Anal. Chem. 1893, 65,3238-3243
3238
Determination of Traces of Nickel(I I) at a Perfluorinated Ionomer/Dimethylglyoxime Mercury Film Electrode Jyh-Myng Zen*and Miao-Ling Lee Department of Chemistry, National Chung-Hsing University, Taichung, Taiwan 400, Republic of China
The feasibility of fabricating nickel-sensitive chemically modified electrodes (CMEs) for practical use in analysis was investigated. This new CME system consisted of a Nafion-coated thin mercury film electrode containing appropriate amounts of dimethylglyoxime (DMG). Since DMG is fairly soluble in ethanol, it is easy to mix with Nafion and form the coating solution. The coating solution was then spin-coated on the surface of a glassy carbon electrode. Subsequently, mercury was plated onto the electrode. The main advantages of the modified electrode are improved resistance to interference from surface-active compounds and better mechanical stability of the mercury film. The CME surface could be regenerated either with exposure to acid or by electrochemical means. The sensitivity increased with preconcentration time. After a 300-s preconcentration period, a blank of 0.1 ppb nickel could be detected. Linear response was observed from 1to 80 ppb. Excess concentration of Al, Cd, Ca, Mn, Pb, Hg, and Bi did not influence the nickel response, but Zn and Cu were found to interfere at a 100-fold excess and Pd was found to interfere at a 10-fold excess. The most serious interference was from cobalt(II), only a 1-fold excess could affect the nickel signal. However, since the reduction peaks were well-separated, the CME could actually be used for simultaneous determination of nickel(I1) and cobalt(I1). Satisfactory accuracy was obtained for nickel(11) in a variety of reference materials. INTRODUCTION In recent years an increasing demand for environmental monitoring has stimulated analytical chemists in their efforts to develop sensitive and selective detection systems. The substances of interest in environmental analysis are mainly heavy metals and organic compounds. Based on a recent report, the necessary detection limits for the most interesting heavy metals-Ni, Cr, As, Cd, and Hg-are less than 10 ppb.1 To achieve such a low detection limit, the electroanalytical stripping procedures provide high sensitivity by analyte accumulation prior to measurement. However, due to the lack of selectivity in the stripping procedures, the utilization of the techniques for practical analysis problems has been extremely limited. Hence, the development of electroanalytical techniques analogous to the stripping approach but possessing more selective analyte collection mechanisms represents an extremely enticing goal. Although the electroanalyticalstripping techniques provide very sensitive routes to the quantitation of many trace metals, ~~~~
~~
~
~
* Author to whom correspondence should be addressed. (1) Niessner, R. Trends Anal. Chem. 1991, 10, 310.
0003-2700/93/0385-3238504.0010
there are difficulties encountered in determining nickel(I1) via anodic stripping voltammetry with electrodeposition. Numerous early attempts have shown that the nickel(I1) reduction is irreversible, requiring a deposition potential of at least -1200 mVvs SCE.2J Furthermore, the reduced nickel shows a strong tendency to form intermetallic compounds with other metallic species co-deposited a t the very negative potentials required. Since this reaction yields complex, matrix-dependent stripping patterns, the stripping procedure is not well suited for the quantitation of nickel(II).3 As a result, alternate preconcentration procedures for nickel(1I) have been suggested in place of the usual reductive deposition of the elemental metal. These include severalstudies in which nickel(I1) is accumulated on a bare mercury or graphite surface, following the addition of dimethylglyoxime (DMG) to the sample solution, as a film of insoluble dimethylglyoximate.4-9 A nickel-sensitivechemically modified electrode (CME) was also fabricated based on DMG-containing carbon paste.10Jl We report here the construction of a new CME system consisting of a Nafion-coated thin mercury film electrode containing appropriate amounts of DMG. Ndion has been used for electrode coatings in a variety of electrochemical studies, mostly in conjunction with the immobilization of positively charged redox couples within the film.12J3 The Ndion polymer is chemically inert, nonelectroactive, hydrophilic, and insoluble in water and, thus, possesses almost ideal properties for the preparation of modified electrodes. Since DMG is fairly soluble in ethanol, it is easy to mix with Nafion to form a coating solution. The coating solution is then spincoated on the surface of a glassy carbon electrode. Subsequently, mercury is plated onto the electrode by the method reported by Hoyer et al.I4 In this way, we can take advantage of three effects: the preconcentration effect, of the ionexchange polymer (Nafion), the selective chelating effect of the chelating agent (DMG), and the accumulation effect of the electrode (Hg). The other advantages of the modified electrode are improved resistance to interference from surfaceactive compounds and better mechanical stability of the mercury film. The electrodes thus developed were found to possess the properties anticipated and to be suitable for (2) Olson, C.; Adams, R. N. Anal. Chim. Acta 1963,29,358. (3) Vydra, F.; Stulik, K.; Julakova, E. Electrochemical Stripping Analysis; Ellis Horwood Limited Sussex,England, 1976. (4) Flora, C. J.; Nieboer, E. Anal. Chem. 1980,52, 1013. (5) Adeloju, S. B.; Bond, A. M.; Briggs, M. H. Anal. Chim. Acta 1984, -IM. - -, 181. - - -. (6) Torrance, K. Analyst 1984,109,1035. (7) Locatelli, C.; Fagioli, F.; Garai, T. Anal. Chem. 1991, 63, 1409. (8)Basu, B. J.; RnjGopalan, S. R. Analyst 1992, 117, 1623. (9) Economou, A.; Fielden, P. R. Analyst 1993,118,47. (10)Baldwin, R. P.; Christensen, J. K.; Kryger, L. Anal. Chem. 1986, 58, 1790.
(11) Thomsen, K. N.; Kryger, L.; Baldwin, R. P. Anal. Chem. 1988,60, 151. (12) Martin, C. R.; Rubinstein, I.;Bard, A. J. J. Am. Chem. SOC. 1982, 104,4817. (13)White, H.S.; Leddy, T.; Bard, A. J. J. Am. Chem. SOC. 1982,104, 4811. (14) Hoyer, B.; Florence, T. M.; Batley, G. E. Anal. Chem. 1987,59, 1608. 0 1993 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993
voltammetric quantitation of traces of nickel(II), which is preconcentrated selectivelyon the CME surface from a variety of complex chemical systems. EXPERIMENTAL SECTION Reagents and Chemicals. Double-distilled deionized water was used to prepare all solutions. Nafion perfluorinated ionexchange powder, 5 wt % solution in a mixture of lower aliphatic alcohols and 10% water, was obtained from Aldrich. 2,3Butanedione dioxime (or commonly called dimethylglyoxime), mercury(I1) nitrate, Triton-X 100,and sodium acetate were of analytical grade. All buffers and supporting electrolyte solutions were prepared from Merck Suprapur reagents and doubledistilled deionized water. Nickel(I1) standard solution (lo00 ppm) and all other standard metal solutionsused in interference studies were obtained from Merck. Apparatus. All square-wavevoltammetry experimentswere performed with a BAS-100Belectrochemicalanalyzer. The threeelectrode system consists of a Nafiion/DMG thin mercury film CME working electrode, a saturated Ag/AgCl reference electrode, and a platinum wire counter electrode. Procedure. The glassy carbon disk electrode (A = 0.0685 cm2, BAS) was first polished with the BAS polishing kit to a shiny surface. It was then rinsed with deionizedwater and further clean ultrasonically in 1:l nitric acid and deionized water successively. Unless otherwise stated, Nafiion/DMG coating solutions were spin-coated on the glassy carbon electrode surface at 2800 rpm. The total volume of the coating solution pipeted onto the electrode surface was always 4 pQ there were different ratios between Ndion and DMG. A uniform thin film was formed after about 5 min of spinning. The performance of the electrode was optimized by improving the coating solutionsin three stages: (1)2 mL of Nafiion + 1 mL of DMG (0.01-2 cut % in ethanol). Note that DMG does not dissolve in ethanol to greater than 2 wt % . The best condition in the first stage was then used in the second stage. (2)2.2-1 mL of Nafion + 1mL of DMG (2wt % in ethanol). The best condition in the second stage was further optimized by changing the film thickness in the third stage. (3) 1.7 mL of Nafiion + 1 mL of DMG (2 wt % in ethanol) (spin-coating at 2800-1OOO rpm). Mercury was deposited onto the glassy carbon/(Ndion,DMG) substrate by adding 5 X l W M mercury(I1) nitrate to the supporting electrolyte medium at -800mV vs Ag/AgCl for 6 min as described earlier.” The supporting electrolyte medium contains 0.07 M acetate buffer (pH 4.0) prepared from sodium acetate and nitric acid. Unless otherwise stated, a medium containing pH 9.0 ammonia-ammonium chloride buffer was used in the electrochemical experiments. Solutionsand sampleswere deaerated with nitrogen for at least 5 min prior to the measurements. Underground water was collected from the campus of ChungHsing University located about 5 km from several electroplating facilities. fn order to fit into the linear detection range, squarewave analysis of the sample consisted of first diluting 2.5 mL of underground water with pH 9.0 ammonia-ammonium chloride buffer and then adjusting the pH to 9.0 with drops of 1M NH4OH and 1M HC1 to a final volume of 20 mL. The underground water used for measurement was thus diluted by a factor of 8. Electroplatingwaste solution was first acidified to pH < 2.0 with an appropriate amount of H2SO4 and then filtered with a 0.45pm sterilized membrane. Following the same diluting procedure as mentioned above, the diluting factor of electroplatingwaste solution used for measurement was 500.
9239
on the CME by cyclic voltammetric experiments. The absence of a peak in the anodic branch of the cyclic voltammograms indicates that the reduction of the complex is an irreversible process. When cycling was done between -600 and -1100 mV vs Ag/AgCl, the voltammograms remained essentially unchanged, except that the NiA2 reduction signal decreased considerably from cycle to cycle. This indicates that the reduction of the NiAz is exhaustive during the preceding scan. Since the signal was found to increase with nickel(I1) concentration and chemical deposition time, it was attributed to the following reaction: NiA,
+ 2H+ + 2e-
-
Ni + 2HA
The electrochemical behavior of nickel(I1) on the CME is similar to most of the other studies.”” Optimization of t h e NiAz Reduction Signal. In order to arrive a t the optimum conditions for nickel(I1) determination, there are two aspects that should be considered the electrode and the detection. As to the electrode aspect, the principal factors governing the performance of the Ndion/ DMG thin mercury film CME are the concentration of DMG in the film, the thickness of the Nafion/DMG film, and the deposition of mercury. As to the detection aspect, the factors consist of the solution pH, the preconcentration time, the preconcentration potential, and the square-wave parameters. Electrode Aspect. Effect of DMG Concentration. CMEs prepared from coating solutions that contain 2 mL of Nafion 1 mL of DMG (0.01, 0.1, 1 , and 2 wt % in ethanol) were examined under identical conditions. In all cases, nickel responses could be obtained and the peak current increased as the content of DMG in the coating solution increased. Apparently, the chelating ability of DMG with nickel(I1) functions properly in the CME. The equilibrium process of the above electrodes was further studied by the following experiment. Peak currents of nickel(I1) were measured a t the CMEs by successive determination every 30 s as shown in Figure 1. Due to the gradual increasing [Ni+2]in the film through exchange between Ndion and nickel(I1)and through chelating between DMG and nickel(II), the observed peak currents are expected to increase with time. On the other hand, since the NiA2 reduction process is irreversible, some of the NiA2 complexes are reduced under successive determinations, and the observed peak currents are expected to decrease with time. Therefore, the occurrence of amaximum peak current is actually a compromise result of these two effects. Beyond the maximum, since almost all of the DMG sites in the film are occupied, only the latter process should be important. The peak currents therefore decrease. Most important of all, the time required to reach a maximum peak current increasesas the content of DMG in the coating solution increases. This phenomenon is apparently once again a good indication of the proper chelating process between DMG and nickel(I1). Unfortunately, since DMG is hard to dissolve in ethanol to greater than 2 w t % ,the condition was further optimized in the second stage by varying the amount of Ndion in the coating solutions as follows: 2.2-1 mL of Nafion + 1mL of DMG (2 wt% in ethanol). As shown in Figure 2, the results indicate that the optimum coating solution is 1.7 mL of Ndion R E S U L T S AND DISCUSSION + 1mL of DMG (2 w t % in ethanol). Electrodes prepared by the above combination of coating solution were therefore Electrochemical Behavior of Nickel(I1) on t h e CME. used in all subsequent work. The formation of the NiA2(HA = DMG) complex was clearly observed under the mildly basic conditions. The observed Effect of Film Thickness. The thickness of the Ndion/ peak potential of -1.01 V vs Ag/AgCl for the Ndion/DMG DMG film directly controls the amount of DMG in the film and hence the electrode performance. Nevertheless, the thin mercury f i i CME was similar to that previously reported for the reduction of NiAz adsorbed onto m e r ~ u r y . 4Nu~ ~ ~ ~ optimum film thickness apparently depends on both the diffusion process of NiA2 in the film and the maximum NiA2 merous prior works have shown that the nickel(I1) reduction loading that does not affect the adhesion of the film to the is irreversible.2ea The same phenomenon was also observed
+
ANALYTICAL CHEMISTRY, VOL. 05, NO. 22, NOVEMBER 15, 1993
3240
I
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0
2
1
3
5
4
8
7
1000
1500
2000
3000
2500
r.p.m.
Time (min)
Figure 3. Effect of fllm thickness to the peak current of nlckeI(I1) determlnatlon obtained at the CME. Conditions are the same as in Figure 2.
86-
86-
D
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,Y
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4-
L
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I -, L 1 - - l d .- l -
0
2
4
6
8
10
12
14
16
18
20
22
Deposition Time (min) Flgure 4. Effect of mercury deposition time to the peak current of nlckeI(1I ) determinationobtainedat the CME. Conditionsare the same as In Figure 2.
T
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v 0
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.
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NafiodDMG ( v h ) Flgure 2. Effect of the coating solution In preparing the CME-voiume ratios between Naflon and DMQ (2 wt % In ethanolpto the peak current of nlckel(I1)determlnation. [N12+]= 0.1 ppm, preconcentration time (6) = 3 mln, and preconcentratlon potential @p) = -0.6 V vs Ag/AgCI.
glassy carbon surface. The film thickness was varied by preparing the electrodes with the optimum coating solution, 1.7 mL of Nafion + 1 mL of DMG (2 w t % in ethanol), at differentspin-coatingrates. The results of the thickness effect to the electrode performance are shown in Figure 3. As can be seen, the peak current levels off around 2200 rpm and starts to decrease rapidly when the spin-coating rate is lower than 2000 rpm. Since the film thickness increases as the spin-coating rate decreases, this phenomenon might be explained by the films gradually losing their adhesion to the glassy carbon surface due to the overloading of NiAz. Electrodes prepared with the optimum coating solution at
the 2800 rpm spin-coating rate were therefore used in all subsequent work. Effect of Mercury Deposition. Mercury electrodeposited onto the glassy carbon/(Nafion, DMG) substrate from mercury(I1)-containingsolutionscould clearly be seen as agrayish deposit formed. The amount of mercury plated onto the CME depends on the deposition time. The results of mercury deposition on the electrode performance are shown in Figure 4. As can be seen,the peak current increases as the deposition time increases with a linear range between 2 and 10 min and reaches a maximum after 15 min. It decreases rapidly when the deposition time is longer than 16 min. It was reported earlier that the reduction of the mercuric ions and the growth of the mercury phase might take place at the glassy carbon/ Nafion interface.14 On the other hand, there is still a possibility that the mercury may fill up the pores of the Nafion/DMG coating and extend beyond it. We do have a reproducibility problem for electrodes prepared with a mercury deposition time of longer than 8 min. This may have something to do with the process discussed above. Therefore, a deposition time of 6 min, which lies in the linear range, was used in all subsequent work. Detection Aspect. Effect of pH. In saturated DMG solutions buffered to a pH value between 7 and 11,a total nickel concentration larger than about 0.5 pM w i l l cause NiAz to precipitate. Below pH 7,the solubility increases to such an extent that, at pH 4,even millimolar nickel(I1) can exist in solution. The pH of the preconcentration solution was found to exert a significant but largely predictable effect on the CME process. For the CME, the optimum conditions for nickel(I1) accumulation were observed when the sample was
ANALYTICAL CNMISTRY, VOL. 85, NO. 22, NOVEMBER 15, 1993
8241
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9
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0
12
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PH Flgurr 5. Effect of pH on the peak current of nlckei(I1). Conditions are the same as in Figure 2, except $, = 5 min.
8
10
12
14
Preconcentration Time (min)
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slightly basic, pH 9-11, as shown in Figure 5. The pH region was slightly different from the reported pH 8-10 for the DMGcontaining carbon paste electrode.10 This change is because the SOs- sites of Nafion can attract protons and hold them inside the polymer matrix, an effect which is apparently equal to lowering the pH of the solution. Effect of Preconcentration Timeand Potential. The effect of preconcentration potential on the square-wave response for nickel(I1) is shown in Figure 6. As can be seen, between 0 and -0.7 V vs AgIAgCl, the peak current increases as the potential of the electrode becomes more negative. This behavior is explained by the fact that nickel(1D bears a positive charge; as a result, the adsorption of nickel(I1) is favored at more negative potentials. However, the peak current drops rapidly as the potential is more negative than -0.7 V. One possibility is that as the preconcentration potential moves closer to the redox potential of the NiAz couples, more of the deposited NiAz was reduced, causing a decrease in peak current. A preconcentrationpotential of -0.6V was therefore chosen in all subsequent work. The effect of preconcentration time on the square-wave response for nickel(I1) is shown in Figure 7. For higher concentration (10ppb) of nickel(II), the peak current increases as the preconcentration time increases and starte to level off around 6 min. For a lower concentration (0.1 ppb) of nickel(111, it takes about 15 min for the peak current to level off. This phenomenon is as expected and further confirms the chelating process between DMG and nickel(I1) in the CME. Apparently,since electrodes were prepared at 2800 rpm spincoating rate with 4 pL of the optimum coating solution, which is composed of 1.7 mL of Ndion + 1mL of DMG (2 w t 5%
10
15
20
25
30
Preconcentration Time (min)
0.2
Preconcentration Potential (volt) Flgurr 6. Effect of preconcentratlon potential to the peak current of nickel(I1)determlnatlon obtained at the CME. [NI”] = 0.01 ppm, $, = 5 mln, and & = -0.8 V vs Ag/AgCI.
5
Flgwr 7. Effect of prmcentrationtlmetothe peak current of nick& (11) determination obtalned at the CME. [Nil+]: (A) 10 ppb; (B) 0.1 ppb, A, = -0.8 V VS Ag/AgCi.
in ethanol), the amount of DMG on the CME was actually fixed. Therefore, in order to chelate with all the DMG sites available, a longer time is needed for the lower concentration of nickel(11). Effect of Square- Wave Parameters. The square-wave parameters that were investigated were the frequency, the pulse height, and the pulse increment. These parameters are interrelated and have a combined effect on the response, but here only the general trends will be examined. The response for Ni increases with square-wave frequency, but at frequencies higher than 100-Hz sloping background current, renders the measurement difficult. To achieve higher frequencies, the pulse width is shortened. As a result, the measurement is taken at a time when the capacitive current is still significant and contributes to the measured reaponse.OJ6 Increase in the pulse height causes an increase in the Ni peak up to 25 mV and similar behavior was observed in earlier studies.OJ6 The peak potential shifta to the positive direction with increasing frequency. The scan increment (SI) together with the frequency cf, define an effective scan rate (v) according to the relationship: v = SIf hence an increase of either the frequency or the scan increment results in an increase in the effective scan rate. By fixing the frequency as 80 Hz,the effect of scan increment was studied. At scan incrementa greater than 10 mV, too few points are sampled and the peaks are slightlydistorted, whereas at small scan incrementa (1-2 mV) the response is more accurately recorded, but higher frequency noise is also present. Since the reduction of NiAz is an irreversible process, an increase ~~~
(16) Oetapczuk, 1986,214, 51.
P.;Valenta, P.;”berg,
H.J. Electround. Chem.
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ANALYTICAL CHEMISTRY, VOL. 65. NO. 22, NOVEMBER 15, 1093
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0 1000E-05
ranging from 0.1 to 100 ppb. The nickel(I1) concentrations calculated from the observed signals and the slope of the regression line were in good agreement with the experimental concentrations,thus indicating essentiallyconstant sensitivity from 1to80ppbnickel(II). Thesensitivitystartedtodecrease when the concentration of nickel(I1) was higher than 80 ppb. The detection limit is defined as the concentration of the analyte resulting in a signalthree times the standard deviation of the blank, as recommended by IUPAC.1' For a preconcentration time of 5 min, the detection limit was 0.1 ppb for nickel(I1). The sensitivity is much better than conventional CSV methods, e.g., 10 ppba and 2.93 ppb.10 Of course, with shorter preconcentration times, higher concentrations were required for detection. On the contrary, an even lower detection limit could be achieved for nickel(I1)provided that the preconcentration time is longer than 5 min as indicated by Figure 7. Interferences. Any electroactive species that forms precipitate with DMG might present an obstacle in the determination of nickel(II). The metals commonly cited as forming stable DMG complexesare copper(II), palladium(II), cobalt(111, and bismuth(II1). However, the chemical conditions in effect during the preconcentration step might easily be optimized to favor the formation of the nickel(I1) complex over that of most of the other metals as reported by Baldwin et al.10 Therefore, the number of species interacting in this manner is rather limited. For 0.01 ppm nickel(II),the results show that over 1000-fold excess concentration of aluminum(1111, cadmium(II), calcium(II), magnesium(II), lead(II), mercury(II), and bismuth(II1) did not influence the nickel response. Zinc(I1) and copper(I1) were found to interfere at a 100-fold excess, and palladium(I1) interfered at a 10-fold excess. The most serious interference was from cobalt(I1); only a 1-fold excess could affect the nickel signal. However, since the reduction peaks were well-separated,the CME could actually be used for simultaneous determination of nickel(I1) and cobalt(I1). The interference effects caused by surface-active compounds in ASV by using MFE are well recognized.14 Coating of the MFE with permselective membranes has been introduced as a means of circumventing the organic interfemces in ASV.14 The function of the membrane is to prevent the organic interferences from reaching the interface at which the deposition/stripping takes place. Triton X-100 was used to simulate the effect of a typical nonionic surfactant, and Figure 8 shows how the 0.01 ppm nickel(I1) stripping peak current is affected for different concentrations of Triton X-100. As can be seen,the detection can tolerate the presence of Triton X-100 up to about 0.5 ppm. Compared to similar experiments carried out with HMDE and MFE: the tolerance was largely improved. Practical Applications. The analytical utility of the CME was assessed by applying it to the determination of nickel(11) in laboratorystandard, underground water, and electroplating waste solutions. The results are summarized in Table I, indicating good accuracy for all the samples. Real values of
1000E-04
1000E-03
Log([Triton X-1001 /
0 01
01
$1)
Flgurr 8. Effect of the surfactant Trlton X-100 at different concentrations on the squarewave stripping response for 0.01 ppm nickeI(I1) on the CME, = 5 mln, &, = -0.6 V vs Ag/AgCl.
in the effectivescan rate results in a shift of the peak potentials to the negative direction. Overall, the best instrumental settings for square-wave stripping voltammetry were as follows: modulation amplitude, 25 mV; modulation frequency, 80 Hz; effective scan rate 320 mV/s. Electrode Renewal. For the CME, the optimum conditions for nickel(I1) accumulation were observed when the sample was slightly basic, pH 9-11. This behavior proved in fact to be quite useful, as i t provided a chemical method-simply exposingthe electrode surface to dilute nitric acid-for removal of the NiAz deposit. A 5-5 immersion into 1M nitric acid was generally sufficient to remove all visual and electrochemical evidence of the preceding nickel(I1) deposition. The CME can also be regenerated by keeping the electrode potential at -1.2 V vs Ag/AgCl in pH 9 buffer solutionfor 2 min as described earlier.16 Both methods showed an excellent reproducibility of the measurementa, usually around 3% in terms of percent relative standard deviation for 10 repetitive preconcentration/reduction/renewalexperiments. Apparently, during the cleaning procedures, nickel(11)is reduced to metallic nickel and just diffuses away from the electrode. Analytical Characterizations of CMEs. Sensitivity, Linear Range, and Detection Limits. Calibration data were obtained for ammonia buffers spiked with nickel(I1) following 3- and 5-min preconcentrations. Fresh sample solutionswere used for each individual concentration and preconcentration time and at least six determinations for each data point. For 5-min preconcentration experiments, the RSD was 1-3% ; while for 3-min preconcentration experiments, the RSD was 4-8 96. The calibration plots thus obtained show a very linear behavior with slope (pA/ppm),intercept (pA), and correlation coefficients of 218.0, -0.9983, and 0.9978 for 3-min preconcentration and 283.2, 0.8219, and 0.9999 for 5-min preconcentration, respectively. The linear range for 5-min preconcentration was investigated by adding nickel(II)concentrations
Table I. Determination of Ni in Underground Water, Electroplating Waste Solutions, and Laboratory Standard detected value,' detected value," sample original (ppm) spike (ppm) after spike (ppm) recovery ( % 1 real value (ppm) underground water electroplating waste solution
" n = 6.
0.014i 0.001 0.031 i 0.001
* *
0.020 0.030
0.034 0.004 0.060 0.004
100 96.7
0.112& 0.08 16.6 i 0.5
sample
added (ppb)
found" (ppb)
laboratory standard
30.0 4.0
36.0 2.0 4.3 0.2
*
ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993
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nickel(I1) content in underground water and electroplating waste solutions were calculated from the diluting factors of each sample during preparation. The relative high content of Ni in underground water may be related to Ni emissions associated with the local electroplating facilities.
possesses good selectivityand can be easily regenerated either with exposure to acid or by eiectrocbemical means. The mechanical stability of the mercury film is also improved.
CONCLUSIONS
The authors wish to acknowledge gratefully the financial support from the National Science Council of the &public of China under Granta NSC 82-042O-B-005057-MO8and NSC 82-0208-M-005-053.
The resulta show that application of the Ndion/DMG thin mercury film electrode in the determination of traces of nickel(11)is very promising. The electrode modification not only offers considerablyhigher resistance to organic interferences than the MFE but also yields higher sensitivity when used in conjunction with square-wave voltammetry. The CME (16) Ekonomou, A.; Fielden, P. R. Anal. Chim. Acta 1993,273, 27. (17) Miller, J. N. Analyst 1991,116, 3.
ACKNOWLEDGMENT
RECEIVED for review May 26, 1993. Accepted August 6, 1993.'
* Abstract publiihedin Advance ACS Abstracte, September 15,1993.
