Feature Article

Preparation of Silver Surface for Mercury Film Electrode ofProlonged Analytical ApplicationBogus¯aw Bas¬,*a Zygmunt Kowalski a,b

a Department of Analytical Chemistry, Faculty of Materials Engineering and Ceramics, University of Mining and Metallurgy,A. Mickiewicza 30, 30-059 Cracow, Poland; e-mail: bas@ßuci.agh.edu.pl

b Regional Laboratory of Physicochemical Analysis and Structural Research, Ingardena 3, 30-060 Cracow, Poland

Received: July 23, 2001Final Version: December 10, 2001

AbstractThe surface of a silver wire of a fibrous texture, pretreated by storing in Hg(Ag) is converted into a fine crystallinesolid amalgam in the form of a passive skin layer. Applied as the base surface for film electrode, but containing someamount of silver, this layer replaces very well the pure mercury as an electroactive liquid layer.The apparatus forcyclic renovation of the film thickness from 50 nm to 70 nm is described. The promising analytical parameters such asthe reproducibility and low residual current are presented.

Keywords: Voltammetry, Silver amalgams, Mercury film electrodes, Intermetallic compounds

1. Introduction

In several earlier studies silver was recommended as thebase metal for preparingMFE [1 ± 5]. Very good wettabilityof silver by mercury and low solubility of silver in mercurywere the main advantages which attracted the attention ofelectrochemists. Very soon it was noticed, however, that theproperties of such an electrode are not stable even within ashort period of time. It was found that the thickness of thelayer affects the reproducibility.In articles published later the authors paid attention to

some details of the construction of the electrode [6 ± 8],preparation procedures of themercury layer:mechanical [9,10] or by electrochemical deposition [11 ± 14]. It wasobserved, however, that the mechanically prepared amal-gamate surface of the electrode was usually not of uniformthickness. Electrodeposition of the mercury enables esti-mation of the amount of mercury in the layer [11, 15] beforeelectroanalytical experiments. Yet the chemical interactionof mercury with the substrate leads to transformation offluid mercury into solid silver amalgam which implies achange in the thickness of the layer. Someattentionhas beendevoted to the signal to noise ratio [4, 16], resolution of thevoltammetric peaks [17] and the influence of the interme-tallic compounds on analytical results [14, 18]. Shape andsize of the electrode were considered from different view-points.The cylindrical form of an electrode was the object of

interest for some time, but the effect of flowing down of theexcess of mercury in such an electrode creates someexperimental problems and errors in analytical application[19]. Yet such an electrode seems to be very convenient inpractice considering its preparation. In this article theapplication of a cylindrical silver base mercury film elec-

trode is presented. In order to eliminate the undesirable,earlier described troubles, a special apparatus for regener-ation of the mercury layer has been developed and isdescribed here.To obtain the desirable reproducibility of the results, in

our approach the refreshingof the liquid layer is takingplaceimmediately before each measurement cycle.Preparation of the silver surface, preceding the mercury-

plating step, was also a special object of this experimentalstudy and is described here in detail.

2. Experimental

2.1. Apparatus

All experiments were performed with an EA9 electro-chemical analyzer (MTM-ANKO, Poland). The measure-ments were carried out in a three-electrode system usingsilver based cylindrical mercury film electrode as theworking electrode, Pt as the auxiliary electrode and Ag/AgCl as the reference electrode. Mercury film electrodewas produced and renewed using a special device descri-bed earlier [21] and improved [22] (MTM-ANKO, Po-land).

2.2. Reagents and Materials

All solutions were prepared from analytical reagent gradesalts. The silver electrode was prepared from a wire 0.05 cmindiameter andof 99.99%purity (GoodfellowSciencePark,England).

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2.3. Procedure

The most recent version applied is presented in Figure 1a,which shows the apparatus before electrochemical experi-ment.The electrode is in operation, when the silver cylinder is

displaced down and placed in the solution. The newmercurylayer is laid on when the silver wire is plunged in mercuryand the excess of mercury is removed by anO-ring (8) and amultiloop coil (7) (Fig. 1a, b). Figure 1b illustrates theelectrode configuration ready for measurements.

3. Results and Discussion

The applicability of the cyclic method of mercury renova-tion for analytical purpose was tested starting with obser-vations of the mechanical properties of the cylindricalelectrode after manifold mechanical refreshing of the layer.Using a few kinds of silver wire, the experiments werestartedwith placing the initial mercury layer on the polishedsilver surface.In most experiments, consisting in conditioning the silver

wire by pure mercury, already after a few hours of experi-ments some part of the surface was visibly corroded and atthe same time the observed thickness of the mercury filmwas not uniform.When the experiment was prolonged, the silver wire used

as the substrate metal became very breakable. Suchundesirable effects were observed to a smaller extent onlywhen the silver wire used in the experiments had a fibroustexture.On theother hand, the corrosionof the silverwire offibrous texture caused by mercury was absolutely minimalwhen pure mercury was replaced by an amalgam systemcontaining about 1%w/w silver. This amount of silver in theamalgam was determined using AAS methods. In furtherexperiments only this kind, i.e., fluidal Hg(Ag) amalgamwas taken into consideration. Our experimental study wasextended by including metallographic observations as themost reliable way of determining the structure of a givensilver material and the physical condition of the solidproducts of the interaction of silver-mercury in time.The samples of silver wire after conditioning in pure

mercury and in mercury containing dissolved silver wereimmersed in acrylic resin and next, after cutting andpolishing, were observed in a metallographic microscope.Photos of the cross-section of the silver base wire

conditioned in the pure mercury and the silver amalgamare shown in Figure 2.Micrographs of silver-base rods (cross-section) are shown

in Figure 2. Example (2a) relates to the crystalline grade of

Fig. 1. Schematic diagram of the assembly for producing thecylindrical mercury film electrode on silver substrate metal: a)waiting configuration, b) ready to measurement configuration.Apparatus before the experiment: 1) piston pin with Agcylindrical electrode at the end, 2) housing with gas inlet, 3) pilotslide, 4) O-ring, 5) silver amalgam (5 to 20 �L), 6) O-ring, 7) Ag-multiloop coil, 8) O-ring.

Fig. 2. Micrographs of the silver-base wire (cross-section): a) coarse grain silver structure after conditioning in pure mercury, b) fibroustexture silver structure after conditioning in pure mercury, c) as b), but conditioning made in the dilute amalgam of silver containing1% w/w silver.

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silver, conditioned in pure mercury for 30 days (200�magnification).Similar results of an experiment, but carried out with

silver of fibrous texture, are shown in Figure 2b. The surface

of the electrode here is not so corroded and the crystals ofsolid amalgam are not observed, either. Increase in thediameter of the electrode in this case was also very small incomparison with the previous experiment. Figure 2c showsmicrographs of the silver electrode of fibrous texture afterconditioning for 90 days, but in a fluidal Hg(Ag) amalgam.As has been found in such conditions when, instead of

pure mercury, dilute amalgam of silver is used during thetime of conditioning, the surface of pure silver is trans-formed into a solid fine crystalline silver amalgam in theform of a passive skin layer. In this conditioning experimentthe layer reached a thickness of only 5 �m to 20 �m.The substrate prepared as described above retains the

breakage strength of the electrode even for more than 500cyclic refreshing procedures. Within this time the thicknessof the solid skin silver amalgam has increased, but did notexceed 50 nm to 70 nm.These passive skin layers did not show any tendency to

intercrystalline cracking as was in the case when the silverwire had a crystalline structure, other than a fibrous one. Itmust be particularly stressed here that soaking of thesubstrate of the mercury was observed in only a very smalldegree. For this reason in our further experiments only thisprocedure of preparing the silver surface was used, and puremercury was replaced by amalgam containing 1% excess ofsilver Hg(Ag)FE. The sort of the silver wire was carefullyselected, taking into consideration only the sort of a fibroustexture.Electrochemical measurements applying such a substrate

surface and liquid amalgam film containing 1% silver,Hg(Ag)FE have given very promising results.In Figure 3 qualitative results concerning the useful

ranges of the potential are shown. Ten cyclic current voltagecurves for different pHwere recorded usingHMDE (curvesa) andHg(Ag)FE (curves b) generated in theway describedabove without preliminary electrochemical pretreatment,and that is why some background current is seen.As it can be seen, when the Hg(Ag) layer is applied, the

useful ranges of the potential, only to a small extent, differfrom the situation when pure mercury is applied. In thisexperiment the HMDE surface area was close to the filmelectrode, i.e., about 6.5 mm2. Figure 3 reflects only theuseful potential ranges of the film electrode in comparisonwith pure Hg and with various pH. This difference is notsignificant for many analytical applications.Current-voltage scans in the potential range which corre-

spond to the double-layer region illustrated in Figure 4, showhowthe lowrangeof current canbeutilized in theexperiments.The residual current for the Hg(Ag) layer here is only

twice as great as that for puremercury.Toobtain such shapesof the curves and such low residual current, electrochemicalpretreatment is needed. In this experiment precathodiza-tion at � 1.0 V for 10 s eliminated the effects of adsorbedoxygen on the electrode surface. However, in this stepdeposition of some metallic impurities could take place, atthe second step, with polarization at � 0.1 V for 20 s, thedeposited metallic contaminations were stripped. Aftersuch electrochemical procedure, the electrode was used to

Fig. 3. Comparison of current-voltage curves for a) HMDE andb) Ag-based Hg(Ag) film electrode in KNO3�HNO3 or KNO3�KOH media. Potential scan rate 0.025 Vs�1. Electrodes were0.056 cm2 in geometric area.

Fig. 4. Residual-current curves corresponding to the doublelayer region obtained with: a) HMDE, and b) Ag-based Hg(Ag)film electrode after precathodization. Solution: 0.1 KNO3, pH 3,potential scan rate 0.025 Vs�1. Arrows show the direction ofpolarization.

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record cyclic curves in the nano-current scale. Reproduci-bility of the electrode is illustrated in Figure 5. Here, 20cyclic curves were recorded, immediately after successiverefreshing of the layer, using the method described above.Overlapping of the cyclic curve demonstrates qualitativereproducibility of the size of the electrode surface as well asthe thickness of the film. In Table 1 these features of theelectrode are illustrated quantitatively. To show theutility ofthe electrode, the stripping experiments were carried outwith subsoil water from mining areas containing traces ofzinc, lead, cadmium, copper (Fig. 6). Mean value of theheavy metal found: 60 ppb of copper, 123 ppb of lead,3.5 ppb cadmium and 730 ppb zinc; the coefficient ofvariation was successively 2.6%, 1.4%, 3.2%, 2.1%. Eachcycle was recorded on a renewed film electrode.In the earlier investigations of the film electrodewith silver

substrate metal not enough attention has been given to suchessential problems as the structure of silver and its influenceon the future properties as a base for mercury film layer.As it follows from our experimental study, the majority of

disadvantages of this type of electrode, have their origin inthe undesirable crystalline structure of silver. The initial

structure undergoes further transformation under mechan-ical or thermal treatment,which in turn successively changesthe size of the crystallite as well as its shape. Alsointercrystalline microcracks occur.Contact of the silver substrate with mercury provokes

migration of the mercury inside the grain boundary space,which next brings about the destruction of the metalstructure. Crystallizing of the solid amalgam increases thebulkof the silver basewire.All these chemical and structuraltransformations make the thickness of the mercury layerboth unknown and not uniform.A relatively lowmigration ofmercury into the deep of the

silver observed in the case of its fibrous structure encouragesone to apply this type of material as a base.Pretreatment of this kind of silver and forming the passive

skin layer of the solid fine crystalline silver amalgamsadditionally consolidates the surface of the base metal. Tokeep the surface in a desirable condition during exploitationof the electrode, the dilute amalgam of silver as the liquidlayer is recommended instead of pure mercury.As could be expected, any undesirable effects using such

an electrode have not been observed. In consequence, for

Fig. 5. Cyclic current-voltage curves for 0.1 mmol dm�3 Pb and0.2 mmol dm�3 Cd recorded with Ag-based Hg(Ag) film electrode(20 scans, each one after film regeneration). Solution: 0.1 moldm�3 KNO3, pH 3, potential scan rate 0.025 Vs�1.

Table 1. Reproducibility of the current and potentials. LSV, potential scan rate 0.025 V s�1. Solutions: 10�4 mol dm�3 Pb(II), 2� 10�4mol dm�3 Cd(II) in 0.1 mol dm�3 KNO3, pH 3.

Peak current Peak potential Peak width[mV]

Cathodic Anodic Cathodic Anodic[�A] [�A] [mV] [mV]

Pb 0.477� 0.002 2.80� 0.01 � 448� 2 � 429� 2 39� 3Cd 0.735� 0.004 1.85� 0.02 � 632� 2 � 609� 2 46� 3

Fig. 6. Differential pulse stripping voltammograms for Zn, Cd,Pb and Cu in subsoil water at pH 3, containing 0.1 mol dm�3

KNO3. Deposition time 90 s (Zn, Pb, Cu) and 3 min (Cd). a)sample of water, b) and c) after addition of reference solutions,pulse amplitude 20 mV.

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preparing the substrate wire before the production of thelayer the authors advise successively: 1) mechanical surfacetreatment, rotationally using polishing alumina 0.5 �m(Polishing Kit PK-4; BAS, USA); 2) degreasing the surfacein acetone (no chemical treatment can be used); 3) in orderto fabricate a passive skin layer of a solid amalgam, the wirehas to be kept in amalgam containing 1% w/w of silver for acertain time, and next, after removing the excess of theliquid amalgam, thewire is ready to further use as a substratematerial. The base for the electrode, prepared using thismethod, can be exploited continuously for months.The layer thickness produced using our apparatus is

precisely reproduced immediately before eachmeasurementcycle. Since this process is mechanical in character, its layerthickness can be estimated, e.g., using a method proposedearlier [19], by observation of the anodic and the cathodicpeakpotential at a knownpotential scan rate.Thismethod, ashas been found, enables receiving the layer thickness of about50 nm to 70 nm.Worthy of recommendation, in the case of along break in the measurements, is to keep the electrode inneutral atmosphere to avoid oxidation of its surface.

4. Acknowledgement

The study was financially supported by the Ministry ofEducation, Poland, Cracow (BW-10.342.165).

5. References

[1] J. M. Skobec, L. S. Berenblum, N. N. Atamanko, Zavodsk.Lab. 1948, 14, 13.

[2] J. M. Skobec, J. D. Panczenko, W. P. Rjabokon, Zavodsk.Lab. 1948, 14, 1307.

[3] K. W. Gardiner, L. B. Rogers, Anal. Chem. 1953, 25, 1393.[4] W. D. Cook, Anal. Chem. 1953, 25, 215.[5] V. A. Jgolinski, A. G. Stromberg, Zavodsk. Lab. 1964, 30,

658.[6] J. F. Le Meur, J. Courtot-Coupez, Bull. Soc. Chim. France

1973, 3, 939.[7] K. Micka, Chem. Listy 1961, 35, 474.[8] V. A. Jgolinski, O. N. Gurjanowa, Elektrokhimja 1975, 11, 57.[9] V. A. Jgolinski, Elektrokhimja, 1975, 11, 57.[10] Z. P. Riley, H. Gu, Anal. Chim. Acta 1981, 130, 199.[11] A. N. Doronin, D. L. Kabanowa, Elektrokhimja, 1966, 4,

1460.[12] Z. Stojek, Z. Kublik, J. Electroanal Chem. 1975, 60, 349.[13] Z. Stojek, Z. Kublik, J. Electroanal Chem. 1977, 77, 205.[14] P. Ostapczuk, Z. Kublik, J. Electroanal Chem. 1977, 83, 1.[15] E. Bishop, R. G. Dhaneshwar, Analyst 1963, 88, 424.[16] Z. Stojek, Z. Kublik, Chem. Anal. (Warsaw) 1985, 30, 363.[17] Z. Stojek, Z. Kublik, J. Electroanal Chem. 1979, 105, 247.[18] J. Wang, Stripping Analysis ± Principle, Instrumentation, and

Application, VCH, Deerfield Beach 1985.[19] Z. Stojek, Z. Kublik, Chem. Anal. (Warsaw) 1979, 24, 347.[20] W. Lund, M. E. Poer, J. Electroanal. Chem. 1970, 25, 19.[21] Z. Kowalski, Polish Patent No P-319 984 1997.[22] B. Bas¬, PhD Thesis, University of Mining and Metallurgy,

Cracow, Poland 2000.

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