Deposition and electrochemical stripping of mercury ions on polypyrrole based modified electrodes

181

J. Electroanal. Chem., 246 (1988) 181-191 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

DEPOSITION AND ELECIROCHEMICAL ON POLYPYRROLE BASED MODIFIED

M.D. IMISIDES and G.G. WALLACE Chemistry Department,

STRIPPING OF MERCURY ELECTRODES

IONS

l

Universiry of Wollongong. P.O. Box 1144, Wollongong, N.S. W. 2500 (Aurwalia)

(Received 31st August 1987; in revised form 7th December 1987)

ABSTRACT The ability of polypyrrole to act as a conducting electrode modifier for various electroanalytical applications has been demonstrated previously. In this work, it is used as a substrate for the deposition and stripping of mercury ions from solution. Aspects of the deposition and stripping behaviour, bdth for the polymer in its underivatised state, and for the poly(pyrrole-N-carbodithioate) moiety are compared with the response on bare platinum. The polymer electrodes exhibit significant advantages over the bare platinum surface. The improved stability and reproducibility of surface states has been demonstrated.

INTRODUCTION

In the search for more sensitivity and/or selectivity several electroanalytical chemists have explored the use of chemically modified electrodes (CMEs) in recent years [l-6]. Using CMEs the chemistry in the electron transfer region can be controlled rigorously since the electrode substrate is purposefully modified. The implications are that this chemistry can be used to preconcentrate analytes into the electron transfer region [1,2], to provide electrocatalysts [3,4] which result in more rapid or more extensive electron transfer processes, or to exclude possible interferents [5,6] from the electron transfer region. Unfortunately, the application of modified electrodes to analytical problems is not as simple as one might first suspect. High concentrations of reactants on the electrode surface may introduce some unusual phenomena due to interaction of the electroactive sites [7]. Furthermore, the chemical species present in the modifier layer may also take part in the charge transfer process via counterion diffusion - again complicating the overall process [8].

* To whom correspondence should be addressed.

0022-0728/88/$03.50

0 1988 Elsevier Sequoia S.A.

182

Polypyrrole electrodes have been studied extensively by previous workers [9-131. These electrodes are easily synthesized by electrochemical oxidation of the monomer. To date, however, the deployment of these modified electrodes as chemical sensors has been limited, perhaps due to the rigorous requirements which must be met to ensure sensitivity and reproducible results. Factors which influence signal reproducibility including electrode preparation, matrix effects throughout the analytical procedure, and the presence of other analytes become extremely important when using modified electrodes. We have previously reported the possibility of using a poly(pyrrole-N-carbodithioate) electrode for mercury determinations [14]. Given the interest in mercury determinations, due to its toxicity [15] and also the widespread interest in mercury thin film electrodes as electrochemical sensors [16-201, this work has since been considered in more detail. The poly(pyrrole-N-carbodithioate) electrode has been investigated with a view towards developing a mercury analysis system as well as new mercury containing electrode substrates. Some of the factors influencing the deposition and stripping processes from these modified electrodes have been considered. The possibility of using the mercury plated electrodes as new substrates has been addressed. EXPERIMENTAL

Reagents

and standard solutions

All chemicals used were A.R. grade purity unless otherwise stated. Pyrrole was Purum ex Fluka and carbon disulfide was obtained from BDH. The pyrrole was distilled before use. L.R. grade benzene and 2-propanol were obtained from Ajax. HPLC grade acetonitrile was obtained from Mallinckrodt. Metal stock solutions were prepared by dissolving appropriate salts in distilled deionised water. Platinum wire was obtained from Johnson and Matthey. The wire was cleaned in aqua regia before use and between polymer coatings. Tetraethylammoniumperchlorate (TEAP) was purum ex Fluka. Polypyrrole film formation

The polymer was formed by electropolymerisation at platinum from an acetonitrile (0.5 A4 TEAP) solution containing 0.1 M pyrrole. Current densities in the range OS-5 mA/cm* were employed. The polymer is weakly adsorbed to the substrate, and it may easily be wiped off with a tissue when wet. When dry it adheres to the substrate considerably more strongly. Preparation

of poly(pyrrole-N-carbodithioate)

Dithiocarbamate previously [14].

electrode

groups were grafted onto the polymer

surface as described

Instrumentation

A Princeton Applied Research (PAR) Model 173 potentiostat in conjunction with a PAR Model 175 function generator, a PAR 178 electrometer probe, and a PAR

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Princeton

Applied Research

Model no. K0060

-

Resin

INVERTED COUNTER ELECTRODE POLYMERISATION APPARATUS (I.C.E.P.A) Fig. 1. Inverted counter electrode polymexisation apparatus.

Model 179 coulometer were employed. A Metrohm 646 VA Voltammetric Analyser was employed for square wave (SW) voltammetry. An Ag/Ag+ reference electrode was employed for work in acetonitrile solution and an Ag/AgCl (sat. NaNO,) reference electrode was employed for work in aqueous solution. A specially modified electrochemical cell (Fig. 1) was employed for polymer plating. This design minimised problems associated with cell resistance. Cold vapour mercury analyses were performed using a Varian AAS Atomic Absorption Spectrometer in conjunction with a Varian model 65 cold vapour generation unit. RESULTS AND DISCUSSION

In order to elucidate the behaviour of the process under investigation the deposition and stripping of mercury ions from platinum and from a platinum coated polypyrrole electrode were investigated initially. Behaviour of platinum As reported by other workers [16,17] the electrodeposition of mercury ions onto a platinum substrate is readily achieved. At low mercury surface coverage the behaviour is relatively simple. A cyclic voltammogram is shown in Fig. 2a with an oxidation response due to the electrochemical stripping of mercury ions observed at - +0.50 V. At higher surface coverage (Fig. 2b) or longer deposition times the behaviour is more complicated, with multiple stripping peaks being observed. Other workers have reported the presence of different Pt-Hg layers after electrochemical

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Fig. 2. (a) Cyclic voltammogram following uptake from 0.5 mg/l Hg2+ (as NO; ) for 10 min at platinum wire electrode. Scan rate = 50 m V/S. (b) Cyclic voltammogram following uptake from 0.5 mg/l Hg2+ (as NO; ) for 50 min at platinum wire electrode. Scan rate = 50 mV/s.

deposition [17], which presumably give rise to the multiple peaks observed in this work. Behaviour at polypyrrole

coated platinum electrodes

Preliminary experiments on polypyrrole coated platinum electrodes indicated that reproducible mercury stripping responses could be obtained only if the electrode preparation procedure was strictly controlled. It was found that adherent films which gave reproducible results were obtained if the electrode was: (1) Coated with an oxide film by immersion in chromic acid, followed by rinsing in 10 M HNO,. (2) Stripped of the oxide by cathodic electrolysis (Eapp = 0.18 V vs. Ag/AgCl until no further current decay). (3) Stripped of adsorbed hydrogen by anodic electrolysis (Eari, = 0.07 V vs. Ag/AgCl until no further current decay). This procedure results in a platinised platinum surface as described by other workers [21]. Other workers [22] have shown that the morphology of polypyrrole can be controlled by varying the electroplating conditions. In this work polymers produced by both high current density (HCD) plating (id = 5 mA/cm*) and low current density (LCD) plating (id = 0.5 mA/cm*) were investigated. Although the current density was varied over an order of magnitude, the charge density was identical, resulting in a direct comparison of polymers of identical thickness but different morphology. A typical cyclic voltammetric response for the latter is shown in Fig. 3. Given that mercury ions are electrodeposited at potentials where the polymer is

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TABLE

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Polymerisation

for polypyrrole coated platinum wire electrode. Deposition at - 0.1 V of 1 mg/l Hg2+ (as NO; ) for 20 mm. Scan rate = 50 mV/s.

1

Characteristics

of the stripping

response

of various

Platinum

Surface reproducibility Pretreatment Stripping peaks

poor

Maximum Faradaic signal (M.F.S.) Reproducibility of MFS on subsequent scans Subsequent reusability of electrode Substrate longevity

1st scan

Yes number and position vary with [Hg2+ ] between 0.14 and 0.64 V vs. Ag/AgCl

poor: must be pretreated prior to use

’ PW = peak width at half height.

I

E/V(vs.Ag/AgCI)

Fig. 3. Cyclic voltammetric response for polypyrrole coated platinum wire electrode. current density = 0.5 mA/cm2. Polymer thickness = 0.75 pm. Scan rate = 50 m V/s. Fig. 4. Mercury stripping response vs. Ag/AgCl from stirred solution

I

-.I

electrode

substrates

Polypyrrole

Poly-@yrrolsNcarbodithioate)

no 2-e- oxidation: 0.51 V vs. Ag/AgCl PW”=49mV 0.60 V vs. Ag/AgCl PW “=17mV l-e- reduction: - 0.37 V vs. Ag/AgCl PWB=117mV 1st scan

no 2-e- oxidation: 0.51 V vs. Ag/AgCl PW’=49mV 0.60 V vs. Ag/AgCl PW’=17mV l-e- reduction: - 0.37 V vs. Ag/AgCl PW’=117mV 5-10 scans (“break in”)

- 2 weeks

> 3 months in chelated form

186 TABLE 2 [Hg *+ I B in solution after indicated time. I&, solution) Time/min

0 5 10 15 20 a [Hg2+

[Hg”

= - 0.1 V vs. Ag/AgCl,

deposition time = 20 min (stirred

]/mg 1-l

Platinum

Polypyrrole

Poly(pyrrole-Ncarbodithioate)

3.00 2.11 2.40 2.38 2.38

3.00 2.78 2.22 2.10 2.10

3.00 2.16 2.14 2.14 2.14

] determined using cold vapour AAS.

conductive (-0.6 to 0.5 V vs. Ag/AgCl) and consequently more porous, it is possible that metallic mercury is electrodeposited on either the platinum substrate or the polymer. Using the LCD coatings at an applied potential of -0.10 V, mercury ions were electrodeposited. The electrochemical stripping response (Fig. 4) provides evidence for this. Characteristics of this stripping response for the various electrode substrates examined are compared in Table 1. Monitoring of mercury solutions which contained the electrodes under investigation indicated that the coated electrode removed more mercury ions from solution more rapidly than a bare platinum electrode (Table 2). As a result, the stripping responses obtained with the polypyrrole electrode were much larger in magnitude. To ascertain if this was simply a surface area phenomenon, linear sweep voltammograms of a standard K,Fe(CN), oxidation response were recorded at platinum and polypyrrole-platinum electrodes. The limiting currents obtained were identical, indicating that the electroactive surface areas were similar. Consequently, the polypyrrole must facilitate the trapping process in some way. Other characteristics of the mercury stripping responses are similar on platinum and polypyrrole coated platinum electrodes. The response decreases with the number of scans as would be expected as the mercury metal is depleted on the electrode surface. It is envisaged that the stripping response is due to oxidation of mercury metal as observed at a platinum substrate. These polymer coated electrodes can be reused for mercury deposition immediately after stripping. The coated electrodes are stable for at least 2 weeks. They provide reproducible mercury deposition and stripping responses over this time period without intermediary treatment. This is much more reproducible than any previously employed solid electrode. With HCD polymers no responses are observed after uptake at -0.10 V vs. Ag/AgCl.

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A po~fpyrro~e-~-carb~fhiou~e~ electrode Following the derivatisation procedure outlined previously [14], the cyclic voltammetric data obtained for the derivatised polypyrrole was essentially the same as that obtained prior to derivatisation. The electrodes were investigated for their ability to take up mercury ions. With LCD derivatised electrodes the mercury is removed from solution at a greater rate than the non-derivatised at the potentials investigated (Table 2). This would suggest that with the polyfpyrrole-N-carbodithioate) electrode the sulfur containing groups are involved in the uptake process. Note that particularly in the early stages of deposition the rate of removal is markedly greater. Although most of the characteristics of the stripping response are similar to those discussed above for the polypyrrole electrode, there are some significant differences. Firstly, rather than decrease with subsequent cycles, the stripping response initially increases (Fig. 5), stays constant at a maximum value for another 4 or 5 cycles, and then decreases. The factors determining the number of cycles required to establish the maximum response, and the number of cycles for which the response stays at a maximum value are not well understood and are currently being investigated. This “break-in” process may be attributed to the establishing of conduction bands through the polymer, or possibly solvent wetting [19] as postulated by other workers. Alternatively, it may be due to the electrochemical reduction of mercury ions complexed rather than ele~trodeposited onto the modified electrode surface.

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Fig. 5. Voltammogram of “break-in” process. Deposition on poly(pyrrole-N-carbodithioate) platinum wire electrode at -0.1 V vs. Ag/AgCl from stirred solution of 2 mg/l Hg2+ (as NO,) min. Scan rate = 50 mV/s. Mercury stripping response for scans l-14. Fig. 6. Mercury stripping response vs. Ag/AgCl from stirred solution

I

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coated for 10

for polypyrrole coated platinum wire electrode. Deposition at - 0.1 V of 1 mg/l Hg2+ (as NO, ) for 30 min. Scan rate = 50 mV/s.

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This reduction would be achieved during cycling as the potential was scanned more negative. Another significant difference in behaviour is the fact that the derivatised polymers will not take up mercury ions after the stripping response has disappeared. (It appears that the oxidised mercury is trapped in the polymer and prevents access of new mercury ions.) The trapped mercury (visible as a grey film) was not electroactive. Electrochemical oxidation did not remove the mercury metal trapped on the polymer. The reasons for this are not clear, and are currently under investigation. Consequently, a procedure aimed at displacing the mercury ions via competitive complexation was devised. Soaking in a saturated Zn(NO,), solution for a period of ten days resulted in an electrode capable of mercury ion uptake which was reusable, and which gave excellent reproducibility. Potential cycling in a zinc ion solution accelerated this regeneration procedure, in which case it could be achieved in a matter of minutes. The derivatised polymer electrode suffers from instability compared to the unmodified polypyrrole electrode if stored in its non-chelated form. Although still displaying a consistent cyclic voltammogram for the polypyrrole backbone, mercury cannot be electrodeposited. If left in the mercury complexed form and regenerated prior to deposition, this problem does not occur; in fact, the electrode is then stable for at least 3 months. The HCD derivatised polymer electrode was also previously shown [14] to trap mercury ions from solution. The rate of removal compares well with the LCD polymers. However, the electrochemical responses of the HCD polymers were more pronounced. Both polymer electrodes manifest an additional oxidation response (Fig. 6) at +0.60 V vs. Ag/AgCl. Scanning to these more positive potentials results in a rapid decrease in the first oxidation response. In fact, both responses decrease with increasing number of scans. The rapid loss of electrochemical activity made it impossible to characterise these responses. The electrochemical stripping response Preliminary experiments indicate that waveforms such as differential pulse (DP) or square wave voltammetry may also be employed to consider the stripping response for mercury analytically. DP voltammetry results in enhanced sensitivity by a factor of 4-5 compared to linear sweep voltammetry. Square wave voltammetry gave no enhancement of the mercury response but did result in a flat background over the potential range investigated. The higher frequency waveform appears to discriminate against polypyrrole background responses. More extensive studies are currently under way. Applications

The coated electrodes showed an analytically useful dependence on [Hg”] at low mercury concentrations. It is envisaged that the linear range could be extended

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[Hg’+] I mg I-’

Fig. 7. Calibration curve for the mercury stripping response at poly(pyrrole-N-carbodithioate) coated platinum electrode. Polymer thickness = 0.75 pm. Deposition time = 20 min. Deposition potential = - 0.1 V vs. Ag/AgCl. Scan rate = 50 mV/s.

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E / V(vs.Ag/AgCl 1 Fig. 8. Cyclic voltammogram following deposition of (CH,),Hg onto poly(pyrroleN-carbodithioate) electrode. Deposition time = 30 min from stirred solution of 100 mg/l (CH,),Hg. Applied potential = -0.1 V vs. Ag/AgCl. Scan rate = 50 mV/s.

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by adjusting the polymer thickness. Figure 7 shows the stripping response observed for a derivatised electrode as a function of [Hg”] (NaNO, supporting electrolyte). Uptake time was 20 min from a stirred solution. The current limit of detection is 200 pg/l for both types of modified electrode, although it is envisaged that as a result of the optimisation of the various parameters involved, the sensitivity of the method may be extended. Speciation The determination of both phenyl and dimethyl mercury ions at the modified electrodes was investigated. No response could be obtained for phenyl mercury. However, for dimethyl mercury a response at Ep = + 0.40 V vs. Ag/AgCl, which is slightly removed from the free mercury ion response, was observed (Fig. 8) on the poly(pyrrole-IV-carbodithioate) electrode. This response did not exhibit break-in behaviour as was observed for mercury ions. Other workers have shown that dithiocarbamate resins may be employed to trap organomercury species and that dithiocarbamate ligands are useful in speciating mercury ions [23,24]. CONCLUSIONS

The deposition and electrochemical stripping of mercury from polypyrrole based electrodes has been studied in detail. With CMEs the number of parameters influencing the sensitivity and reproducibility of the analytical signal are multiplied compared to a conventional solid substrate. A method for regenerating poly(pyrrole-N-carbodithioate) electrodes has been designed and tested. The feasibility of speciation work using the sulfur containing electrode has/been demonstrated. The mercury coated electrodes discussed in this work are currently being evaluated as thin film electrodes for determination of other species. REFERENCES 1 G.T. Cheek and R.F. Nelson, Anal. L&t., 5 (1978) 393. 2 D.M.T. G’Riordan and G.G. Wallace, Anal. Chem., 53 (1986) 128. 3 S.M. Geraty, D.W.N. Arrigan and J.G. Vos in M.R. Smyth and J.G. Vos (Eds.), Electrochemistry Sensors and Analysis, Elsevier, Amsterdam, 1986, p. 303. 4 R.M. Ianniello and A.M. Yacynych, Anal. Chem., 53 (1981) 2090. 5 J. Wang and L.D. Hutchins-Kumar, Anal. Chem., 58 (1986) 402. 6 G. Gittampalam and G.S. Wilson, Anal. Chem., 55 (1983) 1608. 7 A.H. Schroeder, F.B. Kaufman, V. Pate1 and E.M. Engter, J. Electroanal. Chem., 113 (1980) 193. 8 D.A. Buttry and F.G. Anson, J. Electroanal. Chem., 130 (1981) 333. 9 R.A. Bull, F.R. Fan and A.J. Bard, J. Electrochem. Sot., 129 (1982) 1009. 10 R.C.M. Jakobs, L.J.J. Janssen and E. Barendrecht, Reel. Trav. Chim. Pays-Bas, 105 (1984) 275. 11 P. Pfluger and G.B. Street, Polym. Film Prep., 23 (1982) 122. 12 S. Kuwabata, H. Yoneyama and H. Tamura, Bull. Chem. Sot. Jpn., 57 (1984) 2247.

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L.L. Miller, B. Zinger and Q.X. Zhou, J. Am. Chem. Sot., 109 (1984) 2267. M.D. Imisides, D.M.T. G’Riordan and G.G. Wallace in ref. 3, p. 293. L. Hodges, Environmental Pollution, Holt, Reinhart and Wilson, New York, 1973. Z. Yoshida, Bull. Chem. Sot. Jpn., 54 (1981) 556. Z. Yoshida, Bull. Chem. Sot. Jpn., 54 (1981) 562. Z. Stojek and Z. Kublik, J. Electroanal. Chem., 77 (1977) 205. G.E. Bately and T.M. Florence, J. Electroanal. Chem., 55 (1974) 23. W.E. Van der Linden and J.E. Dieker, Anal. Chim. Acta, 119 (1980) 1. R.N. Adams, Electrochemistry at Solid Electrodes, Marcel Dekker, New York, 1969. S. Asavapiriyanont, G.K. Chandler, G.A. Gunawardena and D. Pletcher, J. Electroanal. (1984) 229. 23 T. Braun and M.N. Abbas, Anal. Chim. Acta, 131 (1981) 311. 24 L.H.J. Lajunen, E. Eijarvi and P. Nieme, Finn. Chem. Lett., 6 (1984) 146.

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