amalgam electrodes for electroanalysis

Review

Amalgam Electrodes for ElectroanalysisÈyvind Mikkelsen, Knut H. Schr˘der*

Norwegian University of Science and Technology, Department of Chemistry, N-7491 Trondheim, Norway* e-mail: [email protected]; [email protected]

Received: September 27, 2002Final version: November 11, 2002

AbstractLiquid mercury is a unique material for the indicator electrode in voltammetry. One reason for this is the highovervoltage for hydrogen formation, thus extending the actual potential window. Diluted amalgams are importantreaction products in voltammetric (polarographic) processes, however liquid amalgams are rarely used directly aselectrode material for analytical purposes. Because of the fact that voltammetry is very suitable for field and remotemonitoring, issues concerning the use of mercury electrodes in environmental analyses have led to considerableresearch effort aimed at finding alternative tools with acceptable performance. Solid electrodes are such alternatives.Different types of electrodes are reviewed. In particular, solid amalgam electrodes are very promising, withacceptable low toxicity to be used for field measurements. Solid amalgam electrodes are easy and cheap to constructand are stable over a reasonable time up to several weeks. Assessment of the toxicity risk and the long time stabilityfor remote and unattended monitoring is discussed. The differences between solid dental amalgam electrodes, madeby using techniques known from dental clinical practice, and mercury film or mercury layer electrodes on solidsubstrates are reviewed. In particular the dental technique for constructing solid amalgam electrodes gives advantagebecause it×s fast and inexpensive. Also the technique for making dental amalgam has been explored and optimizedover years by dentists, giving advantage when the same technique is used for constructing electrodes. Dental amalgamelectrodes has been found to act similar to a silver electrodes, but with high overvoltage towards hydrogen. This makeit possible to use the dental amalgam electrode for detection of zinc, cobalt and nickel in additions to other metals likelead, copper, thallium, cadmium, bismuth, iron etc. Also the use for reducible organic compounds is expected to bepromising.

Keywords: Voltammetry, Polarography, Stripping, Dental, Amalgam, Electrode, Mercury, Heavy metals, Mediumexchange

1. Introduction

Voltammetry is known as a very useful method, not only formore fundamental studies of electrode reactions, but alsofor applied analytical purposes. Important examples ofapplications are in the field of environmental pollutionmonitoring and to control industrial products and effluents.The popularity of this technique has varied very much overtime since Professor Jaroslav Heyrovsky invented thepolarographic method, which dates from 1922. This is onone side due to the increasing number of alternativemethods, but also on the other side the stepwise improve-ment in electronics and instrumentation. The concern ofpollution and pollution control during the last decades hasalso been important in this regard.On the other hand, and aswill be discussed later, the uniqueness of using the toxicliquid mercury as electrode material is a serious limitation.Discussions about the extent of the health risk of usingmercury for voltammetric purposes take place. However,for off-laboratorymeasurements this cannot be accepted, atleast of psychological and political reasons.

1.1. Advantages Using Voltammetric Analyses

During the last years many new analytical methods havebeen developed [1]. Regarding this, the present and futurerole of voltammetry has to be considered. In a well-equipped laboratory with a great number of samples to beanalyzed, normally other methods will be preferred. How-ever, there are reasons to use voltammetry, even in alaboratory: The method is easy to use for many actualmeasurements, in particular for compounds of environ-mental interest. In contrast tomost othermethods, chemicalspeciation and not only the total amount can be measured.Finally, if it is needed to use mercury, the health risk will benegligible using controlled laboratory routines [2]. How-ever, the psychological and political reason not to usemercury should not be underestimated.The main interest for voltammetry is when no other

analytical methods are applicable. Continuous and remotemeasurements and monitoring in the field are typical taskswhere very few or no other methods are appropriate at lowconcentration levels. For such purposes, classical analyticaltechniques are not sufficiently sensitive, and upcountry tolocate huge instrumentswith the required infrastructure andat several locations is very inconvenient, and normally this isnot possible at all.

679

Electroanalysis 2003, 15, No. 8 ¹ 2003 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim 1040-0397/03/0805-0679 $ 17.50+.50/0

2. Liquid Electrodes

The advantages of using liquid electrodes in voltammetryare obvious. A new and reproducible surface is easilyformed without any interference from previous measure-ments or from contaminants, and cleaning or refreshingprocedures are not required. On the other hand, handling ofdrop systems is technically somewhat more complicatedthan using solid electrodes.

2.1. Liquid Mercury Electrodes

During the history of polarography, the use of otherelectrode materials than liquid mercury is very unusual.This is due to the unique properties of mercury, with highovervoltage for hydrogen evolution reaction. This willextend the measuring window, enabling one to measurecompounds appearing in the more negative potential range,like zinc.On theother hand, due to theoxidationofmercury,such electrodes cannot be used at more positive potentials.Another advantage using mercury is that diluted, liquid

amalgams are formed with many metals during the reduc-tion process. This is important in stripping techniques andcontributes to better-defined voltammetric curves.The third advantage appears due to the liquid state, as

pointed out above.

2.2. Liquid Amalgam Electrodes

The word ™amalgam∫ has different meanings in differentcontexts and in different languages, from the more generalterm ™united into one body∫ to themore specific term™alloyof mercury and one or more metals∫. The latter definition isused in voltammetry. The direct use of liquid, i.e., diluted,amalgam electrodes in analytical voltammetry is verylimited, and is mainly used to investigate electrode proc-esses [3]. However, such electrodes are frequently beingformed starting from pure mercury during the reductionprocess. Due to the high extent of dilution, this liquidamalgam possesses almost the same mechanical propertiesand toxicity as liquid mercury itself. This is of importancefrom an analytical point of view being the second step involtammetric andpotentiometric stripping techniqueswhenthe diluted amalgam is reoxidized to mercury and metalions.

2.3. Liquid Mercury and Amalgams: ToxicologicalConsequences

There has, been a growing concern about the general use ofmercury, due to its toxicity. This includes the use of puremercury as an electrode material in voltammetry. Even forlaboratory use, restrictions are expected to appear in thefuture. Therefore it is of great interest to find alternativeelectrode materials to the liquid mercury and the liquid

amalgam electrodes. The use of dilute amalgams andamalgam films [4, 5] electrodes, or a mercury meniscus onsilver electrode may reduce the amount of used mercurygreatly. However, even for these electrodes the risk due tothe toxicity of mercury in off-laboratory handling is noteliminated.

3. Solid Electrodes

A serious limitation in analytical voltammetry is thereduction of hydrogen at the indicator electrode with theformation of hydrogen gas. This narrows the measurementwindowat negative potentials,makingdeterminationof zincand several other species impossible, due to the current fromthe formation of hydrogen gas. For a platinum electrode,used in an acid solution, the actual potential range is in theregion� 1 to �0.3 V (vs. SCE) [6]. Oxidation of theelectrode material, the matrix to be investigated or theformation of oxygen gas limits the measurement window atpositive potentials, correspondingly. However, the latterlimitation is normally less important because most of actualmeasurements, like most heavy metals etc., happen to takeplace in the negative potential region. An overview ofpotential ranges for some solid electrodes used in voltam-metry is given by Wang [7].

3.1. Mercury-on-Solid-Substrate Electrodes

In 1947 Airey [6] observed that the actual most negativepotential range for the above mentioned platinum elec-trode, as well as with a silver electrode, was extended from�0.3 V to �2.1 V by amalgamation with mercury. It isstated in that article that the best results are obtained withthe platinum electrode. The amalgamation of platinum willmake a very thin film of Pt/Hg only [8], thus frequentlyrepeated preparations will be needed.Below it is differed between electrolytically deposited

mercury film and film formed by dipping into liquidmercury. Obviously, this gives very little, if any, differencesbecause a sufficiently thin film is not strictly a solid or aliquid.

3.1.1. Electrolytically Deposited Mercury Film Electrodes

A great number of substrates have been used for mercurydeposition. This includes the following examples: Silver [9],platinum [10, 11], iridium [12, 13], gold [14] and carbon indifferent forms [15]. Extensive work for in situ measure-ments has been performed by Luther III and co-workers[16 ± 23], using solid gold microelectrodes plated withmercury. Another interesting approach for in situ studiesis worked out by Buffle and his group [24 ± 31]. They use amicro array system, with an iridium matrix plated withmercury. To increase measuring stability by avoidingelectrode fouling they protect the electrodeusing an agarosegel membrane. This acts as a dialysis membrane by allowing

680 È. Mikkelsen, K. H. Schr˘der

Electroanalysis 2003, 15, No. 8

the diffusion of metal ions and complexes and by hinderingthe diffusion of colloids and macromolecules. However, aproblemwith deposition ofmercury ontometal substrates isthe stability over time, due to partly formations of amalgams[32].Several methods are worked out for analyses of heavy

metals, also applicable for seawater, one example is [33].The use of microelectrode arrays with mercury deposit oncarbon fibers is an interesting approach for increasedsensitivity of stripping analyses [34]. Mercury salts can alsobe mixed with the electrode material to generate a mercuryfilm [35]. Finally, amalgam formation can be used forchemical speciation studies [36, 37].

3.1.2. Electrodes Prepared by Dipping into LiquidMercury

An alternative to electrochemical plating of mercury ontometal substrate electrode is simply to dip the electrodedirectly inmercury as suggested byNovotny¬ and co-workers[38 ± 46], that have further developed such electrode sys-tems, initially proposed by Airey [6].In their first versions [38], the electrodes were designed as

given in Figure 1, by using a glass capillary of about 5 mmouter diameter and 1 mm inner diameter. After insertion ofa silver wire of a diameter of about 0.5 mm into the capillary,the glass was conically narrowed and the space between thesilver wire and the glass was tightened with an acrylic resin.After hardening of the resin, themetallic cross-section of thecapillary was polished and dipped into a beaker with liquidmercury, kept under water. Subsequently, the excess ofmercury sticking to the silver disk was gently knocked off.The electrode was activated by cyclic polarizations beforeeach major series of experiments.In the later versions [44], in some extent the design of the

electrode was modified, as shown in Figure 2. Silver orcopper powder was packed into a glass capillary with aninner diameter of about 0.5 mm; electric contact was madewith a platinum wire inserted into the powder. The metalpowder was then amalgamated by dipping the unit intoliquid mercury. The resulting solid amalgam disk waspolished and covered by a mercury meniscus by a newdipping into a small volume of liquid mercury for 10 ± 20 s.Then the electrode was electrochemically activated andfurther treated as above.Voltammetric analyses using such electrodes are conven-

ient, as very little liquid mercury is required. As expected,they behave analytically very similar to mercury electrodes,asmercury is the active part, being in contact with the actualsolution. It is of interest to compare the earlier versions ofthe electrode [38] with the later version [44]. In the earlierversions, silver and copper are used as a substrate for liquidmercury, by dipping the metal electrode into mercury, andthe excess of mercury sticking to the silver disk wasafterwards knocked off. A thin liquid mercury film is thenformed. Then a gradual change of themercury filmwill takeplace due to a slow formation of an amalgam film on thesurface. Such electrodes cannot beused for determinationof

metals at trace levels only, but also for analytical work onreducible organic and biological compounds likeDNA [38 ±46].As pointed out in [44], the earlier version is actually a

nonequilibrium electrode with less stability. The newerversion, however, is not using a metal substrate directlyconnected to the mercury layer, but this is performed in twosteps, the electrode with the metal powder is dipped intoliquid mercury, and then polished after some time, and thendipped into liquid mercury once more. Consequently, wehave the following situation: The platinum wire inside theelectrode is in contact with silver powder, somewhat hard-ened due to some reaction with mercury from the firstdipping sequence. This amalgamation, however, is notcomplete. At the polished end of the electrode, theamalgamation has proceeded further, and this polishedsurface is brought in contact with mercury from the seconddipping sequence. This means that the outer mercury isusing more silver, or slightly amalgamated silver, assubstrate for the earlier versions of the electrode. But forthe newer version, amixture of silver,mercury and amalgamwill be the substrate surface being in contact with the activemercury layer. Consequently, the newer version will givebetter performance and the electrode can be used for aboutaweekbefore renewing is neededbydipping it intomercury.This is due to the slower disappearance of the active part ofthe electrode, the liquidmercurymeniscus, over time [38]. Inthat article it is mentioned: ™They (the electrodes) are basedon solidmetal amalgam phase of the −dental type× accordingto need modified by a very small amount of mercury∫.However, as the active part of the electrode is mercury, andthe idea by the duplicate dipping procedure is to maintainthat mercury, this is actually a convenient design of a

Fig. 1. A mercury/silver electrode; 1) electrode body; 2) Ag-wire; 3) Hg-meniscus or drop; 4) Ag-disk; 5) special adhesivematerial. From [38] by permission.

Fig. 2. Scheme of the used metal solid amalgam electrodes:1) electrode body; 2) Pt-wire; 3) mercury meniscus; 4) solid amal-gam. From [44] by permission.

681Amalgam Electrodes


mercury electrode and not a −dental type× electrode. Suchelectrodes will be discussed, and compared with in thefollowing.

3.2. Dental Amalgam Electrodes

The use of electrodes in voltammetry with more than oneactive compound has not previously been verymuch studiedin voltammetry. It seems in general that it has been assumedthat such an electrode should be pure, consisting of oneelement only. As pointed out above, mercury is unique aselectrode material in analytical voltammetry. Due to therestrictions of usingmercury, in particular for off-laboratoryuse, the present authors attempted to find alternativematerials, dental amalgam being an actual candidate.In dental amalgam, mercury is bound up as a stable Ag2

Hg3 alloy in an excess of silver,making it totally nontoxic foranalytical purpose. The uses of such small electrodes for soiland groundwater analyses are obviously without anyenvironmental hazard. Investigations of groundwater closeto cemeteries [47] have not shown any increased levels ofmercury, illustrating its stability in the environment. Also,not to forget, hundred of millions of people are today livingwith such fillings in their mouths.A great advantage of using dental amalgam as electrodes

in analytical voltammetry is that the technique dealing withthe material is well established from dental clinical practiceand advanced equipment for preparation of proper amal-gams is available commercially. As pointed out above,formation of the amalgam can be a slow process, but this canbe carried out in minutes using modern dental amalgammixing equipment and a proper ratio between silver andmercury [48].With such equipment, air bubbles trapped intothe solidified amalgam are avoided, as this will increase thebackground current.A suitable ratio between silver andmercury should be 1 :1

by weight (mole ratio 1.85). In our laboratories we normallyuse 2.5 g fine silver powder (�50 �) and 2.5 g of analyticalgrade mercury. This mixture is transferred into a mixingcapsule by first introducing the mercury and then the silverpowder. The capsule is then installed in a dental amalgammixer (e.g., ESPE ROTOMIX) and mixed for 12 secondsand centrifuged for 3 seconds. Immediately after the mixingprocess is ended the amalgam paste is filled into theelectrode holder as shown in Figure 3, just as filling a toothcavity, and a copper wire is inserted in the upper part of thematerial as electrical connector. After some minutes theamalgam has solidified, but should be kept for at least oneday for completing the reaction before polishing to amirror-like surface.Several years ago the dentistsmixed the silver or the silver

alloy and mercury using a mortar and pestle, with a verynonhomogeneous amalgamas the result.Obviously, thiswillbe even more the case with a dipping sequence with nomixing, as mentioned above [44], this being analogous tofilling a tooth cavity with silver powder followed by sealingwith a drop of mercury.

Our first presentation, after patenting, of the dentalamalgamelectrodewas given at theESEACmeeting in June2000 [49], not after the Czech appearance of [39] inDecember 2000, as it is written in [44].Preliminary experiments were carried out using lost

dental fillings from the present author [50]. This indicatedthat the dental amalgam had sufficient overvoltage vs.hydrogen formation to be useful analytically. However,most of the material for dental purposes also containsadditionalmaterial, like zinc etc. This is done to improve thedental filling properties. The starting alloy was thereforemodified to be silver only [51 ± 55], this also being anapproved alloy for making dental amalgam.Results from determination of zinc for one of the first

measurements with a standard dental alloy (high coppernon-gamma 2) are given in Figure 4. As found from the

Fig. 3. Cross-section of the dental amalgam electrode. From [50]by permission.

Fig. 4. Detection of zinc on a working electrode containing anon-zinc (high copper non-gamma 2-alloy) amalgam for dentaluse. Analyses performed as differential pulse anodic strippingvoltammetry, scan rate 10 mV/s, pulse height 70 mV, depositiontime 120 s. The concentrations were 100 ppb, 300 ppb, 400 ppb,1000 ppb and 1500 ppb. From [50] by permission.



figure, the overpotential is shifted to allow these measure-ments.A simultaneous determination of zinc, lead and copper in

tap water using pure silver only in amalgamation is shown inFigure 5.Measurements of thallium are very convenient by using a

dental amalgam electrode. This is important due to fre-quently high concentration of that element in wastewaterfrom metallurgical industry. The amalgam electrode hasbeen found to give a very good sensitivity towards thallium,with a detection limit in the low ppb to ppt range with 180 sdeposition time. The peak potentials for thallium is slightlymore positive than that of cadmium, therefore if cadmiumand thallium are both present in a solution cadmiummay beseparated by complexing the cadmium with CN�.

The results reviewed above were carried out using anodicstripping voltammetry. By using cathodic stripping oradsorptive stripping techniques, the applications of dentalamalgam electrodes can be further expanded. Analyses ofnickel and cobalt were performed as adsorptive cathodicstripping voltammetry by use of dimethylglyoxime andiminodiacetate in ammonium buffer solutions. In thissolution nickel and cobalt yielded well-defined peaks withgood resolution from hydrogen evolution. Figure 6 shows asimultaneous detection of nickel and cobalt at differentconcentrations.As an example of using dental amalgam electrodes for

measuring heavy metals in beverages a procedure wasintroduced in [54]. In contrast to measurements usingHMDE, the method of medium exchange cannot be useddirectlywhen adental amalgamelectrode is applied becausethis causes an unpredictable loss of material due tooxidation. We therefore introduced: Online Medium Ex-change (OME) with the stripping medium, by performingthe exchange in a flow system maintaining the depositpotential [54]. Obviously, this technique can also be applieddirectly for flow systems.Dental amalgam voltammetry for remote control seems

very promising and is now applied for continuous environ-mental monitoring. Figure 7 shows a typical example from areal sample from the outlet air scrubbingwastewater, from awaste incineration plant in Trondheim, Norway.As found from the results discussed above, the properties

of dental amalgam electrodes are very similar to silverelectrodes, but with one very essential exception, theovervoltage for formation of hydrogen is much higher.This is not as high as for mercury, but sufficiently high for avoltage window to measure zinc and other compounds.It is of interest to compare the dental amalgam electrode

with the electrodes developed by Novotny¬ and co-workers.They wanted to construct an electrode covered with a stable

Fig. 5. Measurement of heavy metals in old tap water by differential pulse stripping voltammetry. Solid line shows the real sample,dashed line shows same sample after addition of 100 ppb Zn, 50 ppb Pb and 500 ppb Cu. Predeposition was 120 s at �1450 mV, followedby a scan to �20 mV. Modulation pulse: 50 mV, scan rate: 15 mV/s. Zinc observed at �1100 mV, lead at �450 mV and copper at�150 mV.

Fig. 6. Adsorptive cathodic stripping voltammetry detection ofNi(II) and Co(II) (10, 20, 30 ppb) as dimethylglyoxime complex inNH4Cl buffer solution. Iminodiacetate was added to enhance thecobalt response. Deposition times 30 s at �0.7 V, scan from �0.7to �1.25 V, pulse height �50 mV. From [52] by permission.



mercury layer or a stable mercury meniscus as the activepart. In their earliest versions this stabilitywas not as goodaswanted because slow amalgamation of the mercury menis-cus from reaction with the substrate being silver wire.However, in their newer versions this reaction proceedsmuch slower because the substrate, a mixture of silver,mercury and amalgam already contains a more completeamalgam and is stable towards reacting with more mercury.On the other hand, the dental amalgam electrode hasproperties more like the silver electrode with extendedpotentialwindow. It can also be polishedwithout any changein its properties. As discussed next, a silver/mercury alloywith a few percent ofmercury only, also behaves like a silverelectrode, with an analogous extension of the potentialwindow [56]. This further supports this view.The dental amalgam seems to combine some of the

properties of the silver electrode with those of the mercuryelectrode. For instance, the dental amalgam material (Ag2Hg3 in silver) is a solid electrode, and solid deposits areformed. On a mercury drop, mercury film or mercurymeniscus electrode, however, liquid mercury amalgams areformed. Since the dental amalgam electrode is a solidelectrode, formations of intermetallic compounds areobserved to a greater extent than for liquid mercury. Forinstance, in solutions containing both zinc and copper, theformation of zinc-copper intermetallic compound is ob-served and contemporary a significant decrease in theresponse for zinc is observed. However, by addition ofchloride to the solution, this intermetallic compound hasbeen avoided, due to formation of a copper (I) chloridecomplex, which prevents zinc to form an intermetalliccompound.As known, the solubility of cadmium is greater than the

solubility of lead in mercury, and consequently a betterresponse for cadmium is observed when diluted amalgamsare formed in the mercury drop electrode [52]. However, atthe dental amalgam electrode, where solid deposits are

formed, lead shows a better response than cadmium. This isa typical silver behaviour, and this unique affinity seems tobe maintained in the dental amalgam alloy.At a potential of about �200 mV, an anodic current is

observed using dental amalgam electrodes, most likely dueto oxidation of the dental amalgam.As solid, nontoxic material, the dental amalgam can be

used in remote online analyses in the field. After optimizingof the procedures, the detection limits for the metals areassumed to be in the same range as for the mercury andmercury film electrode. It can also be used repeatedly over along period of time without any maintenance, which isessential for remote, online and field apparatus.

3.3. Other Solid Electrodes

Several articles have appeared dealing with alternativeelectrode materials to extend the potential window. Asexamples are the use of bismuth film [57] and boron dopeddiamond [58]. Further discussion of such electrodes isbeyond the scope of the present article.It seems to have been an established opinion that one-

component electrodes should be superior for voltammetricanalyses. The use of alloys, adding some percent of a secondmetal, is not extensively studied in order to obtain anextended potential window. In the battery industry, how-ever, more component alloys are frequently used in order toavoid hydrogen gas formation.The present authors have studied silver electrodes with a

few percent of mercury, bismuth and lead oxide [56, 59], allwith the effect of extending the actual potential window, andit is reasonable to assume that othermainmetals than silver,with small quantities of other components will havecorresponding effect. Discussion of the use of otheradditives than mercury will be beyond the scope of thepresent article.

Fig. 7. Plot from continuous remote monitoring of zinc, cadmium and lead from Heimdal Waste Incineration Plant in Norway. To thescrubbing wastewater was added NH4Cl (to 0.05 M). Differential pulse anodic stripping voltammetry, 120 s deposit time at �1300 mV,scan rate 15 mVs�1, differential pulse 50 mV.



The technique for making such alloys with a few percentofmercury differs substantially from theway tomake dentalamalgam electrodes because dental clinical procedurescannot be applied, as no homogeneous solidification willtake place after the mixing. One way is to take the silverpowder and the actual quantity of mercury in a quartz tube,seal it by heating the upper part, followed by melting andmixing the alloy. After cooling, the alloy is made as a smallcylinder, being the active part of the electrode after polish-ing.Figure 8 shows measurements of zinc by use of a silver

electrode containing 4%mercury. As found from the figure,the potential window is shifted towards a sufficientlynegative value to allow the determination of zinc and otherspecies.Addition of 400 �gL�1 cadmium to solutions mention

above did not show any significant influence on the responseof the zinc peak.Further results [59] indicate that the extension of the

potential window increases with the fraction of mercury orother additives, with pure mercury at present with thelargest negative value. Thus the amount of the additive canbe tailored for the actual analytical problem. However,much more work still remains in this field.

4. Summary and Future Directions

Dental amalgam (Ag2Hg3 as the main compound) seems tobe a promising electrode material for use in voltammetry.The solid dental amalgam electrode combines the proper-ties frommercury with some of the properties of silver. Thealloy has a high overvoltage towards hydrogen, and showsparticular good sensitivity towards zinc, lead and iron. But

also other metals like, e.g., nickel, cobalt, cadmium, andthallium can be detected in the low ppb to sub ppb area. Thedeposition of metals onto the dental amalgam surface isgreatly characterized by the fact that it is a solidmaterial andshows therefore a great difference from the pure mercuryelectrode, e.g., in respect to formation of inter metalliccompounds. However the most common intermetalliccompounds may be avoided by addition of a third elementor compound. At �200 mV an anodic current is observed,most likely due to a decomposition of the dental amalgam.Dental amalgam is a solid, nontoxic material, and this

makes it possible to be used in online analyses in the field.Thedetection limits for themostmetals are assumed to be inthe same range as for themercury ormercury film electrode.The stability over time has been found to be good (severalweeks) without any maintenance, which is essential foronline and field apparatus. Also, it is easy and cheap tomanufacture such electrodes, using techniques well estab-lished for dental clinics.Potentiometric stripping analysis (PSA) is a simple, but

very sensitive and important part of electroanalyticalchemistry. PSA was presented several decades ago [60],but it has not obtained as much attention as it deserves. Thestripping procedure is in general performed in two differentways, 1. By oxidation using chemicals, most frequently usedwith the presence of oxygen, or 2. By constant currentoxidation. The latter is normally preferred because morestandardized conditions are obtained.Amalgam electrodes are very useful for PSA analyses for

the same reasons as given for voltammetry. This method isvery convenient for remote and off-laboratory use. Prelimi-nary investigations indicate dental amalgam to be a verypromising material for such purposes and further develop-ments are in progress in our laboratories.

Fig. 8. Detection of zinc on silver with 4% mercury alloy electrode. The zinc concentration was 100, 200, 300 and 400 �gL�1. All scanswere performed in DPASV mode in NH4Ac (0.05 M). Scan rate: 15 mVs�1, pulse height: 70 mV, and deposition time: 2 min. From [51]reproduced by permission of the Royal Society of Chemistry.



A general disadvantage by using amalgam electrodes isthe short potential window in the positive voltage range.Certainly, most of the actual measurements are at negativepotentials, but many are also at more positive values, butdeveloping cathodic as well as anodic procedures can inmany cases solve this. Future directions are expected tobe inthe development of new techniques, with new electrodematerials like two and more component alloys, and con-ducting polymers. Another promising direction, verifiedfrom our preliminary experiments, is to modify the dentalamalgam by adding small amounts of ion exchanger beforeit solidifies. Increase in sensitivity and selectivity is thenachieved.A great number of analytical methods have been devel-

oped during the last years, but very few of thosemethods areactually in daily use to solve real problems. In particular thisis the case for off-laboratory applications. It is a greatdistance from developing a method used in a laboratorytested on pure materials, to continuous outdoor measure-ments. For that reason such monitoring is still limited to afew and very simple parameters. Voltammetry and PSA arevery convenient for such monitoring, but the long-termstability is a critical factor. Thin-film electrodes, likemercury film, are useful for many applications, but forcontinuous measurements the homogeneous electrodes,like silver anddental amalgam, haveadvantages due to long-term stability. It has been found that the dental amalgamelectrode in most cases can be used to detect the sameanalyte in the samematrix as for themercury dropelectrode,therefore it is expected that the dental amalgamelectrode aswell as the alloy electrodes also will be suitable for use inanalytical work on reducible organic and biological com-pounds as, e.g., DNA. This will be investigated later.Asmentioned above, such dental amalgamelectrodes and

alloy electrodes are quite stable over time without anymaintenance. It is only needed to polish the electrodesmonthly for some seconds with a filter paper whenmonitoring drinking water, and more frequently for pol-luted water, in order to avoid electrode fouling. However,for remote monitoring it is of interest to extend this period.Again a technology with a scaling developed for dentalpractice can be applied. From time to time, such homoge-neous electrodes can be automatically treated like removalof tooth stones and plaque in dentistry [61]. We haverecently started investigations in these lines, with verypromising results. This will be reported later.Finally, additionally to applying low frequency sound of

about 80 ± 100 Hz [49, 62] it is very convenient for off-laboratory use with increased sensitivity.Better understanding of the nature of overpotential and

underpotential reactions is of great interest, not only from atheoretical point of view, but also as a tool for improvedelectrode development. Finally, the development of devicesfor miniature measurements will be important in the future.

5. References

[1] P. Vany¬sek, Modern Techniques in Electroanalysis, Vol. 139,Chemical Analysis, Wiley, New York, 1996.

[2] P. Zuman, Anal. Lett. 2000, 33, 163.[3] D. A. Payne, A. J. Bard, J. Electrochem. Soc. 1972, 119, 1665.[4] M. Lovric, Sœ. Komorsky-Lovric, A. M. Bond, J. Electroanal.

Chem, 1991, 319, 1.[5] L. K. Bieniasz, J. Electroanal. Chem, 2002, 527, 21.[6] L. Airey, Analyst 1947, 72, 304.[7] J. Wang, Analytical Electrochemistry, 2nd ed, Wiley-VCH

2000, pp. 107.[8] G. T. Cheek, T. Graham, W. E. O×Grady, Proc. Electrochem.

Soc. 1994, 94.[9] M. Ciszkowska, M. Donten, Z. Stojek, Anal. Chem. 1994, 66,

4112.[10] K. R. Wehmeyer, R. M. Wightman, Anal. Chem. 1985, 57,

1989.[11] M. A. Baldo, S. Daniele, C. Bragato, G. A. Mazzocchin,

Anal. Chim. Acta 2002, 464, 217.[12] S. P. Kounaves, W. Deng, J. Electroanal. Chem. 1991, 301, 77.[13] R. R. De Vitre, M. L. Tercier, M. Tsacopoulos, J. Buffle,

Anal. Chim. Acta 1991, 249, 419.[14] W. Lu, A. S. Baranski, J. Electroanal. Chem. 1992, 335, 105.[15] M. Ciszkowska, Z. Stojek, J. Electroanal. Chem. 1985, 191,

101.[16] P. J. Brendel, G. W. Luther III, Environ. Sci. Technol. 1995,

29, 751.[17] G. W. Luther III, C. E. Reimers, D. B. Nuzzio, D. Lovalvo,

Environ. Sci. Technol. 1999, 33, 4352.[18] G. W. Luther III, P. J. Brendel, B. L. Lewis, B. Sundby, L.

LefranÁois, N. Silverberg, D. B. Nuzzio, Limnol. Oceanogr.1998, 43, 325.

[19] M. Huettel, W. Ziebis, S. Forster, G. W. Luther III, Geochim.Cosmochim. Acta. 1998, 62, 613.

[20] M. E. Dollhopf, K. H. Nealson, D. M. Simon, G. W. Luther -III, Mar. Chem. 2000, 70, 171.

[21] M. Taillefert, G. W. Luther III, D. B. Nuzzio, Electroanalysis2000, 12, 401.

[22] G. W. Luther III, T. F. Rozan, M. Taillefert, D. B. Nuzzio, C.DiMeo, T. M. Shank, R. A. Lutz, S. C. Cary, Nature 2001,410, 813.

[23] P. Anschutz, B. Sundby, L. LefranÁois, G. W. Luther III, A.Mucci, Geochim. Cosmochim. Acta 2000, 64, 2751.

[24] M.-L. Tercier, J. Buffle, F. Grazoittin, Electroanalysis 1998,10, 355.

[25] C. Belmont-He¬bert, M.-L. Tercier, J. Buffle, G. C. Fiaccab-rino, N. F. deRooij, M. Koudelka-Hep, Anal. Chem. 1998, 70,2949.

[26] M.-L. Tercier, J. Buffle, Anal. Chem. 1996, 68, 3670.[27] R. R. DeVitre, M.-L. Tercier, M. Tsacopoulos, J. Buffle,

Anal. Chim. Acta 1991, 249, 419.[28] M.-L. Tercier, N. Parthasarathy, J. Buffle, Electroanalysis

1995, 7, 55.[29] C. Belmont, M.-L. Tercier, J. Buffle, G. C. Fiaccabrino, M.

Koudelka-Hep, Anal. Chim. Acta 1996, 329, 203.[30] M.-L. Tercier, J. Buffle, Electroanalysis 1993, 5, 187.[31] O. C. Keller, J. Buffle, Anal. Chem. 2000, 72, 936.[32] B. Bas, Z. Kowalski, Electroanalysis, 2002, 14, 1067.[33] E. Fischer, C. M. G. van den Berg, Anal. Chim. Acta 1999,

385, 273.[34] D. A. Fungaro, C. M. A. Brett, Anal. Chim. Acta 1999, 385,

257.[35] S. B. Khoo, S. X. Guo, Electroanalysis 2002, 14, 813.[36] E. Fischer, C. M. G. van den Berg, Anal. Chim. Acta 2001,

432, 11.



[37] J. Galvez, K. H. Schr˘der, J. Electroanal. Chem. 1993, 361,121.

[38] L. Novotny¬, L. Havran, B. Yosypchuk, M. Fojta, Electro-analysis 2000, 12, 960.

[39] L. Novotny¬, B. Yosypchuk, Chem. Listy 2000, 94, 1118.[40] B. Yosypchuk, L. Novotny¬, US-CZ Workshop on Electro-

chemical Sensors, Czech Chemical Society, Prague 2001, p. 26.[41] K. Peckova, J. Barek, M. Drevinek, T. Navratil, L. Novotn¬ , B.

Yosypchuk, S. Vaingatova, J. Zima, US-CZ Workshop onElectrochemical Sensors, Czech Chemical Society, Prague2001, p. 32.

[42] B. Yosypchuk, L. Novotny¬, Crit. Rev. Anal. Chem. 2002, 32,141.

[43] B. Yosypchuk, L. Novotny¬, Electroanalysis 2002, 14, 1138.[44] B. Yosypchuk, L. Novotny¬, Talanta 2002, 56, 971.[45] B. Yosypchuk, L. Novotny¬, Chem. Listy 2002, 96, 756.[46] F. Jelen, B. Yosypchuk, A. Kourilova¬ , L. Novotny¬, E.

Palecek, Anal. Chem. 2002, 18, 4788.[47] M. Eiskjaer, D. Arenholdt-Bindslev, J. Dent. Res. 1994, 73,

981.[48] E. H. Greener, J. K. Harcourt, E. P. Lautenschlager, Materi-

als Science in Dentistry. Williams & Wilkins, Baltimore 1972.ch. 8.

[49] È. Mikkelsen, K. H. Schr˘der, The 8th Int. Conf. Electro-Analysis (ESEAC 2000). Bonn, Germany, 2000. http://www.chem.ntnu.no/� knusch/bonnposter.pdf

[50] È. Mikkelsen, K. H. Schr˘der, Analy. Lett. 2000, 33, 3253.[51] È. Mikkelsen, K. H. Schr˘der, Electrochemistry and Inter-

facial Chemistry in Environmental Clean-Up and GreenChemical Processes, Coimbra, Portugal, 2001.

[52] È. Mikkelsen, K. H. Schr˘der, T. A. Aarhaug, Collection ofCzechoslovak Chemical Communications 2001, 66, 465.

[53] È. Mikkelsen, K. H. Schr˘der, Environmental and HealthAspects of Mining, Refining and Related Industries, Skukuza,Kruger National Park, South Africa, 2001.

[54] È. Mikkelsen, K. H. Schr˘der, Anal. Chim. Acta 2002, 1, 249.[55] È. Mikkelsen, K. H. Schr˘der, 9th International Conference

on Electroanalysis European Society for ElectroanalyticalChemistry, Krako¬w, Poland, 2002.

[56] È. Mikkelsen, K. H. Schr˘der. Analyst 2000, 125, 2163.[57] J. Wang, J. Lu, Electrochemistry Communications 2000, 2,

390.[58] Y.-C. Tsai, B. A. Coles, K. Holt, J. S. Foord, F. Marken, R. C.

Compton, Electroanalysis 2001, 13, 831.[59] È. Mikkelsen, K. H. Schr˘der, M. Skogvold, L. T. Findalen,

9th International Meeting on Chemical Sensors. Boston USA,2002. http://www.chem.ntnu.no/� knusch/alloyposter.pmd

[60] D. Jagner, Trends Anal. Chem. 1983, 2, 53.[61] http://www.dentalnet.no/produkt/EMS[62] È. Mikkelsen, K. H. Schr˘der, Electroanalysis 1999, 11, 504.



amalgam electrodes for electroanalysis

Documents