Effect of humic acid on the underpotential deposition-stripping voltammetry of copper in acetic acid soil extract solutions at mercaptoacetic acid-modified gold electrodes

Analytica Chimica Acta 511 (2004) 137–143

Effect of humic acid on the underpotential deposition-stripping voltammetry of copper in acetic acid soil extract solutions at mercaptoacetic acid-modified gold electrodes Grégoire Herzog, Valerio Beni, Patrick H. Dillon, Thomas Barry, Damien W.M. Arrigan∗ NMRC, University College, Lee Maltings, Prospect Row, Cork, Ireland Received 29 October 2003; received in revised form 8 January 2004; accepted 22 January 2004

Abstract Electrochemical measurements were undertaken for the investigation of the underpotential deposition-stripping process of copper at bare and modified gold electrodes in 0.11 M acetic acid, the first fraction of the European Union’s Bureau Communautaire de Références (BCR) sequential extraction procedure for fractionating metals within soils and sediments. Gold electrodes modified with mercaptoacetic acid showed higher sensitivity for the detection of copper than bare gold electrodes, both in the absence and in the presence of humic acid in acetic acid solutions, using the underpotential deposition-stripping voltammetry (UPD-SV) method. In the presence of 50 mg l−1 of humic acid, the mercaptoacetic acid modified electrode proved to be 1.5 times more sensitive than the bare gold electrode. The mercaptoacetic acid monolayer formed on the gold surface provided efficient protection against the adsorption of humic acid onto the gold electrode surface. Variation of the humic acid concentration in the solution showed little effect on the copper stripping signal at the modified electrode. UPD-SV at the modified electrode was applied to the analysis of soil extract samples. Linear correlation of the electrochemical results with atomic spectroscopic results yielded the straight-line equation y (␮g l−1 ) = 1.10x − 44 (ppb) (R = 0.992, n = 6), indicating good agreement between the two methods. © 2004 Elsevier B.V. All rights reserved. Keywords: Underpotential deposition-stripping voltammetry; Modified gold electrode; Mercaptoacetic acid; Acetic acid; Copper; Humic acid

1. Introduction The need for measurement of soil pollution has led to the development of numerous heavy metal extraction procedures, which generally use a sequence of solvents to release metals from the soil matrix [1–4]. The European Union’s Bureau Communautaire de Références (BCR) published a three-step sequential extraction scheme in order to facilitate interlaboratory comparison of data [1]. In the first step of this scheme, 0.11 M acetic acid is used to extract water and acid soluble species contained in the soil sample. In the second step, 0.1 M hydroxylamine hydrochloride (acidified to pH 2) is used to extract species held within reducible matter (typically iron and manganese oxyhydroxides). In the third step, addition of hydrogen peroxide permits the release of metal ions bound to be oxidizable, mainly organic, matter. After the removal of excess of hydrogen peroxide, 1 M ∗ Corresponding author. Tel.: +353-21-4904079; fax: +353-21-4270271. E-mail address: [email protected] (D.W.M. Arrigan).

0003-2670/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2004.01.032

ammonium acetate acidified to pH 2 with concentrated nitric acid is added. This multi-step extraction procedure enables characterization of the heavy metal pollution in soils according to the fraction in which they are detected. Water and acid soluble metal species, contained in the first fraction, are more mobile in the environment and potentially more toxic [1–4]. Atomic spectroscopic analytical techniques are generally used for quantitative determination of heavy metals in these soil extracts [5–7]. Although recently some electrochemical methods have been proposed [8–11], they all are based on mercury electrodes. Because of the high toxicity of mercury, gold electrodes among other alternative materials, have been the subject of a number of investigations [12–18]. Beni et al. investigated the voltammetric behaviour of copper and lead at gold electrodes in the three BCR sequential extraction media [12]. The electroanalytical determination of copper was possible in BCR soil extracts without alteration of the matrix (addition of electrolyte, mercury salts or adjustment of pH) using the technique of underpotential deposition-stripping voltammetry (UPD-SV) [12]. UPD-SV was introduced

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by Kirowa-Eisner and co-workers[16–22] as a means of achieving very low detection limits at solid electrodes, thus removing the necessity for mercury electrode use. The excellent capabilities of UPD-SV for determination of lead [16,19–21], copper [18,22], mercury [17,22], and cadmium [16,21] in drinking water [16,18,19,21], waste water [17,22], sea water [18], river water [20], and urine [17] was demonstrated. However, to overcome the problem of organic matter present in natural samples, which adsorbs onto the electrode surface and inhibits the electrochemical reactions, extensive sample preparation was often required [17,18,20,21]. Humic acid is a natural substance present in soils and the aquatic environment. Humic acid, which is water soluble at pH >2 [23], could be present in the first BCR extract solution (i.e. 0.11 M acetic acid), depending on the nature and content of soil being extracted. Because of the large number of phenolic and carboxylic acid functions, humic acid offers numerous binding sites to metallic ions. Carbon paste electrodes modified with humic acid have been used for the voltammetric detection of Pb2+ , Cu2+ , and Hg2+ [24], and Fe2+ , Ni2+ , and Cu2+ [25]. Conductivity measurements have shown that, in KCl solutions, copper forms strong associations with humate substances [26]. Moreover, Jeong et al. [24] observed that the optimal pH range for the detection of copper, via complexation with humic acid, was 3.5–7.5. For pH <3.5, the oxidation peak of copper was much smaller, indicating a limited complexation of copper with humic acid in such acidic conditions. Therefore, in the first fraction of the BCR sequential extraction procedure (0.11 M acetic acid; pH 2.75), copper and humic acid should not complex in a significant manner. However, humic acid is a natural surfactant, and under acidic conditions was found to adsorb onto gold [27], bismuth film [28], and mercury [29] electrodes. Wang et al. demonstrated that the voltammetric response of lead and cadmium at a bare bismuth film electrode decayed when the humic acid concentration was increased [28]. Labuda et al. investigated the anodic stripping voltammetry of heavy metals at a hanging mercury drop electrode [29]. They found that the presence of 10 mg l−1 of humic acid in the solution resulted in a lowering of the sensitivity for the detection of copper and zinc in a 0.01 M HCl solution (pH 2). UV-digestion, designed to eliminate humic acid, improved the quality of stripping voltammograms of copper, lead, cadmium and zinc. The aim of the work reported in this paper was to investigate the behaviour of short chain alkane thiol-modified gold electrodes in BCR soil extract solutions, and to examine the ability of these short chain thiols for prevention of humic acid adsorption onto the gold electrode surface. Previous work in this laboratory [15,30] has shown that short hydrocarbon chain thiols with bulky endgroups (e.g. mercaptoacetic acid (MAA), mercaptoethane sulfonic acid (MES)) formed disorganized monolayers which allowed metal cations through to the underlying electrode surface, but prevented the adsorption of synthetic surfactants onto

the gold working electrode [15,30]. If such behavior can be extended to natural surfactants present in environmental samples, then there is scope for simplifying aspects of the electrochemical analysis of environmental samples, such as removing the need for extensive sample pre-treatment or digestion. The secondary aim of this work was to study the underpotential deposition (UPD) of copper at gold electrodes in acetic acid and to see if the behaviour in this medium is different from the sulfuric acid UPD experiments. Using these two sets of data, an electroanalytical methodology for determination of copper in soil extracts was evaluated with a number of acetic acid soil extract samples.

2. Experimental All electrochemical measurements were performed using a CHI620A (CH Instruments, from IJ Cambria Scientific, Burry Port, Wales, UK). A standard three-electrode cell was used. The working electrode was a polycrystalline gold disc (diameter 2 mm). The counter electrode was a platinum wire and the reference electrode was an Ag|AgCl|KCl (3 M) electrode (all from CH Instruments). All potentials are reported with respect to this reference electrode. Prior to electrochemical measurements, the gold disc was polished mechanically with alumina powder (1, 3, and 0.05 ␮m) aqueous suspension. The working electrode was then cycled between 0 and 1.5 V in 0.1 M H2 SO4 solution until reproducible gold oxide formation and reduction were observed. The electrode was then rinsed with de-ionized water and was ready for experiments or surface modification. To form the MAA monolayer-modified electrode, the clean electrode was immersed in aqueous 5 mM MAA in 0.1 M HClO4 for 5 min at open-circuit potential, as previously employed [15,30–32]. Electrochemical studies were performed using cyclic voltammetry (CV) and linear sweep anodic stripping voltammetry (LSASV), in the presence of dissolved oxygen and in unstirred solution. The scan rate used was 10 mV s−1 for CV and 100 mV s−1 for LSASV. LSASV experiments were done in triplicate. All chemicals, of the highest purity commercially available, were purchased from Sigma–Aldrich. De-ionized water of resisitivity 18 Mohm cm was used for preparation of solutions. Two sets of experiments, in the presence of humic acid, were performed for UPD-SV experiments. For the first series, set A, the solutions containing copper and humic acid in 0.11 M acetic acid were prepared and the UPD-SV experiments were carried out immediately. Solutions of the set B were prepared and left to stand for 18 h to equilibrate before the UPD-SV experiments were carried out. The soil samples were collected from a former mining region within the European Union. These were subjected to the first stage of the BCR extraction scheme (0.11 M acetic acid) and metals content determined by ICP-OES or ICP-MS. The data and extracts were provided by Professor

G. Herzog et al. / Analytica Chimica Acta 511 (2004) 137–143 1.0 0.5

Current /µ A

0.0

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0.1

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E (vs. Ag|AgCl) / V

1.0 0.5 0.0 Current /µ A

M. Valiente, Barcelona. The extracts were stored at 4 ◦ C and then analyzed using the following procedure. After equilibration at room temperature, the sample (2.0 cm3 ) was placed into the electrochemical cell and the three electrodes inserted. UPD-SV was performed by applying the deposition potential (0.00 V) for the chosen time and then stripping the underpotential deposit from the electrode surface by application of linear sweep voltammetry (sweep rate 100 mV s−1 , step height 1 mV). The deposition time was chosen individually for each sample by carrying out the UPD-SV experiment initially at 0 s deposition time, and then increasing the deposition time until an acceptable stripping peak signal was obtained. Sample solutions were neither stirred during the deposition step, nor was de-aeration carried out to remove dissolved oxygen. The stripping voltammogram was calibrated by two standard additions of aliquots of 1 mM Cu2+ in 0.11 M acetic acid. All measurements were in triplicate (i.e. three measurements of the sample, three measurements after the first standard addition, and three measurements after the second standard addition), giving nine measurements in total, contributing to the calculation of the sample’s Cu2+ concentration. In between repetitions, the electrodes were removed from the solution and then re-inserted immediately, to ensure that fresh solution was present at the electrode surface prior to each measurement.

139

(a)

-0.5 -1.0 -1.5

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-2.0 -2.5 0

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0.3 E (vs. Ag|AgCl) / V

Fig. 1. CV at a bare gold electrode (a) in the presence and (b) in the absence, of 0.1 mM of CuSO4 in (A) 0.025 M sulfuric acid and (B) 0.11 M acetic acid. Scan rate: 10 mV s−1 .

3. Results and discussion 3.1. Voltammetry at bare gold electrode in acetic acid CVs of copper in 0.025 M sulfuric acid, 0.11 M acetic acid and in mixtures of these two reagents, with varying concentration of acetic acid but constant sulfuric acid, were performed. This allowed a direct comparison of the Cu-UPD process in these electrolyte solutions. Fig. 1 shows typical CVs for this series of experiments. On the forward scan (negative direction), the broad peak corresponds to the formation of the underpotential deposit of copper on the gold surface. On the backward scan, the anodic peak is due to the oxidation (stripping) of the deposited copper atoms. The charges

for the copper stripping peak in these reagent mixtures were similar (Table 1), indicating little influence of the media on the UPD-SV of copper at polycrystalline gold electrodes. For experiments in mixtures of acetic acid and sulfuric acid, the concentration of sulfuric acid was kept constant, 0.025 M, whereas, the acetic acid concentrations were varied (0.0011, 0.011, and 0.11 M). No noticeable change was observed in terms of the shape of the voltammograms, stripping peak potential, current or charge (Table 1), across this variation in acetic acid concentration in sulfuric acid solution, suggesting little interaction of acetic acid with either the copper (metal or ions) or with the gold electrode surface. However, adsorption of acetate on single-crystalline Pt electrodes has

Table 1 Electrochemical characteristics at a bare gold electrode in aqueous solutions of sulfuric acid, acetic acid and their mixtures Solutionsa

0.025 M sulfuric acid 0.11 M acetic acid 0.025 M sulfuric acid + 0.0011 M acetic acid 0.025 M sulfuric acid + 0.011 M acetic acid 0.025 M sulfuric acid + 0.11 M acetic acid

CV experiments

UPD-SV experiments

Ep,a (V)

ip,a (␮A)

Qp,a (␮C)

Γ t =180s (10−9 mol cm−2 )b

0.24 0.27 0.28 0.25 0.25

0.76 0.70 0.73 0.84 0.79

9.25 10.01 9.58 10.66 10.73

1.91 1.76 1.80 1.77 1.94

± ± ± ± ±

0.003 0.048 0.002 0.010 0.022

Saturation time (s) 60 45 60 60 60

Ep,a : anodic peak potential; ip,a : anodic peak current; Qp,a : anodic peak charge; Γ t =180s : surface coverage after a deposition time of 180 s. a pH values of these solutions were in the range of pH 1.57–1.60. b precision given is the standard deviation, n = 3.

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Γ =

Q nFA

(1)

where Q is the stripping charge measured from the copper stripping peak (in C), n the number of electrons exchanged during the stripping process, F the Faraday constant (in C mol−1 ), and A is the geometrical surface area of the electrode (in cm2 ). As UPD is the formation of a monolayer (or less) of metal on a solid substrate, the surface coverage value reached a maximum when the deposition time applied was sufficiently long, corresponding to full monolayer coverage of the electrode surface. In all the media investigated, saturation of the electrode surface was reached at surface coverages below 2 × 10−9 mol cm−2 , the theoretical value for a monolayer of a metal on a substrate [37]. The experimental surface coverage values at deposition time t = 180 s are given in Table 1. The surface coverage was slightly higher in 0.025 M H2 SO4 than in 0.11 M acetic acid. However, when the concentration of acetic acid in 0.025 M H2 SO4 was increased, no particular trend was observed for the surface coverage values. Moreover, the deposition time necessary to obtain the full coverage of the electrode surface, i.e. saturation time, was very similar across the five media investigated (Table 1). These observations are in agreement with the information obtained by CV and indicate that Cu-UPD-SV at a polycrystalline gold electrode in acetic acid is equivalent to that in sulfuric acid.

8

6 Current /µA

been investigated [33,34]. Fukuda et al. observed a wave attributed to the adsorption of acetic acid even at pH <2 [34]. The wave potential decreased with increasing acetic acid concentration. Moreover, at a Pt(1 1 1) electrode, two competing Cu-UPD-SV processes were observed in a mixture of 0.1 M sulfuric acid and 1 mM acetic acid [35]. One UPD-SV process occurred at the expense of the other, taking place at the same potential as in 0.1 M sulfuric acid alone. A mixture of 0.1 M acetic acid and 0.1 M sodium acetate was found to hinder mercury-UPD at a Au(1 1 1) electrode [36]. It was suggested that the origin of this hindrance might be a complexation between Hg and acetate ions or the adsorption of acetate onto the gold surface. Note that all these studies were made at single-crystal electrodes. The highly ordered surface structure of single-crystal electrodes is very sensitive to the adsorption and deposition processes that occur at the electrode. However, differences of the Cu-UPD-SV in the different sulfuric acid and acetic acid media were not observed, possibly because of the irregular characteristics of a polycrystalline surface, or simply because there is not a strong interaction between the acetic acid and the gold surface or either form of copper present during the UPD process. Single crystal gold electrode studies are planned in order to elucidate this further. UPD-SV experiments were performed at a bare gold electrode in the five different media. The deposition time was varied between 0 and 180 s. The surface coverage, Γ (mol cm−2 ), was calculated according to Eq. (1):

Stripping Peak Current /µA

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8 6 4 2 0 -2 0.0

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E (vs. Ag|AgCl) / V 0 0

50

100

150

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Fig. 2. Evolution of the stripping peak current with deposition time. Detection at a MAA-modified electrode by LSASV of 0.1 mM of copper in 0.11 M acetic acid. Deposition potential: 0.0 V, scan rate: 100 mV s−1 . When not visible, errors bars are smaller than the size of the symbol. Inset: LSASV of 0.1 mM of copper in 0.11 M acetic acid at a MAA-modified electrode. Voltammograms, from bottom to top, were obtained following 0, 5, and 15 s deposition time.

3.2. Cu-UPD-SV at a MAA-modified electrode Cu-UPD-SV in 0.11 M acetic acid was investigated at a MAA-modified electrode (Fig. 2). Increasing the deposition time resulted in an increase of the stripping peak current up to a plateau region at 30 s deposition time. At shorter deposition times, the stripping peak of copper obtained was well defined (inset, Fig. 2) and the surface coverage values were similar to those obtained at a bare gold electrode in 0.11 M acetic acid. For a deposition time of 15 s, in 0.1 mM Cu2+ , the surface coverage at both bare and modified electrode was 1.3 × 10−9 mol cm−2 . Despite having part of the electrode covered by the MAA monolayer, sufficient surface area is still available for the UPD-SV of copper, indicating the possibility of using MAA-modified gold electrodes for the detection of copper in 0.11 M acetic acid. The application of deposition times greater than 60 s produced a broadening of the stripping peak resulting in a decrease of the stripping peak current (Fig. 2). The reason for this decay of the copper stripping current, observed at both bare and modified electrodes in 0.11 M acetic acid, is not yet understood. However, by maintaining a short deposition, well-defined stripping peaks were obtained, permitting an analytical interpretation of the signal. 3.3. Protection of the gold surface from humic acid adsorption Comparison of the effect of the presence of humic acid on the voltammetric response to copper at bare and MAA-modified electrodes was studied (Fig. 3). At the bare gold electrode in the absence of humic acid, the response observed for Cu-UPD-SV was well defined (Fig. 3A, curve a). In the presence of 50 mg l−1 of humic acid, the stripping peak was of a lower intensity and shifted to a more positive potential (Fig. 3A, curve (b)). Such results indicate the hindering of the Cu-UPD-SV process by the presence of

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Fig. 4. Evolution of the stripping peak current of 40 ␮M of copper in 0.11 M acetic acid with the concentration of humic acid at (䉬) a MAA-modified electrode and at (䊏) a bare gold electrode. UPD-SV results obtained with deposition potential: 0.0 V, deposition time: 15 s, scan rate: 100 mV s−1 . When not visible, error bars are smaller than the size of the symbol.

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Fig. 3. UPD-SV of 20 ␮M of copper in 0.11 M acetic acid (a) in the absence and (b) in the presence of 50 mg l−1 of humic acid at (A) a bare gold electrode and (B) a MAA-modified electrode. Deposition potential: 0.0 V, deposition time: 15 s, scan rate: 100 mV s−1 .

humic acid at a bare gold electrode. The same experiments were carried out at the MAA-modified electrode (Fig. 3B). The stripping peak potential of copper remained constant, whether humic acid was present or not. A slight decrease of the stripping peak current was observed in the presence of humic acid compared to in the absence of humic acid. Although the stripping response at the MAA-modified electrode was influenced by the presence of humic acid in solution, the MAA-covered electrode offered a better shaped stripping peak and a greater intensity than the bare gold electrode. The ability of the MAA molecular monolayer to protect the gold electrode surface, was also investigated by UPD-SV with solutions of different humic acid concentration. The copper concentration was kept constant, at 40 ␮M, while the humic concentrations selected were 1, 5, 10, and 50 mg l−1 (Fig. 4). It can be seen that the stripping current at the bare gold electrode was lower than that at the MAA-modified electrode. The mean value at the bare electrode was 2.58 (±0.26) ␮A, whereas, at the modified electrode it was 3.17 (±0.18) ␮A. [The mean values were calculated using the stripping peak currents obtained at all the humic acid concentrations studied. The precision given is the standard deviation (n = 12).] Generally, an increase of the concentration of humic acid resulted in a greater decrease of the stripping signal for copper at the bare electrode

than at the MAA-modified electrode. In the latter case, a decrease was evident only at the highest concentration of humic acid (50 mg l−1 ) (Fig. 4). These results demonstrate that the MAA monolayer contributes to an improvement of the analytical response and offers protection of the gold surface from the fouling activity of humic acid on the electrode surface. 3.4. Quantitative determination of copper Calibration curves for copper in 0.11 M acetic acid were constructed at bare and MAA-modified electrodes using UPD-SV. To obtain a well-defined peak of reasonable height, a deposition time of 15 s was selected. This short deposition step enabled both avoidance of electrode surface saturation and use of short analysis times. The copper concentration range studied was from 0 to 40 ␮M. In the absence of humic acid, the copper stripping signal at the MAA-modified electrode was greater than at the bare gold electrode. The calibration curve obtained at the modified electrode exhibited better sensitivity and linearity in these experimental conditions (Table 2). For calibration curve experiments in the presence of humic acid, two sets of copper solutions containing 50 mg l−1 humic acid in 0.11 M acetic acid were prepared. The solutions of Set A were prepared just before performing the electrochemical experiments. The second set of solutions, Set B, was allowed to stand for 18 h to enable equilibration, if necessary. At both types of electrode studied, the difference in calibration plot slopes for the two sets of solutions was not significant (Table 2). This implies weak interactions between copper and humic acid at low pH (the pH measured for the solutions was 2.8), as Jeong et al. suggested [24]. In the presence of 50 mg l−1 of humic acid, the slope of the calibration curve at the MAA-modified electrode decreased by 15% compared to the case in the absence of humic acid. The same trend was observed at the bare gold electrode, but the decrease was 22%. The adsorption of humic acid on the

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Table 2 Comparison of the sensitivity to copper at the bare and the MAA-modified gold electrodes in the absence and presence of humic acid Solution

Bare electrode Slope

No humic acid 50 ppm of humic acid, Set A 50 ppm of humic acid, Set B

(␮A ␮M−1 )a

0.067 ± 0.003 0.052 ± 0.001 0.045 ± 0.001

Modified electrode R

Slope (␮A ␮M−1 )a

R

0.9925 0.9989 0.9976

0.082 ± 0.001 0.070 ± 0.001 0.072 ± 0.001

0.9997 0.9997 0.9997

Copper concentration range: 0–40 ␮M. Same experimental parameters as for Fig. 5. Set A: Solutions were prepared just before undertaking the experiments. Set B: Solutions were allowed to equilibrate for 18 h before the experiments were carried out. a Precision given is the standard deviation (n = 8), calculated according to the method described in [38].

gold surface was not totally prevented by the presence of the MAA disorganized monolayer. However, the difference between the slope at the bare electrode and at the modified electrode increases with the addition of humic acid to the solution, indicating that the MAA monolayer on the gold electrode surface improves the conditions for copper determination in the presence of humic acid. It can also be noted from Table 2 that the average slope at the MAA-modified electrode in the presence of humic acid is 1.5 times that at the bare electrode (in presence of humic acid), indicating the enhanced sensitivity achievable at the monolayer modified electrode. 3.5. Soil extract sample analysis The efficiency of the MAA-modified electrode for the UPD-SV detection of copper in soil extracts was tested by the analysis of six 0.11 M acetic acid soil extract samples in conjunction with the standard additions calibration method. The extracts were analyzed by deposition at 0.0 V for between 5 and 180 s, depending on the copper concentration, in unstirred solution. Six samples were analysed, yielding relative standard deviations of between 2 and 11% (the average % R.S.D. was 5%). The electrochemical results were compared to those obtained by atomic spectroscopic methods (Fig. 5). Excellent agreement was found between the

Fig. 5. Comparison of the electrochemical and spectroscopic results for the determination of copper in soil extract (0.11 M acetic acid) samples. For the electrochemical results, the precision given is the standard deviation (n = 9), calculated according to the method described in Ref. [38]. When not visible, error bars are smaller than the size of the symbol.

results obtained by the two approaches, over a wide range of copper concentration, from low parts per billion (␮g l−1 ) to low parts per million (␮g l−1 ) levels. The equation of the line of best fit between the two methods is y = 1.10x − 44 (R = 0.992, n = 6). The slope obtained was close to 1, which would correspond to a perfect agreement between the electrochemical and spectroscopic method results. The high correlation coefficient was also indicative of the high degree of agreement between the two methods. Intentionally spiking a soil extract sample with humic acid, to a concentration of 50 mg l−1 , allowed assessment of the affect of humic acid in such a sample matrix. The concentration of copper found in this humic acid-spiked sample was 54 (±4) ␮g l−1 (n = 9), which was in good agreement with the results of the same sample before spiking, 55 (±3) ␮g l−1 (n = 9) and both electrochemical results were in good agreement with the atomic spectroscopic data for the sample, 46 (±43) ␮g l−1 . 4. Conclusions Detection of copper in the first fraction of the BCR extraction procedure at a disorganized MAA monolayer-modified gold electrode, using the underpotential deposition-stripping voltammetry approach, has been reported. It was found that the underpotential deposition of copper at polycrystalline gold was equivalent in 0.11 M acetic acid and 0.025 M sulfuric acid. The MAA monolayer on the gold surface provided good protection against the adsorption of humic acid onto the electrode surface. Quantitative determination of copper was possible in the absence and the presence of humic acid. Although some decrease of the analytical signal at the modified electrode in the presence of humic acid was observed, better results were obtained than at the bare gold electrode. The analysis of a number of acetic acid soil extract samples gave results in good agreement with those obtained by a reference technique, indicating the applicability of UPD-SV at disorganized monolayer-coated gold electrodes for the determination of copper in environmental matrices. A wide range of copper concentrations were determined by this approach, facilitated by choice of appropriate deposition time for the UPD-SV protocol. Moreover, addition of humic acid to a known environmental extract did not affect the analytical result.

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Acknowledgements The Royal Society of Chemistry is thanked for a PhD studentship (GH) via the “RSC/EPSRC Analytical PhDs” scheme. The European Commission is acknowledged for financial support via the 5th Framework Programme Energy Environment and Sustainable Development Programme, project DIMDESMOTOM (Development of Improved Detection Systems for Monitoring of Toxic Heavy Metals in Contaminated Groundwaters and Soils, contract EVK1-CT-1999-00002). We thank Professor M. Valiente, Universitat Autonoma de Barcelona, for providing the soil extract samples and analytical data. PHD thanks the US National Science Foundation (DMR-00706097) for funding his summer undergraduate research studentship in Cork.

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