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Voltammetric method for direct determination of nickel in natural waters in the presence of surfactants
Talanta 53 (2000) 679 – 686 www.elsevier.com/locate/talanta
Voltammetric method for direct determination of nickel in natural waters in the presence of surfactants Mieczyslaw Korolczuk * Faculty of Chemistry, Maria Curie Sklodowska Uni6ersity, 20 -031 Lublin, Poland Received 25 April 2000; received in revised form 4 August 2000; accepted 11 August 2000
Abstract Voltammetric procedures for direct Ni(II) determination in natural water samples are described. The procedures are based on nickel deposition to the metallic state and then its oxidation in the presence of dimethylglyoxime with the formation of the complex adsorbed on the electrode. Reduction of the complex is exploited in the detection step. Due to the application of a sufficiently negative deposition potential the interference from surfactants is minimized. The detection limits for Ni(II) are 2 ×10 − 9 and 2 ×10 − 10 mol l − 1 for deposition times of 30 and 120 s, respectively. The influence of foreign ions is also presented. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Nickel determination; Voltammetry; Natural water; Surfactants
1. Introduction Adsorptive stripping voltammetry (AdSV) is a suitable technique for determination of traces of nickel and cobalt in environmental samples because of its high sensitivity and low cost of instrumentation used. Many procedures have been proposed for nickel determination using HMDE [1–12], mercury film [13 – 20] and chemically modified electrodes [21,22]. An automated voltammetric system for determination of Ni(II) also has been reported [23]. Although different complexing agents have been employed for accu-
* Fax: + 48-81-5333348. E-mail address: [email protected] (M. Korolczuk).
mulation of nickel and cobalt, dimethylglyoxime (DMG) has gained wider use. The mechanism of the reduction of Ni(II) in the presence of DMG has also been investigated [24–26]. In most papers it was stated that the surface active compounds cause a decrease or decay of the nickel peak [2,16,17,19,27–29], so efficient decomposition methods of natural samples must be applied before determination. Decomposition methods are time consuming, so various approaches have been proposed to overcome interference from surface active substances in Ni(II) determination, for example, an addition to the sample of fumed silica [27,28], coverage of the electrode by Nafion film [28] or using the renewable-reagent adsorptive stripping sensor [29]. These approaches led to higher detection limit [28,29] or lower precision of determinations [28]. Additionally, the application
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of natural water samples without pretreatment and reasonable reduction in the sensitivity. In the method proposed nickel was deposited to the metallic state and then was oxidized to Ni(II) in the presence of DMG with formation of the complex adsorbed on the electrode. Reduction of the complex is used in the detection step as in the case of direct accumulation of Ni(DMG)2 complex by adsorption [1–29]. Due to deposition of nickel at a relatively negative potential according to literature data [30], the interference from surface active compounds on the deposition step was minimized. Fig. 1. Square wave voltammograms obtained for various concentrations of Ni(II) following the procedure exploiting the deposition of nickel from 0.2 mol l − 1 HCl + 0.2 mol l − 1 KSCN. Added Ni(II) concentration: (a) 0; (b) 5 ×10 − 9; (c) 2 × 10 − 8; (d) 5×10 − 8 mol l − 1. Deposition at − 1.1 V for 30 s.
of silica in flowing systems can cause a blockage of tubing and valves. The purpose of this paper is to report a sensitive voltammetric method for nickel determination in the flow system which allows the analysis
2. Experimental
2.1. Apparatus The measurements were performed using Autolab PGSTAT10 analyser (Netherlands) and controlled growth static mercury drop electrode in the HMDE mode, made by MTM Cracow, Poland. A three-electrode flow cell consisting of an Hg electrode, a Pt electrode and an Ag/AgCl reference electrode as described before [31–33]
Table 1 Relative signals for Ni(II) in the presence and absence of foreign ionsa Ion
Fe(III) Zn(II) Co(II) Cr(III)
Mn(II) Cd(II) Cu(II) Pb(II)
Fold excess
100 1000 100 1000 10 100 10 100 1000 1000 1000 100 1000 100 1000
Deposition from Acidic solution at −1.1 V for 30 s
Alkaline solution at −1.35 V for 120s
Alkaline solution at −1.65 V for 120 s
1.05 0.97 1.04 0.53 1.04 0.72 1.0 0.95 0.97 1.03 1.03 0.96 0.51 0.97 0.32
0.98 0.85 0.97 1.01 1.03 0.35 1.04 0.99 1.02 1.01 0.98 1.03 0.64 1.04 0
1.03 0.65 0.98 1.03 1.02 0.33 1.05 drop falls – 1.04 1.01 0.96 0.61 1.03 0
a Ni(II) concentrations were 1×10−7 and 2×10−8 mol l−1 of Ni(II) for deposition from acidic and alkaline solutions, respectively.
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Fig. 2. Relative signals for Ni(II) in the presence and absence of synthetic and natural surfactants for procedures exploiting in the accumulation step: (a) adsorption of Ni(DMG)2 complex from 0.2 mol l − 1 ammonium buffer +1 × 10 − 4 mol l − 1 DMG + 2× 10 − 8 mol l − 1 Ni(II) for 30 s; (b) deposition of metallic Ni from 0.2 mol l − 1 HCl+ 0.2 mol l − 1 KSCN+ 1 ×10 − 7 mol l − 1 Ni(II) for 30 s; (c) deposition of metallic Ni from ammonium buffer containing 5 ×10 − 4 mol l − 1 DMG +2 ×10 − 8 mol l − 1 Ni(II) at potential −1.35 V for 120 s; (d) as (c) but deposition potential was changed to −1.65 V. Added surfactants: (A) Triton X-100; (B) sodium dodecylsulfate; (C) cetyltrimethylammonium bromide; (D) humic acid.
was used. The volume of the cell was about 0.5 ml. The surface area of mercury drop was 0.85 mm2. The solutions were delivered using a peristaltic pump PPa-12A (Emko, Poland). The flow rates of each solution were about 16 ml min − 1. In comparative determinations of Ni(II) UV digester (Mineral, Poland) was used for the destruction of organics present in water samples.
2.2. Reagents Ammonium buffer (2.0 mol l − 1; pH = 9.0) was prepared from Suprapure HCl and Tracepur ammonium hydroxide (Merck). Dimethylglyoxime (DMG; 0.1 mol l − 1 ) was prepared by dissolving an appropriate quantity of reagent (POCh, Poland) in 96% ethanol. Standard solution of
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682
Fig. 3. Relative signals for Ni(II) in the presence and absence of EDTA using various procedures of nickel determination described in legends for Fig. 2.
nickel (1g l − 1; Fluka) was used. KSCN solution (2 mol l − 1) was prepared from pure for analysis reagent (Ubichem, England) and then purified by shaking with Zn powder. Humic acid (sodium salt) was obtained from Aldrich. Other reagents were obtained from POCH, Poland, and used as received. All solutions were made using triply distilled water.
2.3. Standard procedures Two analytical procedures were used depending on the acidity of the solution used in the deposition step. Both procedures differed in the composition of the solutions used for deposition of nickel and
for washing of the cell, and the value of the potential used in the deposition and washing steps. For deposition from the acidic medium a solution 0.2 mol l − 1 KSCN+0.2 mol l − 1 HCl was used as a supporting electrolyte. In this procedure a solution 0.005 mol l − 1 KSCN+0.005 mol l − 1 ammonium buffer (pH= 9.0) containing 1× 10 − 4 mol l − 1 DMG was used as a washing one. In the procedure exploiting the deposition from the alkaline medium nickel was deposited from 0.02 mol l − 1 ammonium buffer containing 5× 10 − 4 mol l − 1 DMG, while a solution 0.01 mol l − 1 ammonium buffer+ 1× 10 − 4 mol l − 1 DMG was used as a washing one. The procedures were performed according to the following scheme. 1. A flow of the washing solution was directed to the cell. 2. A new mercury drop was formed, the stirrer was switched on and the deposition potential was applied. Thereafter the flow of the sample solution with the added supporting electrolyte was directed to the cell and the deposition of nickel to the metallic state was carried out for a fixed time. 3. A flow of the washing solution was started for 90 s and the substances present in the sample, which could interfere in the detection step were removed from the cell. Five seconds before the end of this step the stirrer was switched off. During this step the potential of the electrode was the same as in the deposition step. 4. The flow of the solution 0.2 mol l − 1 ammonium buffer and 1×10 − 4 mol l − 1 DMG was
Table 2 Results of Ni(II) determinations (in nmol l−1) in tap and river water samples using various accumulation proceduresa Accumulation procedure
Sample
Tap water Czecho´wka river water a
Adsorption of Ni(DMG)2 complex at −0.7 V from:
Deposition of nickel from:
Untreated sample
UV-irradiated
Acidic solution
Alkaline solution
sample
at −1.1 V
at −1.35 V
at −1.65 V
6.6 (4.0) 11.6 (3.4)
6.3 (11) 12.0 (7.6)
6.0 (4.2) 7.8 (5.2)
5.8 (4.7) 7.9 (3.9)
6.3 (3.9) 8.2 (3.1)
Figures in parentheses are the relative standard deviations in% from three measurements.
M. Korolczuk / Talanta 53 (2000) 679–686
directed to the cell for 45 s. Then the potential was changed to − 0.075 V and nickel was oxidized for 45 s. The complex of Ni(II) with DMG was formed and it was adsorbed on the electrode. Then after a rest period of 5 s at potential − 0.75 V a voltammogram was recorded using a cathodic square wave voltammetric scan (− 0.75 to − 1.2 V, frequency 1 kHz, pulse height 50 mV, scan increment 4 mV). 5. After the measurement the flow of the washing solution was started for at least 30 s before the next measurement. All solutions used were not deaerated.
3. Results and discussion To minimize the interference from organic substances present in natural water samples on voltammetric determination of nickel, a several-step procedure in the flow system was proposed. The procedure consists of three main steps: deposition of the metallic Ni on the mercury electrode; oxidation of Ni0 to Ni(II) in the presence of DMG with formation of the complex adsorbed on the electrode; reduction of Ni – DMG complex in ammonium buffer. For deposition of nickel two electrolytes were used: acidic and alkaline. The last step had already been described in detail in papers [e.g. 2,5] so the optimization of the overal procedure was directed to the first two steps.
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V were chosen. Using the deposition potential − 1.65 V, foreign ions can have greater effect on Ni(II) signal, however, at such a negative potential the interference of surfactants should be completely eliminated.
3.2. Deposition time For procedure exploiting the deposition from acidic solution the effect of the deposition time was studied for deposition potentials − 1.0 and −1.1 V. It was found that the nickel peak increases linearily with the deposition time only to 30 s, when the deposition potential of − 1.1 V was chosen. Prolongation of the deposition time from 40 to 120 s does not influence the nickel peak. However, when the deposition potential was changed to − 1.0 V, the nickel peak increased linearily up to 90 s, and then increased nonlinearily up to 240 s. Such dependences can be explained by blockage of the electrode by hydrogen formed on Ni clusters at a more negative potential. Such different dependences for deposition potentials −1.0 and −1.1 V indicate that to obtain a lower detection limit following the deposition of nickel from acidic solutions a deposition potential − 1.0 V should be chosen. However, in such a case surfactants can have a greater effect on the nickel signal. The effect of deposition time in procedure exploiting the deposition of nickel from alkaline solutions was studied for deposition potential −1.35 V and Ni(II) concentrations 1×10 − 8 and 1× 10 − 7 mol l − 1, and it was found that the nickel peak increases linearily with the deposition time to 900 and 180 s, respectively.
3.1. Deposition potential 3.3. Oxidation potential and oxidation time According to literature data [30], to overcome the interference from surface active substances on the deposition step a sufficiently negative deposition potential should be used. For deposition from acidic solutions deposition potentials of − 1.0 and −1.1 V were chosen, because at a more negative potential the efficiency of nickel deposition decreases, probably due to a simultaneous reduction of the hydrogen ions on Ni clusters formed onto mercury surface. For deposition from alkaline solutions deposition potentials − 1.35 and − 1.65
For oxidation of nickel the solution 0.2 mol l − 1 NH4Cl+ NH4OH (pH = 9.0) containing 1× 10 − 4 mol l − 1 DMG and potential of −0.075 V were chosen. At such a potential not only metallic nickel but also nickel in the form of intermetallic compound such as Ni–Cu [34], formed after deposition of nickel from real samples, should be oxidized fast. The oxidation time of 45 s was chosen to reduce a possible interference connected with the formation of intermetallic compounds.
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3.4. Calibration graph, precision and detection limit The calibration graph for procedure exploiting the deposition from acidic solution at potential −1.1 V for 30 s was linear from 5×10 − 9 to 1× 10 − 7 mol l − 1 and obeyed equation y= 104 x +144, where y and x are the peak current (nA) and Ni(II) concentration (nmol l − 1), respectively. The linear correlation coefficient r =0.998. The relative standard deviations (RSDs) from five determinations were 8.1 and 6.2% for Ni(II) concentrations 5×10 − 9 and 1 ×10 − 7 mol l − 1, respectively. The limit of detection estimated from (3s) for low concentration of Ni(II) was about 2× 10 − 9 mol l − 1. The voltammograms obtained in the course of Ni(II) determination following the deposition of nickel from the acidic solution are presented in Fig. 1. The calibration graph for procedure exploiting the deposition from alkaline solution at potential −1.35 V for 120 s was linear from 1×10 − 9 to 1×10 − 7 mol l − 1 and obeyed equation y= 584 x +68 (r =0.9997). The RSD’s from five Ni(II) determinations at concentrations 1×10 − 9 and 2× 10 − 8 mol l − 1 were 7.1 and 5.8%, respectively. The detection limit for deposition time of 120 s was 2 × 10 − 10 mol l − 1 and can be further lowered by prolongation of the deposition time.
3.5. Interferences The influence of foreign ions on the determination of Ni(II) was studied using a fixed concentration of Ni(II), 1× 10 − 7 and 2×10 − 8 mol l − 1 for procedures exploiting the deposition from acidic and alkaline solutions, respectively. The results obtained are presented in Table 1. The results show that foreign ions at concentrations typically present in natural water samples should not influence the signal for Ni(II). The decrease or lack of the Ni(II) signal was observed only in the presence of a large excess of Cu(II) or Pb(II), probably as a result of the formation of intermetallic compound [34] which is not oxidized at potential − 0.075 V, used for oxidation of nickel in the procedure proposed. The influence of various surfactants on the Ni(II) signal was studied for all
three procedures proposed and for the conventional procedure exploiting the adsorption of the Ni(DMG)2 complex from a solution 0.2 mol l − 1 NH4Cl+ NH4OH (pH = 9.0)+1× 10 − 4 mol l − 1, used as a supporting electrolyte. The results obtained are presented in Fig. 2. The results show that in all the procedures proposed the influence of surface active substances on the determination of Ni(II) is significantly lower than in the case of the conventional procedure. For example, interference of Triton X-100 in conventional procedure is observed, starting from its concentration of 0.1 mg l − 1, while in procedure exploiting deposition of nickel from alkaline solution at − 1.65 V the interference is not observed even at Triton X-100 concentration 100 mg l − 1. Smaller differences in relative signals for all procedures used were observed when humic acid was added to the analysed solution. Such influence of humic acid is not connected probably with surface active properties of these compounds but rather with their complexing abilities. The influence of surfactants was also lower as compared to the procedure exploiting addition of fumed silica to remove surfactants from the sample [27,28] or the procedure exploiting microdialysis sampling [29]. The presence of Triton X-100 in the sample solution causes additionally a reduction of the background current when the proposed procedures are used. Such a positive effect of Triton X-100 has been reported in literature before [17] when the measurements were carried out in the presence of dissolved oxygen as in this work. For studying the effect of complexing agents on the Ni(II) peak EDTA was used. The results obtained are presented in Fig. 3. The results show that determination of Ni(II) following the deposition of nickel from the acidic solution is not influenced by EDTA up to concentration of 5 ×10 − 4 mol l − 1. This fact indicates that the procedure can be used for direct determination of total nickel. The conditions used in this procedure can be compared to those used for total Pb(II), Cu(II) and Cd(II) determinations using 0.1 mol l − 1 HCl as the supporting electrolyte [35,36]. It must be noted, however, that total Ni(II) determined by the proposed procedure can sometimes differ from that determined after a digestion of the sample, be-
M. Korolczuk / Talanta 53 (2000) 679–686
cause the complexing ligands present in natural water samples can form more stable complexes than EDTA used as a modelling complexing substance in this work. To obtain total concentration of nickel in the sample following the conventional procedure the organics in the sample should be destroyed using time consuming mineralization procedures, usually UV irradiation. When deposition was carried out from an alkaline solution, the addition of EDTA caused a decrease of the nickel peak (Fig. 3), as in the case of the conventional procedure. Small differences in relative signals for procedures exploiting deposition from alkaline solutions and the conventional procedure are probably connected with different concentrations of the supporting electrolyte and DMG used. A decrease of the nickel peak in the presence of EDTA is in accordance with the literature data. The conventional procedure of direct determination of nickel as well as the procedures exploiting the deposition of nickel from alkaline solutions allow for determination of labile nickel. The main advantage of the procedures exploiting the deposition from alkaline solutions is the possibility of direct labile nickel determination in the presence of surfactants.
3.6. Analyses of water samples A river water sample, filtered using 0.45 mm Milipore membrane filter, and a nonfiltered tap water sample were analysed following the deposition from the acidic and alkaline solution according to the procedures proposed and additionally using the conventional procedure. The results obtained are presented in Table 2. The results of total and labile nickel determinations according to the proposed procedures and the conventional procedure agree well. The proposed procedures were also applied for Ni(II) determinations in a synthetic sea water sample, prepared according to [37]. The sample was spiked with 5 × 10 − 8 mol l − 1 Ni(II) and analysed using the standard additions method. The recoveries for Ni(II) ranged between 96 and 103%, so the proposed procedures can be applied also for analyses of Ni(II) in saline water samples.
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4. Conclusion All the proposed voltammetric procedures allow for direct Ni(II) determinations in the presence of surfactants. The procedure exploiting the deposition of nickel from the acidic solution allows for total Ni(II) determinations, because the complexing agents such as EDTA do not influence the nickel signal. Out of the procedures exploiting the deposition of nickel from alkaline solutions, the procedure in which the deposition potential of − 1.35 V was used is preferable, because a lower interference effect from foreign ions and humic acid was observed. The procedure allows for labile nickel determination in the presence of higher concentrations of surfactants than are usually present in natural water samples.
References [1] J. Golimowski, H.W. Nu¨rnberg, P. Valenta, Lebensmittelchem. Gerichtl. Chem. 34 (1980) 116. [2] B. Pihlar, P. Valenta, H.W. Nu¨rnberg, Z. Fresenius, Anal. Chem. 307 (1981) 337. [3] J. Golimowski, M. Cendrowska, Chem. Anal. 30 (1985) 777. [4] C. Locatelli, F. Fagioli, T. Garai, Anal. Chem. 63 (1991) 1409. [5] S.B. Adeloju, A.M. Bond, M.H. Briggs, Anal. Chim. Acta. 164 (1984) 181. [6] P. Ostapczuk, M. Goedde, M. Stoeppler, H.W. Nu¨rnberg, Z. Fresenius, Anal. Chem. 317 (1984) 252. [7] H. Zhang, R. Wollast, J.C. Vire, G.J. Patriarche, Analyst 114 (1989) 1597. [8] S. Sanllorente, M.C. Oritiz, M.J. Arcos, Analyst 123 (1998) 513. [9] J. Perez-Pena, J.J. Hernandez-Brito, J.A. Herrera-Melian, C. Callado-Sanchez, C.M.G. van den Berg, Electroanalysis 6 (1994) 1069. [10] M.M. Palrecha, Fresenius J. Anal. Chem. 351 (1995) 800. [11] J. Golimowski, M. Lisiewicz, Szynkarczuk, Chem. Anal. 43 (1998) 829. [12] J.R. Donat, K.W. Bruland, Anal. Chem. 60 (1988) 240. [13] H. Braun, M. Metzger, Z. Fresenius, Anal. Chem. 318 (1984) 321. [14] F. Wahdat, R. Neeb, Z. Fresenius, Anal. Chem. 320 (1985) 334. [15] Ch.M.A. Brett, A.M.C.F.O. Brett, J.L.C. Pereira, Electroanalysis 3 (1991) 683. [16] A. Economou, P.R. Fielden, Analyst 118 (1993) 47. [17] A. Economou, P.R. Fielden, Anal. Chim. Acta. 273 (1993) 27.
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[18] A. Economou, P.R. Fielden, Talanta 46 (1998) 1137. [19] S.B. Adeloju, A. Hadjichari, Anal. Sci. 15 (1999) 95. [20] J. Wang, J. Wang, W.K. Adeniyi, S.P. Kounaves, Electroanalysis 12 (2000) 44. [21] R.P. Baldwin, J.K. Christensen, L. Kryger, Anal. Chem. 58 (1986) 1790. [22] B. Chen, R. Deen, G.N. Khang, C.H.L. Sai, L. Kryger, Talanta 49 (1999) 651. [23] E.P. Achterberg, C.M.G. van den Berg, Anal. Chim. Acta. 284 (1994) 463. [24] C.J. Flora, E. Nieboer, Anal. Chem. 52 (1980) 1013. [25] B. Pihlar, P. Valenta, H.W. Nu¨rnberg, J. Electroanal. Chem. 214 (1986) 157. [26] W. Jin, K. Liu, J. Electroanal. Chem. 216 (1987) 181. [27] W. Kubiak, J. Wang, J. Electroanal. Chem. 258 (1989) 41.
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Voltammetric method for direct determination of nickel in natural waters in the presence of surfactants Mieczyslaw Korolczuk * Faculty of Chemistry, Maria Curie Sklodowska Uni6ersity, 20 -031 Lublin, Poland Received 25 April 2000; received in revised form 4 August 2000; accepted 11 August 2000
Abstract Voltammetric procedures for direct Ni(II) determination in natural water samples are described. The procedures are based on nickel deposition to the metallic state and then its oxidation in the presence of dimethylglyoxime with the formation of the complex adsorbed on the electrode. Reduction of the complex is exploited in the detection step. Due to the application of a sufficiently negative deposition potential the interference from surfactants is minimized. The detection limits for Ni(II) are 2 ×10 − 9 and 2 ×10 − 10 mol l − 1 for deposition times of 30 and 120 s, respectively. The influence of foreign ions is also presented. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Nickel determination; Voltammetry; Natural water; Surfactants
1. Introduction Adsorptive stripping voltammetry (AdSV) is a suitable technique for determination of traces of nickel and cobalt in environmental samples because of its high sensitivity and low cost of instrumentation used. Many procedures have been proposed for nickel determination using HMDE [1–12], mercury film [13 – 20] and chemically modified electrodes [21,22]. An automated voltammetric system for determination of Ni(II) also has been reported [23]. Although different complexing agents have been employed for accu-
* Fax: + 48-81-5333348. E-mail address: [email protected] (M. Korolczuk).
mulation of nickel and cobalt, dimethylglyoxime (DMG) has gained wider use. The mechanism of the reduction of Ni(II) in the presence of DMG has also been investigated [24–26]. In most papers it was stated that the surface active compounds cause a decrease or decay of the nickel peak [2,16,17,19,27–29], so efficient decomposition methods of natural samples must be applied before determination. Decomposition methods are time consuming, so various approaches have been proposed to overcome interference from surface active substances in Ni(II) determination, for example, an addition to the sample of fumed silica [27,28], coverage of the electrode by Nafion film [28] or using the renewable-reagent adsorptive stripping sensor [29]. These approaches led to higher detection limit [28,29] or lower precision of determinations [28]. Additionally, the application
0039-9140/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0039-9140(00)00552-X
M. Korolczuk / Talanta 53 (2000) 679–686
680
of natural water samples without pretreatment and reasonable reduction in the sensitivity. In the method proposed nickel was deposited to the metallic state and then was oxidized to Ni(II) in the presence of DMG with formation of the complex adsorbed on the electrode. Reduction of the complex is used in the detection step as in the case of direct accumulation of Ni(DMG)2 complex by adsorption [1–29]. Due to deposition of nickel at a relatively negative potential according to literature data [30], the interference from surface active compounds on the deposition step was minimized. Fig. 1. Square wave voltammograms obtained for various concentrations of Ni(II) following the procedure exploiting the deposition of nickel from 0.2 mol l − 1 HCl + 0.2 mol l − 1 KSCN. Added Ni(II) concentration: (a) 0; (b) 5 ×10 − 9; (c) 2 × 10 − 8; (d) 5×10 − 8 mol l − 1. Deposition at − 1.1 V for 30 s.
of silica in flowing systems can cause a blockage of tubing and valves. The purpose of this paper is to report a sensitive voltammetric method for nickel determination in the flow system which allows the analysis
2. Experimental
2.1. Apparatus The measurements were performed using Autolab PGSTAT10 analyser (Netherlands) and controlled growth static mercury drop electrode in the HMDE mode, made by MTM Cracow, Poland. A three-electrode flow cell consisting of an Hg electrode, a Pt electrode and an Ag/AgCl reference electrode as described before [31–33]
Table 1 Relative signals for Ni(II) in the presence and absence of foreign ionsa Ion
Fe(III) Zn(II) Co(II) Cr(III)
Mn(II) Cd(II) Cu(II) Pb(II)
Fold excess
100 1000 100 1000 10 100 10 100 1000 1000 1000 100 1000 100 1000
Deposition from Acidic solution at −1.1 V for 30 s
Alkaline solution at −1.35 V for 120s
Alkaline solution at −1.65 V for 120 s
1.05 0.97 1.04 0.53 1.04 0.72 1.0 0.95 0.97 1.03 1.03 0.96 0.51 0.97 0.32
0.98 0.85 0.97 1.01 1.03 0.35 1.04 0.99 1.02 1.01 0.98 1.03 0.64 1.04 0
1.03 0.65 0.98 1.03 1.02 0.33 1.05 drop falls – 1.04 1.01 0.96 0.61 1.03 0
a Ni(II) concentrations were 1×10−7 and 2×10−8 mol l−1 of Ni(II) for deposition from acidic and alkaline solutions, respectively.
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Fig. 2. Relative signals for Ni(II) in the presence and absence of synthetic and natural surfactants for procedures exploiting in the accumulation step: (a) adsorption of Ni(DMG)2 complex from 0.2 mol l − 1 ammonium buffer +1 × 10 − 4 mol l − 1 DMG + 2× 10 − 8 mol l − 1 Ni(II) for 30 s; (b) deposition of metallic Ni from 0.2 mol l − 1 HCl+ 0.2 mol l − 1 KSCN+ 1 ×10 − 7 mol l − 1 Ni(II) for 30 s; (c) deposition of metallic Ni from ammonium buffer containing 5 ×10 − 4 mol l − 1 DMG +2 ×10 − 8 mol l − 1 Ni(II) at potential −1.35 V for 120 s; (d) as (c) but deposition potential was changed to −1.65 V. Added surfactants: (A) Triton X-100; (B) sodium dodecylsulfate; (C) cetyltrimethylammonium bromide; (D) humic acid.
was used. The volume of the cell was about 0.5 ml. The surface area of mercury drop was 0.85 mm2. The solutions were delivered using a peristaltic pump PPa-12A (Emko, Poland). The flow rates of each solution were about 16 ml min − 1. In comparative determinations of Ni(II) UV digester (Mineral, Poland) was used for the destruction of organics present in water samples.
2.2. Reagents Ammonium buffer (2.0 mol l − 1; pH = 9.0) was prepared from Suprapure HCl and Tracepur ammonium hydroxide (Merck). Dimethylglyoxime (DMG; 0.1 mol l − 1 ) was prepared by dissolving an appropriate quantity of reagent (POCh, Poland) in 96% ethanol. Standard solution of
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682
Fig. 3. Relative signals for Ni(II) in the presence and absence of EDTA using various procedures of nickel determination described in legends for Fig. 2.
nickel (1g l − 1; Fluka) was used. KSCN solution (2 mol l − 1) was prepared from pure for analysis reagent (Ubichem, England) and then purified by shaking with Zn powder. Humic acid (sodium salt) was obtained from Aldrich. Other reagents were obtained from POCH, Poland, and used as received. All solutions were made using triply distilled water.
2.3. Standard procedures Two analytical procedures were used depending on the acidity of the solution used in the deposition step. Both procedures differed in the composition of the solutions used for deposition of nickel and
for washing of the cell, and the value of the potential used in the deposition and washing steps. For deposition from the acidic medium a solution 0.2 mol l − 1 KSCN+0.2 mol l − 1 HCl was used as a supporting electrolyte. In this procedure a solution 0.005 mol l − 1 KSCN+0.005 mol l − 1 ammonium buffer (pH= 9.0) containing 1× 10 − 4 mol l − 1 DMG was used as a washing one. In the procedure exploiting the deposition from the alkaline medium nickel was deposited from 0.02 mol l − 1 ammonium buffer containing 5× 10 − 4 mol l − 1 DMG, while a solution 0.01 mol l − 1 ammonium buffer+ 1× 10 − 4 mol l − 1 DMG was used as a washing one. The procedures were performed according to the following scheme. 1. A flow of the washing solution was directed to the cell. 2. A new mercury drop was formed, the stirrer was switched on and the deposition potential was applied. Thereafter the flow of the sample solution with the added supporting electrolyte was directed to the cell and the deposition of nickel to the metallic state was carried out for a fixed time. 3. A flow of the washing solution was started for 90 s and the substances present in the sample, which could interfere in the detection step were removed from the cell. Five seconds before the end of this step the stirrer was switched off. During this step the potential of the electrode was the same as in the deposition step. 4. The flow of the solution 0.2 mol l − 1 ammonium buffer and 1×10 − 4 mol l − 1 DMG was
Table 2 Results of Ni(II) determinations (in nmol l−1) in tap and river water samples using various accumulation proceduresa Accumulation procedure
Sample
Tap water Czecho´wka river water a
Adsorption of Ni(DMG)2 complex at −0.7 V from:
Deposition of nickel from:
Untreated sample
UV-irradiated
Acidic solution
Alkaline solution
sample
at −1.1 V
at −1.35 V
at −1.65 V
6.6 (4.0) 11.6 (3.4)
6.3 (11) 12.0 (7.6)
6.0 (4.2) 7.8 (5.2)
5.8 (4.7) 7.9 (3.9)
6.3 (3.9) 8.2 (3.1)
Figures in parentheses are the relative standard deviations in% from three measurements.
M. Korolczuk / Talanta 53 (2000) 679–686
directed to the cell for 45 s. Then the potential was changed to − 0.075 V and nickel was oxidized for 45 s. The complex of Ni(II) with DMG was formed and it was adsorbed on the electrode. Then after a rest period of 5 s at potential − 0.75 V a voltammogram was recorded using a cathodic square wave voltammetric scan (− 0.75 to − 1.2 V, frequency 1 kHz, pulse height 50 mV, scan increment 4 mV). 5. After the measurement the flow of the washing solution was started for at least 30 s before the next measurement. All solutions used were not deaerated.
3. Results and discussion To minimize the interference from organic substances present in natural water samples on voltammetric determination of nickel, a several-step procedure in the flow system was proposed. The procedure consists of three main steps: deposition of the metallic Ni on the mercury electrode; oxidation of Ni0 to Ni(II) in the presence of DMG with formation of the complex adsorbed on the electrode; reduction of Ni – DMG complex in ammonium buffer. For deposition of nickel two electrolytes were used: acidic and alkaline. The last step had already been described in detail in papers [e.g. 2,5] so the optimization of the overal procedure was directed to the first two steps.
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V were chosen. Using the deposition potential − 1.65 V, foreign ions can have greater effect on Ni(II) signal, however, at such a negative potential the interference of surfactants should be completely eliminated.
3.2. Deposition time For procedure exploiting the deposition from acidic solution the effect of the deposition time was studied for deposition potentials − 1.0 and −1.1 V. It was found that the nickel peak increases linearily with the deposition time only to 30 s, when the deposition potential of − 1.1 V was chosen. Prolongation of the deposition time from 40 to 120 s does not influence the nickel peak. However, when the deposition potential was changed to − 1.0 V, the nickel peak increased linearily up to 90 s, and then increased nonlinearily up to 240 s. Such dependences can be explained by blockage of the electrode by hydrogen formed on Ni clusters at a more negative potential. Such different dependences for deposition potentials −1.0 and −1.1 V indicate that to obtain a lower detection limit following the deposition of nickel from acidic solutions a deposition potential − 1.0 V should be chosen. However, in such a case surfactants can have a greater effect on the nickel signal. The effect of deposition time in procedure exploiting the deposition of nickel from alkaline solutions was studied for deposition potential −1.35 V and Ni(II) concentrations 1×10 − 8 and 1× 10 − 7 mol l − 1, and it was found that the nickel peak increases linearily with the deposition time to 900 and 180 s, respectively.
3.1. Deposition potential 3.3. Oxidation potential and oxidation time According to literature data [30], to overcome the interference from surface active substances on the deposition step a sufficiently negative deposition potential should be used. For deposition from acidic solutions deposition potentials of − 1.0 and −1.1 V were chosen, because at a more negative potential the efficiency of nickel deposition decreases, probably due to a simultaneous reduction of the hydrogen ions on Ni clusters formed onto mercury surface. For deposition from alkaline solutions deposition potentials − 1.35 and − 1.65
For oxidation of nickel the solution 0.2 mol l − 1 NH4Cl+ NH4OH (pH = 9.0) containing 1× 10 − 4 mol l − 1 DMG and potential of −0.075 V were chosen. At such a potential not only metallic nickel but also nickel in the form of intermetallic compound such as Ni–Cu [34], formed after deposition of nickel from real samples, should be oxidized fast. The oxidation time of 45 s was chosen to reduce a possible interference connected with the formation of intermetallic compounds.
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3.4. Calibration graph, precision and detection limit The calibration graph for procedure exploiting the deposition from acidic solution at potential −1.1 V for 30 s was linear from 5×10 − 9 to 1× 10 − 7 mol l − 1 and obeyed equation y= 104 x +144, where y and x are the peak current (nA) and Ni(II) concentration (nmol l − 1), respectively. The linear correlation coefficient r =0.998. The relative standard deviations (RSDs) from five determinations were 8.1 and 6.2% for Ni(II) concentrations 5×10 − 9 and 1 ×10 − 7 mol l − 1, respectively. The limit of detection estimated from (3s) for low concentration of Ni(II) was about 2× 10 − 9 mol l − 1. The voltammograms obtained in the course of Ni(II) determination following the deposition of nickel from the acidic solution are presented in Fig. 1. The calibration graph for procedure exploiting the deposition from alkaline solution at potential −1.35 V for 120 s was linear from 1×10 − 9 to 1×10 − 7 mol l − 1 and obeyed equation y= 584 x +68 (r =0.9997). The RSD’s from five Ni(II) determinations at concentrations 1×10 − 9 and 2× 10 − 8 mol l − 1 were 7.1 and 5.8%, respectively. The detection limit for deposition time of 120 s was 2 × 10 − 10 mol l − 1 and can be further lowered by prolongation of the deposition time.
3.5. Interferences The influence of foreign ions on the determination of Ni(II) was studied using a fixed concentration of Ni(II), 1× 10 − 7 and 2×10 − 8 mol l − 1 for procedures exploiting the deposition from acidic and alkaline solutions, respectively. The results obtained are presented in Table 1. The results show that foreign ions at concentrations typically present in natural water samples should not influence the signal for Ni(II). The decrease or lack of the Ni(II) signal was observed only in the presence of a large excess of Cu(II) or Pb(II), probably as a result of the formation of intermetallic compound [34] which is not oxidized at potential − 0.075 V, used for oxidation of nickel in the procedure proposed. The influence of various surfactants on the Ni(II) signal was studied for all
three procedures proposed and for the conventional procedure exploiting the adsorption of the Ni(DMG)2 complex from a solution 0.2 mol l − 1 NH4Cl+ NH4OH (pH = 9.0)+1× 10 − 4 mol l − 1, used as a supporting electrolyte. The results obtained are presented in Fig. 2. The results show that in all the procedures proposed the influence of surface active substances on the determination of Ni(II) is significantly lower than in the case of the conventional procedure. For example, interference of Triton X-100 in conventional procedure is observed, starting from its concentration of 0.1 mg l − 1, while in procedure exploiting deposition of nickel from alkaline solution at − 1.65 V the interference is not observed even at Triton X-100 concentration 100 mg l − 1. Smaller differences in relative signals for all procedures used were observed when humic acid was added to the analysed solution. Such influence of humic acid is not connected probably with surface active properties of these compounds but rather with their complexing abilities. The influence of surfactants was also lower as compared to the procedure exploiting addition of fumed silica to remove surfactants from the sample [27,28] or the procedure exploiting microdialysis sampling [29]. The presence of Triton X-100 in the sample solution causes additionally a reduction of the background current when the proposed procedures are used. Such a positive effect of Triton X-100 has been reported in literature before [17] when the measurements were carried out in the presence of dissolved oxygen as in this work. For studying the effect of complexing agents on the Ni(II) peak EDTA was used. The results obtained are presented in Fig. 3. The results show that determination of Ni(II) following the deposition of nickel from the acidic solution is not influenced by EDTA up to concentration of 5 ×10 − 4 mol l − 1. This fact indicates that the procedure can be used for direct determination of total nickel. The conditions used in this procedure can be compared to those used for total Pb(II), Cu(II) and Cd(II) determinations using 0.1 mol l − 1 HCl as the supporting electrolyte [35,36]. It must be noted, however, that total Ni(II) determined by the proposed procedure can sometimes differ from that determined after a digestion of the sample, be-
M. Korolczuk / Talanta 53 (2000) 679–686
cause the complexing ligands present in natural water samples can form more stable complexes than EDTA used as a modelling complexing substance in this work. To obtain total concentration of nickel in the sample following the conventional procedure the organics in the sample should be destroyed using time consuming mineralization procedures, usually UV irradiation. When deposition was carried out from an alkaline solution, the addition of EDTA caused a decrease of the nickel peak (Fig. 3), as in the case of the conventional procedure. Small differences in relative signals for procedures exploiting deposition from alkaline solutions and the conventional procedure are probably connected with different concentrations of the supporting electrolyte and DMG used. A decrease of the nickel peak in the presence of EDTA is in accordance with the literature data. The conventional procedure of direct determination of nickel as well as the procedures exploiting the deposition of nickel from alkaline solutions allow for determination of labile nickel. The main advantage of the procedures exploiting the deposition from alkaline solutions is the possibility of direct labile nickel determination in the presence of surfactants.
3.6. Analyses of water samples A river water sample, filtered using 0.45 mm Milipore membrane filter, and a nonfiltered tap water sample were analysed following the deposition from the acidic and alkaline solution according to the procedures proposed and additionally using the conventional procedure. The results obtained are presented in Table 2. The results of total and labile nickel determinations according to the proposed procedures and the conventional procedure agree well. The proposed procedures were also applied for Ni(II) determinations in a synthetic sea water sample, prepared according to [37]. The sample was spiked with 5 × 10 − 8 mol l − 1 Ni(II) and analysed using the standard additions method. The recoveries for Ni(II) ranged between 96 and 103%, so the proposed procedures can be applied also for analyses of Ni(II) in saline water samples.
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4. Conclusion All the proposed voltammetric procedures allow for direct Ni(II) determinations in the presence of surfactants. The procedure exploiting the deposition of nickel from the acidic solution allows for total Ni(II) determinations, because the complexing agents such as EDTA do not influence the nickel signal. Out of the procedures exploiting the deposition of nickel from alkaline solutions, the procedure in which the deposition potential of − 1.35 V was used is preferable, because a lower interference effect from foreign ions and humic acid was observed. The procedure allows for labile nickel determination in the presence of higher concentrations of surfactants than are usually present in natural water samples.
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