Ivanka Pižeta
email: pizeta@irb.hr

Gabriel Billon
email: gabriel.billon@univ-lille1.fr

Dario Omanović
email: omanovic@irb.hr

Vlado Cuculić
email: cuculic@irb.hr

Cédric Garnier
email: cgarnier@univ-tln.fr

Jean-Claude Fischer
email: jean-claude.fischer@univ-lille1.fr

Pseudopolarography of lead (II) in sediment and in interstitial water measured with a solid microelectrode

Keywords: Pseudopolarography, Ir solid microelectrode, Sediment, Interstitial water, Speciation, Lead (II)

Pseudopolarograms of lead (II) constructed from the voltammograms measured in situ in the sediment and in the interstitial water by using an Ir solid microelectrode with a thin mercury film have shown as a kind of fingerprints of the sample. Despite shortcomings when compared to measurements with the mercury drop electrode and in model solutions, the measurement procedure was adapted for enough signal repeatability, avoiding to a reasonable extent the memory effect and electrode surface blocking. To make the best use of the information available, besides the classical pseudopolarograms, i.e. besides the dependence of the peak-height on the deposition potential, it is necessary to analyze the peak-area, the peak-position and the half-peak width versus deposition potential, and combine them with the knowledge from various theoretical and model situations. They have shown to contain interesting information about speciation. This information is not always unambiguous, it is often semi-quantitative, and cannot be reached by other methods, however, in combination with other methods it could be useful for the characterization of the sample solution. Pseudopolarograms of lead (II) in different liquid fractions of the sediment were measured and compared, the electrode sensitivity varying from 4 to 20 nA/mol L -1 of lead (II). The differences in half-wave potentials recorded were ranging up to 0.6 V and those in the slopes of pseudopolarograms were three-fold, having interesting relationships with the peak potentials of single voltammetric curves.

Introduction

Pseudopolarography is a method appropriate for distinguishing the labile from the inert complexes of dissolved trace metals with ligands in their natural concentrations . It can be particularly applied for the speciation of trace metals that form amalgams, being based on anodic stripping voltammetry or lately on stripping chronopotentiometry as well [30][31][32][33]. A pseudopolarogram, which is similar in shape to the classical polarogram, is the dependence of peak-heights of a redox reaction on the deposition potential, which changes in subsequent voltammetric measurement cycles [1,3,4,7,8], and which reflects thermodynamic and kinetic properties of metal ions as well as geometric and hydrodynamic characteristics of the working electrode. The main parameters used for the speciation of trace metals using pseudopolarographic data are the half-wave potential and the limiting current. However, we can benefit from the entire shape of the pseudopolarographic curve, which provides a scanning image (fingerprint) of the investigated electrochemical reactions. It was shown that thermodynamic stability constant of the inert metal complex can be obtained from the difference between the standard potentials of the redox reactions of the free and of the inert complexed metals [10]. This theoretical concept is empirically used for the determination of stability constants of inert metal complexes using pseudopolarographic half-wave potentials, and is successfully applied for trace metals speciation of unknown natural aquatic systems [18,19,28,29]. From the ratio of the limiting currents of the labile and inert metal complexes (using known side reaction data) the calculation of the stability constant is possible, as well. The parameters relative to the redox reaction, such as the transfer coefficient and the rate constant, can be obtained through the analysis of the shape of pseudopolarographic wave [34,35].

Theoretical background for the pseudopolarography has been developed for the hanging mercury drop electrode [4,7], for the thin film electrode under different conditions [8,11,36] and for the mercury ultramicroelectrode [9]. The automation of the pseudopolarographic measurement is crucial for its wider employment and has been achieved with the nowadays instrumentation and the existing software [23].

There is often a considerable discrepancy when measuring and examining model solutions with well-defined composition and the measuring of the samples of natural, polluted or pore waters. This is truth already when the working electrode is a mercury hanging drop electrode, with the surface renewed for every cycle of the measurement, but if instead, the working electrode is a solid electrode, more problems could be encountered, something to be worried about. In that case it cannot be counted on single "clear" effects, one by one thoroughly elaborated in the literature, to appear one at a time, instead a superposition of various effects should be expected. Anyhow, voltammetric in situ measurements are restricted to the solid electrodes that can be moved vertically to characterize depth profiles, or they can follow the evolution in time of a single sampling site either by simple repeating of measurements or by gradually changing one of the parameters of the method as in the case of pseudopolarography.

Lead is a toxic, nonessential element, thus its increased concentrations in the media affect living organisms inducing negative physiological effects [START_REF] Kaim | Bioinorganic Chemistry: Inorganic elements in the chemistry of life[END_REF]. During the early diagenetic processes, iron oxides were reduced, which lead to releasing Pb 2+ into pore waters, while the production of sulphides tended to (co)precipitation of Pb 2+ (mainly as PbS or Pb associated with FeS 2 ) [START_REF] Huerta-Diaz | [END_REF]39]. Nevertheless, partial redissolution might occur at the oxic-anoxic interfaces [40,41,[START_REF] Huerta-Diaz | [END_REF], especially when the concentration of lead in high contaminated sediments reaches nearly 1 g kg -1 . Moreover, significant enrichment of trace elements in interstitial waters has been explained by the effect of complexing by organic substances [START_REF] Hong | Interstitial waters[END_REF]. This explains why the concentrations of most of the trace elements in interstitial waters were significantly higher than could have been predicted from their sulphide solubility [START_REF] Huerta-Diaz | [END_REF][START_REF] Billon | [END_REF]. On the other hand, lead is a classical element to be investigated by voltammetry and a mercury drop as a working electrode as it forms amalgams, and even a large number of pseudopolarographic measurements were made in model and natural solutions with lead as the analyte [3,8,10,12,21,24,27,[29][30][31]34].

Pseudopolarograms obtained in the experiments described in this work differ from the theoretical assumptions and calculations, or at least they represent a combination of several ones. Shifts of half-wave potentials and changes in the slope due to the complexation, changes in reversibility and metal-ligand ratios have been elaborated theoretically [7][8][9][10][11]30,32,36], as well as the existence and the effects of the combinations of electrode reactions (E) with coupled homogeneous chemical reactions (C), such as the CEC preceding and following reactions [START_REF] Bard | Electrochemical Methods, Fundamentals and Applications[END_REF].

Since pseudopolarography is here considered as a kind of fingerprint of the measured sample, or the place near the electrode put in situ, retrieving more complex information from this composite voltammetric method by tracing not only the peak-height and the peak-area, but also the peak-potential and the half-peak width versus deposition potential is proposed.

Experimental

Electrode preparation

The experiments were performed with an Ir microelectrode, designed as described elsewhere [START_REF] Pižeta | [END_REF] for the similar Au and Ag microelectrodes. This electrode was 75 m in diameter and the procedure of surface activation differed from the previous works in the part related to the application of mercury. Mercury was applied electrochemically from a 0.01 M solution of Hg(NO 3 ) 2 at -0.4 V versus Ag/AgCl, [KCl] = 3 mol L -1 . A very thin film of mercury was deposited in order to avoid memory effect, which was observed with a hemispheric mercury drop. Under the microscope neither the mercury layer nor the mercury drops were observed. Kept in distilled water, the electrode was stable for several days. The sensitivity of the electrode was checked by the standard addition method with lead (II) and cadmium (II), before and after the experiments.

Instrument and chemicals

A µAutolab 2, multimode polarograph with the software package GPES 4.8 (Ecochemie, Utrecht) was used. A Metrohm Stand 663 was used for mercury deposit and calibration steps, where a facility for proper purging with nitrogen was needed. Purging with nitrogen was also applied when measuring the interstitial water samples in order to prevent any oxidation of the medium. Stirring the solution was not applied in order to approach in situ conditions, cell volume was 20 ml, and supporting electrolyte for calibration measurements was filtered seawater of 31 salinity. The reference electrode was an Ag/AgCl electrode and the counter electrode was a Pt wire. The reference electrode for in situ measurement was a Unisense Ag/AgCl electrode and the counter electrode was a robust Pt rod electrode. The applied methods were square wave voltammetry (SWV) for measuring, and cyclic voltammetry (CV) for stabilizing and cleaning the electrode [START_REF] Pižeta | [END_REF]. All the potentials were measured versus Ag/AgCl. If not stated otherwise the following conditions were applied: for SWV -conditioning potential E cond = -0.1 V, conditioning time t cond = 60 s, deposition potential E dep from -1.3 to -0.9 V or to -0.32 V, deposition time t dep = 60 s, equilibration time t eq = 5 s, frequency f = 50 Hz, step potential st pot = 0.00195 V, SW amplitude A = 0.025 V, initial potential

E init was -1.3 V or -0.9 V, final potential E final = -0.1 V; for CV -start potential -1.8 V, first vertex potential -0.1 V, second vertex potential -1.8 V, step potential 0.00198 V, scan rate 1 V/s, number of scans 20.
The following chemicals were used during the experiments: Hg(NO 3 ) 2 •H 2 O, Merck p.a.; PbCl 2 standard, Merck Titrisol; HNO 3 , Merck s.p. Ultrapure water was generated using Milli-(Q) Plusfilter apparatus (Millipore).

Sample preparation

The site where sediment and water used in this experiment were sampled is located in the River Deûle canal, near the town of Dourge (Northern France), approximately 2.5 km downstream from a former lead and zinc smelter, closed recently in 2003. This area is highly polluted and metal composition of the water and sediment was detailed in a previous work, published recently [46]. About 10 kg of sediment was collected with a Van Veen grab and put in a plastic bag. In the laboratory, the sediment sample was homogenized, put in a plastic box and left for 1 month at room temperature to achieve a new equilibrium. In addition, River Deûle canal water was also sampled at the same time and added on the surface of the sediment to prevent any drying process. After this rest period, quasi in situ measurements have been undertaken at 2 cm depth under the water-sediment interface. Interstitial water was afterwards sampled at this depth as follows: small perpex tubes were introduced in the sediment and the cores were put in a glove box flushed with nitrogen to avoid any oxidation of anoxic sediment. The core slices between the depth of 1.5 and 2.5 cm were put in polycarbonate flasks under nitrogen, centrifuged at 3000 rpm, and the overlaying fluids were recovered carefully. For some experiments, pore waters were afterwards filtered on a 0.45 m membrane filter (Alltech, cellulose acetate). These samples were introduced with a syringe in the purged voltammetric cell, previously set on the Metrohm stand, and used without any other pretreatment.

Results and discussion

The progress in the Ir solid microelectrode surface preparation, and the differences compared to our previous results on silver and gold microelectrodes [START_REF] Pižeta | [END_REF], as well as the adjustment of voltammetric parameters for pseudopolarographic measurement in situ and in interstitial water samples, were tested and checked on natural samples. The electrode sensitivity varied between 4 and 20 nA/mol L -1 , and its detection limit was about 3 × 10 -8 mol L -1 .

Four different systems were measured: filtered interstitial water with added lead (II), non-filtered interstitial water without the addition and with the addition of lead (II) ions and sediments, respectively. In each case a set of anodic stripping square wave voltammograms was recorded, necessary for the construction of a pseudopolarogram. The results will be presented with the emphasis on three main relationships: the peak-height (i p ) (or the peak-area (A p )), the peak-potential (E p ) and the half-peak width (w 1/2 ) versus the deposition potential (E dep ).

Filtered interstitial water sample-testing of the electrode

Filtered interstitial water was used as a matrix for testing the electrode for the stability in pseudopolarographic type of measurements. At its natural pH of 8.6 or at the pH of 2.13, obtained with the addition of HNO 3 , no reactant could be found. Subsequently, the concentration of 5 × 10 -6 mol L -1 of lead (II) ions was added to ensure a signal large enough to test the electrode. Under the project mode of GPES 4.8 program, six cycles for the construction of 64-point pseudopolarograms were measured automatically, one after the other, which lasted 16.5 h altogether. The results for the first and the sixth pseudopolarograms are given in Fig. 1. Comparing the curves of those two cycles of measurements, first to be noticed is a certain peak-height enhancement (Fig. 1A) as well as a complete peak potential shift and a slight peak broadening (Fig. 1B) occurred during the time elapsed. There are many propositions in the literature for the observed shift, and it could be ascribed to the contamination with organic material [9]. To avoid that, or to diminish at least, in the subsequent analyses a protocol based on cyclic voltammetry was established (20 cycles between -1.8 and -0.1 V with the scan rate of 1 V/s) before a new pseudopolarogram, which proved helpful in "unblocking" the electrode. However, each peak-current pseudopolarogram was regular, with no gradient in the limiting current region for the first curve, showing no heterogeneous inert complexes present in the solution (only one-step pseudopolarogram) and, what was important to us, a stable behaviour of the solid electrode. The peak-position pseudopolarograms (Fig. 1B) shows an excursion of about 30 mV in the positive direction when the deposition potential approached the half-wave potential of the redox reaction. This was also shown by Town and van Leeuwen [31] in the case of Pb(II) alone, and it is an indication that the filtered and acidified sample had no inert complexes present in the solution, at least not in the potential window of the method. The half-width of the peaks, as well as the shape of the single voltammograms, indicate simple and reversible electrode reaction (Fig. 1A inset).

Non-filtered interstitial water sample

The following experiment witnesses the instability of the interstitial water sample despite the precautions taken according to the protocol, as well as its promptitude for changes. Interstitial water, freshly prepared under nitrogen atmosphere was transferred to the voltammetric cell. A few first SWV measurements showed a peak at about -0.6 V, which disappeared shortly afterwards. (The same reaction will be more stable in the sediment). Applying cyclic voltammetry, cleaning the electrode at -2 V or washing with distilled water did not help with the reappearance of the peak. After lowering the pH from 8.60 to 4.07 with the addition of HNO 3 , which induced certain redistribution of species, the peak appeared again. Three pseudopolarograms were recorded, each of them constructed from 38 voltammograms. The pH of the solution after the first pseudopolarogram rose to 4.90, and after the third one the pH obtained was 5.86, which could be ascribed to the buffering capacity of the non-filtered water sample. The first pseudopolarogram is presented in Fig. 2. In Fig. 2A interesting peak-current and peak-area pseudopolarograms are presented which decline from the theoretically ideal shapes. The peak-position and the half-width versus the deposition potential curves are presented in Fig. 2B. A three dimensional (3D) presentation of all 38 square wave voltammograms is given in Fig. 2C. Partially similar cases were described in the literature [27,21,31,47] however with better defined model solutions. Current decay at the very negative deposition potentials has not been reported yet, to the best of our knowledge. It was present in all the following cases at different degrees. Current decay might be connected with hydrogen evolution combined with some other source of perturbation on the electrode surface, while in the case of filtered interstitial water sample shown in Fig. 1, where pH was 2.13, such behaviour was not present. This might be due to the blocking of electrode by natural organic matter present in the solution with potential dependent surface-active qualities [48 p. 299]. The peak shift for the entire pseudopolarogram reads ca. 180 mV and its direction is towards the more negative values as the deposition potential advances towards the more positive values. Similar behaviour was ascribed to the effect of ligand concentration at the electrode surface [27,31,47,[START_REF] Mota | [END_REF].

When the pH of the same solution was lowered to 2.13 with the addition of HNO 3 as in the case of filtered interstitial water sample, a peak at -0.6 V disappeared, but another peak at -0.33 V appeared, irregular in shape, i.e. double, but reversible. Finally, 6.3 × 10 -6 M Pb(II) was added to the solution. After a few single measurements and 3 h of equilibration when it was clear that the "free" lead was prevalent and when it was well checked that the cleaning period at -0.1 V for 60 s is sufficient for avoiding memory effects, a new four series of pseudopolarograms were measured. The second pseudopolarogram is presented in Fig. 3. The pseudopolarogram looks like those of the filtered interstitial water by the general shape of peak current, potential and half-width. However, a step at about -0.5 V indicating a non-labile complex is present in the peak current versus deposition potential curve (Fig. 3A). The peak potential vs. deposition potential curve has a more complex but reproducible shape in all four repetitions. It should be noticed that the peak potential shifts towards the more negative values in the region of inert complex reoxidation, as in the case of interstitial water at natural pH shown in Fig. 2, and it shifts towards the more positive values in the region of free lead reoxidation, which was the case with the behaviour of lead in filtered solution (Fig. 1B).

Sediment

The following measurements were performed in the sediment, prepared as described previously in Section 2. The reference and the counter electrodes were put into the sediment and the microelectrode was positioned with a micromanipulator. The depth profiles were not the subject of this presentation, but the signal considerably changed with the depth, dependent on the concentration of oxygen, and also with the position inside the sediment. Certain unhomogeneity was observed, so the authors incline to the theory of the existence of micro-niches with different characteristics, noticed as well by using the DGT and DET techniques [50]. There were positions where only one reversible peak was found at -0.441 V, the position where two peaks were simultaneously present, one more negative and irreversible at -0.595 V and one more positive and more reversible at -0.444 V. At the position where high response of the more negative peak was found (this peak was huge in comparison with the more positive one), the entire pseudopolarographic measurement, with in-between CV cycles to unblock the electrode, was performed several times. Here, we are especially aware of the heterogeneous nature of the ligands, their possible adsorption on the electrode, and even the uncertainty of the reactant. From other analyses it was known about an increased concentration of lead in the interstitial water, even a few orders of magnitude larger than in the total sediment [46], so the working hypothesis of this experiment was that under the anoxic conditions that we reach with the solid microelectrode, we "see" the part of the dissolved lead inertly complexed with the ligands around, which rapidly changes its speciation when exposed to oxygen and/or low pH values.

One series of the resultant pseudopolarograms are shown in Fig. 4. presenting peak-height, peak-area, peak-position and half-peak width versus deposition potential. The shape of the I p versus E dep is again different with the expressed maximum before the decay of the current. The insight to the individual voltammograms clearly indicates the double nature of the peak implying the existence of different groups of ligands. The peak-potential behaved as in the case of charged interstitial water sample (Fig. 2) and showed an excursion of about 130 mV in the negative direction as the deposition potential became more positive. This shows to the existence of low concentration of strong complexing ligands as explained earlier [27,21,31,47].

Analyses of the signals

The "so-called" logarithmic analysis of the currentvoltage (i-E) curve [48 p. 129], in our case pseudopolarograms from Figs. 1234, given by the expression: log(i di/ i) = αnF/RT (E -E 1/2 ) = αn/0.058(E -E 1/2 ), where i d is limiting current, α is transfer coefficient, n is number of electrons in the reaction, F = 96500 C, R = 8.314 J mol -1 K -1 , T = 293 K, E 1/2 is half wave potential, potential at which the value of log(i di)/i is zero, is shown in Fig. 5. The obtained half-wave potentials and the slopes of the linearized pseudopolarograms expressed in αn are summarized in Table 1. Besides the differences in the half-wave potential and the reversibility of the reactions, there is a remarkable difference ( E = E p -E 1/2pse ) in the distances of the peak potentials (E p ) and the half-wave potentials (E 1/2pse ) of the pseudopolarograms, which is shown in Fig. 6. This shows to the existence of stronger complexes in the non-filtered interstitial water of higher pH (2a and 2b) and in sediment (4) than those in the filtered interstitial water of lower pH (1, 3a and 3b). This also indicates to the existence of complexes prompt to changes during sampling and off-situ measurements. An inversion occurred as well. The peaks of the SW voltammograms in the sediment were more negative than the other peaks for each of the given deposition potentials (inset in Fig. 6, curve 4). However, the corresponding pseudopolarographic half-wave potential was more positive than in the case of the non-filtered interstitial water, thus the E of this latter is more important (Fig. 6, curves 2a and 2b compared to curve 4). This matches with the perception of higher solubility and mobility of the reactants in anoxic conditions found in the sediments.

The fact that the peak potentials of SW voltammograms of the four measured cases differ so much, besides variations within each pseudopolarogram, is an information about the complex structure of the matrix, causing the complication of the reaction with the preceding and the following reactions that implement the complex reaction schemes [START_REF] Bard | Electrochemical Methods, Fundamentals and Applications[END_REF] in both, the accumulation (reduction) and the stripping (oxidation) step. It would be interesting to compare those peak potentials with e.g. the peak potentials of the measurements of Rozan et al. [29] where no such information was given and where it was presumed that the peaks were at the potential of the free metal oxidation. There are, however, manifold differences between our case and the case just mentioned (reactant concentration range, type of the electrode), so a direct comparison could not be possible. Making an estimation of thermodynamic stability constants based on the differences between half-wave potentials of the pseudopolarograms of free and inertly complexed reactant, like those given in the literature [10,18,19,28,29] would be too uncertain, when the metal and the ligand side reactions are not known, as well as the half-wave potential of the uncomplexed reactant. At this stage we should acquiesce with witnessing and mapping the processes.

Conclusion

Microelectrodes that enable in situ measurements open new possibilities for detecting and characterizing metalligand complexes, especially those in anoxic conditions (very difficult to preserve during sampling), where the solubility of metal ions is significantly increased and their complexation with available ligands takes place. Pseudopolarography is a suitable method for the speciation -discerning between the free -labile and the inert metal complexes. Besides the peak-height versus deposition potential, the peak-area, the peak-position and the half-peak width versus deposition potential bear as well the information about the electrochemical processes, i.e. the characteristics of the species.

The results of measuring lead (II) in the sediment and its interstitial water characterize the sediment and also propose new challenges for modelling and simulating more complex reactions that occur in the so far rarely reachable conditions. Although the obtained pseudopolarograms differ from the theoretical ones, repeating of their shapes confirms the existence of useful information.

Our further work should concentrate on improving the sensor, i.e. the microelectrode, with respect of its sensitivity and stability, and on intensifying the fieldwork which should open new problematic and ideas to improve the measurement methods.

Fig. 1 .

 1 Fig. 1. Two series of 64-point pseudopolarograms in a filtered interstitial water sample of pH 2.13 with 5 × 10 -6 mol L -1 of added lead (II). (᭹), first and ( ) sixth pseudopolarogram, respectively. (A) Peak-height vs. deposition potential; (B) peak-position and half-peak width vs. deposition potential; inset of A: a single SW voltammogram measured at E dep = -0.430 V. (1) Net current, (2) forward current, (3) backward current.

Fig. 2 .

 2 Fig. 2. A 38-point pseudopolarogram of a non-filtered interstitial water sample of pH 4.9. (A) Peak-height and peak-area vs. deposition potential; (B) peakposition and half-peak width vs. deposition potential; (C) 3D presentation of 38 SW voltammograms used for the construction of pseudopolarograms in (A) and (B); inset of (A): a single SW voltammogram measured at E dep = -1.200 V. (1) Net current, (2) forward current, (3) backward current.

Fig. 3 .

 3 Fig. 3. A 64-point pseudopolarogram of a non-filtered interstitial water sample of pH 2.13 with 6.3 × 10 -6 mol L -1 of added lead (II). (A) Peak-height and peak-area vs. deposition potential; (B) peak-position and half-peak width vs. deposition potential; inset of A: a single SW voltammogram measured at E dep = -0.570 V. (1) Net current, (2) forward current, (3) backward current.

Fig. 4 .

 4 Fig. 4. A 44-point pseudopolarogram of a sediment measured in situ. (A) Peak-height and peak-area vs. deposition potential; (B) peak-position and half-peak width vs. deposition potential; inset of A: a single SW voltammogram measured at E dep = -1.020 V. (1) Net current, (2) forward current, (3) backward current.

Fig. 5 .

 5 Fig. 5. Log analysis of pseudopolarographic curves shown in Figs. 1-4 vs. deposition potential. Numbers correspond to figures; (A) and (B) correspond to two slopes of one pseudopolarogram. The obtained half wave potentials and αn values from the slopes are given in Table1.

Fig. 6 .

 6 Fig. 6. Differences between the peak potentials and the half wave potentials of the pseudopolarograms obtained by log analysis shown in Fig. 5. Inset: dependences of peak potentials on deposition potentials for the pseudopolarograms shown in Figs. 1-4.

Table

Table 1

 1 Half-wave potentials and αn values from the logarithmic analysis of pseudoploragrams from Figs.1-4

		E 1/2 /V ± CI	αn ± CI
	F1	-0.426 ± 0.003	1.026 ± 0.051
	F2a	-1.025 ± 0.005	0.791 ± 0.054
	F2b	-1.002 ± 0.006	2.201 ± 0.335
	F3a	-0.565 ± 0.004	2.182 ± 0.291
	F3b	-0.455 ± 0.003	1.167 ± 0.066
	F4	-0.969 ± 0.002	1.646 ± 0.109
	CI = confidence interval for 95% confidence limits.

Acknowledgements

This research was supported by PNETOX and the Region Nord Pas-de-Calais [CPER and INTERREG (Franco-Flamand Programs)], as well as by COGITO, program of integrated actions by ÉGIDE. I.P. has benefited from the CNRS grant. The financial support of the Ministry of Science, Education and Sports of the Republic of Croatia (through the Grant No. 0098121) is gratefully acknowledged. We would like to thank two anonymous reviewers for their comments and suggestions.


Silje M Ø Skogvold
email: silje.skogvold@chem.ntnu.no

Mikkelsen Øyvind
email: oyvind.mikkelsen@chem.ntnu.no

Billon Gabriel
email: gabriel.billon@univ-lille1.fr

Garnier Cedric

Lesven Ludovic
email: ludoviclesven@univ-lille1.fr

Jean-Francois Barthe
email: jean-francois.barthe@univ-lille1.fr

Mikkelsen

G C Billon

L Garnier
email: cgarnier@univ-tln.fr

J.-F Lesven

Barthe

ORIGINAL PAPER

Keywords: Voltammetry, Solid electrodes, Real samples

Microelectrodes of silver-copper alloys have been evaluated for use in voltammetric analyses. Increased overpotential towards the hydrogen overvoltage reaction (HER) was found as a function of increased copper content in the silver. A study of oxidizing products by cyclic voltammetry (CV) in NaOH solution showed ten anodic and eight cathodic peaks which are described in the present paper. The behaviour of these alloy electrodes is somewhere between pure silver and pure copper electrodes. Differential pulse anodic stripping voltammetry (DPASV) was used to measure zinc, cadmium and lead in ultrapure water only (18 MΩcm), and good linearity was found for all metals (r 2 =0.998) in the range of 0.5 to 5 ppb with a 600-to 1,200-s plating time. It was additionally found that cadmium and lead were better separated on the alloy electrodes compared to pure silver electrodes. Measurements of nickel were carried out on alloy electrodes by use of adsorptive differential pulse cathodic stripping voltammetry (Ad-DPCSV), and good linearity (r 2 =1.000) was found in the range from 0.5 to 5 ppb with an adsorption time of 120 s. The alloy electrodes were also found to be sensitive to nitrate, and good linearity (r 2 =0.997) was found in the range from 1 mg L -1 to 100 mg L -1 using differential pulse voltammetry (DPV) scanning from -450 mV to -1,500 mV. Addition of nitrate in ultrapure water afforded two different peaks related to the successive reductions of nitrate and nitrite. In ammonium buffer solution (pH 8.6) only one peak resulting from reduction of nitrate was observed. Furthermore, the use of alloy electrodes containing 17% Cu was tested in real samples, by installing it in a voltammetric system for monitoring of zinc and lead in a polluted river, the river Deûle, near the town of Douai in northern France. Results were found to be in agreement with parallel measurements carried out by ICP-MS.

Introduction

Increasing focus on environmental surveillance [1, 2] has created a demand for continuous improvements and development of usable field instruments and sensors. Heavy metals represent an important group of contaminants, and monitoring of these species in the field is of great importance to avoid serious environmental damage. Pollution by heavy metals, even in small concentrations, may have deadly influences on the surroundings [3][4][5].

The most frequently used methods in environmental monitoring today are atomic absorption spectroscopy (AAS) and inductively coupled plasma-mass spectroscopy (ICP-MS). However, in order to carry out continuous online monitoring, these methods are too complicated. Electrochemical techniques represent an alternative, fastmeasuring and inexpensive group of methods useful for monitoring heavy metals. Different types of voltammetric methods are particularly suitable [6][7][8][9][10][11][12][13][14][15][16][17], and voltammetric apparatus for online monitoring of metal speciation can be constructed [18].

In order to succeed in making voltammetric field apparatus for online monitoring of heavy metals, a critical factor is to develop usable electrodes. Liquid mercury represents the most frequently used material for electrodes in voltammetry; however, in field apparatus a liquid mercury electrode is difficult to install, and solid electrodes represent an easier alternative. Intensive research has been carried out in recent years in order to develop useful solid electrodes [18][19][20][21][22][23][24][25][26][27]. However, the main problems with solid electrodes are lack of overpotential towards the hydrogen evolution reaction in water solutions, and problems with the stability over time due to adsorptions of organic or charged compounds on the electrode surface or formation of insulating oxide layers.

A proper combination of chemical and electrochemical cleaning may overcome the problem of passivation to some extent, and increase the time between manual maintenance of the electrodes long enough to make it of potential use in the field [28,29]. However, the problem with low overvoltage towards the hydrogen evolution reaction still has to be dealt with, since most solid metal electrodes actually have a relatively low overvoltage towards hydrogen evolution. This makes them unsuitable for analyses of several important metals like nickel, cobalt, zinc, iron and cadmium. Studies of specific mixed metal electrodes, containing two or more metals, have shown a significant increase in overvoltage towards hydrogen evolution and are promising for use in field apparatus. Of specific interest are electrodes containing silver doped with one or more metals (e.g. mercury, bismuth or copper) which have a high overvoltage towards the hydrogen evolution reaction [30][31][32].

Nitrate is another important compound to measure in environmental monitoring, because of its important influence on the environment. Several papers have been published on the subject of fundamental processes associated with the reduction of nitrate with various metal substrates [48][49][50][51][52][53][54][55][56]. However, only a few papers focus on the analytical aspect, and the possibility of using voltammetric techniques with solid electrodes for determination of nitrate in the environment [57][58][59][60][61][62][63]. Development of sensitive methods for measuring nitrate in the field is therefore of great interest and importance.

Performing voltammetric analyses in the field also requires a proper system for adding supporting electrolyte and stirring the solution. Even though it is possible to construct such a system, it should as easy as possible to build. Measurements with microelectrodes makes it possible to avoid use of supporting electrolyte and in some cases also the agitation of the solution during the plating time in differential pulse stripping voltammetry [33]. Furthermore, the sensitivity is improved owing to an improved ratio of faradic to charging current and signal to noise ratio. Because of this, the use of such small electrodes has great advantages for use in field apparatus. Several papers dealing with microelectrodes have been published [34][35][36][37][38], and silver and gold microelectrodes are of special interest. In this contribution 80-μm electrodes of have been used. These electrodes represent the upper limit regarding the definition of microelectrodes; however, microelectrode behaviour was observed.

In view of the problems discussed above concerning overpotential towards the hydrogen evolution reaction, passivation and formation of oxide layers, and stability over time, voltammetric electrodes should be made in a way as to overcome these problems as best as possible. This paper deals with a study of mixed microelectrodes of silver and copper. Such alloys are noted for their good stability, serving as materials in jewellery for hundred of years. Furthermore, when alloying these metals, structural changes in the surface could improve the overvoltage towards the hydrogen evolution reaction, something which is studied in this paper. Some practical applications of these microelectrodes in real samples of polluted river water are also demonstrated.

Experimental

Analyses were performed with voltammetric equipment from PalmSens (The Netherlands) together with a threeelectrode system using a platinum counter electrode, an Ag/AgCl/KCl (3 M) reference electrode, and silver-copper alloys were used as working quasi-microelectrodes (d = 80 μm). Additionally, as specified in the text below, some analyses were performed with a homemade potentiostat. Oxygen was not removed from any of the samples.

The silver alloys were ordered as wires from K. A. Rasmussen, Hamar, Norway. The alloys tested were silver 830 (83% silver and 17% copper) and silver 925 (92.5% silver and 7.5% copper). Additionally, pure silver wire (99.99%), ordered from Goodfellow Company (UK), was used for making silver microelectrodes for comparative measurements. The electrodes were made in a similar way to gold and silver microelectrodes described elsewhere [39], by gluing together 4-5 cm of the silver or silver alloy wires to a copper wire connector and then sealing the respective wires in polyethylene pipette tips (200 μL) at 500 °C. A second tip enclosing the whole device was finally stuck to the first one and fixed with non-conductive epoxy glue.

The electrodes were first polished on P2,500 and secondly on P4,000 (Struers) grid papers, before polishing with diamond paste (1 μm, Struers). After polishing, the electrodes were checked with an optical microscope (Reichert, Austria) at magnification from 110× to 320×. The electrodes were rinsed under ultrasound for 60 s, first in ultrapure ethanol and then in ultrapure HNO 3 (10%) solution before they were rinsed in ultrapure water (18 MΩcm). Finally, the electrodes were checked by cyclic voltammetry, scanning 9 cycles in ultrapure water (18 MΩcm) between -1,350 mV and -50 mV, starting from -350 mV at a scanning rate of 100 mV s -1 .

Standard solutions of zinc, lead and cadmium (100 μg L -1 , 500 μg L -1 and 1 mg L -1 , 10 mg L -1 ) were prepared by dilution of 1,000 mg L -1 metal solutions (CertiPur, Merck) in deionised water, which was subsequently purified with a Milli-Q Plus apparatus (Millipore, resistivity 18 MΩcm). All other reagents (HNO 3 and HCl (suprapur, Merck), NaOH, NH 4 Cl, NH 3 , dimethylglyoxime, KNO 3 , C 2 H 5 OH) were of analytical grade.

Studies of overvoltage towards the hydrogen evolution reaction were performed by linear scan voltammetry in HCl (0.1 M) and HNO 3 (0.01 M) solutions with a scan rate of 15 mV s -1 . Oxide products were studied by cyclic voltammetry in NaOH (1 M) solutions. The electrodes were scanned 9 times between -1,300 and 800 mVat a scan rate of 100 mV s -1 , starting at -50 mV.

Laboratory measurements of zinc, cadmium and lead in ultrapure water (18 MΩcm) using the alloy electrodes were performed in a 30-mL cell system by differential pulse anodic stripping voltammetry. Plating time was 600 s and 1,200 s at -1,250 mV, the scan rate was 15 mV s -1 and modulation pulse was 70 mV. Similarly, nickel was determined by use of adsorptive differential pulse cathodic stripping voltammetry (Ad-DPCSV) with a working electrode consisting of silver containing 17% copper (silver 830). Standard additions of nickel were carried out in ammonium chloride solution adjusted to pH 8.6 with NH 3 , and dimethylglyoxime (210 μM).

Measurements of nitrate were performed with a silver electrode containing 17% copper, both in ultrapure water and in ammonium buffer solution (pH 8.6), using differential pulse voltammetry by scanning from -450 mV to -1,500 mV at a scan rate of 15 mV s -1 . Nitrate was added as KNO 3 . Prior to each scan, the electrode was kept at -450 mV for 20 s to stabilize the initial current and improve the baseline.

Measurements of zinc and lead in real samples were carried out in the river Deûle, a polluted river near the town of Douai, France. The sampling point was about 500-m upstream from a former metallurgical industry site (Metaleurop industry). Measurements were carried out for a period of 3 h using the portable PalmSens potentiostat. Each sample was quantified by using individual standard additions (two-point addition). Manual samples (filtered and unfiltered) were collected and acidified to pH 2 with HNO 3 , for comparative measurement by ICP-MS (X Series, Thermo Elemental). Manual samples (unfiltered) were also used to test repeatability by DPASV. Samples were automatically brought to the cell system using a plastic CO/TECH pump immersed in the river water. The pump was mounted on a floating stage by fixing the pump at a level of 0.5 m below the water surface. The pump supplied two at-line homemade Teflon cells (65-mL internal volume) with water samples. The first cell was used to perform pH measurements, using a pH-combined microelectrode (Radiometer) linked to a pH meter (Meterlab PHM210, Radiometer). The second cell was used for voltammetric analyses of zinc and lead using the PalmSens voltammetric equipment. The working electrode was a silver-copper electrode (silver 830). Both cells were equipped with magnetic stirrers (Radiometer), and were continuously stirred between the voltammetric measurements to prevent sedimentation of particles. The sampling and measurement sequence was initiated by rinsing both cells for 5 min with a continuous flow of river water through the system, while simultaneously cycling the voltammetric working electrode three times between -1,500 and -50 mV at a scan rate of 100 mV s -1 . The sampling pump was then stopped, and the magnetic stirrer installed in the voltammetric system was switched on during a plating time of up to 300 s. After a resting time of an additional 15 s, a DPASV scan was carried out.

Results

Overvoltage towards the hydrogen evolution reaction (HER)

An important aspect in finding useful electrodes for analytical purposes in voltammetry is to develop electrodes from materials with specific electrochemical properties, and of particular importance are properties concerning overvoltage towards the hydrogen evolution reaction (HER).

As seen in Fig. 1, a moderate increase in overvoltage towards HER both in HNO 3 and HCl solutions was found for the silver-copper alloy electrodes. The increase in overvoltage towards hydrogen in acid solutions is somewhat surprising, since silver and copper have similar properties concerning the overvoltage towards the hydrogen evolution reaction [START_REF] Wieckowski | Interfacial electrochemistry, theory, experiments, and applications[END_REF]. However, when an alloy is formed, its physical and chemical properties including the overvoltage towards hydrogen evolution may be significantly different from the pure metals. Changes in the surface chemistry can be explained from changes in the electronic structure of the surface, resulting in modifications of the adsorption energetics and altering the barrier for reactions. This effect may be local to an alloying atom, or it may be longer range. Furthermore, the introduction of different species into a surface creates chemically different sites on the surface. This provides a range of different sites for e.g. adsorption, and hence will also change the distribution of transition states available for reaction or dissociation [41]. The increasing overvoltage towards hydrogen could therefore be explained from a decrease in the total coverage of adsorbed hydrogen on the electrode surface, or due to changes in the activation energies for the hydrogen evolution reaction.

Stability and formation of oxides on the electrode surface

Investigations of the electrochemical behaviour of silver in NaOH have previously been carried out by Abd El Rehim et al. [42]. They found that the anodic polarization curve of silver in NaOH is characterized by five anodic peaks, and that the cathodic curve shows four peaks, one anodic and three cathodic. Studies of oxide layer formation on silvercopper alloys in NaClO 4 solutions and in NaOH solutions have shown additional peaks originating from copper and copper oxides in the alloyed electrodes [43,44].

This paper includes a study of silver-copper alloys containing down to 7.5% added copper, and therefore an alloy consisting of only a eutectic α-phase. In the papers referred to above higher amounts of copper were used and those alloys therefore consisted of mixtures of both eutectic α-phase and eutectic β-phases. It is therefore of interest to study if this difference significantly changes the electrochemical properties and the formation of oxide layers. A brief study of possible oxidizing products forming on the silver copper alloy electrodes was therefore carried out in NaOH (1 M) solution with cyclic voltammetry for electrodes of polycrystalline silver, silver containing 7.5% added copper and silver containing 17% added copper. Each electrode was cycled 9 times between -1,300 mV and 800 mV, starting from -50 mV.

The cyclic voltammograms, shown in Fig. 2, are in good agreement with findings in the work referred to above [42,43]. In the resulting voltammograms ten anodic peaks (eight in the anodic direction and two in the cathodic direction), and eight cathodic peaks in the cathodic direction were observed. In the anodic direction A1 to A8 presumably correspond to formation of Cu Due to the solubility data for copper in silver [START_REF] Hansen | Constitution of binary alloys 18[END_REF], the two electrodes studied in this work will have some difference in composition. In the silver electrodes contain-ing7.5% added copper only a eutectic α-phase (solid solution of copper in silver) is present. In the silver electrodes containing 17% added copper the structure will be a mixture of a eutectic α-phase and eutectic β-phase (solid solution of silver in copper).

Several changes were observed in the cyclic voltammograms, both as a function of number of scans and as a function of copper in the silver electrodes. In addition to the peaks A1, A2, A3, C6, C7 and C8, which are directly related to copper, a significant increase in peak A6 and decrease in peak A7 is also observed with increasing copper content in the silver electrode. Furthermore, the peaks A4, A5, C1, C4 and C5 are significantly decreased with an increase of copper in the electrode, and A4, A5, A8 and C1 are more or less not observed at all, while a significant peak C3 appears in the ninth voltammetric scan for the electrode containing 17% added copper as seen in Fig. 2.

The significant changes in peak A5 (corresponding to formation of Ag 2 O monolayer) with increasing copper in the electrode could be a result of copper dissolution overlapping with the A5 peak. This assumption is supported by the fact that the height of peak A5 increased significantly with number of cyclic scans for the electrodes containing added copper compared to pure silver electrodes, and could also explain the C3 peak which could be attributed to bulk deposition of copper. This observation could be explained by a possible increase in the amount of free copper ions building up near the electrode surface. An interesting point is that since copper is oxidized before silver and reoxidized after silver, the ratio of copper and silver ions near the electrode surface could change with increasing number of scans. This means that with an increasing number of scans the electrode surface could be coated by increasing amounts of copper. This assumption is supported by the fact that many of the anodic and cathodic peaks observed for pure silver are significantly decreased or not observed at all in the ninth cyclic scan of the electrodes containing added copper as seen in Fig. 2.

It is also evident that the anodic peaks A9 and A10, which occur in the cathodic direction, are absent in all voltammetric scans for the electrodes containing added copper, which could indicate that the formation of Ag 2 O 3 and further nucleation and growth of Ag 2 O 3 (A9) and Ag 2 O (A10) are suppressed when copper is present in the electrode. This is in agreement with the a report from Zaky et al. [44] in which they found that the dissolution rate of silver from alloys with copper is slower than from pure silver.

On the basis of the papers referred to above, and from the results obtain in this paper we conclude that addition of copper to the silver results in a slower release of silver ions, Fig. 3 DPASV of standard additions of zinc a 3, 5, 7 and 10 μg L -1 , b 0.5, 1, and 2.5 μg L -1 in ultrapure water (18 MΩcm). Used electrode was silver containing 17% added copper, scan rate was 15 mV s -1 , modulation pulse 70 mV and deposition times were 600 s (a) and 1,200 s (b) at -1,250 mV. Relative standard deviation was 3.2% (n=5) a slower passivation and therefore a more stable electrode surface.

Application of Ag-Cu electrodes in differential pulse anodic stripping voltammetry

Differential pulse anodic stripping voltammetry (DPASV) was carried out in ultrapure water to study the sensitivity of the important trace and heavy metals zinc, cadmium and lead. As seen from Figs. 3, 4 and 5, good linearity (r avg = 0.998) was found for all three metals in the sub-and low-μg L -1 range.

Only minor shifts in peak potentials for the studied metals were observed when comparing the different electrodes. As observed, the peaks for lead and cadmium were found to be relatively close. However, compared to pure sliver electrode the alloy electrodes showed a somewhat better separation of cadmium and lead, and the separation was found to slightly increase with amount of copper added. The somewhat improved separation was a result of a slight shift towards a more positive value for the cadmium peak, and a slight shift towards a more negative value for the lead peak. However, the observed shifts in peak potentials were only within a few mV (5 to 10 mV) in each direction. A difference of about 50 mV was observed using the silver electrode containing 17% added copper. An second and important observation was that the sensitivity for lead was significantly better compared to cadmium on the pure silver electrode, making it difficult to detect the cadmium peak when the concentrations of the two metals were equal. On the silver-copper alloy electrodes the sensitivity to lead was decreased with the amount of added copper, and for the silver electrode containing 17% added copper the sensitivity for lead and cadmium was approximately the same.

All peak potentials were further observed to be shifted compared to corresponding measurements using electrodes in the mm scale with added supporting electrolyte. The shift in peak potentials, compared to the condition using conventional electrodes and supporting electrolytes, can be explained from the fact that the measurements were carried out in ultrapure water only. However, some anions could be present in the solutions since all standard solutions were made of diluted metal nitrate solutions, but the contribution of nitrates from the standard additions is assumed to be relatively small or not significant. In addition, chloride may contribute by leakages from the reference electrode, but the presence of anions in the solutions should all in all be low. This was confirmed by separate measurements of the conductivity in samples where a reference electrode was Fig. 4 DPASV of standard additions of cadmium a 1, 3, 5, 7 μg L -1 and b 0.5, 1, 1.5 μg L -1 . All measurements were performed in ultrapure water using a silver electrode containing 17% added copper, scan rate was 15 mV s -1 , modulation pulse 70 mV and deposition times were 600 s (a) and 1,200 s (b) at -1,250 mV. Relative standard deviation was 2.4% (n=5) left for a period of time similar to the measuring time used in the analyses.

Normally, if anions like chloride are present in a significant amount, the potential for the cadmium peak is more negative than the peak potential for lead. The explanation for the significant shift in the peak potentials for cadmium observed here comes from the low ionic strength in the solutions, resulting in an IR drop significantly different from solutions with a higher ionic strength. Due to difference in diffusion constants the studied metals will be influenced to different extents upon the stripping process, and significant shifts in peak potentials are expected compared to solutions with higher ionic strengths [START_REF] Ciszkowska | [END_REF]. It is also important to consider the nature of the different types of electrodes, which plays an important role in the process.

As observed in Figs. 3, 4 and 5 there is a change in the slope of the standard addition curves between the higher concentration series compared to the slope for the low concentration series. However, the overall relative standard deviation for these measurements was on average 2.7%. Taking into account the relative standard deviation in these measurements, the interceptions are slightly outside the standard deviation for cadmium. The slightly positive Fig. 5 DPASV of standard additions of lead a 1, 2, 3, 5, 7, 10 μg L -1 ) and b 0.5, 1, 1.5, 2, 2.5 μg L -1 in ultrapure water using a silver electrode containing 17% added copper. The scan rate was 15 mV s -1 , modulation pulse 70 mV and deposition time were 600 s (a) and 1,200 s (b) at -1,250 mV. Relative standard deviation was 2.5% (n=5) Fig. 6 Measurements of nickel with Ad-DPCSV, using a working electrode of silver containing 17% added copper. Standard addition of 0.5, 1, 2.5, 5, 7.5 and 10 μg L -1 nickel, in ultrapure water containing DMG (210 μM) and ammonium buffer (pH=8.6). The scan rate was 15 mV s -1 , modulation pulse 70 mV and adsorption time 120 s at -450 mV. Relative standard deviation was 3.5% (n=5) interception is difficult to explain, but could be due to small changes in pH over the electrode surface due to the very long deposition time, and some temperature effects could also occur. With a 20-min deposition time, the total time at room temperature for the three standard additions will be over 60 min for each sample parallel, so this could explain some of the positive interception. Some smaller changes in the ionic strength as a result of adding the standards could also have an effect. For lead, a slightly negative interception was observed.

Application of the Ag-Cu electrode in differential pulse cathodic stripping voltammetry Nickel was detected by adsorptive cathodic stripping voltammetry by using dimethylglyoxime (DMG) complexation [47]. Adsorption of the nickel-DMG complex was not straightforward, as it initially was difficult to establish good linearity. However, by including a preparation step which involved applying a potential of -1,350 mV for 25 s on the working electrode just before the adsorption step, good linearity (r 2 =1) was achieved as seen in Fig. 6.

An explanation for the significant increase in linearity which was observed after including the preparative step can be found from the fact that between voltammetric measurements the electrode is rested at a potential where no current flows (open circuit potential). This potential was observed to be around -50 mV for the silver-copper electrode. Under these conditions adsorption of the nickel-DMG complex can occur, resulting in a non-linearity in standard addition analyses. Another explanation could be that oxide layers form on the electrode under these resting conditions and alter the properties of the electrode surface. By conditioning the electrode, by applying a large negative potential before a new measurement is performed, potentially adsorbed species and oxide layers will be removed: a freshly regenerated surface will be created, giving more reproducible results.

As seen from Fig. 6 two peaks were observed in the measurements of nickel. The peak observed at -950 mV can be assigned to increasing presence of nitrate, as the added nickel was from a standard nickel nitrate solution. Addition of nitrate only resulted in an increase in this peak. A further study on the detection of nitrate is described below.

Application of Ag-Cu electrodes for detection of nitrate

Several papers have been published on the subject of fundamental processes associated with the reduction of nitrate with various metal substrates [48][49][50][51][52][53][54][55][56]. However, only a few papers focus on the analytical aspect, and the possibility of using voltammetric techniques with solid Upper left part of the figure shows the voltammograms for several standard additions of nitrate ranging from 1 mg L -1 to 100 mg L -1 with linearity curve below. Upper right part shows the first 10 additions (ranging from 1 mg L -1 to 10 mg L -1 ) of nitrate with a linearity curve for the first 4 standard additions below. DPV scans from -450 mV to -1,500 mV, using a scan rate of 15 mV s -1 and modulation pulse of -75 mV. Measurements were performed within 1 min after addition electrodes for determination of nitrate in the environment [57][58][59][60][61][62][63].

From the study of nickel, it was found that silver-copper alloy electrodes could be useful for detection of nitrate. A study of the ability to determine nitrate on such electrodes was therefore carried out on a silver electrode containing 17% added copper (830 silver) using differential pulse voltammetry. Pretreatment of the electrode was carried out in ultrapure water (18 MΩcm) as described in the "Experimental". A voltammetric scan from -450 mV to -1,500 mV was performed at a scan rate of 15 mV s -1 . The obtained voltammograms for increasing amounts of nitrate are shown in Fig. 7. Three peaks are observed in the voltammogram: two peaks located at -685 mV (A1) and -775 mV (A2) making a plateau in this region are assumed to be connected to the electroreduction of nitrate to nitrite; a third, well-defined peak, observed at -1,070 mV (A3), is assumed to be connected to further electroreduction of nitrite. The observation of a double peak for nitrate could reflect a complex reduction process. However, another explanation is that this double peak could be attributed to reduction of nitrate on both silver-copper sites and pure silver sites on the electrode. Since this work is focussed on the analytical application of the silver-copper alloy electrodes, a more fundamental study of the reduction process of nitrate on these electrodes will be submitted in a later communication.

Figure 7 shows that the slopes of the two linearity curves are different. The linearity curve given in the left part of the figure reports the linearity for all standard additions in the range from 1 mg L -1 to 100 mg L -1 with a slope of 1.5, whereas the curve in the right part reports the linearity for the first four additions only (1 mg L -1 to 4 mg L -1 ) having a slope of 0.5. The difference in the slopes can be explained from the fact that addition of nitrate was started in ultrapure water only. This resulted in a significant change in the conductivity of the sample solution during the first few additions. To demonstrate this, a separate linearity curve was drawn for the first four additions of nitrate.

When comparing Figs. 6 and7 the results indicate that the presence of nitrate may interfere with the measurements of nickel. However, measurements of nickel as dimethylglyoxime complex are carried out in an ammonium buffer solution (pH 8.6). In this matrix only one peak at -950 mV was observed for nitrate, and a distinct separation between nitrate and added nickel was observed as shown in Fig. 8.

Zinc and cobalt may be expected to interfere in detection of nitrate in natural or acid solutions. However, the concentration of these metals must then be several mg L -1 and several orders of magnitude higher than concentrations found in environmental samples like river water and lakes.

Application in real samples (polluted river water) A voltammetric system, as described in the "Experimental", was used for detection of zinc and lead in the polluted river Deûle near the town of Douai (France). The measuring site in the river was located 500-m upstream from a former metallurgic industrial area (Metaleurop industry). Measurements were carried out over a period of 3 h. Manual samples (filtered and unfiltered) for later analyses with ICP-MS were collected for comparison each time a new measurement was performed.

As shown in Fig. 9, significant amounts of zinc and lead were found as expected due to former and present activity in the area. In addition to zinc and lead, a third peak probably corresponding to iron is observed at -800 mV. Furthermore, an unidentified peak at -300 mV is observed as well. It is beyond the scope of this paper to explain the amounts of metals found in the river water; however, there is no doubt that several processes will influence the present metal concentration in the river, including temperature, precipitation and complexation or adsorptions on particles. General activities in the area like former industrial activity, the significant traffic of cargo boats on the river, nearby highways, and the geology in the area will all be important factors in this complex system. In this contribution the discussion will be limited to presentation of results from voltammetric analyses together with parallel data from ICP-MS to verify the results and to study the difference in electro-labile and total amounts of the metals.

As seen in Fig. 9, comparable measurements with ICP-MS on manually collected and filtered river samples are in good agreement with the voltammetric at-line measurements. The concentration of lead seems to increase slightly during the period, whereas the concentration of zinc seems to slightly decrease. The same trend was found in the ICP-MS measurements of the river water. The pH was found to be 7.9±0.1. The slight changes in zinc and lead concentrations are most likely due to interactions with organic compounds, particles and the biological activity in the river water.

As mention above, ICP-MS measurements of manually sampled river water were carried out. It is interesting to . This figure shows that concentrations obtained from ICP-MS slightly exceed the amounts found by voltammetry, and that the results follow the same trend. Concentrations found in unfiltered samples reflect the total amount of the respective metals. The differences in concentrations between the unfiltered and the filtered samples reflect the amount of metals strongly bound in complexes or in particles. The results from the unfiltered samples showed that the total amount of metals decreased through the test period. Comparison of the filtered and unfiltered samples showed that the amount of metals bound in particles was 60% for zinc and almost 80% for lead. One interesting aspect was also that the amount of metal bound in particles varied much more than the labile and complexed amount.

Oxygen was not removed since the intention was to perform the measurements in a sample as similar as possible to the original sample. Tests performed in samples purged with nitrogen did not show any significant changes.

Repeatability of DPASV

Stability over time was tested by repetitive measurement in the same sample every 0.5 h during a 16-h period, using a homemade potentiostat. No decrease in sensitivity was observed during this period as seen in Fig. 10.

Conclusions

The electrochemical properties and abilities of silvercopper alloy electrodes for use in the field have been studied. Their properties have been examined according to results obtained by applying different modes of voltammetry.

Results from cyclic voltammetry and differential pulse stripping voltammetry show that silver electrodes containing added copper increase their overvoltage towards hydrogen with increasing amounts of copper in the electrode. Studies of oxide products interestingly showed that the mixed electrodes behaved as a combination of pure copper and pure silver electrodes; however, silver was found to dissolute more slowly from the alloy compared to pure silver, and the alloy electrodes were found to be more stable than pure silver electrodes.

Linearity was found for zinc, cadmium and lead in the sub-and low-μg L -1 range in DPASV mode, and for nickel in DPCSV mode. Good linearity was also found for nitrate. The electrodes were stable for a long period of time and the results found in real samples were in good agreement with results obtained by ICP-MS. The combination of a higher overpotential towards the hydrogen evolution reaction and

Fig. 1

 1 Fig. 1 Linear scan of pure silver, silver 830 and silver 925 in a HNO 3 (0.01 M) and b HCl (0.1 M). Scan rate was 15 mV s -1 , and the scan was performed after a 5-s resting time

  2 O (A1), formation of Cu(OH) 2 (A2), formation of CuO (A3), dissolution of silver as soluble [Ag(OH) 2 ] -complex (A4), formation of Ag 2 O monolayer (A5), further nucleation and growth of multilayer of Ag 2 O (A6), electrooxidation of Ag 2 O and Ag to AgO (A7), and formation of Ag 2 O 3 (A8). The two anodic peaks observed in the cathodic

Fig. 2

 2 Fig. 2 Cyclic voltammetry of a the first scan and b the ninth scan (magnified on the right) of silver, silver containing 7.5% added copper and silver containing 17% added copper electrodes in NaOH (1 M). Scan going from -50 mV to 900 mV to -1,300 mV, scan rate was 100 mV s -1 . Peaks are marked with asterisks. The cathodic peaks in the cathodic scan are assumed to correspond to electroreduction of Ag 2 O 3 and AgO to Ag 2 O (C1), electroreduction of different Ag(I) species, Ag 2 O to Ag (C4 and C5), AgO -to Ag (C6), reduction of CuO to Cu 2 O (C7), and reduction of CuO 2 and Cu(OH) 2 to Cu (C8). Chloride ions possibly leaking from the reference electrode could also attribute to the C5 peak

Fig. 7

 7 Fig. 7 Differential pulse voltammetry of nitrate in ultrapure water (initially 18 MΩcm).Upper left part of the figure shows the voltammograms for several standard additions of nitrate ranging from 1 mg L -1 to 100 mg L -1 with linearity curve below. Upper right part shows the first 10 additions (ranging from 1 mg L -1 to 10 mg L -1 ) of nitrate with a linearity curve for the first 4 standard additions below. DPV scans from -450 mV to -1,500 mV, using a scan rate of 15 mV s -1 and modulation pulse of -75 mV. Measurements were performed within 1 min after addition

Fig. 8

 8 Fig.8Standard additions of 3.2, 6.5, 13 and 26 mg L -1 nitrate and then 5 μg L -1 nickel in ammonium buffer solution (pH 8.6). Differential pulse voltammetry scanning from -450 mV to -1,200 mV with a scan rate of 15 mV s -1 , and a modulation pulse of -75 mV

Fig. 9

 9 Fig.9Left comparison of voltammetric measurements with ICP-MS (filtered samples adjusted to pH 2 with added HNO 3 ) results. Right DPASV atline measurements, plating time was 300 s, scan rate 15 mV s -1 , and modulation pulse 50 mV. The last scan was performed in river water containing 30 ppb added Zn and 30 ppb added Pb. Relative standard deviation for ICP-MS was within 1%

a better stability greatly extends the range of analytical applications and opens possibilities for online use in the field.