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Trace determination of mercury by anodic stripping voltammetry at the rotating gold electrode
Analytica Chimica Acta 424 (2000) 65–76
Trace determination of mercury by anodic stripping voltammetry at the rotating gold electrode Y. Bonfil, M. Brand, E. Kirowa-Eisner∗ School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel Received 21 March 2000; received in revised form 27 June 2000; accepted 12 July 2000
Abstract A simple and highly reliable method for the determination of mercury on a rotating gold disk electrode is reported. The signal is linear with concentration over a wide concentration range (0.2–400 nM). The stability of the electrode is excellent. No mechanical polishing between runs is required and a simple electrochemical pretreatment is applied about once in 100 runs. The detection limit in synthetic solutions, applying the subtractive mode of anodic stripping voltammetry (SASV) is 50 pM for a 120 s deposition time at 5000 rpm and 4 nM in urine sample for 180 s deposition time. The reproducibility of the analytical signal is better than 2% in solutions containing 1 nM Hg(II). The applicability of the method in urine analysis was demonstrated with the use of certified samples. No interference by lead, copper, cadmium, chromium or selenium was found at concentrations corresponding to their toxic occurrence in urine. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Anodic stripping voltammetry; Rotating gold electrode; Mercury; Urine
1. Introduction Many papers on anodic stripping analysis of mercury have been published in recent years. The lowest reported detection limit is 5 × 10−14 M, achieved by Meyer and coworkers [1], employing a glassy carbon electrode and thiocyanate as the supporting electrolyte [2]. Turyan and Mandler [3] reported a detection limit of about 10−12 M on a glassy carbon electrode, the surface of which has been modified by a crown ether, chosen to fit the size of the mercury ion. A detection limit of 5×10−12 M, employing a micro-fabricated array of iridium micro-discs has also been reported [4]. The excellent features of the modified disposable thick ∗ Corresponding author. Tel.: +972-3640-8239; fax: +972-3-5406553 E-mail address: [email protected] (E. Kirowa-Eisner).
film graphite electrodes (MDTFGEs) with respect to sensitivity, selectivity and reproducibility was reported by Stojko and coworkers [5,6]. This type of electrode was reported to function well even in the presence of small amounts of surfactants. Numerous interesting works on impregnated graphite, carbon paste and glassy carbon — with and without modifying agents — have been reviewed in the literature [1,5,7]. Different types of gold electrodes for the determination of mercury have been used. Solid gold electrodes [8] are less commonly used than fiber gold [9] and plated gold electrodes [7,10,11]. The main disadvantage of solid gold electrodes is the wellknown phenomenon of structural changes of the gold surface, caused by amalgam formation, and the time-consuming and complex electrochemical pretreatments that are needed to achieve reproducibility [10]. This, however, only applies when bulk deposition
0003-2670/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 0 0 ) 0 1 0 7 4 - 6
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of mercury takes place [12]. There is a great difference in the behavior of the gold electrode when bulk deposition and when anodic-stripping voltammetry are concerned. In the latter, the fractional surface coverage during the deposition step is typically less than 1% [13–15]. Under such experimental conditions some metal ions form a uniform adlayer, a process referred to as underpotential deposition (UPD) [16]. Thus, during repetitive deposition and dissolution of mercury on gold electrodes (as is the case when anodic stripping is applied under typical conditions of concentrations below 10−7 M and relatively short deposition times, or by deposition from higher concentrations of mercury in the UPD region), excellent reproducibility is reached starting from the first run. This indicates that there are no structural changes under these conditions. The absence of structural changes has also been demonstrated by Inukai et al. [12], by applying in-situ scanning tunneling microscopy and cycling the electrode in the potential region in which a UPD layer is formed. The high reproducibility of anodic stripping voltammetry in the determination of lead on silver electrodes and of copper on gold electrodes [13–15] was shown to be due to the involvement of UPD. In the present work we also find excellent long-term stability, reproducibility and sensitivity of the gold electrode used for the determination of mercury, in the UPD region. Mechanical polishing was applied only to newly fabricated electrodes. A simple 1 min electrochemical pretreatment was found to be of utmost importance. However, more than 100 measurements can be performed between pretreatments. It is of importance to point out that the electrochemical pretreatment employed in this work is exactly the same as that successfully used earlier in our laboratory, in the determination of copper on gold [15]. The detection limit of 5 × 10−11 M for mercury, at a deposition time of 120 s, reached in the present study, compares well with published work, considering the short deposition time applied. In view of the simplicity of the procedure and the fact that electrochemical pretreatment has to be repeated only about once in a hundred determinations, this method has great advantages in practice. It competes favorably with existing electrochemical and non-electrochemical methods. The method has been successfully applied for the determination of mercury in urine.
2. Experimental 2.1. Instrumentation A Autolab II (EcoChemie, The Netherlands) with GPES software and a homemade rotating disk electrode were used. 2.2. Chemicals and glassware All solutions were prepared in Type 1 reagent grade water (18.3 M cm, resistivity, obtained with Barnstead EASYpureTM RF water purification). A 0.1 mM HgO (Merck, GR) solution in 10 mM HNO3 was used for daily preparation of the standard solutions. HNO3 65% (Merck, Suprapur), NaCl (Merck, Suprapur) and KCl (Merck, Suprapur) were used for preparations of samples. Plastic (PTFE) and glassware were cleaned prior to use by immersion for a few hours in (a) liquid cleaner (MICRO, Cole Parmer), (b) 1:1 aqueous solution of HNO3 and rinsed by copious amounts of reagent grade water. Economo glass vials (Packard) for liquid scintillation counting were used for the urine samples wet digestion analysis. Urine reference standards (batch H-99-01, H-99-03), Interlaboratory Comparison Program, Quebec, Canada, were used. The samples were stored at −4◦ C. 2.3. Cell A 10 ml quartz cell (Fig. 1 in ref. [17]) was used in a three-electrode configuration. The working electrode was a homemade rotating gold electrode. The counter electrode was a platinum wire (20 mm length and 0.6 mm diameter). The reference electrode was a homemade Ag/AgCl, 1 M KCl electrode (ref. [15]). The rinsing of the cell between separate experiments was performed without dismantling or touching the cell. The cell content was discarded using a 1 mm i.d. PTFE tube, connected to a rubber-membrane air pump. 2.4. Preparation of the rotating disc gold electrode (Au-RDE) The gold electrode (6.61 mm2 area) was a homemade disc electrode embedded in PTFE. A newly prepared electrode was rinsed in ethanol, and then
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the anodic stripping voltammograms before and after activation (cf. Fig. 1). Whenever the proper functioning of the electrode deteriorates, the electrochemical pretreatment is repeated. An anodic stripping voltammogram of 60 nM Hg(II) was used for testing the functioning of the electrode. When the analytical response (peak height or slope) decreases by more than 10%, the electrochemical pretreatment is applied. Only rarely was it needed to resort to repolishing of the electrode with alumina powder. 2.5. The ASV procedure
Fig. 1. Subtractive anodic stripping voltammetry (SASV) of Hg(II) on a non-activated and on an activated gold electrode without removal of oxygen. Solution: 10 nM Hg(II), 10 mM HNO3 , 10 mM NaCl. Edeposition : −0.4 V; tdeposition : 30 s; rotating rate: 5000 rpm. SW mode, square-wave amplitude: 10 mV; step amplitude: 2.5 mV; frequency: 25 Hz. Activation of the electrode according to Section 2.4.1.
polished successively with 600-grit EMERY paper and with 0.3 and 0.05 m alumina powder on a polishing wheel (TEXMET polishing cloth, Buehler catalog No. 40-7618) to mirror-like finish. The electrode was then rinsed with water, immersed in an ultrasonic bath for 3 min and rinsed again. A homemade rotator with adjustable rate of rotation in the range 400–9000 rpm was used. While not in use the electrode was stored in pure water. 2.4.1. Activating the gold electrode A freshly polished gold electrode displays an ill-defined anodic stripping voltammogram for mercury. The electrode is activated by applying a current of 150 mA cm−2 for 60 s between the working and the counter electrodes, dipped in the supporting electrolyte (10 mM HNO3 and 10 mM NaCl). The gas bubbles formed at the gold electrode are removed by occasionally rotating the electrode. The activation applied is identical to that used in the determination of copper at the gold electrode [15]. The importance of the activation procedure is clearly seen by comparing
The analysis was performed without removal of oxygen in a 5.00 ml aliquot, containing 10 mM HNO3 and 10 mM NaCl. The ASV procedure consisted of the following steps performed in an uninterrupted sequence: (a) Conditioning of the electrode: the working electrode was rotated while applying a potential in order to destroy the diffusion layer formed from previous experiment and to ensure dissolution of remaining deposits: E conditioning = 0.9 V, t conditioning = 10 s. The stirring of the solution during the conditioning step is of utmost importance when subsequent anodic stripping voltammograms are performed in a fast sequence. A systematic increase in the peak height was observed if this step is deleted. (b) The deposition step: Edeposition , in the range of (−0.4)–(+0.5) V; 15 ≤ t deposition ≤ 360 s; 1000 ≤ N ≤ 9000 rpm; (c) The rest step (as step (b), but without rotating the electrode): t rest = 10 s; (d) The stripping step was performed with square wave voltammetry: square-wave amplitude (peak to peak)=10 mV; step amplitude = 2.5 mV; frequency = 25 Hz; E initial = in the range of (−0.4)–(+0.5) V; E final = 0.9 V. 2.6. The subtractive anodic stripping voltammetry, SASV, procedure In applying the subtractive ASV method it is of utmost importance to perform the consecutive runs and the standard additions under a rigid regime with a rapid sequence of all steps involved. The procedure of recording a subtractive anodic stripping voltammogram was as follows: The regular anodic stripping voltammogram was followed in a non-interrupted sequence by a blind anodic stripping
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voltammogram (zero deposition time) using the ‘Project’ feature of the GPES-Autolab software. The conditions for the blind anodic stripping voltammogram were as those for the normal anodic stripping voltammogram. By subtracting the blind voltammogram from the normal anodic stripping voltammogram, the subtractive anodic stripping voltammogram was obtained. 2.7. Evaluation of the analytical signal of the ASV and of the SASV The sum of the absolute values of the slopes at the two inflection points of the peak was used as measure for the analytical signal of the anodic stripping voltammogram. It was shown in previous works [13–15] that the slope at the inflection point is best suited when the base line is asymmetric around the peak. The slope is independent of the way the background is chosen, while in the measurement of the peak current or of the charge it is strongly dependent on it. At concentrations higher than 5 nM the peaks are symmetrical
and the peak current can also be used as a measure for the analytical signal. 2.8. Construction of standard addition curves and calibration curves The curves were constructed in a rigid, noninterrupted sequence using the ‘Project’ feature of the GPES-Autolab software. Performing the standard additions in the shortest possible time is of utmost importance in the sub-gl−1 concentration range. Keeping the time of analysis short decreases the chances of: (a) contamination of the test solution, (b) changes in the magnitude of the signal due to temperature variations and (c) changes in the response of the electrode. A new aliquot of mercury is introduced immediately after the recording of the previous curve and the time of the conditioning step of the regular SWASV (performed while rotating the working electrode) was extended to 20 s to allow uniformity of the solution. The quantitative determinations were performed using the standard additions method.
Fig. 2. Comparison of the potential range in which the electroactivity of mercury is displayed at low and at high concentrations. Supporting electrolyte: 10 mM HNO3 , 10 mM NaCl. The SASV plots are performed under conditions defined in Fig. 1. Concentration of Hg(II) from bottom to top: 9.9, 19.6, 29.1, 38.4, 47.6, 56.6, 65.4, 74.0, 82.5, 90.9, 99.1 nM. The RDE plots are voltammograms recorded in absence of oxygen. Rotation rate: 5000 rpm; potential sweep: 10 mV s−1 .
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2.9. Pretreatment of the urine samples The pretreatment, intended to destroy organic complexants and organic surfactants, was performed by the wet digestion method. Urine samples (2.5 ml) were transferred to glass vials. To each vial, 1 ml concentrated nitric acid was added and the samples were heated and let to boil gently for 10 min. Then 1 ml of concentrated perchloric acid was added to each sample and the vials were transferred to a muffle furnace and kept at 250◦ C for 45 min. The vials were let to cool to room temperature. The volume of the samples in a vial was about 1 ml. Supporting electrolyte (10 mM NaCl, 10 mM HNO3 ) was added to a final volume of 5 ml. A portion of 1.00 ml was transferred to the cell that contained 4 ml supporting electrolyte and analyzed. The analysis of the samples was performed with three standard additions.
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well-studied phenomenon [12,18]. One of the most important features of the mercury UPD on gold on view of its analytical applications is that the electrochemical characteristics of the electrode is not affected by repeated deposition and dissolution of mercury and in order to achieve good reproducibility no pretreatment of the gold between experiments is required. This is in contrast with experiments in which bulk deposition of mercury is involved. At bulk deposition, the morphology of the electrode is changed due to the formation of amalgam and can be restored only with mechanical electropolishing, while at underpotential deposition, the adatom layer of mercury does not penetrate into the matrix of the gold and does not change its crystalline structure. It is of interest to note that while excellent reproducibility is reached with subsequent anodic stripping experiments, the reproducibility of the experiments at high concentration of mercury was poor in view of location and shape of the waves (but good in respect of the limiting current). Only with thorough repolishing
3. Results and discussion 3.1. Under potential and bulk deposition of mercury on gold The anodic stripping characteristics of mercury is displayed in a potential region, which is few hundreds of millivolts more positive than the reversible potential of Hg(II)/Hg in the presence of chlorides. As seen in Fig. 2 the peak potential of the anodic stripping voltammograms at nanomolar concentrations of Hg(II) at a gold electrode is 0.62 V, while the direct reduction of Hg(II) at much higher concentrations (millimolar concentrations) at rotating disc electrode is displayed at potential more negative than 0.3 V. The characteristics of the voltammograms at high concentrations is peculiar (appearance of a maximum and the split of the reduction wave) and to the best knowledge of the authors has not been reported in the literature. The electrochemical behavior at the low concentration range corresponds to the underpotential deposition/dissolution phenomena, while at the high concentration corresponds to overpotential deposition. The characteristics of the anodic stripping voltammograms reflect the underpotential deposition/ dissolution properties (UPD) of mercury, which is a
Fig. 3. SASV plots of 60 nM Hg(II) and 60 nM Cu(II) in 10 mM HNO3 and different concentrations of NaCl: (1) 0.5 mM, (2) 1.5 mM, (3) 2 mM, (4) 10 mM, (5) 20 mM, (6) 37 mM, (7) 135 mM. The SASV plots are performed under conditions defined in Fig. 1.
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of the electrode after each run could good repeatability be obtained. The adatom layer of mercury, formed during the deposition step of an anodic stripping experiments, covers only a small fraction of the electrode. In typical experimental conditions, 0.01–1% of monolayer is formed in 0.2–200 nM of mercury for the respective deposition times of 15–200 s at a rotating rate of 5000 rpm. The value of the fraction coverage, θ, is obtained according to θ = (q dep /q monolayer )/R F , where q(= j dep · t dep ) is calculated from Levich equation for the RDE and the roughness factor RF of the electrochemically pretreated electrode is 2 according to the procedure described in Section 3.2.4 in ref. [15]. 3.2. Anions effect on the analytical signal The electrochemical dissolution of mercury from the gold electrode is strongly influenced by the supporting electrolyte. According to Gil and Ostapcuk [7], the ASV determination of mercury on gold electrodes is to be carried out in the presence of species capable
of complexing Hg(II). They showed that the presence of 2–20 mM chloride is adequate for the analysis. The peak height or slope of the mercury dissolution peak at gold depends on the concentration of chlorides (Figs. 3 and 4). The peak reaches maximum value at 10 mM Cl− . The peak width is almost independent of the chloride concentration. On the modified disposable thick film graphite electrodes, the peak maximum is reached at 50 mM Cl− and is four times as large than in absence of chlorides [6]. The fact that the maximum at different electrodes is reached at different concentrations of chlorides, is an indication of the strong involvement of chlorides in the formation of the adatom layer of mercury. The effect of the chloride concentration on the peak potential of mercury and copper is shown in Figs. 3 and 5. The higher the chloride concentration, the closer the peaks. While the mercury peak is shifted to negative potentials with increasing chloride concentration, the copper peak is shifted to positive potentials. The copper peak decreases continuously with increasing chloride concentration.
Fig. 4. The analytical response of the subtractive anodic stripping voltammograms peak (the slope) in 10 mM HNO3 and in different concentrations of NaCl, NaF, Na2 SO4 , NaNO3 , HClO4 . The SASV plots are performed under conditions defined in Fig. 1.
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The effect of fluoride on the peak slope is similar to that of the chloride (Fig. 4). The peak slope of mercury exhibits a maximum at 25 mM F− . The peak slope of mercury decreases continuously with increasing concentrations of ClO4 − , NO3 − and SO4 2− (Fig. 4). Acidifying of the solution to be analyzed has been found beneficial in view of peak shape and reproducibility of the mercury peak. Gil and Ostapcuk [7], reported good reproducibility in both HClO4 and H2 SO4 , but inferior in HNO3 . In the conditions of our work no such difference was observed. The apparent discrepancy is probably due to the different modes of pretreatment of the gold electrodes. 3.3. Analytical characteristics of ASV and SASV of Hg(II) The anodic stripping voltammogram (and the subtractive anodic stripping voltammogram) peak potential at mercury concentrations of 0.2–100 nM, in
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10 mM NaCl and 10 mM HNO3 is about +0.65 V versus Ag/AgCl, 1 M KCl. It is positive by more than 0.5 V in respect to the reversible potential of the Hg(II)/Hg couple, as it is related to the underpotential deposition/dissolution phenomenon of mercury adatoms. Being deposited at an underpotential, the adatoms are distributed uniformly on the electrode surface and under the experimental conditions in the nanomolar concentration range, the coverage is less than 0.1%. 3.3.1. Anodic stripping voltammetry versus subtractive anodic stripping voltammetry ASV and SASV scans of 5 to 20 nM Hg(II) and Cu(II) solutions are presented in Fig. 6. The advantage of the SASV is clear: (i) the peaks are better defined, (ii) the background currents are smaller and calibration plots with higher correlation coefficients are obtained. At concentrations higher than 20 nM both stripping methods (ASV and SASV) yield good analytical results. At concentrations lower than 15 nM
Fig. 5. Peak potential of the subtractive anodic stripping voltammograms at different concentrations of NaCl, NaF, Na2 SO4 , NaNO3 , HClO4 . Conditions as in Fig. 4.
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Fig. 6. Anodic stripping voltammograms and subtractive anodic stripping voltammograms at the Au-RDE. 4, 8, 12 and 20 nM Hg(II) and 5, 10, 15, 20 nM Cu(II) in 10 mM HNO3 , 10 mM NaCl solution. Condition of electrodeposition: Edeposition : −0.4 V; tdeposition : 30 s; 5000 rpm. SW mode, square-wave amplitude 10 mV; step amplitude: 2.5 mV; frequency: 25 Hz.
only the subtractive anodic stripping voltammetry is suitable for analytical uses. 3.3.2. Correlation of the analytical signal with deposition potential, concentration, deposition time and speed of rotation Stripping voltammograms with different deposition potentials were recorded at 60 nM Hg(II). The repeatability and the magnitude of the analytical signal were found to be independent of the deposition potential in the potential range of (+0.55)– (−0.4) V. A linear relationship is observed between the analytical signal and (a) CHg(II) in the range of 0.2–400 nM (cf. Figs. 7 and 8), (b) time of deposition in the tested time range of 15–360 s (cf. Fig. 8) and (c) rate of rotation in the range of 1000–9000 rpm. The quality of the respective plots is very good. The correlation coefficient is better than 0.999.
Deviation from linearity occurs when the product CHg(II) ·N1/2 ·tdeposition exceeds the value of 1×106 nM rpm1/2 s (N is the rotation rate in rpm). The product CHg(II) ·N1/2 is proportional to the mass transport limiting current of deposition of mercury. The mercury coverage on the electrode at the point of break from linearity corresponds to about 5% of a monolayer (the estimation is based on calculations similar to those performed in Section 3.1). Similar behavior was observed in the case of determination of lead on a silver electrode [13,14] and copper on gold [15]. The condition CHg(II) ·N1/2 ·t deposition < 1 × 106 nM rpm1/2 s is useful for estimating the upper limit values for given CHg(II) ·N and tdeposition at which linearity is maintained. slope
3.3.3. The sensitivity, SHg , and the detection limit slope
The sensitivity, SHg , of the method is determined from the calibration plots as the slope of the analytical
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Fig. 7. Subtractive square wave anodic stripping voltammetry, SASV, of Hg(II) at the Au-RDE and the respective calibration curves. The values in the calibration curves are corrected for the respective blanks. Solution composition: 10 mM HNO3 , 10 mM NaCl and Hg(II). Condition of electrodeposition: Edeposition : −0.4 V; 5000 rpm; SW mode, square-wave amplitude: 10 mV; step amplitude: 2.5 mV; frequency: 25 Hz.
response versus concentration obtained in the supporting electrolyte used in this work (10 mM HNO3 and NaCl): slope
SHg
= (5 ± 0.5)AV−1 cm−2 M−1 s−1 rpm−1/2 , slope
where SHg
=
analytical response (slope) AAu CHg2+ tdep N 1/2
and AAu is the electrode area of the gold electrode. The detection limit was determined as three times the standard deviation of the analytical signal of a
0.4 nM in Hg(II) solution. The detection limit for a 120 s electrodeposition at 5000 rpm was 0.05 nM. 3.3.4. Short and long term stability of the Au-disc electrode The repeatability of the SASV, expressed as the relative standard deviation of seven consecutive experiments performed in a rigid, non-interrupted sequence using the project feature of the GPES-Autolab software is less than 1% for a 1 nM solution at 90 s deposition time.
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Fig. 9. Subtractive anodic stripping voltammogram at the Au-RDE of a mixture of Pb, Bi, Cu and Hg. Supporting electrolyte: 10 mM HNO3 , 10 mM NaCl. Concentration of the metal ions: 10 nM each. tdeposition : 30 s. Other conditions for SASV as in Fig. 7. Fig. 8. SASV Calibration plots for Hg(II) at the Au-RDE for different deposition times, as marked on plots. Other conditions as in Fig. 7.
Over long periods of time the analytical signal fluctuates to about 10%. 3.3.5. Proper functioning of the electrode The following four criteria can be used to determine the functioning of the electrode in 10 mM NaCl, 10 mM HNO3 . In case one of them is not fulfilled, the electrode should be pretreated electrochemically with or without mechanical polishing. 1. The width of the peak at half height: 55 ± 5 mV. 2. The repeatability: below 2% for the 1–200 nM concentrations range of Hg(II). slope 3. The sensitivity, SHg : (5±0.5) AV−1 cm−2 M−1 s−1 rpm−1/2 . 4. The correlation coefficient of a calibration curve contains at least three nines. 3.3.6. Determination of mercury in the presence of copper, lead and bismuth The determination of mercury is unaffected by the presence of copper, lead and bismuth in the solution. The linearity of the calibration curve of mercury is maintained also in presence of 100 nM of copper, lead
and bismuth. A subtractive anodic stripping voltammogram of an equimolar mixture (10 nM) of the four elements is shown in Fig. 9. The peak of mercury is sharp and very well defined compared to the other peaks. The peaks of mercury and copper are well separated while the copper and bismuth peaks are overlapping. 3.3.7. Determination of mercury in urine samples A pretreatment of the sample prior to the analysis of mercury is required [9,19], (a) to release the metals bound as organic complexes and (b) to disrupt the organic ligands in order to minimize the problem of background currents and fouling of the working electrode. The pretreatment applied in this work consists of wet digestion with nitric and perchloric acids (cf., Section 2.9). The validity of the wet digestion method was tested by the degree of recovery of mercury from five samples of urine to which a known concentration of Hg(II) (100 or 200 nM) was added. The average recovery was 95% with a R.S.D of 5%. The analytical determination of the certified urine sample (H-99-03) is displayed in Fig. 10. The stripping curves are very well defined, although the
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Fig. 10. Determination of Hg(II) in certified urine sample (H-99-03) using the method of standard additions. The sample was pretreated and prepared according to Section 2.9. Each standard addition consists: 50.0 l, 1.00 M Hg(II) into a 4.00 ml sample. Edeposition : −.4 V; tdeposition : 180 s; N: 5000 rpm.
background currents are increased. The results of the mercury analysis for the certified samples are summarized in Table 1. The deviations in respect to the certified values are within 10%. The detection limit of mercury in urine samples for 180 s deposition time at 5000 rpm is 4 nM. This is considerably higher than the detection limit in synthetic samples in 10 mM NaCl, and HNO3 (without digestion) and is due to (a) impurities in the perchloric acid and (b) differences in matrix.
Interferences. Different metal ions were tested for possible interferences. The cations and their concentrations were chosen according to their toxic occurrence in urine. No interference was found up to the following limited concentrations: 2 M Pb(II), 2 M Cu(II), 0.5 M Cd(II), 0.5 M Cr(III) and 5 M Se(IV). Determination of mercury in waste water. The same method described above has been applied for the determination of Hg(II) and Cu(II) in waste water [20].
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Table 1 Determination of mercury in certified urine samples by ASV (six replicates) Urine Sample
Certified value of Hg (nM)
Mercury found (nM)
H-99-01 H-99-03
330 115
362 ± 21 108 ± 8
Acknowledgements This work was supported in part by the Israeli Ministry of the Environment. The authors thank Dr. Ralph Shain from the National Institute of Occupational and Environmental Health and Mr. Ronen Alkalay from the Dan Region for fruitful discussions. References
4. Conclusions The involvement of underpotential deposition in the anodic-stripping analysis of mercury has beneficial effects on the analysis. The coverage of the mercury adatoms on the gold electrode is in the range of 0.01–1%, depending on the concentration of mercury in the sample and on the duration of electrolysis. As a result, there are no structural changes on the gold electrode, as opposed to electrochemical measurements performed at high concentrations of mercury, where amalgamation causes profound changes. Thus, without the need of mechanical or electrochemical pretreatment, the reproducibility of the analytical signal and the stability of the electrode are excellent over long periods of use. The importance of the subtractive mode of anodic stripping voltammetry is demonstrated in this work. It improves the quality of the stripping curves and the detection limit by more than an order of magnitude. The detection limit of 50 pM for 120 s deposition time, can easily be improved by prolonging the time of electrodeposition, as is generally the practice. We have not done this because short electrolysis times decrease the contamination from the environment and permit analysis under normal laboratory conditions, without the need of clean rooms. The method was successfully applied to the analysis of Hg(II) in urine.
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Trace determination of mercury by anodic stripping voltammetry at the rotating gold electrode Y. Bonfil, M. Brand, E. Kirowa-Eisner∗ School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel Received 21 March 2000; received in revised form 27 June 2000; accepted 12 July 2000
Abstract A simple and highly reliable method for the determination of mercury on a rotating gold disk electrode is reported. The signal is linear with concentration over a wide concentration range (0.2–400 nM). The stability of the electrode is excellent. No mechanical polishing between runs is required and a simple electrochemical pretreatment is applied about once in 100 runs. The detection limit in synthetic solutions, applying the subtractive mode of anodic stripping voltammetry (SASV) is 50 pM for a 120 s deposition time at 5000 rpm and 4 nM in urine sample for 180 s deposition time. The reproducibility of the analytical signal is better than 2% in solutions containing 1 nM Hg(II). The applicability of the method in urine analysis was demonstrated with the use of certified samples. No interference by lead, copper, cadmium, chromium or selenium was found at concentrations corresponding to their toxic occurrence in urine. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Anodic stripping voltammetry; Rotating gold electrode; Mercury; Urine
1. Introduction Many papers on anodic stripping analysis of mercury have been published in recent years. The lowest reported detection limit is 5 × 10−14 M, achieved by Meyer and coworkers [1], employing a glassy carbon electrode and thiocyanate as the supporting electrolyte [2]. Turyan and Mandler [3] reported a detection limit of about 10−12 M on a glassy carbon electrode, the surface of which has been modified by a crown ether, chosen to fit the size of the mercury ion. A detection limit of 5×10−12 M, employing a micro-fabricated array of iridium micro-discs has also been reported [4]. The excellent features of the modified disposable thick ∗ Corresponding author. Tel.: +972-3640-8239; fax: +972-3-5406553 E-mail address: [email protected] (E. Kirowa-Eisner).
film graphite electrodes (MDTFGEs) with respect to sensitivity, selectivity and reproducibility was reported by Stojko and coworkers [5,6]. This type of electrode was reported to function well even in the presence of small amounts of surfactants. Numerous interesting works on impregnated graphite, carbon paste and glassy carbon — with and without modifying agents — have been reviewed in the literature [1,5,7]. Different types of gold electrodes for the determination of mercury have been used. Solid gold electrodes [8] are less commonly used than fiber gold [9] and plated gold electrodes [7,10,11]. The main disadvantage of solid gold electrodes is the wellknown phenomenon of structural changes of the gold surface, caused by amalgam formation, and the time-consuming and complex electrochemical pretreatments that are needed to achieve reproducibility [10]. This, however, only applies when bulk deposition
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of mercury takes place [12]. There is a great difference in the behavior of the gold electrode when bulk deposition and when anodic-stripping voltammetry are concerned. In the latter, the fractional surface coverage during the deposition step is typically less than 1% [13–15]. Under such experimental conditions some metal ions form a uniform adlayer, a process referred to as underpotential deposition (UPD) [16]. Thus, during repetitive deposition and dissolution of mercury on gold electrodes (as is the case when anodic stripping is applied under typical conditions of concentrations below 10−7 M and relatively short deposition times, or by deposition from higher concentrations of mercury in the UPD region), excellent reproducibility is reached starting from the first run. This indicates that there are no structural changes under these conditions. The absence of structural changes has also been demonstrated by Inukai et al. [12], by applying in-situ scanning tunneling microscopy and cycling the electrode in the potential region in which a UPD layer is formed. The high reproducibility of anodic stripping voltammetry in the determination of lead on silver electrodes and of copper on gold electrodes [13–15] was shown to be due to the involvement of UPD. In the present work we also find excellent long-term stability, reproducibility and sensitivity of the gold electrode used for the determination of mercury, in the UPD region. Mechanical polishing was applied only to newly fabricated electrodes. A simple 1 min electrochemical pretreatment was found to be of utmost importance. However, more than 100 measurements can be performed between pretreatments. It is of importance to point out that the electrochemical pretreatment employed in this work is exactly the same as that successfully used earlier in our laboratory, in the determination of copper on gold [15]. The detection limit of 5 × 10−11 M for mercury, at a deposition time of 120 s, reached in the present study, compares well with published work, considering the short deposition time applied. In view of the simplicity of the procedure and the fact that electrochemical pretreatment has to be repeated only about once in a hundred determinations, this method has great advantages in practice. It competes favorably with existing electrochemical and non-electrochemical methods. The method has been successfully applied for the determination of mercury in urine.
2. Experimental 2.1. Instrumentation A Autolab II (EcoChemie, The Netherlands) with GPES software and a homemade rotating disk electrode were used. 2.2. Chemicals and glassware All solutions were prepared in Type 1 reagent grade water (18.3 M cm, resistivity, obtained with Barnstead EASYpureTM RF water purification). A 0.1 mM HgO (Merck, GR) solution in 10 mM HNO3 was used for daily preparation of the standard solutions. HNO3 65% (Merck, Suprapur), NaCl (Merck, Suprapur) and KCl (Merck, Suprapur) were used for preparations of samples. Plastic (PTFE) and glassware were cleaned prior to use by immersion for a few hours in (a) liquid cleaner (MICRO, Cole Parmer), (b) 1:1 aqueous solution of HNO3 and rinsed by copious amounts of reagent grade water. Economo glass vials (Packard) for liquid scintillation counting were used for the urine samples wet digestion analysis. Urine reference standards (batch H-99-01, H-99-03), Interlaboratory Comparison Program, Quebec, Canada, were used. The samples were stored at −4◦ C. 2.3. Cell A 10 ml quartz cell (Fig. 1 in ref. [17]) was used in a three-electrode configuration. The working electrode was a homemade rotating gold electrode. The counter electrode was a platinum wire (20 mm length and 0.6 mm diameter). The reference electrode was a homemade Ag/AgCl, 1 M KCl electrode (ref. [15]). The rinsing of the cell between separate experiments was performed without dismantling or touching the cell. The cell content was discarded using a 1 mm i.d. PTFE tube, connected to a rubber-membrane air pump. 2.4. Preparation of the rotating disc gold electrode (Au-RDE) The gold electrode (6.61 mm2 area) was a homemade disc electrode embedded in PTFE. A newly prepared electrode was rinsed in ethanol, and then
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the anodic stripping voltammograms before and after activation (cf. Fig. 1). Whenever the proper functioning of the electrode deteriorates, the electrochemical pretreatment is repeated. An anodic stripping voltammogram of 60 nM Hg(II) was used for testing the functioning of the electrode. When the analytical response (peak height or slope) decreases by more than 10%, the electrochemical pretreatment is applied. Only rarely was it needed to resort to repolishing of the electrode with alumina powder. 2.5. The ASV procedure
Fig. 1. Subtractive anodic stripping voltammetry (SASV) of Hg(II) on a non-activated and on an activated gold electrode without removal of oxygen. Solution: 10 nM Hg(II), 10 mM HNO3 , 10 mM NaCl. Edeposition : −0.4 V; tdeposition : 30 s; rotating rate: 5000 rpm. SW mode, square-wave amplitude: 10 mV; step amplitude: 2.5 mV; frequency: 25 Hz. Activation of the electrode according to Section 2.4.1.
polished successively with 600-grit EMERY paper and with 0.3 and 0.05 m alumina powder on a polishing wheel (TEXMET polishing cloth, Buehler catalog No. 40-7618) to mirror-like finish. The electrode was then rinsed with water, immersed in an ultrasonic bath for 3 min and rinsed again. A homemade rotator with adjustable rate of rotation in the range 400–9000 rpm was used. While not in use the electrode was stored in pure water. 2.4.1. Activating the gold electrode A freshly polished gold electrode displays an ill-defined anodic stripping voltammogram for mercury. The electrode is activated by applying a current of 150 mA cm−2 for 60 s between the working and the counter electrodes, dipped in the supporting electrolyte (10 mM HNO3 and 10 mM NaCl). The gas bubbles formed at the gold electrode are removed by occasionally rotating the electrode. The activation applied is identical to that used in the determination of copper at the gold electrode [15]. The importance of the activation procedure is clearly seen by comparing
The analysis was performed without removal of oxygen in a 5.00 ml aliquot, containing 10 mM HNO3 and 10 mM NaCl. The ASV procedure consisted of the following steps performed in an uninterrupted sequence: (a) Conditioning of the electrode: the working electrode was rotated while applying a potential in order to destroy the diffusion layer formed from previous experiment and to ensure dissolution of remaining deposits: E conditioning = 0.9 V, t conditioning = 10 s. The stirring of the solution during the conditioning step is of utmost importance when subsequent anodic stripping voltammograms are performed in a fast sequence. A systematic increase in the peak height was observed if this step is deleted. (b) The deposition step: Edeposition , in the range of (−0.4)–(+0.5) V; 15 ≤ t deposition ≤ 360 s; 1000 ≤ N ≤ 9000 rpm; (c) The rest step (as step (b), but without rotating the electrode): t rest = 10 s; (d) The stripping step was performed with square wave voltammetry: square-wave amplitude (peak to peak)=10 mV; step amplitude = 2.5 mV; frequency = 25 Hz; E initial = in the range of (−0.4)–(+0.5) V; E final = 0.9 V. 2.6. The subtractive anodic stripping voltammetry, SASV, procedure In applying the subtractive ASV method it is of utmost importance to perform the consecutive runs and the standard additions under a rigid regime with a rapid sequence of all steps involved. The procedure of recording a subtractive anodic stripping voltammogram was as follows: The regular anodic stripping voltammogram was followed in a non-interrupted sequence by a blind anodic stripping
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voltammogram (zero deposition time) using the ‘Project’ feature of the GPES-Autolab software. The conditions for the blind anodic stripping voltammogram were as those for the normal anodic stripping voltammogram. By subtracting the blind voltammogram from the normal anodic stripping voltammogram, the subtractive anodic stripping voltammogram was obtained. 2.7. Evaluation of the analytical signal of the ASV and of the SASV The sum of the absolute values of the slopes at the two inflection points of the peak was used as measure for the analytical signal of the anodic stripping voltammogram. It was shown in previous works [13–15] that the slope at the inflection point is best suited when the base line is asymmetric around the peak. The slope is independent of the way the background is chosen, while in the measurement of the peak current or of the charge it is strongly dependent on it. At concentrations higher than 5 nM the peaks are symmetrical
and the peak current can also be used as a measure for the analytical signal. 2.8. Construction of standard addition curves and calibration curves The curves were constructed in a rigid, noninterrupted sequence using the ‘Project’ feature of the GPES-Autolab software. Performing the standard additions in the shortest possible time is of utmost importance in the sub-gl−1 concentration range. Keeping the time of analysis short decreases the chances of: (a) contamination of the test solution, (b) changes in the magnitude of the signal due to temperature variations and (c) changes in the response of the electrode. A new aliquot of mercury is introduced immediately after the recording of the previous curve and the time of the conditioning step of the regular SWASV (performed while rotating the working electrode) was extended to 20 s to allow uniformity of the solution. The quantitative determinations were performed using the standard additions method.
Fig. 2. Comparison of the potential range in which the electroactivity of mercury is displayed at low and at high concentrations. Supporting electrolyte: 10 mM HNO3 , 10 mM NaCl. The SASV plots are performed under conditions defined in Fig. 1. Concentration of Hg(II) from bottom to top: 9.9, 19.6, 29.1, 38.4, 47.6, 56.6, 65.4, 74.0, 82.5, 90.9, 99.1 nM. The RDE plots are voltammograms recorded in absence of oxygen. Rotation rate: 5000 rpm; potential sweep: 10 mV s−1 .
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2.9. Pretreatment of the urine samples The pretreatment, intended to destroy organic complexants and organic surfactants, was performed by the wet digestion method. Urine samples (2.5 ml) were transferred to glass vials. To each vial, 1 ml concentrated nitric acid was added and the samples were heated and let to boil gently for 10 min. Then 1 ml of concentrated perchloric acid was added to each sample and the vials were transferred to a muffle furnace and kept at 250◦ C for 45 min. The vials were let to cool to room temperature. The volume of the samples in a vial was about 1 ml. Supporting electrolyte (10 mM NaCl, 10 mM HNO3 ) was added to a final volume of 5 ml. A portion of 1.00 ml was transferred to the cell that contained 4 ml supporting electrolyte and analyzed. The analysis of the samples was performed with three standard additions.
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well-studied phenomenon [12,18]. One of the most important features of the mercury UPD on gold on view of its analytical applications is that the electrochemical characteristics of the electrode is not affected by repeated deposition and dissolution of mercury and in order to achieve good reproducibility no pretreatment of the gold between experiments is required. This is in contrast with experiments in which bulk deposition of mercury is involved. At bulk deposition, the morphology of the electrode is changed due to the formation of amalgam and can be restored only with mechanical electropolishing, while at underpotential deposition, the adatom layer of mercury does not penetrate into the matrix of the gold and does not change its crystalline structure. It is of interest to note that while excellent reproducibility is reached with subsequent anodic stripping experiments, the reproducibility of the experiments at high concentration of mercury was poor in view of location and shape of the waves (but good in respect of the limiting current). Only with thorough repolishing
3. Results and discussion 3.1. Under potential and bulk deposition of mercury on gold The anodic stripping characteristics of mercury is displayed in a potential region, which is few hundreds of millivolts more positive than the reversible potential of Hg(II)/Hg in the presence of chlorides. As seen in Fig. 2 the peak potential of the anodic stripping voltammograms at nanomolar concentrations of Hg(II) at a gold electrode is 0.62 V, while the direct reduction of Hg(II) at much higher concentrations (millimolar concentrations) at rotating disc electrode is displayed at potential more negative than 0.3 V. The characteristics of the voltammograms at high concentrations is peculiar (appearance of a maximum and the split of the reduction wave) and to the best knowledge of the authors has not been reported in the literature. The electrochemical behavior at the low concentration range corresponds to the underpotential deposition/dissolution phenomena, while at the high concentration corresponds to overpotential deposition. The characteristics of the anodic stripping voltammograms reflect the underpotential deposition/ dissolution properties (UPD) of mercury, which is a
Fig. 3. SASV plots of 60 nM Hg(II) and 60 nM Cu(II) in 10 mM HNO3 and different concentrations of NaCl: (1) 0.5 mM, (2) 1.5 mM, (3) 2 mM, (4) 10 mM, (5) 20 mM, (6) 37 mM, (7) 135 mM. The SASV plots are performed under conditions defined in Fig. 1.
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of the electrode after each run could good repeatability be obtained. The adatom layer of mercury, formed during the deposition step of an anodic stripping experiments, covers only a small fraction of the electrode. In typical experimental conditions, 0.01–1% of monolayer is formed in 0.2–200 nM of mercury for the respective deposition times of 15–200 s at a rotating rate of 5000 rpm. The value of the fraction coverage, θ, is obtained according to θ = (q dep /q monolayer )/R F , where q(= j dep · t dep ) is calculated from Levich equation for the RDE and the roughness factor RF of the electrochemically pretreated electrode is 2 according to the procedure described in Section 3.2.4 in ref. [15]. 3.2. Anions effect on the analytical signal The electrochemical dissolution of mercury from the gold electrode is strongly influenced by the supporting electrolyte. According to Gil and Ostapcuk [7], the ASV determination of mercury on gold electrodes is to be carried out in the presence of species capable
of complexing Hg(II). They showed that the presence of 2–20 mM chloride is adequate for the analysis. The peak height or slope of the mercury dissolution peak at gold depends on the concentration of chlorides (Figs. 3 and 4). The peak reaches maximum value at 10 mM Cl− . The peak width is almost independent of the chloride concentration. On the modified disposable thick film graphite electrodes, the peak maximum is reached at 50 mM Cl− and is four times as large than in absence of chlorides [6]. The fact that the maximum at different electrodes is reached at different concentrations of chlorides, is an indication of the strong involvement of chlorides in the formation of the adatom layer of mercury. The effect of the chloride concentration on the peak potential of mercury and copper is shown in Figs. 3 and 5. The higher the chloride concentration, the closer the peaks. While the mercury peak is shifted to negative potentials with increasing chloride concentration, the copper peak is shifted to positive potentials. The copper peak decreases continuously with increasing chloride concentration.
Fig. 4. The analytical response of the subtractive anodic stripping voltammograms peak (the slope) in 10 mM HNO3 and in different concentrations of NaCl, NaF, Na2 SO4 , NaNO3 , HClO4 . The SASV plots are performed under conditions defined in Fig. 1.
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The effect of fluoride on the peak slope is similar to that of the chloride (Fig. 4). The peak slope of mercury exhibits a maximum at 25 mM F− . The peak slope of mercury decreases continuously with increasing concentrations of ClO4 − , NO3 − and SO4 2− (Fig. 4). Acidifying of the solution to be analyzed has been found beneficial in view of peak shape and reproducibility of the mercury peak. Gil and Ostapcuk [7], reported good reproducibility in both HClO4 and H2 SO4 , but inferior in HNO3 . In the conditions of our work no such difference was observed. The apparent discrepancy is probably due to the different modes of pretreatment of the gold electrodes. 3.3. Analytical characteristics of ASV and SASV of Hg(II) The anodic stripping voltammogram (and the subtractive anodic stripping voltammogram) peak potential at mercury concentrations of 0.2–100 nM, in
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10 mM NaCl and 10 mM HNO3 is about +0.65 V versus Ag/AgCl, 1 M KCl. It is positive by more than 0.5 V in respect to the reversible potential of the Hg(II)/Hg couple, as it is related to the underpotential deposition/dissolution phenomenon of mercury adatoms. Being deposited at an underpotential, the adatoms are distributed uniformly on the electrode surface and under the experimental conditions in the nanomolar concentration range, the coverage is less than 0.1%. 3.3.1. Anodic stripping voltammetry versus subtractive anodic stripping voltammetry ASV and SASV scans of 5 to 20 nM Hg(II) and Cu(II) solutions are presented in Fig. 6. The advantage of the SASV is clear: (i) the peaks are better defined, (ii) the background currents are smaller and calibration plots with higher correlation coefficients are obtained. At concentrations higher than 20 nM both stripping methods (ASV and SASV) yield good analytical results. At concentrations lower than 15 nM
Fig. 5. Peak potential of the subtractive anodic stripping voltammograms at different concentrations of NaCl, NaF, Na2 SO4 , NaNO3 , HClO4 . Conditions as in Fig. 4.
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Fig. 6. Anodic stripping voltammograms and subtractive anodic stripping voltammograms at the Au-RDE. 4, 8, 12 and 20 nM Hg(II) and 5, 10, 15, 20 nM Cu(II) in 10 mM HNO3 , 10 mM NaCl solution. Condition of electrodeposition: Edeposition : −0.4 V; tdeposition : 30 s; 5000 rpm. SW mode, square-wave amplitude 10 mV; step amplitude: 2.5 mV; frequency: 25 Hz.
only the subtractive anodic stripping voltammetry is suitable for analytical uses. 3.3.2. Correlation of the analytical signal with deposition potential, concentration, deposition time and speed of rotation Stripping voltammograms with different deposition potentials were recorded at 60 nM Hg(II). The repeatability and the magnitude of the analytical signal were found to be independent of the deposition potential in the potential range of (+0.55)– (−0.4) V. A linear relationship is observed between the analytical signal and (a) CHg(II) in the range of 0.2–400 nM (cf. Figs. 7 and 8), (b) time of deposition in the tested time range of 15–360 s (cf. Fig. 8) and (c) rate of rotation in the range of 1000–9000 rpm. The quality of the respective plots is very good. The correlation coefficient is better than 0.999.
Deviation from linearity occurs when the product CHg(II) ·N1/2 ·tdeposition exceeds the value of 1×106 nM rpm1/2 s (N is the rotation rate in rpm). The product CHg(II) ·N1/2 is proportional to the mass transport limiting current of deposition of mercury. The mercury coverage on the electrode at the point of break from linearity corresponds to about 5% of a monolayer (the estimation is based on calculations similar to those performed in Section 3.1). Similar behavior was observed in the case of determination of lead on a silver electrode [13,14] and copper on gold [15]. The condition CHg(II) ·N1/2 ·t deposition < 1 × 106 nM rpm1/2 s is useful for estimating the upper limit values for given CHg(II) ·N and tdeposition at which linearity is maintained. slope
3.3.3. The sensitivity, SHg , and the detection limit slope
The sensitivity, SHg , of the method is determined from the calibration plots as the slope of the analytical
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Fig. 7. Subtractive square wave anodic stripping voltammetry, SASV, of Hg(II) at the Au-RDE and the respective calibration curves. The values in the calibration curves are corrected for the respective blanks. Solution composition: 10 mM HNO3 , 10 mM NaCl and Hg(II). Condition of electrodeposition: Edeposition : −0.4 V; 5000 rpm; SW mode, square-wave amplitude: 10 mV; step amplitude: 2.5 mV; frequency: 25 Hz.
response versus concentration obtained in the supporting electrolyte used in this work (10 mM HNO3 and NaCl): slope
SHg
= (5 ± 0.5)AV−1 cm−2 M−1 s−1 rpm−1/2 , slope
where SHg
=
analytical response (slope) AAu CHg2+ tdep N 1/2
and AAu is the electrode area of the gold electrode. The detection limit was determined as three times the standard deviation of the analytical signal of a
0.4 nM in Hg(II) solution. The detection limit for a 120 s electrodeposition at 5000 rpm was 0.05 nM. 3.3.4. Short and long term stability of the Au-disc electrode The repeatability of the SASV, expressed as the relative standard deviation of seven consecutive experiments performed in a rigid, non-interrupted sequence using the project feature of the GPES-Autolab software is less than 1% for a 1 nM solution at 90 s deposition time.
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Fig. 9. Subtractive anodic stripping voltammogram at the Au-RDE of a mixture of Pb, Bi, Cu and Hg. Supporting electrolyte: 10 mM HNO3 , 10 mM NaCl. Concentration of the metal ions: 10 nM each. tdeposition : 30 s. Other conditions for SASV as in Fig. 7. Fig. 8. SASV Calibration plots for Hg(II) at the Au-RDE for different deposition times, as marked on plots. Other conditions as in Fig. 7.
Over long periods of time the analytical signal fluctuates to about 10%. 3.3.5. Proper functioning of the electrode The following four criteria can be used to determine the functioning of the electrode in 10 mM NaCl, 10 mM HNO3 . In case one of them is not fulfilled, the electrode should be pretreated electrochemically with or without mechanical polishing. 1. The width of the peak at half height: 55 ± 5 mV. 2. The repeatability: below 2% for the 1–200 nM concentrations range of Hg(II). slope 3. The sensitivity, SHg : (5±0.5) AV−1 cm−2 M−1 s−1 rpm−1/2 . 4. The correlation coefficient of a calibration curve contains at least three nines. 3.3.6. Determination of mercury in the presence of copper, lead and bismuth The determination of mercury is unaffected by the presence of copper, lead and bismuth in the solution. The linearity of the calibration curve of mercury is maintained also in presence of 100 nM of copper, lead
and bismuth. A subtractive anodic stripping voltammogram of an equimolar mixture (10 nM) of the four elements is shown in Fig. 9. The peak of mercury is sharp and very well defined compared to the other peaks. The peaks of mercury and copper are well separated while the copper and bismuth peaks are overlapping. 3.3.7. Determination of mercury in urine samples A pretreatment of the sample prior to the analysis of mercury is required [9,19], (a) to release the metals bound as organic complexes and (b) to disrupt the organic ligands in order to minimize the problem of background currents and fouling of the working electrode. The pretreatment applied in this work consists of wet digestion with nitric and perchloric acids (cf., Section 2.9). The validity of the wet digestion method was tested by the degree of recovery of mercury from five samples of urine to which a known concentration of Hg(II) (100 or 200 nM) was added. The average recovery was 95% with a R.S.D of 5%. The analytical determination of the certified urine sample (H-99-03) is displayed in Fig. 10. The stripping curves are very well defined, although the
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Fig. 10. Determination of Hg(II) in certified urine sample (H-99-03) using the method of standard additions. The sample was pretreated and prepared according to Section 2.9. Each standard addition consists: 50.0 l, 1.00 M Hg(II) into a 4.00 ml sample. Edeposition : −.4 V; tdeposition : 180 s; N: 5000 rpm.
background currents are increased. The results of the mercury analysis for the certified samples are summarized in Table 1. The deviations in respect to the certified values are within 10%. The detection limit of mercury in urine samples for 180 s deposition time at 5000 rpm is 4 nM. This is considerably higher than the detection limit in synthetic samples in 10 mM NaCl, and HNO3 (without digestion) and is due to (a) impurities in the perchloric acid and (b) differences in matrix.
Interferences. Different metal ions were tested for possible interferences. The cations and their concentrations were chosen according to their toxic occurrence in urine. No interference was found up to the following limited concentrations: 2 M Pb(II), 2 M Cu(II), 0.5 M Cd(II), 0.5 M Cr(III) and 5 M Se(IV). Determination of mercury in waste water. The same method described above has been applied for the determination of Hg(II) and Cu(II) in waste water [20].
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Table 1 Determination of mercury in certified urine samples by ASV (six replicates) Urine Sample
Certified value of Hg (nM)
Mercury found (nM)
H-99-01 H-99-03
330 115
362 ± 21 108 ± 8
Acknowledgements This work was supported in part by the Israeli Ministry of the Environment. The authors thank Dr. Ralph Shain from the National Institute of Occupational and Environmental Health and Mr. Ronen Alkalay from the Dan Region for fruitful discussions. References
4. Conclusions The involvement of underpotential deposition in the anodic-stripping analysis of mercury has beneficial effects on the analysis. The coverage of the mercury adatoms on the gold electrode is in the range of 0.01–1%, depending on the concentration of mercury in the sample and on the duration of electrolysis. As a result, there are no structural changes on the gold electrode, as opposed to electrochemical measurements performed at high concentrations of mercury, where amalgamation causes profound changes. Thus, without the need of mechanical or electrochemical pretreatment, the reproducibility of the analytical signal and the stability of the electrode are excellent over long periods of use. The importance of the subtractive mode of anodic stripping voltammetry is demonstrated in this work. It improves the quality of the stripping curves and the detection limit by more than an order of magnitude. The detection limit of 50 pM for 120 s deposition time, can easily be improved by prolonging the time of electrodeposition, as is generally the practice. We have not done this because short electrolysis times decrease the contamination from the environment and permit analysis under normal laboratory conditions, without the need of clean rooms. The method was successfully applied to the analysis of Hg(II) in urine.
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