electrodo de amalgama dentista

7/27/2019 Electrodo de Amalgama Dentista

1/8

Analytica Chimica Acta 458 (2002) 249256

Voltammetry using a dental amalgam electrode for heavymetal monitoring of wines and spirits

yvind Mikkelsen, Knut H. Schrder

Department of Chemistry, Norwegian University of Science and Technology, N-7491 Trondheim, Norway

Received 20 June 2001; received in revised form 30 November 2001; accepted 11 December 2001

Abstract

We have introduced a non-toxic electrode material similar to dental amalgam for use in voltammetry. Its electrochemicalproperties are like a silver electrode. However, it possesses a higher overvoltage towards hydrogen than silver, and thereforeenables detection of metals like zinc, nickel and cobalt. As such solid electrodes are found to give stable results over severalweeks, without any maintenance, and because this method greatly facilitates monitoring of heavy metals, attempts to applysuch methods to various samples have been are carried out. The present paper deals with the determination of zinc and leadat nanogram per milliliter levels in wines and spirits with only minor treatment of the samples. The procedure may easily beadapted to continuous monitoring.

We have previously found that audible sound may greatly increase the voltammetric signal using liquid mercury as well assilver as electrode material. This is also applied to the actual systems.

Finally, model determinations of thallium in brandy with the dental amalgam electrodeare compared with atomic absorptionspectrometric (AAS) measurements. It was found that the electrode could be used repeatedly, without fouling, and with resultsclose to those found by the AAS method. 2002 Elsevier Science B.V. All rights reserved.

Keywords:Voltammetry; Dental amalgam electrode; Wine; Online; Medium exchange; Zinc; Cadmium; Thallium; Lead

1. Introduction

Voltammetric methods are very attractive for deter-mining heavy metals at trace levels. Pure liquidmercury or liquid mercury amalgams are superioras electrode materials in voltammetry for analyticalpurposes [13]. This is mainly due to the high over-voltage for hydrogen, which renders possible a wideworking potential range for the electrode. There has,however, been a growing concern about the generaluse of mercury because of its toxicity. This includesthe use of pure mercury as an electrode material in Corresponding author. Tel.: +47-73596205.

E-mail address: [email protected] (K.H. Schrder).

voltammetry. Even for laboratory use, restrictionsare expected to appear in the future. Therefore, itis of great interest to find new alternative electrode

materials for use in voltammetry.Numerous papers have been published dealing withalternative electrodes, but all these electrodes havelimited analytical value because they cannot operateat potentials more negative than 900 mV due to theirlow hydrogen overpotential. This is a great drawbacksince important metals like zinc, nickel and cobalthave half-wave potentials at more negative values,and therefore cannot be detected by the use of theseelectrodes. However, some interesting results usingmercury-free film electrodes have been reported [47],

0003-2670/02/$ see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S 0003-26 70(01 )0160 6-3


2/8

250 . Mikkelsen, K.H. Schrder / Analytica Chimica Acta 458 (2002) 249256

but the practical applications of these electrodes arelimited for on-line use.

The present authors have recently introduced the use

of the solid dental amalgam electrode for zinc, cad-mium, lead and thallium determination by differen-tial pulse anodic stripping voltammetry (DPASV), andadsorptive cathodic stripping voltammetry of nickeland cobalt as dimethylglyoxime complexes [3,8]. Suchdeterminations are important for field and online anal-yses of pollutants in soil and groundwater, and theelectrode may be used repeatedly. Further improve-ments may be obtained by optimising the composi-tion of the alloy and electrolyte, and by applicationof sound to the electrode system [2,9]. Preparation ofsolid amalgam electrodes is fast and simple, e.g. byusing techniques well established in dental practice[10], and in addition they are non-toxic [11]. Recently,we have also reported that other silver alloys with afew percent of mercury, lead, bismuth, etc. also canbe used as an electrode material [12].

Several techniques are available to measure heavymetals in wine and spirits. Our intention is to evaluatemethods to be used for on-line and in situ monitor-ing; this excludes the use of laboratory methods basedon atomic absorption spectrometry (AAS) and induc-tively coupled plasma mass spectrometry (ICP-MS).

The remaining methods are electroanalytical, usingDPASV [13,14], and potentiometric stripping [1521].

Analytical voltammetry of heavy metals at tracelevels, using solid dental amalgam electrodes or othersilver alloys, can be applied for various purposes likein situ drinking water quality control and continuousmonitoring of effluents from the mining industry. Rel-evant methods have been worked out in our laborato-ries [22]. Analyses of food and beverages are also ofgreat interest. The present paper deals with analysingwines and spirits using dental amalgam electrodes.

Also, results obtained with the voltammetric amalgamelectrode system and by AAS has been compared.In several papers [2325] it is demonstrated that a

solution can be used in the stripping sequence in ASVdifferent to the test solution used in the plating se-quence, in order to overcomeinterferences. In complexsolutions where, e.g. organic compounds are presentthis medium exchange (ME) technique may giveimprovements of the voltammetric signal and a betterbaseline. However, this method is mostly used withliquid mercury electrodes as drops or deposits. With

such electrodes, the heavy metal deposited is dissolvedin the liquid mercury phase and diffuses into the bulkof that mercury. Consequently, during the ME process

only a minor fraction of the dissolved metal will bepresent on the electrode surface and available for ox-idation during the time when the electrical circuit isbroken. By using solid electrodes the metal depositis assumed to be a partially covered monolayer, eas-ily oxidised by air or by other compounds when theelectrical circuit, with its reducing capability, is bro-ken. For that reason an alternative ME technique isintroduced where the reduction potential is maintainedwhile the electrolyte is replaced.

By use of the simple equation:

c = c0e(vt/V0)

(1)where c0is the initial concentration, V0the cell volumeandvthe flow rate, the concentration at a given time atequilibrium can be computed. This corresponds to aneffective deposition time during the exchange given by

teffective =V0

v(1 e(vt/V0)) (2)

which is essential to compute when exchanging solu-tion under the deposition step.

2. Apparatus

The analyses were performed by DPASV, usinga three-electrode system. The counter electrode wasa platinum wire and potentials were measured againsta silver/silver chloride/saturated silver chloride/satu-rated potassium chloride reference electrode. Theworking electrode was a solid silver amalgam elec-trode, as shown in Fig. 1. Details of the preparationof the silver amalgam electrode, characterisation and

verification have been given elsewhere [3,8,12].Unless otherwise stated, the solutions were bubbledwith nitrogen gas prior to the measurements, and nitro-gen was purged above the cell during the experiments.All the analyses were performed in NH4Ac (0.05M,100 ml), unless otherwise stated.

Prior to each measurement the electrode wascleaned electrolytically by applying a potential of200mV for 120s.

The voltammetric equipment was constructed by,and is available from, the company OCEANOR,


3/8

. Mikkelsen, K.H. Schrder / Analytica Chimica Acta 458 (2002) 249256 251

Fig. 1. Construction of the dental amalgam electrode.

Trondheim, Norway, in collaboration with the authors.It is made as a flexible instrument to be remotely con-trolled via the Internet with the results to be presentedon the World Wide Web.

The dental silver amalgam electrode was preparedby techniques well known from dental practice, as de-scribed previously [3] by mixing one part by weight of

pure silver crystals for dental use (particles


4/8


Table 1Results of DPASV analyses directly in white and red wine sampleswith added 0.05M NH4Aca

Heavy metals Red wineb White wineb

Zinc (ngml1)c 369.1 (3.1%) 646.7 (1.1%)Lead (ngml1) 56.7 (3.6%) 9.6 (5.1%)

a Scan rate: 15 mVs1, pulse height: 50 mV, pre-deposition at1300mV in 120s.

b Mean (S.D.) (n = 10).c Cadmium was not deleted.

et al. [14], who particularly studied the relationshipbetween the total and free amounts of metals in wines.The measurements for zinc in white wine were re-peated with standard addition of the metal ions, with

Fig. 2. Detection of zinc and lead in white wine by the standard addition method. DPASV analyses with a pre-deposition step of 120 s at1300mV, scan rate 15mVs1, and pulse height 50 mV. Three hundred microliter standard solution with a concentration of 50g ml1

was added twice (200 + 200ngml1). All analyses were performed directly in the sample with addition only of 0.05 M NH4Ac. The lead

peak is enlarged in the inserted frame up in the right corner.

Table 2Comparison of analyses for zinc and lead in wines using sound exposure versus magnetic stirring a

Metal/wine Magnetic stirringpeak height (A)

Sound exposurepeak height (A)

Difference, sound vs.magnetic stirrer (%)

Zinc, red wine 4.38 9.20 +110Lead, red wine 3.28 3.4 +2.7Zinc, white wine 39.45 62.50 +58.4Lead, white wine 7.20 5.85 18.8

a Sound frequency was 80 Hz and the intensity 2 W in the deposition step and 0.5 W in the scanning step. S.D.


5/8


Fig. 3. Voltammetric scan of () a pure red wine sample (pH = 3.5), and (---) red wine with added NH4Ac (pH = 4.7). DPASV analyseswith a pre-deposition step of 120s at 1300mV, scan rate 15mVs1, pulse height 50 mV.

metals in wine are not bound to matrix componentswhen strong acid is added, so that this will enablevoltammetric determination of the total metal content

[14]. For that reason, hydrochloric acid was added tosome samples prior to the measurements and left forabout 3 days for equilibration. Just before the analy-ses, NH4Ac was added. No significant changes in thedetected metal concentration were observed for winesamples within pH 2 and 5, a common pH range formost wines. However, with a pH


6/8


Fig. 4. DPASV scan of zinc in red wine. Dashed line shows scan after 120s deposition in red wine with added 0.1M HCl and 0.05MNH4Ac (pH = 2). Solid line shows a similar scan from the same solution after medium exchange with 0.05M NH4Ac (pH = 6) asdescribed in the text.

In OME large volumes of the exchange solution areneeded. According to Eqs. (1) and (2), a 90% exchange

of a solution of 68 ml will require a flow of 156.6 mlof the exchange solution. If this is carried out in 60 s,this corresponds ideally to an effective extension ofthe deposition time of 23.5 s, due to the correctionfor the successive dilution. For the present studies,with an exchange volume of 300 ml added in 60 s, anda cell volume of 68 ml, the effective extension of thedeposit time is 13.4s, corresponding to an exchangewith the remaining 1.21% of the original solution.

A much more suitable way, therefore, is to use wa-ter only during the OME, and add solid ammonium

acetate prior to the stripping sequence. The results aregiven in Fig. 4. The procedure can also be furtheroptimized by improvements of the cell design, usingsmaller volumes. In the present paper the intention isnot to report a complete flow system, but rather to showwhat kind of problems that may occur for mediumexchange when a solid electrode is used, and solu-tions for these problems are suggested. The use of an-other medium than the sample solution in the strippingsequence effectively eliminates problems with, e.g.electroanalytically active organic compounds and the

pH may be changed to a more suitable value than inthe original sample. The problem is to do the exchange

of the medium without any oxidation of the depositedmaterial, and the present technique seems to solve thisproblem. The technique can easily be implemented ina flow system.

3.4. Measurements of thallium in brandy

Determination of heavy metals in beverages may beof importance for forensic evidence. For a High Courtcriminal case the authors carried out a large modelexperiment involving over 1000 analyses to investi-

gate how different amounts of thallium sulphate thatwas added to a bottle of brandy would dissolve whenthe bottle was used as in real life. Two bottles werestudied, to which were added respectively 0.5023 and2.1882 g solid thallium(I) sulphate. Over about threeweeks, drinks of 20 ml were taken out of each bottleand analysed for thallium. The bottles were treatedas in real life as if they were kept in a liquor cabinet.Both voltammetric and AAS analyses were performedon diluted samples and the results were compared.Surprisingly small amount of the thallium(I) sulphate


7/8


Table 3Measurements of thallium (mg/20 ml) in brandya

Days Bottle 1 Bottle 2

Voltammetry Flame Voltammetry Flame

1 3.6 1.1 3.1 1.53 5.8 6.4 6.8 5.95 10.0 10.0 15.2 16.85 13.2 11.16 16.7 14.2 23.2 19.47 18.2 18.1 27.7 26.110 26.9 26.2 40.4 37.210 32.4 33.611 30.9 29.5 38.0 45.513 40.8 36.1 47.9 46.613 54.4 58.317 47.2 48.8 56.6 69.6

18 47.0 48.719 44.0 43.9 55.8 55.0Total dissolved 304.3 298.4 401.5 415.5Rest in bottle and brandy glass 224.6 1848.8Total found 528.9 523.0 2250.3 2264.3Total added 502.3 2188.2Differenceb (%) 5.2 4.1 2.8 3.4

a Voltammetric results compared with AAS results. DPASV scan from 900 to 350 mV. Pre-concentration step of 60 s at 900 mV.Scan rate 15 mVs1 and pulse height 50 mV, NH4Ac (0.05 M) added directly to diluted brandy sample. (Dilution under analysis: DPASV1:5000, AAS 1:25). S.D.


8/8


References

[1] J. Wang, Analytical Electrochemistry, 2nd Edition, Wiley-VCH, New York, 2000.

[2] . Mikkelsen, K.H. Schrder, Electroanalysis 11 (1999)401.

[3] . Mikkelsen, K.H. Schrder, Anal. Lett. 33 (2000)3253.

[4] J. Andersen, E.H. Hansen, Fresenius J. Anal. Chem. 362(1998) 77.

[5] K.Z. Brainina, G. Henze, N. Stojko, N. Malakhova, C. Faller,Fresenius J. Anal. Chem. 364 (1999) 285.

[6] G. Wittstock, A. Strubing, R. Szargan, G. Werner, J.Electroanal. Chem. 444 (1998) 61.

[7] J. Wang, J. Lu, .A. Kirgz, S.B. Hocevar, B. Ogorevc, Anal.Chim. Acta 434 (2001) 29.

[8] . Mikkelsen, K.H. Schrder, T.A. Aarhaug, Collect. Czech.Chem. Commun. 66 (2001) 465.

[9] . Mikkelsen, K.H. Schrder, Electroanalysis 13 (2001)687.

[10] Dental materials. Alloys for dental amalgam, InternationalStandard ISO, 1559:1995(E)/NS-ISO 1559.

[11] Statement on the toxicity of dental amalgam, Committee onToxicity of Chemicals in Food, Consumer Products and theEnvironment, Department of Health, UK, 1997.

[12] . Mikkelsen, K.H. Schrder, Analyst 125 (2000) 2163.

[13] M. Saftic, M. Tkalcec, I. Piljac, Prehrambeno-Tehnol. Rev.15 (1977) 83.

[14] M.T. Arcos, M.C. Ancin, J.C. Echeverria, A. Gonzalez, J.Julian, J. Agric. Food Chem. 41 (1993) 2333.

[15] Z.J. Suturovic, N.J. Marjanovic, Nahrung 42 (1998) 36.[16] J. Garrido, B. Ayestaran, P. Fraile, C. Ancin, J. Agric. Food

Chem. 45 (1997) 2843.[17] P. Ostapczuk, H.R. Eschnauer, G.R. Scollary, Fresenius J.

Anal. Chem. 358 (1997) 723.[18] L. Toeben, P. Ostapczuk, Wein-Wiss. 50 (1995) 123.[19] G.N. Chen, G.R. Scollary, V.A. Vicente-Beckett, Am. J. Enol.

Vitic. 45 (1994) 305.[20] C. Marin, P. Ostapczuk, Fresenius J. Anal. Chem. 343 (1992)

881.[21] S. Daniele, M.A. Baldo, P. Ugo, G. Mazzocchin, Anal. Chim.

Acta 219 (1989) 9.[22] . Mikkelsen, K.H. Schrder, in: Proceedings of Environmin

2001, Environmental and Health Aspects of Mining, Refiningand Related Industries, Skukuza, South Africa, July 2001,p. 64.

[23] M. Ariel, V. Eisner, S. Gottesfeld, J. Electroanal. Chem. 7(1964) 307.

[24] R.P. Baldwin, K.N. Thomsen, L. Kryger, Anal. Chem. 60(1988) 151.

[25] E. Alonso Alvarez, M. Callejn Mochn, J.C. JimnezSnchez, M. Ternero, Electroanalysis 10 (1998) 917.

electrodo de amalgama dentista

Documents