Optimisation of mercury film deposition on glassy carbon electrodes: evaluation of the combined effects of pH, thiocyanate ion and deposition potential

Analytica Chimica Acta 503 (2004) 203–212

Optimisation of mercury film deposition on glassy carbon electrodes: evaluation of the combined effects of pH, thiocyanate ion and deposition potential Sandra C.C. Monterroso, Helena M. Carapuça∗ , João E.J. Simão, Armando C. Duarte Department of Chemistry, Campus de Santiago, University of Aveiro, 3810-193 Aveiro, Portugal Received 17 February 2003; received in revised form 2 June 2003; accepted 13 October 2003

Abstract The combined effects of pH, thiocyanate ion and deposition potential in the characteristics of thin mercury film electrodes plated on glassy carbon surfaces are evaluated. Charges of deposited mercury are used as an experimental parameter for the estimation of the effectiveness of the mercury deposition procedure. The sensitivity of the anodic stripping voltammetry (ASV) method for the determination of lead at in situ and at ex situ formed thin mercury films are also examined. It was concluded that, in acidic solutions (pH 2.5–5.7) and fairly negative deposition potentials, e.g. −1.3 to −1.5 V, thiocyanate ion promotes the formation of the mercury film, in respect both to the amount of deposited mercury and to the mercury deposition rate. Also, the mercury coatings produced in thiocyanate solutions are more homogeneous, as depicted by microscopic examinations. In the presence of thiocyanate there is no obvious advantage of using high concentrations of mercury and/or high deposition times for the in situ and ex situ preparation of the mercury film electrodes. The optimised thin mercury film electrode ex situ prepared in a 5.0 mM thiocyanate solution of pH 3.4 was successfully applied to the ASV determination of lead and copper in acidified seawater (pH 2). The limit of detection (3σ) was 6 × 10−11 M for lead and 2 × 10−10 M for copper for a deposition time of 5 min. Relative standard deviations (R.S.D.s) of <1.2% were obtained for determinations at the nanomolar of concentration level. © 2003 Elsevier B.V. All rights reserved. Keywords: Mercury film electrode; Thiocyanate; Lead; Copper; Anodic stripping voltammetry

1. Introduction The application of thin mercury film electrodes (TMFEs), in situ or ex situ produced on glassy carbon (CG), to the anodic stripping voltammetry (ASV) quantification of trace heavy metals is widespread [1–13]. As a general rule, the mercury plating is performed in acidic medium, at fairly negative potentials, e.g. −0.7 to −1.0 V (versus Ag/AgCl) but there are almost as many plating protocols as the number of users. The film thickness ranges from 1 to 10 nm for in situ formed TMFEs and from 0.1 to 1 ␮m for ex situ formed TMFEs [14]. In what concerns in situ TMFEs their use may be quite complicated due to the eventual irreproducibility of the mercury film morphology, to the incomplete removal of the film at the end of an experiment or ∗ Corresponding author. Tel.: +351-234-370732; fax: +351-234-370084. E-mail address: [email protected] (H.M. Carapuça).

0003-2670/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2003.10.034

even to the formation of insoluble mercury(I) compounds [5,8,9,15–18]. Often, the performance of the mercury film electrode is lower than it would be expected and the regular use of these electrodes is rather tedious due to the frequent polishing/cleaning procedures required. Also, mechanical deterioration had been reported after excessive hydrogen evolution during deposition in acidic solutions [9,15,17]. In the last decade, a modified procedure for plating/removal of mercury on GC was reported [19], in which the in situ mercury deposition and removal were aided by complexation with thiocyanate ion, resulting in an improved electrode performance for the CSV determination of nickel. The mercury deposition step was performed at a highly negative potential (−2.0 V). In fact, thiocyanate was already known as a suitable electrolyte for the ASV determination of mercury at graphite and GC electrodes [20,21]. The usefulness of thiocyanate was also tested in the formation and removal of mercury films on iridium microelectrodes [16] and on the analytical performance of mercury films on cylindrical

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carbon fibre microelectrodes [22]. The thiocyanate assisted mercury plating/removal procedure on GC was optimised and applied to the ASV determination of ultra trace levels of lead and cadmium in seawater using the in situ TMFE methodology without any apparent drawback due to the presence of chloride ion [12]. On the other hand, the nature of mercury coatings on GC has been the subject of several studies [1,9,13,14,17,18,21,23,24] since their initial application. However, studies on the influence of experimental parameters on the morphology, on the amount of deposited mercury and on the analytical characteristics of thin films are limited. Wu [14] studied the influence of the deposition potential on the nature and stability of thin mercury coatings in situ and ex situ plated on GC in acidic medium, including the determination of amounts of deposited mercury and optical microscopic examinations. In addition, the role of the GC surface morphology, e.g., its microstructure, and of the polishing procedure was shown to be important in the production of homogeneous mercury films [9,24]. In fact, Stuliková [23] had already concluded that for low overpotentials the deposition of mercury occurred mainly on defect sites while a homogeneous mercury film of closely packed Hg microdroplets of rather equal size is obtained at fairly negative deposition potentials. This effect was assigned to the occurrence of hydrogen evolution simultaneously to the elemental mercury deposition but the mechanism by which that process occurs has not been established. However, in a study by other authors [15], changes in the background current in LSV measurements with in situ TMFEs, prepared in neutral medium at deposition potentials more negative than −1.1 V, were assigned to modifications in the morphology of the films due to extensive hydrogen evolution. As far as we know, for mercury films on glassy carbon electrodes produced in the presence of thiocyanate there are no systematic studies on the effect of the above mentioned experimental parameters on their overall characteristics and analytical performance. The main objectives of the present paper are the exploitation and discussion of the effects of some common experimental parameters namely pH and deposition potential on the reproducibility, surface morphology, stability and sensitivity of thin mercury film electrodes (thicknesses (∼nm)), plated on GC, in acidic solutions of thiocyanate. Also, the role of pH in the film formation process at very negative potentials is considered. The charge associated to the linear scan stripping peak of mercury is determined and used for estimating the amounts of mercury deposited and deposition rates, for different experimental conditions. Optical microscopy is applied for morphological comparisons. The analytical performance of in situ and ex situ formed thin mercury film electrodes towards lead is also examined. The analytical use of the optimised thin mercury film electrode, ex situ formed in thiocyanate solution is tested on the quantification of lead and copper in seawater.

2. Experimental 2.1. Instrumentation All the voltammetric experiments were performed with a computer controlled potentiostat (PGSTAT-12 controlled by GPES software from EcoChemie, the Netherlands) connected, via an IME-663 module, to an electrode stand (663 VA-Stand, Metrohm, Switzerland). The three-electrode configuration was used comprising a TMFE plated onto a rotating GC disc (1.9 mm diameter, Metrohm) as the working electrode, a GC rod counter electrode and a double junction Ag/AgCl (3 M KCl, saturated AgCl, and 3M KCl in the bridge) reference electrode. All potentials quoted are relative to this Ag/AgCl reference electrode. All experiments were done at room temperature. A combined glass electrode (Orion 9104SC) connected to a pH-meter (Cole Parmer, Model 05669-20) was used for pH measurements. The microscope examinations were carried out with an optical lenses system (magnifying lens Nikon SMZ800 coupled to a Nikon H-II Power 1) connected to a camera (Nikon FDX-35). 2.2. Reagents and solutions All solutions were prepared from analytical-reagent grade chemicals (ammonium acetate and ammonium thiocyanate, both from Riedel-de-Haën, hydrochloric acid, 37% (trace select, Fluka), nitric acid and sodium hydroxide, both from Merck) using deionised water (18.2 M cm, Milli-Q system, Millipore-aters, Milford, MA, USA). Stock solutions of ammonium acetate pH buffer (1 M NH4 Ac–0.5 M HCl) and of thiocyanate 1.0 M were prepared monthly and used without further purification. Solutions of metal ions, Hg, Cu and Pb, were prepared by appropriate dilution of the corresponding 1000 ppm AA-Spectrosol standards (BDH). Nitric acid and sodium hydroxide solutions were used for pH adjustments. Hydrochloric acid was used for pH adjustments of the seawater samples. Contaminating heavy metal ions were removed from the sodium hydroxide solution by shaking with a manganese(IV) oxide (Merck) suspension (10 mg dm−3 MnO2 ), followed by filtration through a 0.45 ␮m cellulose acetate membrane filter (Millipore, Bedford, MA) [25]. Dispensing pipettes (5–50, 50–200 and 200–1000 ␮l, Proline, Biohit) equipped with disposable tips were used for the appropriate dilutions. 2.3. Procedure Just before every experiment, the GC electrode was polished with aluminium oxide (grain size = 0.3 ␮m, Metrohm, Switzerland), on a microcloth polishing pad. Then the GC electrode surface was rinsed with deionised water and cleaned by sonication, for 60 s, to obtain a renewed electrode surface. Afterwards, an electrochemical pre-treatment was carried out using multicycle voltammetric

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scanning (50×) between −0.8 and +0.8 V at 0.1 V s−1 , in the NH4 Ac–HCl buffer solution [12]. These polishing and electrochemical pre-treatments were repeated daily. When not in use the GC electrode was stored dry in a clean atmosphere. The electrochemical experiments with the in situ thin mercury film electrodes were carried out in a 20 ml deionised water sample buffered with 0.01 M of NH4 Ac–HCl, to which appropriate amounts of thiocyanate and mercury ion were added in order to give a thiocyanate concentration of 5.0 mM and mercury(II) concentrations of 3.00, 6.00 or 12.0 × 10−5 M. The concentration of thiocyanate was set at 5.0 mM based on an optimisation study by other authors [12]. However, for experiments with the higher mercury concentration, 12.0 × 10−5 M, the thiocyanate concentration was doubled up in order to maintain a ca. 100-fold excess in solution. All solutions were purged with nitrogen for 5 min before the voltammetric measurements. The mercury deposition step lasted from 1 to 5 min at a deposition potential, Edep , within the interval −0.5 to −2.0 V, whilst the electrode was rotated at a rotational frequency of 25 s−1 . After a 10 s quiescent time, the stripping step was initiated from an initial potential corresponding to Edep (upper limit: −1.3 V) to a final potential of +0.8 V. For the measurement of the charge under the voltammetric stripping peak of Hg a linear scan of 0.25 V s−1 was applied, in accordance to the method described by Wu [14]. That charge is an estimate of the amount of deposited mercury on the GC electrode and is calculated by electronic integration of the linear scan voltammetric mercury peak corresponding to the stripping back of the mercury previously deposited onto the GC electrode surface. The linear scan stripping voltammograms of mercury in the presence of SCN− presented one anodic peak at ca. +0.34 V. In the ASV experiments for the measurement of the Pb-stripping peak, a square-wave (SW) scan was used (SW parameters: frequency, 50 Hz; amplitude, 25 mV; and potential step, 5 mV). The cell concentration of lead(II) was 3.0 × 10−8 or 3.00 × 10−7 M. After each stripping scan, an electrochemical cleaning step was performed at a conditioning potential, Econd = +0.6 V, to assure the complete removal of the mercury film. The cleaning period was either 15 s (experiments with thiocyanate) or 30 s (experiments without thiocyanate). For the optical microscopic examinations the mercury coatings were obtained at the above-mentioned experimental conditions, the electrochemical cell was turned off and the electrode was removed and carefully cleaned with water by running the water down the side of the electrode. Then the electrode surface was dipped in ultra pure water for the time span before the optical examination, in order to preserve the integrity of the film structure. Photographs were taken at magnifications of 30× and 63×. For the determination of total quantities of lead and copper in seawater samples, the thin mercury film electrodes were produced ex situ at −1.3 V for 60 s, at a rotation rate of 1500 rpm, using a solution of 5.0 mM thiocyanate and

205

3.0 × 10−5 M mercury(II), adjusted to pH 3.4. Afterwards, the electrode was carefully rinsed and placed in the seawater sample aliquot, where the ASV–differential pulse (DP) measurements were carried out with an accumulation step of 5 min at −0.8 V, with a rotation rate of a rotational frequency of 25 s−1 . The DP parameters were: amplitude, 25 mV; potential step, 5 mV; and scan rate, 10 mV s−1 . Determinations with ASV–DP at the HMDE were also performed for comparison, using the same accumulation step parameters. In all the determinations, the standard addition method was used and all peak currents are mean values of five replicate measurements. The seawater sample was collected near the shore at Aveiro, Portugal (salinity: 35.2‰) and was filtered through a 0.45 ␮m membrane filter (Millipore), acidified to pH 2 and stored at 4 ◦ C in an acid-cleaned polyethylene bottle for 1 week. The sample was brought to room temperature before the ASV measurements. The seawater sample presented very low dissolved organic matter (COD: <1 mg l−1 ) and, therefore, the acidified sample allows the measurement of total quantities of dissolved heavy metal ions [4].

3. Results and discussion 3.1. Electrochemical removal of the mercury film One common problem associated to the application of TMFEs is its incomplete electrochemical removal leading to erroneous results and requiring the use of tedious mechanical cleaning procedures. In order to establish an adequate cleaning procedure for the in situ thin mercury film electrodes produced in the presence of thiocyanate, preliminary experiments were carried out using a solution of 5.0 mM thiocyanate and 3.00 × 10−5 M mercury(II), adjusted to pH 3.4. The deposition time was 60 s at Edep = −1.5 V. After the application of the appropriate voltammetric scan for either the determination of the voltammetric charge corresponding to the amount of deposited mercury, QHg , or the square wave ASV stripping peak of lead, Ip(Pb) , a conditioning potential, Econd , was applied for a given time. The Econd and the cleaning period were varied between +0.8 and +0.6 V and 10–60 s, respectively, and a 20 mV s−1 linear scan was performed, between 0.0 and +0.5 V, to confirm the absence of any mercury stripping current. The optimum Econd and cleaning period were set at +0.6 V and 15 s, respectively. Moreover, 10 successive measurements of QHg and Ip(Pb) , using the optimised cleaning procedure, produced good quality results (relative standard deviation (R.S.D) of 0.68% for QHg and of 0.61% for Ip(Pb) measurements). For the in situ thin mercury film electrodes produced in the absence of thiocyanate, the cleaning period had to be extended to 30 s (Econd = +0.6 V) but no further mechanical (polishing) procedures had to be used because there were no evidences of residual mercury on the electrode surface. No damaging effects resulting from the formation of calomel at the working electrode due to an eventual leaking

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Table 1 Values of peak height for lead and corresponding relative standard deviation (N = 10), on different days, with and without thiocyanatea for mercury films produced after the polishing plus multicycle scanning pre-treatments Day

1 2 3 4 5

Without SCN−

With SCN−

Ip (␮A)

R.S.D. (%)

Ip (␮A)

R.S.D. (%)

16.0 15.5 16.2 15.6 15.9

2.6 3.3 1.4 2.3 3.3

17.8 17.7 17.8 17.9 17.8

0.6 0.3 0.3 0.3 0.7

Deposition time 60 s at −1.3 V; [PbII ] = 3.0 × 10−8 M; see text for other experimental details. a

of chloride anion from the salt bridge of the reference electrode were detected. Conditioning potentials higher than +0.65 V should be avoided, because oxygen evolution takes place at the GCE and, even using a high rotation rate, gas bubbles remained affixed to the electrode surface, precluding the proper formation of the subsequent mercury film. 3.2. Glassy carbon polishing and activation procedures Another experimental factor under consideration is the effect of the manual polishing procedure of the GCE surface on the reproducibility of the results, especially if comparisons are to be made from results obtained in different days. The determination of Ip(Pb) for in situ thin mercury film electrodes produced after performing the polishing and electrochemical pre-treatments in 5 successive days revealed no significant variations on the current signal (cf. Table 1), especially for the films produced in thiocyanate solution. During 1-day term, after more than 60 successive measurements of Ip(Pb) , there was a small decrease in the electrode performance, i.e. the peak current decreased 7%. Also, the cyclic voltammograms of the recently polished glassy carbon electrode in the buffer solution were always similar. In fact, the manual polishing of the glassy carbon electrode with 0.3 ␮m alumina produced reproducible surface areas of 3.06 mm ± 0.09 mm (mean value of four polishing experiments each one with four replicate determinations; measured by cronoamperometry, with a 1.00 × 10−3 M hexacyanoferrate(III) solution in KCl 0.5 M). The obtained electrochemical area is 10% higher than the geometric one (2.78 mm2 ) evidencing that the present polishing procedure produces moderate roughened glassy carbon surfaces [26]. Addition-

ally, the cyclic voltammetry of hexacyanoferrate(III) gave indication of a reasonable activated carbon surface: the cathodic to anodic peak separation ranged between 64 and 67 mV and the difference between the peak potential and the half-wave potential varied between 50 and 52 mV (scan rate: 20 mV s−1 ). These results are a clear indication that the manual polishing procedure with alumina (φ = 0.3 ␮m) does produce reproducible and activated glassy carbon surfaces with an adequate microstructure. It should be noted that a highly polished electrode is unsuitable because elemental mercury will not adhere properly to the GC surface [9]. On the other hand, if the polishing pre-treatment is performed only on the first day of the week (in the subsequent days only the multicycle scanning is performed) the peak intensity of lead(II) on the fifth day decreases 20% comparing with the results obtained on the first day. These results show that the deterioration of the electrode performance is related to the actual GC surface condition, which degrades faster if the polishing treatment is skipped at the beginning of the working day. Consequently, we decided to use a daily combined pre-treatment, i.e., polishing plus multicycle scanning in order to compare the results obtained in different occasions. In between measurements with the in situ thin mercury film electrodes or after the utilisation of an ex situ film, no wiping off the mercury film with a tissue or electrochemical removal at drastic oxidation conditions or even polishing is required. The removal of the mercury film in a solution of thiocyanate at +0.6 V is the best option. This last statement agrees with the observations made by other investigators [12,18,19]. 3.3. Mercury deposition process Table 2 presents the characteristic charge parameters of the present mercury films, compared to those of similar films produced in the absence of thiocyanate. The charge values were higher for the films produced in solutions of thiocyanate and the reproducibility was considerably improved. Further, QHg increased linearly with the deposition time. The slope of the plots QHg versus time is an estimate for the rate of the mercury deposition process. In the present experimental conditions (constant deposition potential and stirring) the rate of deposition is expected to be a function only of the mercury concentration [14]. However, as shown in Table 2, the rate of deposition decreases to some extent (11%) in the absence of thiocyanate at the same bulk concentration of

Table 2 Characteristic parameters of the thin mercury filmsa

With SCN− Without SCN− a b

QHg (␮C)b tdep = 60 s

R.S.D. (%)b

Slope (rate) (␮C s−1 )

r (N)

Deposition charge density (␮C mm−2 s−1 )

70.7 60.6

0.8 4.7

0.145 0.129

0.998 (5) 0.998 (5)

0.0523 0.0466

Deposition times from 60 to 300 s, at −1.3 V; Hg(II) concentration of 3.0 × 10−5 M; see text for other experimental details. Five replicate measurements.

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anion promotes the formation of the mercury film, in respect both to the amount of deposited mercury and to the mercury deposition rate. Moreover, the ASV peak current of lead is increased. Furthermore, there is no obvious advantage of using high concentrations of mercury for the in situ preparation of the mercury films in the presence of thiocyanate. 3.4. Combined effects of pH, thiocyanate and the deposition potential

Fig. 1. Variation of the ASV-SW peak current of lead as a function of the mercury concentration at in situ thin mercury film electrodes, for two different deposition times. Films produced in the absence (diamonds) and in the presence of 5.0 mM thiocyanate (black circles) in ammonium acetate buffer solutions of pH 3.4. Concentration of Pb(II): 30 nM; deposition times: 60 and 300 s, at −1.3 V.

mercury (II) and same pH. Hence, the chemical equilibrium involving the thiocyanate-mercury complexation has an effect on the actual mercury concentration at the electrode surface, improving the overall deposition process. Moreover, for the films produced in the presence of thiocyanate, QHg , increased linearly with the concentration of mercury (II), as expected (slope: 0.54 ± 0.01 ␮C dm3 mol−1 ; r = 0.998, N = 3). On the other hand, the reproducibility of the mercury film electrodes decreases slightly as the concentration of mercury increases (R.S.D. rises from 0.7 to 1.0%). Other authors [12,16], based solely on the ASV responses to metal cations, demonstrated the advantageous use of thiocyanate. In fact, as shown in Fig. 1, the peak current for lead, Ip(Pb) increased in the presence of thiocyanate (circles). Moreover, in the present experimental conditions (Edep = −1.3 V, pH 3.4) there was no significant variation of Ip(Pb) with the concentration of mercury(II), in spite of the observed increase in QHg . In addition, the relative standard deviations were constant over the mercury concentration range used. Also, in the presence of thiocyanate, Ip(Pb) increased linearly with the deposition time (r = 0.997; N = 5), as expected for a ASV measurement. Assuming a thin-film condition, the thickness of the film can be estimated from the amount of charge of mercury using the Faraday law and assuming the Hg atomic radius as 1.44 Å [19]. For the present mercury films (QHg = 70.7 ␮C cf. Table 2) the average mercury loading was 25.3 ␮C mm−2 , which is equivalent to a film thickness of ca. 2 nm. This value corresponds to the formation of a very thin film [1,11,14] and is 4.5× lower than the estimative for the films produced by Wu [11,14] who used mercury loadings of 115 ␮C mm−2 . Comparison with other studies on TMFEs, particularly for films produced in thiocyanate solution, cannot be made easily because of the lack of information on the actual mercury loadings used. Hence, from all the above results it may be stated that, at pH 3.4 and Edep −1.3 V, highly reproducible thin mercury films can be produced in the presence of thiocyanate. This

In order to investigate the simultaneous effects of pH, deposition potential and thiocyanate ion on the characteristics of the thin mercury film electrodes, a series of experiments was performed at three different pH values (3.4, 4.2 and 5.7), with and without the addition of thiocyanate, and at a deposition potential varying between −0.5 and −2.0 V. The deposition time was fixed at 60 s. The Hg(II) concentration was 3.00 × 10−5 M. For the ASV determinations of Pb, the cell solution was spiked with Pb(II), to a final cell concentration of 3.00×10−7 M. Both the mercury charge, QHg , and the square wave ASV stripping peak of lead, Ip(Pb) , were measured. All peak currents and voltammetric charges are mean values of ten replicate measurements. Fig. 2(A)–(C) shows the mercury voltammetric charges, QHg , as a function of the deposition potential, Edep , for three different pH values, in the absence and in the presence of 5.0 mM thiocyanate. At all pH values, QHg increased in the presence of thiocyanate and the larger effect occurred for pH 5.7. On the other hand, regardless the solution pH and the presence of thiocyanate, the variation of QHg with Edep follows the same pattern, reaching a maximum at −1.5 V. This maximum value, QHg(max) , depended on pH (cf. Table 3), increasing for lower pH values. Hence, the film formation process (nucleation plus droplet grow), monitored via the amount of deposited mercury, is facilitated by a high overpotential, which promotes the nucleation process, as expected from energetic considerations [23]. Further, the maximum QHg values are observed within a potential region where the hydrogen evolution reaction occurs at the GC electrode. However, for potential values more negative than −1.5 V, the high-energy hydrogen evolution might damage the mercury film and/or prevent a proper nucleation by limiting the surface area available for deposition, leading to the observed decrease of QHg . This damaging effect occurs regardless of the solution pH and of the presence of thiocyanate. Fisher Table 3 Maximum values of the mercury charge, QHg(max), and relative standard deviation (N = 10), for thin mercury films produced at −1.5 V, at different pH values, with and without thiocyanate pH

3.4 4.2 5.7

Without SCN−

With SCN−

QHg(max) (␮C)

R.S.D. (%)

QHg(max) (␮C)

R.S.D. (%)

65.9 53.4 13.9

6.5 9.2 7.5

74.3 61.9 53.9

0.8 0.8 0.9

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(A)

100

100

(B)

80

80

60

60

60

40 20 0 -0.5 -0.9 -1.3 -1.7 -2.1 E dep (V)

Q Hg (uC)

80 Q Hg (uC)

Q Hg (uC)

100

40 20

(C)

40 20

0 -0.5 -0.9 -1.3 -1.7 -2.1 E dep. (V)

0 -0.5

-0.9 -1.3 -1.7 -2.1 E dep (V)

Fig. 2. Mercury voltammetric charges as a function of the deposition potential at in situ thin mercury film electrodes produced at different pH (ammonium acetate buffer solutions): 3.4 (A), 4.3 (B) and 5.7 (C), in the absence of thiocyanate (diamonds) and in the presence of 5.0 mM thiocyanate (black circles). Deposition time: 60 s; mercury concentration: 30.0 ␮M.

and van den Berg [12] advanced a similar conclusion based on the ASV current responses of lead and cadmium. Also, the in situ optical inspection of thin mercury films on a GC electrode produced in acidic solutions of hydrochloric acid at −1.8 V showed the ready formation of hydrogen bubbles after 5–10 s of electrolysis [17]. Fig. 3 presents photographs of mercury coatings on the GC electrode produced at pH 3.4 and 5.7, with and without the addition of thiocyanate. The film is more homogeneous for depositions in thiocyanate solutions. (At the magnifications used the size of the droplets cannot be measured.) For the mercury films formed in the absence of thiocyanate, the film condition is always poorer exhibiting regions with lower coating (cf. Fig. 3A and D). In fact, Sahlin et al. [17] showed that for depositions in hydrochloric acid solutions at potentials in the range −0.6 to −1.1 V, the electrode surface was multi-coloured with an uneven distribution of the mercury droplets. Therefore, we may assume that in solutions of thiocyanate and regardless of the pH value (within 3.4–5.7) the nucleation process is promoted producing more uniform mercury films. The reason why this occurs might be related to the kinetic control of the actual mercury concentration at the electrode surface due to the complexation by thiocyanate leading to a regular deposition. On the other hand, the fact that higher amounts of mercury are deposited (and overall deposition rates are observed) may be correlated also to the effect of the anion-induced deposition of mercury, initially on the raw GC surface promoting nucleation and, in a second stage, at the pre-formed Hg nucleus. The phenomenon of anion-induced deposition of metal ions at mercury electrodes has long been studied and occurs particularly with anions such as thiocyanate, amongst other inorganic and organic species [27]. Stuliková [23], based on optical microscopic studies and on the effect of the deposition potential on the mercury film formation in acidic solutions considered that the concomitant hydrogen evolution might be responsible for attaining highly homogeneous thin mercury films with a more even drop size and drop distribution. Nothing was said, however,

about the mechanism by which that effect occurs. Fisher and van den Berg [12], based on the peak current responses of lead and cadmium, proposed that the combination of thiocyanate with the atomic hydrogen formation would facilitate the nucleation of mercury at glassy carbon surfaces. Again, no detailed mechanism was advanced. Conversely, the deposition of elemental mercury is hindered at GC sites with oxygen-containing groups [9,17,24]. Additional experiments performed in more acidic solution (see Fig. 4) showed that for pH <2.5 the amount of charge used for the mercury deposition in the presence of thiocyanate increases slightly reaching a plateau. Surprisingly, in that pH region much higher deposition charges were observed without thiocyanate in the plating solution. Hence, for pH <2.5 thiocyanate is no longer promoting the mercury deposition. Yet, there is an actual effect of pH, associated with that of the deposition potential, clearly expressed by the increasing amounts of QHg for pH <2.5. Hence, we may suggest that the effect of pH on the mercury film formation at rather negative deposition potentials might be related to the reduction of GC surface C–O functionalities in proton dependent reactions, leading to the unblocking of the active sites for nucleation. This mechanism would be non-operative in SCN− solution because thiocyanate ion itself is regulating the deposition of mercury due to its ready adsorption at the electrode surface, preventing, to some degree, the occurrence of H+ dependent reactions. Note that for the entire pH region under consideration thiocyanate is not expected to be protonated (pKa = −1.1 [28]). Another important consideration about the thin mercury films produced in the presence of thiocyanate is the substantial improvement in the repeatability, expressed by the R.S.D. values (cf. Table 3) which decreased from 7–9% to 0.8–0.9% in the presence of thiocyanate. There were no evidences of erratic current baselines and film degradation upon use, opposite to the observations made by others [15]. The effects of pH, thiocyanate and Edep on Ip(Pb) were also examined. As can be seen in Fig. 5(A)–(C) for deposition potentials more negative than –0.5 V, Pb was efficiently

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209

Fig. 3. Optical micrographs of thin mercury film electrodes at GCE produced at different pH (ammonium acetate buffer or nitric acid) with and without the addition of thiocyanate. (A) and (D) pH 3.4, no thiocyanate; (B) and (E) pH 3.4, 5.0 mM [SCN− ]; (C) and (F) pH 5.7, 5.0 mM [SCN− ]. Deposition: 60 s at −1.3 V; mercury concentration: 30.0 ␮M. Magnification: (A)–(C) 30×; (D)–(F) 63×.

400

Q Hg (uC)

300 200 100 0 1.5

3.5

5.5

pH

Fig. 4. Variation of the mercury voltammetric charges as a function of pH (ammonium acetate buffer or nitric acid solutions) at in situ thin mercury film electrodes, in the absence of thiocyanate (diamonds) and in the presence of 5.0 mM thiocyanate (black circles). Deposition: 60 s at −1.3 V; mercury concentration: 30.0 ␮M.

accumulated at the thin mercury film electrode. The general trend of the curves is similar at all pH values and resembles that of the variation of QHg (cf. Fig. 2). The greatest effect of the presence of thiocyanate on Ip(Pb) was observed at pH 5.7 in agreement with the behaviour in QHg (see also Table 4). The Ip(Pb) values were always higher in thiocyanate solution irrespectively of pH. This is probably related to the characteristics of the mercury films plated in thiocyanate solution where very homogeneous deposits were observed (cf. Fig. 3). As occurred for the mercury charge values, the precision of the peak current values is enhanced when the thin mercury film electrodes are produced in the presence of thiocyanate: the R.S.D. values (cf. Table 4) decreased from 1–6 to 0.4–0.9%. Considering the overall results, we may state that homogeneous thin mercury electrodes of good reproducibility can be prepared in buffered acidic Hg(II) solutions in the presence of thiocyanate. In what concerns the mercury film

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20

(A)

12

Ip (Pb) uA

Ip (Pb) uA

16

8

20

(B)

16

16

12

12

Ip (Pb) uA

210

8

(C)

8

4

4

4

0 -0.5 -0.9 -1.3 -1.7 -2.1 E dep (V)

0 -0.5 -0.9 -1.3 -1.7 -2.1 E dep (V)

0 -0.5

-0.9

-1.3 -1.7 E dep (V)

-2.1

Fig. 5. ASV-SW peak currents for Pb as a function of the deposition potential at in situ thin mercury film electrodes produced at different pH (ammonium acetate buffer solutions): 3.4 (A), 4.3 (B) and 5.7 (C), in the absence of thiocyanate (diamonds) and in the presence of 5.0 mM thiocyanate (black circles). Deposition time: 60 s; mercury concentration: 30.0 ␮M; lead concentration: 0.3 ␮M.

characteristics, the best results are obtained at moderate pH values and relatively low concentrations of mercury (i.e., pH 3.4 and 0.03 mM Hg(II)) for coatings produced at rather negative deposition potentials (−1.3 to −1.5 V) and relative short deposition times (60 s). Regarding the ASV results for Pb(II), if in situ thin mercury film electrodes are to be used, the balance between repeatability and sensitivity must be considered because for higher pH values, Ip increases (25%, cf. Table 4) but the precision decreases (R.S.D. increases by 75%). 3.5. Ex situ produced Hg thin films Several studies of the effect of chloride anion on the behaviour of in situ mercury film electrodes [5,8,9,14,17,18,22] led to the conclusion that the performance of TMFEs is disturbed if anodic polarization in chloride media takes place. This occurs because a film of mercury(I) chloride (calomel, Hg2 Cl2 ) forms on the electrode surface. For that reason the ASV in chloride medium with in situ formed TMFEs gave unstable results and noisy high background currents, especially in the potential region of copper. However, Fisher and van den Berg showed that in situ TMFEs produced in the presence of thiocyanate do not suffer from those disadvantages [12]. Nevertheless, in the present work we decided to test the application of the thin mercury film electrode ex situ formed in thiocyanate solution for ASV determinations in seawater samples. Note that the present Table 4 Maximum values of peak height for lead, Ip(max) , and corresponding relative standard deviation (N = 10), at different pH values, with and without thiocyanatea pH

3.4 4.2 5.7 a

Without SCN−

With SCN−

Ip(max) (␮A)

R.S.D. (%)

Ip(max) (␮A)

R.S.D. (%)

11.6 10.7 6.6

5.6 4.6 1.1

12.7 13.6 15.9

0.5 0.4 0.9

Deposition time 60 s at −1.3 V.

ex situ formed mercury film is considerably thinner than the usual ones that present film thicknesses typically 0.1–1 ␮m [7,8,10,14,20]. Hence, ex situ thin mercury film electrodes were produced using the optimised conditions stated in Section 3.4, i.e. in 5.0 mM thiocyanate and 3.00 × 10−5 M Hg(II) solutions of pH 3.4, with a deposition time of 60 s at Edep −1.3 V. Firstly, we compared the analytical response, in ammonium acetate buffer, of the ex situ thin mercury film electrode with that of the in situ thin films, both prepared using the optimised procedure. The mean value of Ip(Pb) for 10 replicate measurements (lead concentration: 3.00 × 10−8 M) was 0.89 ␮A (R.S.D. = 0.7%) and 0.66 ␮A (R.S.D. = 0.9%), respectively, for the ex situ and in situ films, indicating no significant differences in the sensitivity and precision for both methodologies. Secondly, the repeatability of the ASV determination of lead at the same ex situ formed mercury film was checked in 30 successive SW-ASV determinations of Pb in a 3.0 × 10−8 M standard solution (mean peak current: 0.86 nA, R.S.D. = 4.4%). This is indicative of good film stability. Brainina et al. [7] pointed out the importance of a low redox solution potential in the improvement of the lifetime of ex situ plated mercury film electrodes, especially for determinations in acidified seawater. In the present work, we applied the ex situ formed thin mercury film to measurements in a seawater sample acidified to pH 2. In this medium, there were no apparent modifications of the electrode system upon comparison with the data obtained in ammonium acetate buffer; no significant decrease of precision was observed, the baseline magnitude and slope were retained as well as the Pb peak width. Hence, there were no evidences for film deterioration in chloride medium conditions. Additionally, at the present experimental conditions, the stripping peak of mercury occurs at positive potentials (>+0.20 V) and did not have an effect on the baseline for copper. For these reasons, we considered the present thin mercury films ex situ produced in the presence of thiocyanate to be very stable in both ammonium acetate pH buffer and in acidified seawater, within a 1-day term.

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Table 5 Characteristic parameters of ASV measurementsa in a seawater sample with the ex situ thin mercury film electrodeb (ex situ TMFE) and with the HMDE Methodology

Pb Slope

Ex situ TMFE HMDE a b c

Cu (A dm3

11.7 ± 0.3 0.54 ± 0.01

mol−1 )

r (N = 5)

l.o.dc

0.9994 0.9998

6.4 × 10−11 1.1 × 10−10

(M)

Slope (A dm3 mol−1 )

r (N = 5)

l.o.dc (M)

2.79 ± 0.04 0.22 ± 0.01

0.9997 0.9995

2.2 × 10−10 6.2 × 10−10

Accumulation time 5 min at −0.8 V (a rotational frequency of 25 s−1 ). Pre-formed in acidic solution of thiocyanate at Edep = −1.3 V, 60 s (see text for experimental details). Calculated as 3σ.

3.6. Determination of total dissolved lead and copper in seawater Anodic stripping voltammetry was applied to the analytical determination of trace levels of lead and copper in seawater using a 5 min. accumulation period at a thin mercury film electrode pre-prepared (ex situ) in the presence of thiocyanate (see Section 3.3). Fig. 6 presents the anodic stripping voltammograms for the seawater sample using the ex situ thin mercury film electrode. The determined metal concentrations were (5.69 ± 0.24) and (9.39 ± 0.20) nM for Pb and Cu, respectively. The HMDE was also used for comparison and the metal concentrations obtained were (5.68 ± 0.05) nM for Pb and (9.29 ± 0.36) nM for Cu. The slopes and the limits of detection of the standard addition calibration curves using both methodologies are presented in Table 5. The sensitivity and the reproducibility were higher with the TMFE methodology for both heavy metals quantified. The values of R.S.D. (N = 5) achieved for Pb and Cu within the present concentration range were <1.2%. For the HMDE the values of R.S.D. were higher (5.8% for Pb and 6.7% for Cu). According to the results, the ASV at thin mercury films pre-formed in thiocyanate solutions of moderate pH can be considered a very sensitive and reproducible methodology for the determination of lead and copper in seawater samples.

Fig. 6. Differential pulse anodic stripping voltammograms of an acidified seawater sample (pH 2) at the ex situ formed thin mercury film electrode. The curves correspond to the sample and two standard additions (2.0 and 4.0 nM for lead and 4.0 and 8.0 nM for copper). Deposition: 300 s at −0.8 V. See text for other experimental conditions.

4. Conclusions The examination of the effects of pH, deposition potential, deposition time and concentration of mercury(II) in the formation of thin mercury film electrodes on GC in acidic solutions of thiocyanate showed that the combination of a rather negative deposition potential with thiocyanate facilitates the formation of homogeneous deposits of high reproducibility. The study of the effect of several experimental parameters on the mercury voltammetric charge gave evidence to the role of thiocyanate in the promotion of the formation of the mercury film, in what concerns the amount of deposited mercury, the deposition rate and also the film homogeneity. It may be proposed that in the actual experimental conditions (moderate acidic solution and rather negative deposition potentials, i.e., pH values within 2.5–5.7 and Edep −1.3 to −1.5 V) thiocyanate does promote the nucleation process. But, for more acidic solutions (pH <2.5) thiocyanate is no longer aiding the mercury deposition process. Moreover, in the presence of thiocyanate there is no obvious advantage of using high concentrations of mercury and/or high deposition times for the in situ and ex situ preparation of the mercury film electrodes. Hence, reproducible and stable thin mercury films can be easily prepared (with mercury loadings of about 25 ␮C mm−2 ). The present investigation gives additional support to the findings of other authors, adding highly relevant information on the characteristics of the mercury films produced in acidic solutions of thiocyanate. Also, we found that thin mercury films ex situ produced in the presence of thiocyanate are very stable in acidified seawater, within a 1-day term. The ASV methodology with an ex situ ultra-thin mercury film electrode prepared in a 5.0 mM thiocyanate solution of pH 3.4 was successfully applied to the determination of lead and copper in acidified seawater without any apparent drawback due to the actual chloride medium. The limit of detection (3σ) was 0.6 × 10−10 M for lead and 2 × 10−10 M for copper (R.S.D. <1.2%) using a deposition time of 5 min.

Acknowledgements Sandra Monterroso acknowledges also to the University of Aveiro for a PhD grant. The authors are grateful to Prof.

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