Modified Composite Electrode Containing HgO asBuilt-In Mercury Precursor for Anodic StrippingAnalysis of Trace Metals

Kyoungwon Seo, Sunghyun Kim, and Jongman Park*

Department of Chemistry, Basic Science Research Institute, Kon-Kuk University, 93-1 Mojin-dong, Kwangjin-ku,Seoul 143-701, Korea

A HgO-modified composite electrode was prepared byadapting a composite electrode technique, which is com-patible to a mercury thin-film electrode in anodic strippinganalysis of heavy metals. The preparation method andutilization of the HgO-modified composite electrode inanodic stripping analysis are described. The built-inmercury precursor HgO could be utilized feasibly forgeneration of surface mercury droplets by in situ electro-chemical treatment or pretreatment without the necessityof a Hg2+ solution. Mercury microdroplets generated onthe electrode surface were stable to meet ordinary re-quirements for stirring of solution or rotation of electrode.Hydrogen gas evolution perturbed the stability of thesurface mercury severely. The hydrogen reduction over-potential was directly dependent on the content of mer-cury oxide in the electrode matrix. The electrodes con-taining more than 10 wt % mercury oxide were fairly stableso that the deposition potential could be extended up to-1.2 V for most applications. The electrode revealedimproved renewability and stability of surface mercury aswell as reproducibility of analysis. The convenience ofvoltammetric analysis will be greatly improved if thiselectrode technique is applied in conjunction with a fastscanning stripping technique, in which the deoxygenatingstep may be skipped. Application of this electrodetechnique is very promising for environmental or indus-trial field analysis because of its simplicity of treatments.

Anodic stripping analysis has been known as one of the mostsensitive techniques and is widely used for trace heavy metalanalysis in various samples because of its capability of precon-centration of analytes on the surface of the electrode.1-14 Analyti-

cal instruments for stripping analysis are relatively simple andinexpensive compared to other instrumental methods such asatomic absorption spectroscopy or inductively coupled plasmaatomic emission spectroscopy. They can be easily built in compactsize for field analysis of environmental samples.

In most cases, mercury electrodes such as the hangingmercury drop electrode (HMDE) or thin-film mercury electrode(TFME) have been widely used for anodic stripping analysisbecause of their outstanding characteristics. However, use of thehanging mercury drop electrode is complicated by the difficultiesin handling liquid mercury and disposing of used mercury. Useof the mercury thin-film electrode is preferred because of itsimproved convenience and sensitivity.6-14 If it is used along withfast scan stripping voltammetric techniques, the cumbersomedeoxygenating step can be skipped.9,10 Although the improvedsensitivity and analytical convenience of the mercury thin-filmelectrode may still attract analysts to choose the stripping analysistechnique for trace heavy metal analysis in laboratories in the field,a film generation step either from a solution of Hg2+ or in situ byaddition of Hg2+ to sample solutions should be always involved.A drawback of the mercury thin-film electrode is instability of themercury thin film formed at the surface of glassy carbonelectrodes, which affects the reproducibility of analysis. Althoughthere have been extensive studies on the improvement of stabilityand the characterization of the surface morphology change of themercury film, it is not yet satisfactory.10-14

Here we report on a new modified composite electrodecontaining HgO as a built-in mercury precursor, which is compat-ible to the mercury thin-film electrode in analytical characteristics.The electrode reveals improved performance in anodic strippinganalysis in the aspects of reproducibility, stability, surface renew-ability, and convenience. The HgO-modified composite electrodewas developed by adapting a modified composite electrodetechnique.15-17 The electrode contains highly dispersed mercury* Corresponding author: (phone) +82-2-450-3438; (fax) +82-2-3436-5382;

(e-mail) jmpark@kkucc.konkuk.ac.kr.(1) Kissinger, P. T., Heineman, W. R., Eds. Laboratory Techniques in Electro-

chemistry; Marcel Dekker: New York, 1984; pp 499-538.(2) Whitnack, G. C.; Sasselli, R. Anal. Chim. Acta 1969, 47, 267-274.(3) Sinko, I.; Dolezal, J. J. Electroanal. Chem. Interfacial Electrochem. 1970,

25, 299-306.(4) Gillain, G.; Duyckaerts, G.; Disteche, A. Anal. Chim. Acta 1979, 106, 23-

37.(5) Ostapczuk, P.; Valenta, P.; Nurnberg, H. W. J. Electroanal. Chem. Interfacial

Electrochem. 1986, 214, 51-64.(6) Florence, T. M. J. Electroanal. Chem. Interfacial Electrochem. 1970, 27, 273-

281.

(7) Brainina, Kh. Z. Talanta 1971, 18, 513-539.(8) Lund, W.; Salberg, M. Anal. Chim. Acta 1975, 76, 131-141.(9) Wojciechowski, M.; Balcerzak, J. Anal. Chem. 1990, 62, 1325-1331.

(10) Wu, H. P. Anal. Chem. 1994, 66, 3151-3157.(11) Wu, H. P. Anal. Chem. 1996, 68, 1639-1645.(12) Brainina, K. Z.; Vilchinskaya, E. A.; Khanina, R. M. Analyst 1990, 115, 1301-

1304.(13) Wang, J.; Tian, B. Anal. Chem. 1992, 64, 1706-1709.(14) Carra, R. G. M.; Sanchez-Misiego, A.; Zirino, A. Anal. Chem. 1995, 67,

4484-4486.

Anal. Chem. 1998, 70, 2936-2940

2936 Analytical Chemistry, Vol. 70, No. 14, July 15, 1998 S0003-2700(97)01211-0 CCC: $15.00 © 1998 American Chemical SocietyPublished on Web 06/11/1998

oxide particles through the electrode matrix as the mercuryprecursor. Fine particles of HgO exposed on the surface of theelectrode can be reduced electrochemically into microdroplets ofmercury, which are adhered on the surface of the electrode andrelatively stable during analysis. The electrode surface can beeasily renewed by a simple polishing process whenever a freshsurface is desired. No Hg2+ solution is required for the generationof mercury thin film. So further simplification of the anodicstripping analysis process is possible by employing a HgO-modified composite electrode. Details in the preparation proce-dure and characteristics of the electrode are described below.

EXPERIMENTAL SECTIONAll chemicals used in this work were reagent grade unless

otherwise mentioned. Ketjenblack EC 600JD from Akzo Chemiewas semigraphitic carbon. Styrene (Junsei) and divinylbenzene(55%, Wako) were vacuum distilled and stored in a freezer beforeuse. Azodiisobutyronitrile (AIBN, 2,2-azobis(2-methylpropinoni-trile) (from Aldrich) was used as radical initiator for polymeriza-tion. Commercial standard stock solutions of metals (fromMallinckrodt) for atomic absorption spectroscopy were used forpreparation of metal ion solutions. Deionized water (18 MΩ) wasused for preparation of solutions. A BAS 100W electrochemicalanalyzer from Bioanalytical Systems with a C2 cell stand (a faradaycage equipped with spin rate controllable magnetic stirrer) wasused for voltammetric measurements. A rotator from PineInstruments (AFMSRX) was used for electrode spinning inrotating disk electrode experiments. An Ag/AgCl (3M NaCl)reference electrode was used. A JEOL JSA-840A scanningelectron microscope (SEM) was used for surface morphologyobservation.

Preparation of Electrodes. HgO-modified composite elec-trodes were prepared by adapting the preparation methods ofmodified composite electrodes, which can be found elsewhere.15-17

A typical procedure for the preparation of an electrode containing16.7 wt % HgO is described below briefly. Mercuric chloride(8.35) was dissolved in 200 mL of ethanol. Two grams of carbonblack was added to this solution, and the mixture was sonicatedto disperse the carbon black thoroughly. The relative amount ofmercuric chloride to carbon black was changed to control thecontent of mercuric oxide in the electrode matrix. Deionizedwater was added slowly with vigorous stirring until the contentof ethanol was 20-30%. Ethanol was used to disperse theagglomerated hydrophobic carbon black well into fine particles.The mixture was titrated with 1.5 M NaOH solution to precipitateHg2+ into HgO. It should be noted that the precipitation processis an important step affecting the performance of modifiedelectrodes. Precipitated mercuric oxide should be adsorbed wellon the surface of dispersed carbon black so that the color of themixture become black without any yellowish color of mercuricoxide precipitates. When the yellowish color of mercuric oxidewas observed, the precipitates were dissolved back by addingsome acid solution and then reprecipitated. The mixture wasfiltered, washed with deionized water thoroughly, and then driedat 60-70 °C. It was ground well into fine powder, using a

commercial coffee bean grinder, and then stored in a sealedcontainer.

The mixture was incorporated into a polymeric matrix asfollow; a solution of styrene and divinylbenzene (4:1 ratio)containing AIBN as radical initiator was added slowly to themixture of carbon black and mercuric oxide until a thick pasteformed. The content of carbon black was controlled at 5.2 wt %in order to ensure proper electrical conductivity of the resultingcomposite materials while the mercuric oxide content was variedto get electrodes containing different amounts of modifier. Theamount of AIBN was kept to ∼1% of the mixture. The paste waspolymerized in sealed glass tubing at 70 °C for more than 4 h.The composite materials were 3.1 mm in diameter. They werefabricated into proper types of electrodes by either sealing in glasstubing with commercial epoxy resin for ordinary stripping analysisor press-fit into Teflon body for rotating disk electrode experi-ments. The electrodes were ground with SiC abrasive paper andthen finally polished with 0.05-µm alumina. The electrodes werewashed thoroughly with deionized water between each step.

RESULTS AND DISCUSSIONHgO-modified composite electrodes revealed the proper physi-

cal and mechanical properties for fabrication. The polishedsurface was smooth and shiny. Since fresh HgO modifier particlesare exposed on the surface upon polishing, they can be reducedelectrochemically to fine droplets of mercury during the condition-ing process or deposition process of analytes. The characteristicsof the electrodes must be very dependent on the amounts ofsurface mercury as well as on the distribution of droplets. Thesurface morphology of an electrode containing 16.7 wt % HgOwas observed using a SEM, which is shown in Figure 1.Microcrystalline mercury oxide particles (white and blurry whitespots) were distributed randomly along with carbon black ag-glomerates (gray) in the polymeric matrix (background). Themercuric oxide particles were not uniform in size, but it did notexceed 3 µm. Figure 1b shows the surface morphology afterelectrochemical conditioning at -0.40 V vs Ag/AgCl for 100 s.Mercury droplets were randomly distributed over the surface.They were mostly smaller than 1.5 µm. Most mercury dropletsfilled and stuck in hollow spots with large wetting angles. Someothers were partially exposed on the surface because the precur-sor particles were partially buried in the electrode matrix.However, it should be recognized that the sizes of mercurydroplets pictured must be somewhat smaller than those initiallyconditioned because of the instability of mercury droplets underthe high vacuum of SEM. The picture was taken ∼20 s afterturning on the vacuum system of SEM. During observation ofthe surface morphology through the CRT screen, the gradualdisappearance of small mercury droplets was noted. The blurrywhite spots are thought to be HgO precursors buried completelyin the electrode matrix, which could not be reduced to mercurydroplets. It is thought that large surface interaction between theelectrode matrix and fine mercury droplets plays an importantrole in stabilizing the mercury droplets on the electrode surface.Large agglomerates of mercury oxide particles were observed insome batches containing a large amount of modifier, whichbecome large droplets of mercury by conditioning. Large dropletswould be fallen easily from the surface even with weak distur-bance. Eventually the stripping analysis signals were affected by

(15) Shaw, B. R.; Creasy, K. E. J. Electroanal. Chem. Interfacial Electrochem.1988, 243, 209-217.

(16) Park, J.; Shaw, B. R. Anal. Chem. 1989, 61, 848-852.(17) Chung, H.; Park, J. Bull. Korean Chem. Soc. 1997, 18, 952-957.

Analytical Chemistry, Vol. 70, No. 14, July 15, 1998 2937

the presence of large mercury droplets. Often the first strippingsignals obtained just after the electrode conditioning process weresomewhat larger than others when the electrodes were rotatedduring the deposition steps (see Figure 4). So the precipitationprocess of HgO should be carefully controlled to avoid formationof large HgO agglomerates. However, satisfactory results couldbe obtained if any yellowish color of mercuric oxide was notobserved during the precipitation process.

The mercuric oxide content in the matrix is an important factoraffecting on the analytical characteristics of the electrodes.Hydrogen evolution overpotential, stability and reproducibility ofthe electrode were dependent on the surface concentration ofreduced mercury. The surface concentrations of mercury wereestimated from the charges required to oxidize Hg(0) to Hg(II).Anodic peaks (Ep ) +0.75V vs Ag/AgCl) of linear sweepvoltammograms were integrated to get the charges after condi-tioning at -1.0 V for 120 s. In most case the surface concentrationof mercury increased linearly with slope n = 2 as the mercuricoxide content increased. The amount of mercuric oxide exposedon the surface might increase drastically because of not only theincrement of mercuric oxide content but also the decrease ofpolymeric content. Typically the estimated surface mercuryconcentration of the electrode containing 16.7 wt % HgO was 1.3

× 10-8 mol/cm2, which was about the same order of magnitudeas those reported for ordinary mercury thin-film electrodes.10 Thesurface concentrations of mercury were not altered apparentlyalthough the electrodes were sonicated for 1 min before condition-ing. The mercuric oxide particles must be held strongly on thesurface by the polymeric matrix.

Fast-scan anodic stripping voltammetry was performed for Cu,Pb, and Cd using the electrode containing 16.7 wt % HgO todemonstrate the possibility of its utilization (Figure 2). Theelectrode was conditioned at -0.4 V for 400 s in 0.1 M KCl, 0.01M HCl solution to form mercury droplets and then cleaned at 0.0V for 20 s. Metals were deposited at -1.0 V for 60 s, equilibratedfor 10 s, and then stripped at 2.0 V/s. The solution was stirredwith a magnetic stirrer at constant rate during the conditioning,cleaning, and deposition steps. Between the measurements, theelectrode was cleaned at 0.0 V for 20 s. The deoxygenating stepwas skipped by employing the fast-scan stripping technique asmentioned earlier.9,10 The stripping peak currents for Pb and Cdincreased linearly with the concentration increase. Meanwhile,the linearity for Cu was inferior to those of others because ofsevere band broadening owing to the complicated oxidationprocess of copper.

Since the surface of the electrode is not completely coveredby mercury, unlike the mercury drop electrode or mercury thin-film electrode, it is clear that hydrogen evolution overpotentialmust be dependent on the HgO content in the electrode matrix.Low overpotential will cause not only a narrow potential windowbut also a decrease in sensitivity and reproducibility of analysis.Stripping analysis was performed with four electrodes, each havingdifferent modifier contents, for a 50 ppb Pb2+ solution to investigatethe effect of modifier content on the electrode characteristics. Thedeposition potential was varied from -0.7 to -1.3 V. The

Figure 1. Surface morphology of a HgO-modified compositeelectrode containing 16.7 wt % HgO: polished surface (a) beforeelectrochemical conditioning and (b) after conditioning at -0.4 V vsAg/AgCl, 0.10 M KCl and 0.01 M HCl for 100 s.

Figure 2. Anodic stripping voltammograms of Cu, Pb, and Cd: (a)background, (b) 5 ppb of each metal ion, (c) 10 ppb, (d) 15 ppb, and(e) 20 ppb in 0.1 M KCl and 0.01 M HCl solution. The electrode (16.7wt % HgO) was conditioned at -0.4 V for 300 s; Cleaning at 0 V for20 s; deposition at -1.00 V for 60 s; equilibration for 10 s; strippedat 2 V/s. The solutions were stirred at constant rate.

2938 Analytical Chemistry, Vol. 70, No. 14, July 15, 1998

electrode surface was renewed by polishing between the measure-ments. Figure 3 represents the stripping peak current plotsagainst deposition potential change. The error bars represent (1s of the response, which means reproducibility of analysis relatedto surface renewal. Deposition at -0.7 V seems not to be cathodicenough since the stripping peak currents were neither high norreproducible. In the case of the electrodes containing lessmodifier (6.7 and 10.0 wt %), the stripping peak currents decreasedas the deposition potential became more negative. It is thoughtthat hydrogen evolution was not suppressed effectively at highcathodic potential because of the low surface concentration of themercury formed. The electrode surface might rather reveal somecharacteristics of an unmodified carbon composite electrode.Active evolution of hydrogen on the electrode surface may disturbnot only the mass transfer of analyte ion during the depositionstep but also the stability of the mercury droplets. So the strippingcurrent decreased and the reproducibility became poor. On theother hand, the stripping currents for the electrodes containing ahigher amount of the modifier did not decrease even at highlynegative deposition potentials up to -1.3 V and the reproducibilitywas better.

Stripping analysis was performed while the electrodes wererotated using a rotator at controlled rates, i.e., 500, 1000, and 2000rpm, to test the physical stability of the mercury droplets formedon the surface of electrodes, and the results are shown in Figure4. In most measurements, the anodic stripping peak currents didnot change apparently, which means the surface mercury dropletswere stable enough to endure the centrifugal force exerted byelectrode spinning. However, it was not true for the electrodecontaining the lowest amount of HgO (6.7 wt %). Gradualdecrease of the stripping current was observed, which wasdependent on the spin rate. If it is assumed that the centrifugal

force was the major factor affecting the stability of the surfacemercury droplets, the electrode containing the highest amountof HgO should be most unstable. It is likely that there must beother factors affecting the stability of the surface mercury dropletsrather than the centrifugal force. Since the major difference inthe electrodes is the content of modifier, it is believed that theeffect of hydrogen reduction overpotential would be responsiblefor the instability of the surface mercury droplets. Moreover,since accumulation of hydrogen gas on the surface of the electrodeis not assumed under high spin rates, it is difficult to say that thehydrogen gas accumulated on the surface disturbed the masstransfer of analyte ions. So we concluded that the stability of thesurface mercury droplets was perturbed by an active gas evolutionprocess. The hydrogen reduction overpotentials of the electrodescontaining higher amounts of HgO were negative enough to keepthe droplets stable. It is likely that the remarkable stability ofthe surface mercury droplets would be a result of the extraordi-nary surface morphology of the electrodes. As mentioned earlier,the mercury droplets were filled and stuck in small hollow spotswith large contact angles so that large surface interaction betweenthe electrode matrix and mercury droplets would improve thestability. So electrodes containing a higher content of mercuryoxide in the electrode matrix are preferred. In addition, it shouldbe remarked that the first stripping anodic peak currents weresomewhat higher apparently than the others in most cases. Suchdifferences were likely due to the instability of large-sized dropletsthat might fall off easily at the initial stage of the experiments.Since it appeared to be reproducible, the first stripping analysisdata might be rejected for reliable analytical results.

The feasibility of sensitivity control of analysis by changingthe deposition time is one of the advantages of the strippinganalysis. We examined the proportionality of stripping peak

Figure 3. Deposition potential dependence of electrode responsesfor 50 ppb Pb in anodic stripping analysis HgO (A) 6.7, (B) 10.0, (C)13.4, and (D) 16.7 wt %. Other conditions are the same as in Figure2, except the deposition potential.

Figure 4. Effect of the spin rate of electrodes on the stability ofsurface mercury droplets represented by anodic stripping currentchange: HgO (A) 6.7, (B) 10.0, (C) 13.4, and (D) 16.7 wt %. Pbconcentration, 50 ppb. Scan rate: (0) 500, (O) 1000, and (4) 2000rpm. Other conditions are the same as in Figure 2.

Analytical Chemistry, Vol. 70, No. 14, July 15, 1998 2939

current versus concentration of Pb2+ ion for several orders ofconcentration ranges, shown in Figure 5. The correlation coef-ficients of the calibration curves appeared to be higher than 0.999.When an electrode simply ground with 2000-grit SiC paper wasused, the linearity of the calibration curves was still high.Typically, the correlation coefficient was 0.998 for the range of0-15 ppb Pb2+. Since fairly reliable results can be obtainedwithout a polishing process, the electrode could be utilized foron-site field analysis of heavy metals with the advantage ofconvenience. Repetitive stripping analysis for a 50 ppb Pb2+

solution was also performed with five kinds of polished electrodesfor 15 times each to evaluate the reproducibility of the anodic

stripping peak current. To circumvent the effect of hydrogenevolution during the deposition process, the deposition potentialwas set at -0.8 V for the electrodes containing 6.7 or 10.0 wt %HgO and -1.0 V for the electrodes containing more HgO. Otherconditions for the stripping analysis were the same as in Figure2. The relative standard deviations of the responses ranged from0.5 to 2.0% maximum. Analysis of lead in laboratory tap waterwas also performed by the standard addition method. The relativestandard deviation of analytical results appeared to be 1.6% at the95% confidence level for seven repetitive trials, which means theanalysis was highly reproducible. Between measurements, theelectrode surface was polished every time.

In summary, adaptation of a modified composite electrodetechnique made it possible to prepare the HgO-modified compositeelectrode, which is compatible with mercury thin-film electrodesin anodic stripping analysis of heavy metals. A distinctive featureof the HgO-modified composite electrode is its built-in mercuryprecursor, HgO, which can be utilized feasibly for the generationof surface mercury by in situ conditioning or by pretreatmentwithout the necessity of Hg2+ solution. The physical stability ofthe surface mercury droplets was very high, which is owing tothe unique surface morphology resulting in enhanced contactbetween the electrode matrix and the surface mercury droplets.Reproducibility of analysis could be improved to less than 2%relative standard deviation. Furthermore, the surface of theelectrode could be renewed easily by simple polishing or grindingso that the utility of the electrode is improved. The convenienceof voltammetric analysis will be greatly improved if this electrodetechnique is applied with the fast scanning stripping technique,in which a deoxygenating step is not required. Studies on theutilization of the electrode for environmental or industrial fieldanalysis are underway in our laboratory.

ACKNOWLEDGMENTWe are deeply grateful to Mr. Lim, Heonseong, for assistance

with the SEM analysis. This work was supported by the KoreanMinistry of Education through the Research Fund (BRSI-96-3444)and Kon-Kuk University.

Received for review November 3, 1997. Accepted April16, 1998.

AC971211Y

Figure 5. Linear calibration curves for various ranges of Pbconcentration. Working electrode, 16.7 wt % HgO. Deposition time:(A) 15 min, (B) 4 min, (C) 1 min, and (D) 20 s. Other conditions arethe same as in Figure 2, except Pb concentration.

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