, AGH – University of Science and Technology, Al. Mickiewicza 30, 30-059 Krako´w, Poland Received 24 September 2007; received in revised form 17 December 2007; accepted 14 January 2008 Available online 19 January 2008

Abstract The application of the silver amalgam film electrode (Hg(Ag)FE) of prolonged analytical applicability to the determination of cobalt and nickel by adsorptive stripping voltammetry is presented in this work. The method is based on adsorptive accumulation of analytes at the Hg(Ag)FE in a supporting electrolyte consisting of 0.1 M ammonia buffer and 50 lM cycloheksanedione dioxime. The reduction current of Co is catalytically enhanced by the presence of 0.4 M nitrite. Several key parameters of the differential pulse modulation were optimized, including pulse amplitude, accumulation potential and accumulation time. The detection limit obtained for 60 s accumulation time are estimated to be 5.8  10–11 M (0.0035 lg/L) Co and 2.2  10–10 M (0.013 lg/L) Ni. The repeatability of the peak current was 5.8% and 5.6% for Co and Ni, respectively. The calibration plots were linear from 0.01 to 7 lg/L Co and from 0.1 to 10 lg/L Ni. Finally, the Hg(Ag)FE was applied to the determination of nickel and cobalt in certified reference material with satisfactory result. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Cobalt; Nickel; Nioxime; Nitrite; Silver amalgam electrode; Catalytic adsorptive stripping voltammetry

1. Introduction There is a significant interest in monitoring the Co and Ni ultratraces in both, environmental samples and in some branches of industry. These elements are essential for humans and other living organisms, whereas at higher concentrations toxic effects have been found to occur. The most common optical methods like spectrophotometry, AAS and ICP AES often do not have the necessary sensitivity, selectivity and freedom from matrix interferences. Modern stripping voltammetric methods belong to the most sensitive analytical techniques thanks to the adsorption or electrochemical accumulation of electroactive substances on the electrode surface, as well as the exploitation of the catalytic reactions. In case of Co and

*

Corresponding author. Tel.: +48 012 617 24 51; fax: +48 012 617 24

52. E-mail address: [email protected] (A. Bobrowski). 0022-0728/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2008.01.007

Ni determination both methods can be applied simultaneously, i.e., accumulation and stripping of the accumulated substance combined with a catalytic reaction. The presence of dioximes (e.g. DMG, a-benzil dioxime (aBD), a-furil dioxime (aFD) or cyclohexanedione dioxime (nioxime)) in the solution during determination of Co(II) and Ni(II) by means of catalytic adsorptive stripping voltammetry (CAdSV) leads to the formation of a complex of a metal with the active ligand, which is accumulated adsorptively at the electrode surface. The analytical signal is the sum of reduction currents of the central metal ion (Co or Ni) and the surrounding two dioxime ligands [1–4]. Accordingly, the reduction of the Co-dioxime and Ni-dioxime complexes from adsorptive state involves an overall 10electron reduction process. The more complex catalytic effect is observed during the reduction of the Co(II)-dioxime complexes in the presence of nitrite ions. A great enhancement of Co(II) voltammetric response and very low detection limit of Co determination is the result of triple amplification effect comprising:

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– adsorptive preconcentration of Co(II)-dioxime complexes, – reduction of the Co(II)-dioxime complexes with ligand participation [1–4], – catalytic reduction of nitrite anion induced by the reduction of composite Co(II)-dioxime- NO2 complexes [1,4–8].

cobalt and nickel by means of the CAdSV technique. For that purpose, the composition of the supporting electrolyte and analytical parameters (i.e., deposition time and deposition potential) have been optimized. 2. Experimental 2.1. Chemicals and reagents

As opposed to the Co(II)-dioxime, the Ni(II)-dioxime complex does not induce the catalytic reduction of nitrite ions [1,4–7]. Due to the triple amplification effect, the overall CAdSV Co(II) response is in the presence of NO2– anion at least two orders of magnitude higher than the AdSV Ni(II) response and four orders of magnitude higher than the voltammetric response for the pure diffusion process of Co(II) ions in ammonia buffer solution in the absence of dioxime and NO2– ions. The elaborated CAdSV system of Co and Ni determination has been developed with utilization of various types of working electrodes. The advantages of usage of electrodes based on mercury are obvious. The exploitation of hanging mercury drop electrode (HMDE) has proved their great analytical performance [4–24]. However, the increased risk associated with the use, manipulation and disposal of metallic mercury has led to seek an alternative sensor. As a result, bismuth film electrodes [25–28], and lead film electrode [29–31] have been applied in Co and Ni voltammetric and chronopotentiometric stripping [28] determination exploiting the mentioned catalytic effect. It is also possible to devise an electrode that would have advantages similar to those of the HMDE, and at the same time minimize the hazards connected with the usage of liquid Hg. Such an alternative sensor would utilize Hg either in the safe form of an amalgam, or in very small amounts, making it less hazardous. A non-toxic dental amalgam electrode developed by the Trondheim research group [32–34] was found to be suitable for the determination of Zn, Cd, Pb, Tl, Cu, Ni, Co in stripping voltammetry. Alternative working electrodes based on solid amalgams were also developed by a research group from Prague (the so-called polished solid amalgam electrodes p-AgSAE or, after modification of their surface by a mercury meniscus, mAgSAE) [35,36]. A new, cylindrical silver-based amalgam film electrode (Hg(Ag)FE) of prolonged analytical application has recently been introduced for voltammetric measurements and has been applied for the determination of Pb, Zn, Cd, Cu and Cr by means of voltammetry [37,38] and stripping chronopotentiometry [39]. The electrode is designed in such a way that the thin liquid layer can easily be regenerated before each measurement cycle. Such a procedure ensures good reproducibility of results. A small amount of the silver amalgam, tightly sealed inside the electrode corpus makes the electrode safe to use both in the laboratory and in on-site conditions. The aim of the investigation was to examine whether the novel Hg(Ag)FE could be used for the determination of

All reagents were of analytical grade purity. Ammonia buffer (2 M) was prepared by mixing the corresponding amounts of ammonium chloride and ammonia solution (both SuprapurÒ, Merck). Cyclohexanedione dioxime (nioxime) solution (1  102 M) was prepared by dissolving an appropriate amount of the reagent in 96% ethanol. Nioxime and sodium nitrite were purified by recrystallization from ethanol and water, respectively. Standard stock solutions of Co and Ni were obtained from Merck, and diluted as required. All the solutions were prepared using deionised water produced in an ion-exchange purification system (Cobrabid-Aqua, Warsaw, Poland). Argon (99.99%) was used to remove dissolved oxygen from the solutions prior to analysis. 2.2. Apparatus and instrumentation All the adsorptive stripping voltammetry (AdSV) measurements were carried out with an Autolab PG STAT 2 apparatus (Ecochemie, Utrecht, The Netherlands) operated by GPES 4.5 software (from the same manufacturer). Magnetic TeflonÒ-coated bar was used for stirring during the accumulation period. A three-electrode cell was used, comprising: a cylindrical silver-based amalgam film electrode (Hg(Ag)-FE) with a surface area of 8.7 mm2 as the working electrode, Pt wire as the auxiliary electrode, and a self-made Ag/AgCl electrode with 3 M KCl as the reference electrode. The detailed description of the manual refreshment of the electrode mercury layer and a schematic diagram of the assembly which enables the refreshing of the amalgam film was presented in previous papers on this topic [37–40]. The manual refreshment of the electrode liquid layer precedes each measurement cycle and this is why we are able to obtain results with satisfactory reproducibility. As it was earlier found [41] the reproducibility of the surface of the investigated electrode was better than 2%. 2.3. Analytical procedure The electrochemical cell was filled with the analyzed solution and the solutions were bubbled with argon for 7 min prior to analysis. When the silver cylinder was shifted down and placed in the solution, the differential pulse voltammetric procedure was started. Before each measurement, the electrode surface was restored mechanically, i.e. by pulling the silver wire electrode inside, across the mercury chamber and then pushing it back, outside the elec-

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trode corpus. Hence, every time, a refreshed electrode surface was at our disposal. All measurements were carried out in a supporting electrolyte of 0.1 M ammonia buffer (pH 9.2) containing 0.05 mM nioxime and 0.4 M nitrite. In order to avoid the suppression of the signal by surface-active compounds, the determination of cobalt and nickel in real samples was preceded by the UV mineralization step (4 h irradiation with the addition of 10 ll 30% H2O2 to each 10 ml of the sample). Appropriately diluted samples (five times for Co and three times for Ni quantification) of certified reference material (TMRain – 95) were analyzed by means of the standard addition method. 3. Results and discussion 3.1. Optimization study To obtain suitable conditions for the CAdSV determination of Co and Ni, the influence of deposition time, deposition potential, pulse amplitude, nioxime and nitrite concentration effect was investigated. The effect of the preconcentration time on the stripping peak heights for Co and Ni in the solution containing: 5  1010 M of Co and 3  109 M of Ni in the range of 0–300 s is illustrated in Fig. 1A. Initially the signals increase linearly, and at higher deposition times the plot for Ni slightly levels off, indicating that the saturation surface metal-complex concentration is gradually reached. In further experiments, the accumulation time of 60 s was used, which ensures high sensitivity of Co and short time of analysis. The next investigated factor, accumulation potential has decisive influence on the effectiveness of the metal–nioxime adsorption at the electrode surface. The dependence of the accumulation potential on the stripping current was shown in Fig. 1B. The peak current for Ni remains stable in the potential interval from 0.3 to 0.8 V, then decreases. Adsorption efficiency for Co

A

increases dramatically while the deposition potential was changed from 0.3 to 0.5 V then reaches plateau and rises again in the range from 0.8 to 0.9 V, what can be attributed to nickel–nioxime complex reduction at such a negative cathodic potential. When Co is being quantified in the presence of a Ni excess, the accumulation potential of 0.9 V can be recommended. However, for the simultaneous precise and sensitive determination of Ni and Co, the value of 0.7 V would be more suitable. For further study an accumulation potential 0.7 V was chosen. The influence of the pulse amplitude in differential pulse mode was studied in the range from 5 to 100 mV. The Ni and Co peak CAdSV peak currents increased linearly from 5 to 50 mV. After that the increase became negligible. The value of 50 mV was chosen for further study, since it ensured the highest peak currents for both investigated elements. The effect of the nioxime concentration on the Co and Ni peak heights was examined in the range from 0.5  106 to 150  106 M. The results are presented in Fig. 2A. The peak currents, both for Co and Ni, rise with the increasing concentration of ligand. In case of cobalt, the slope of the plot decreases gradually. Unlike cobalt, peak height for nickel increases significantly in the narrow region up to 1  105 M nioxime, leveling off at higher ligand concentration. The last step of optimization comprised examination of the influence of nitrite concentration on the cobalt peak current (Fig. 2B). The CAdSV cobalt peak current is strongly affected by the concentration of nitrite. It increases linearly with increasing nitrite concentration up to 0.1 M, and then more slowly, up to 0.75 M. The concentration of 0.4 M was selected for further experiments, as it was a good compromise between the sensitivity of the Co response and the blank value for Ni and Co, which increases together with the nitrite concentration. An example of the catalytic effect of the reduction of the

B

0.7

3

0.40 0.35 0.30

Ip / 10-6 A

-6

Ip / 10 A

0.5

0.4

0.25 0.20 0.15

0.2

0.10 0.05

0.0 0

50

100 150 tdep/ s

200

250

300

-0.2

-0.4

-0.6

-0.8

-1.0

E/V

Fig. 1. The effect of accumulation conditions – deposition time (A) and deposition potential (B) – on the voltammetric stripping response for 5  1010 M Co (j – solid line) and 3  109 M Ni (A) and 1  109 M (B) Ni (N – dashed line). Supporting electrolyte: 0.1 M ammonia buffer (pH 9.2); 5  105 M nioxime; 0.4 M nitrite, Edep =  0.7 V (A), tdep = 60 s (B).

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A

0.5

B 0.3

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Ip / 10-6 A

Ip / 10-6 A

0.4

0.2

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0.1 0.0 0

50

100

150

0.0 0.0 0.1

200

10-6 nioxime / M

0.2 0.3 0.4 0.5 nitrite / M

0.6 0.7

0.8

Fig. 2. The dependence of voltammetric stripping response for 5  1010 M Co (j – solid line) and 6  109 M Ni (N – dashed line) on nioxime concentration in a solution containing 0.1 M ammonia buffer (pH 9.2) and 0.4 M nitrite (A) and effect of nitrite concentration on Co response in a solution containing 0.1 M ammonia buffer (pH 9.2) and 5  105 M nioxime (B). Edep:  0.7 V, tdep = 60 s.

Co

Ni 0.0

-1.0 -6

10 A

-6

-0.7 -1.0

-1.5

b

3.0

-2.5

2.5

1.5

2.0 -6

-6

10 A

-1.3

2.0

1.0

-1.0 E/V

-1.1

Fig. 3. DP-CAdSV voltammograms of 1  109 M Co and 1  108 M Ni in supporting electrolyte containing 0.1 M ammonia buffer (pH 9.2) and 5  105 M nioxime before (curve a), and after addition (curve b) of 0.4 M nitrite addition. Edep = 0.7 V, tdep = 60 s.

Co–nioxime complex in the presence of nitrite ions is presented in Fig. 3. 3.2. Analytical characteristics The dependence of stripping peak currents on the concentration of Co and Ni is shown in Fig. 4, which presents the DP CAdSV voltammograms for successive increases of Co and Ni concentrations. The voltammetric curves of Co and Ni, obtained after 30 s of accumulation, were well Co shaped and separated (ENi p ¼ 0:90 V; E p ¼ 1:02 V; DEp ¼ 120 mV). The sensitivity of CAdSV determination of Co and Ni was 2170 nA n M1 for Co and 16.7 nA n M1 for Ni. The calibration plots under optimized conditions were linear from 1.7  10–10 to

1.5 1.0

0.5 0.0

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10 A

-2.0

-1.7 -0.8

Co

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Ni

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a

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30

60 90 [Ni] / 10 -9 M

-0.8

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120

-0.9

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0.3

0.6 0.9 [Co] / 10-9 M

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1.2

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E/V Fig. 4. The CAdSV curves of Co and Ni obtained after subtraction of the background curve for successive increases of cobalt and nickel with 0.2 nM Co and 18 nM Ni increment. Inset: calibration plots. Supporting electrolyte: 0.1 M ammonia buffer (pH 9.2), 0.05 mM nioxime and 0.4 M sodium nitrite. Edep = 0.7 V, tdep = 30 s.

1.2  10–7 M (0.01 to 7 lg/L) Co and from 1.7  10–9 to 1.7  10–7 M (0.1 to 10 lg/L) Ni. The determination coefficients (R2) were equal to 0.997 and 0.998 for Co and Ni, respectively. The detections limits (for tdep = 60 s), defined as three times the standard deviation of blank, are estimated to be 5.8  10–11 M (0.0035 lg/L) Co and 2.2  10–10 M (0.013 lg/L) Ni. Using the Hg(Ag)-FE and the catalytic AdSV results in good repeatability of measured signal. Eight successive measurement of Co and Ni peak current show a relative standard deviation of 5.8% and 5.6% for 3.4  10–10 M (0.02 lg/L) and 1.0  10–9 M (0.06 lg/L) of Co and Ni, respectively.

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3.3. Interferences

5

0.0

3.4. Application The accuracy of the elaborated method employing the Hg(Ag)-FE was assessed by determining nickel and cobalt in a rainwater certified reference sample (TMRain – 95). An example of a separate determination of cobalt and nickel in a TMRain – 95 sample using the standard addition procedure is presented in Fig. 5. Due to different sensitivities of the method for Co and Ni, the investigated elements were quantified separately, using various volumes of the CRM sample. The obtained results presented in Table 1 are in good agreement with the reference values for Co and Ni. 4. Conclusions The silver amalgam film electrode (Hg(Ag)FE) of prolonged analytical applicability, combining properties of liquid mercury with some of the properties of silver, is a useful sensor for adsorptive stripping voltammetry. The

-0.1 -0.2

-6

A

-0.3 10

The effect of co-existing ionic species and surface-active substances on the determination of cobalt and nickel was also investigated. Special emphasis was put on the study of possible interferences from Ni and Zn on Co determination. It was documented that a 500-fold excess of nickel did not influence the determination of 8  10–10 M of Co. It was also found that zinc in concentrations up to 15,000 times higher than the level of cobalt did not interfere with the response of cobalt, though the difference in their peak potentials amounted only to 55 mV (EZn p ¼ 1:075 V). The influence of co-existing metal ions was tested for Fe(III), Mn(II), Cu(II) and Pb(II). The determination of Co(II) at a concentration of 2  10–9 M was not affected by the presence of Pb(II) and Fe(III) at concentrations 1000 times higher than the level of Co(II). Likewise, no interferences were observed from a 10,000-fold excess of Mn(II) and Cu(II). The effect of the surfactant was investigated in a solution containing 0.05 lg/L (8.5  10–10 M) of Co and 2 lg/L (3.4  10–8 M) of Ni. TritonÒ X – 100 added in the amount of 0.5 mg/L resulted in 71% and 39% depression of the cobalt and nickel response, respectively. The addition of 1.0 mg/L of TritonÒ X – 100 caused a complete elimination of the cobalt signal and a 42% decrease of the nickel signal. Therefore, in the determination of cobalt and nickel in natural samples pre-treatment (e.g., UV-irradiation) is required.

-0.4

Ni

-0.5 -0.6 -0.7

Co

-0.8 -0.7

-0.9

-0.8

-1.0

E/V

-0.9

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-1.1

E/V

Fig. 5. DP adsorptive stripping voltammetric determination of cobalt and nickel in TMRain – 95 certified reference sample. The sample before (dashed line) and after three successive standard additions of Ni and Co in 2.5  10–9 M Ni and 5  10–10 M Co steps. Supporting electrolyte: 0.1 M ammonia buffer (pH 9.2), 0.05 mM nioxime and 0.4 M sodium nitrite. Edep = 0.7 V, tdep = 60 s.

simple and very fast cleaning and regeneration procedures assures complete and efficient removal of all electrochemical and adsorptive products from the electrode surface after each measurement cycle. The amalgam chamber inside of the electrode corpus contains only 10 ll of 1% silver amalgam, and the electrode can be used for several months, providing stable operation for a great number of regeneration cycles without replenishing the amalgam reservoir. Moreover, the thickness of amalgam film amounts to a mere 50 nm, and the Hg(Ag)FE does not introduce any mercury into the analyzed solution. Therefore, the use of Hg(Ag)FE does not cause significant problems with the disposal and toxicity of mercury, as opposed to liquid mercury electrodes. The Hg(Ag)FE electrode has a high overpotential towards hydrogen [39], and enables to adjust the electrode surface area in the range from 1.5 to 12 mm2, which allows to increase the amount of species adsorbed at the electrode surface during the accumulation step and, consequently, the sensitivity of the AdSV method. The results of the investigation also demonstrate that the Hg(Ag)FE can be applied successfully for sensitive determination of nickel and cobalt, by means of catalytic adsorptive stripping voltammetry. The peaks of cobalt and nickel are well defined and separated. The estimated limits of detection are at satisfactory levels and comparable to those obtained using hanging mercury electrodes [5]. The sensitivity of the Co and Ni determination at the

Table 1 Results for the determination of Co and Ni in certified reference material of rain water TMRain – 95 Sample

CRM TMRain – 95

Co concentration

Ni concentration

Found (lg/L)

Certified value (lg/L)

Found (lg/L)

Certified value (lg/L)

0.20 ± 0.04

0.22 ± 0.037

0.78 ± 0.12

0.80 ± 0.17

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Hg(Ag)FE is significantly higher and the limits of detection are much lower than those obtained using the bismuth film electrode [25]. The accuracy and repeatability are satisfactory as well. It was found that electrode stability (several months) was excellent over longer periods, even without any additional pre-treatment. This makes it very useful for application in on-line and field analysis. Overall, it can be stated that the investigated electrode can be successfully exploited as an alternative to hanging mercury electrodes, as it combines safe and convenient operating procedures with very good analytical performance. The additional benefits, such as small dimensions and low costs of construction should also be taken into account. Acknowledgements Financial supports from the Ministry of Science and Higher Education (Project N507 063 32/1767) are gratefully acknowledged. References [1] A. Bobrowski, Anal. Chem. 61 (1989) 2178. [2] L.A.M. Baxter, A. Bobrowski, A.M. Bond, G.A. Heath, R.L. Paul, R.I. Mrzljak, J. Zare˛bski, Anal. Chem. 70 (1998) 1312. [3] F. Ma, D. Jagner, L. Renman, Anal. Chem. 69 (1997) 1782. [4] A. Bobrowski, J. Zare˛bski, Electroanalysis 12 (2000) 1177, and refs. therein. [5] A. Bobrowski, Anal. Lett. 23 (1990) 1497. [6] A. Bobrowski, A.M. Bond, Electroanalysis 3 (1991) 157. [7] A. Bobrowski, A.M. Bond, Electroanalysis 4 (1992) 975. [8] A. Bobrowski, Electroanalysis 16 (2004) 1536. [9] A. Bobrowski, Talanta 41 (1994) 725. [10] A. Bobrowski, J. Anal. Chem. 349 (1994) 613. [11] R.J. Mrzljak, A.M. Bond, T.J. Cardwell, R.W. Catrall, R.W. Knight, O.M.G. Newman, B.R. Champion, J. Hey, A. Bobrowski, Anal. Chim. Acta 281 (1993) 281. [12] M. Vega, C.M.G. van der Berg, Anal. Chem. 69 (1997) 874. [13] J. Golimowski, A. Tykarska, Fresen. J. Anal. Chem. 349 (1994) 620.

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