Analytica Chimica Acta 497 (2003) 227–233

Electrochemical lead separation from copper, copper alloy,silver and silver alloy for isotope ratio determination in

archaeometric investigations

Ilian Iliev a, Ivelin Kuleff a,∗, Jörg Adamb, Ernst Pernickaba Faculty of Chemistry, University of Sofia, 1 J.Bouchier Blvd., BG-1164 Sofia, Bulgaria

b Institute of Archaeometry, TU Bergakademie Freiberg, Gustav-Zeuner Str. 5, D-09596 Freiberg, Germany

Received 22 April 2003; received in revised form 2 July 2003; accepted 2 July 2003

Abstract

The possibility of electrochemical oxidation of Pb2+ to PbO2 as a method of isolation of lead in low concentrations incopper and silver matrixes was studied. Suitable electrolysis cell was developed and the influence of various factors (electrodesurface and state, reaction time, temperature, electrolyte volume, acidity, concentration of indifferent electrolyte, stirring rateand current density) on the efficiency of the process was evaluated. The accuracy and reproducibility of the method wereestimated by determination of the quantity of lead in SRMs by inductively coupled plasma atomic emission spectrometry(ICP-AES) and atomic absorption spectroscopy (AAS). By means of ICP-mass spectrometry (MS), the lead isotopic ratiosin SRMs were determined.© 2003 Elsevier B.V. All rights reserved.

Keywords: Electrochemical lead isolation; Archaeometry; Isotopic ratios; SRM analysis; ICP-AES; ICP-MS

1. Introduction

The association between archaeological metal arti-facts (copper, copper alloy, silver and silver alloy) andpresumed sources of raw material is usually carriedout by lead isotope study. There are severe restrictionson the quantity of metal that can be removed froman artifact and in some cases it could be smaller than1 mm3 (by museum standards samples of 3 mm3 arealready unusually large). For that reason, reliable an-alytical technique for isolation and determination ofthe lead in the very small sample with high sensitivityand reproducibility is needed.

∗ Corresponding author.E-mail addresses: kuleff@chem.uni-sofia.bg (I. Kuleff),pernicka@poptuba.tu-freiberg (E. Pernicka).

The determination of lead isotope ratios is con-ducted by mass spectrometry. The presence of matrixelements (Cu, Ag, Ag–Cu, Sn, Zn, etc.) seriouslydecreases the sensitivity and reproducibility of massspectral methods, so that a preliminary lead isola-tion is required. The most frequently used methodfor lead separation from multicomponent systems isthe ion-exchange chromatography[1–4], and specialPb-selective resins[5].

As an alternative for ion-exchange chromatographyfor lead separation from matrix elements is the elec-trochemical lead isolation. The cathodic reduction ofPb2+ to Pb0 in potentiostatic mode[6–8] finds appli-cations for relatively simple systems, but the accuracyof the determination is comparatively low. By meansof anodic oxidation of Pb2+ to PbO2 [7,9], quantita-tive separation of lead from Al, Cd, Cr, Ni, Ti, Zn, Ag,

0003-2670/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.aca.2003.07.008

228 I. Iliev et al. / Analytica Chimica Acta 497 (2003) 227–233

Cu, Hg and Sn matrix is possible[8,10]. The advan-tages of the electrochemical separation in comparisonwith ion-exchange chromatography are the speed ofthe method and the reduced volume of the sample.

The present study deals with the isolation of lead,using electrochemical oxidation of lead (Pb2+) to leaddioxide (PbO2) and determination of the lead con-centration using inductively coupled plasma atomicemission spectrometry (ICP-AES) or atomic absorp-tion spectroscopy (AAS) and lead isotope ratios byICP-mass spectrometry (MS), applied to archaeomet-ric studies of metal artifacts of copper, silver and theiralloys.

2. Experimental

2.1. Reagents and standard solutions

Single stock solutions of Cu and Ag (100 mg/cm3);Pb (10 mg/cm3); Bi, Co, Ni, Cd and Zn (0.5 mg/cm3)were prepared from analytical-grade reagents (Merck,Darmstadt, Germany) by dissolving in HNO3 and di-luting with 0.01 M HNO3. For the dissolution of PbO2,a mixture of 10% HNO3 and 1% H2O2 was used.

The ICP-AES was calibrated using a multielementstandard solution of Pb, Bi, Fe, Co, Ni, Zn and Cd(Merck; Aldrich, Milwaukee, WI, USA).

Model solutions of Cu, Ag and Ag–Cu with differ-ent concentrations of Pb (0.025, 0.1 and 1 mg/cm3)and Bi, Co, Ni, Fe, Zn and Cd (0.01 mg/cm3) wereprepared from stock solutions.

2.2. Instrumentation

For determination of Pb, Bi, Cu, Co, Ni, Fe, Znand Cd concentrations inductively coupled plasmaatomic emission spectrometry “SpectroFlame 504”,flame atomic absorption spectroscopy (F-AAS)“Perkin-Elmer 1100B”, electrothermal atomic absorp-tion spectrometry (ET-AAS) “Perkin-Elmer Zeeman3030” and inductively coupled plasma mass spec-trometry “VG Axiom” were used.

In this study, a double-focussing magnetic sector-based MC-ICP-MS (AXIOM, VG Elemental) wasused at the TU Bergakademie Freiberg, Germany.For the application described in this paper the samplesolutions were introduced through a microconcentric

nebulizer (CETAC MCN-6000) at elevated tempera-ture with detection limits being reduced by enhanc-ing analyte transport efficiency and reducing solventloading to the plasma. Uptake rate was approximately70�l/min. The dried aerosol passed into a plasmatorch where the sample is evaporated, atomized andhighly ionized. The multi-collector system with nineindependently adjustable Faraday cups enables simul-taneous detection of the different isotopes. Therefore,the variations of signal intensity caused by the in-stability of the plasma are compensated. Both thesimultaneous measurements of different masses andthe characteristically flat-topped peak shapes lead to avery good precision and reproducibility of the isotoperatios.

2.3. Analytical procedure for lead isotopedetermination

The 205Tl/203Tl ratio was measured simultane-ously with the lead isotope ratios and compared tothe value of 205Tl/203Tl= 2.3871 [11]. The de-viation of the measured value from the true valuewas used to correct for the mass discrimination ofthe lead isotopes using an exponential relationship.The technique of thallium doping for correction ofthe mass discrimination of lead was tested on severaloccasions and can now be considered as the standardprocedure for the measurement of lead isotope ratiosby ICP-MS[12–14].

For the detection of all lead and thallium iso-topes, six of the nine Faraday detectors were used.The positions of these detectors were adjusted in away to enable a simultaneous determination of203Tl,204Pb, 205Tl, 206Pb, 207Pb and208Pb. Amplifier gaincorrection was performed regularly.

The sample solutions were analyzed in runs of 20single measurements with 15 s integration time each.Background measurements of 20 s duration were per-formed before and after each run. The reported data inTable 4were calculated as the mean of four runs. Datapoints of each run which exceeded a 2σ error limitwere rejected and the mean calculated again. Thiscyclic rejection routine was applied to a maximumrejection of 15% of the measurements. The in-runprecision of the reported lead isotope measurementswas in the range of 0.001–0.040% (2σ) depending onthe lead isotope ratio considered.

I. Iliev et al. / Analytica Chimica Acta 497 (2003) 227–233 229

3

1 2

1cm

Fig. 1. Cross-section view of the electrolytic cell: (1) platinumwire anode; (2) platinum foil cathode; (3) air tube.

2.4. Dissolution of standard reference materials

Standard reference materials were dissolved in 20%HNO3. To each 10 mg Cu or Ag and 0.2–0.3 cm3 20%HNO3 were added. The dissolution was carried out inan electrolysis cell at elevated temperature. After thecomplete dissolution of the sample, the volume wasadjusted to 3–4 cm3 with bi-distilled water.

2.5. Lead separation

The electrolysis cell consists of a glass test tubewith platinum electrodes soldered in glass tubes(Fig. 1). The anode was made of Pt wire 10 cm longand 0.05 cm thick, wound in a coil with a diameterof 0.5 cm and 0.2 cm step (anode surface= 2.5 ×10−4 cm2). The cathode was made of platinum foil0.01 cm thick and a surface of 1 cm2 (1 cm× 0.5 cm).

The electrochemical oxidation of Pb2+ to PbO2 wasperformed with the apparatus shown inFig. 2, con-sisting of the electrolysis cell, an air pump, a powersupply (0–15 V, 0–2 A) in static current mode, a volt-meter (0–5 V) and a current meter (0–100 mA). Theelectrolysis cell was kept at constant temperature of80◦C with a water bath.

After the separation, the anode was removed fromelectrolysis cell, washed with bi-distilled water andthe precipitate of PbO2 was dissolved in 1 cm3

5

3 4

A

1

6

2

Fig. 2. Schematic illustration of the apparatus for electrochemicallead separation: (1) power supply; (2) current regulator; (3) anode;(4) cathode; (5) electrolytic cell; (6) air from air pump.

HNO3/H2O2 mixture (10% HNO3, 3% H2O2). Af-ter appropriate dissolution (at high amount of Pb), adirect instrumental analysis can be done.

3. Results and discussion

3.1. Cell design

The suggested construction of the electrolytic cellcombines a small sample volume, sufficient stirringduring the electrochemical reaction and negligiblesample loss. The coiled form of the anode enablesbetter adhesion of PbO2 layer to the Pt surface. Thelarge cathode surface facilitates the electrodepositionof compact layer Cu or Ag. This is necessary to avoidcontacting of anode and cathode, as well as detachingof metal parts, engendering mechanical damage andloses of PbO2.

Stirring by air during the process is necessary for theincrease of the efficiency of the reaction as well as theelimination of NO2. The electrochemically depositedPbO2 could be dissolved after reduction by HNO2 toPbO, which decreases the yield.

3.2. Operating conditions

Many factors are responsible for the speed andefficiency of anodic oxidation of Pb2+ to PbO2: cur-rent density, reaction time, temperature, electrolyte

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Table 1Optimal conditions for quantitative oxidation of Pb2+ to PbO2

Matrix Anodesurface(cm2)

Cathodesurface(cm2)

Current(mA)

Anode currentdensity iA(A/dm2)

Temperature(◦C)

Gas rate(ml/min)

Time(min)

Electrolytevolume(cm3)

Max. quantityPb (mg)

Cu 2.5× 10−4 1 12.5 0.8 80 50 30 2–4 4Ag, Ag–Cu 2.5× 10−4 1 8 0.5 80 50 40 2–4 3

volume, acidity, type and concentration of indifferentelectrolyte, stirring rate, electrode surface and state.That requires a very carefully optimization of theworking conditions. The proposed optimal conditionsfor quantitative oxidation of Pb2+ to PbO2 are givenin 7Table 1. No correlation between speed and effi-ciency of Pb2+ oxidation and the electrolyte volumein the range of 2–5 cm3 was observed. It is not neces-sary to optimize the current density in order to obtaina stoichiometric composition of the separated PbO2.

Samples of copper and copper alloy as well as silverand silver alloy were dissolved in HNO3. A mixtureof HCl-NHO3 leads to precipitation of Pb2+, Ag+ andBi3+ (PbCl2, AgCl, BiOCl). HNO3 entirely dissolvesPb, Cu, Ag, Bi, Fe, Ni, Co, Zn and Cd. In the pres-ence of Sn and Sb, H2SnO3 (SnO2·H2O) and HSbO5(Sb2O5·H2O) form, which do not impede the lead sep-aration. On prolonged electrolysis it is a possible thatsome PbO2 is lost as a result of dissolution followedby cathode reduction.

5 10 20 30 40 50 600

20

40

60

80

100

time (min.)

25ug100ug500ug25ug

1000ug100ug

(20 C)o

(80 C)o

(20 C)o

(20 C)o

(80 C)o

(80 C)o

1

2

Fig. 3. Correlation between yield of PbO2 and concentration, time and temperature. Curve 1: 20◦C; curve 2: 80◦C.

At these conditions Bi3+ is present in the form ofBiO+. In the separated PbO2, Bi is quantitatively in-cluded if the concentration ratio of Bi/Pb is less than0.3.

The elevated temperature of the electrolyte is im-portant for the speed and efficiency of the electrodereactions as well as for the composition and the struc-ture of the deposited layer. The results of lead sepa-ration by anodic oxidation of 10 mg Cu in yield/timecoordinates at different temperatures are presented inFig. 3. It is obvious that at 80◦C the oxidation of Pb2+to PbO2 proceeds three times faster than at 20◦C. Theyield of lead at both temperatures is close to 100%at duration of reaction over 35 min. The optimal con-ditions for the electrochemical reaction, determinedfrom Fig. 3, are 80◦C and 30 min., which guaran-tees quantity lead isolation for all amounts studied(0.025–1 mg). If the amount of lead in analyzed sam-ple is less than 0.2 mg the duration of the procedurecould be short to 15 min.

I. Iliev et al. / Analytica Chimica Acta 497 (2003) 227–233 231

The good agreement of lead yield values in the pres-ence and in the absence of NH4NO3 (as an indifferentelectrolyte) allows using of a high concentration ofHNO3 for faster dissolution without heating of sam-ples and later neutralization with NH4OH before elec-trochemical lead oxidation, without changing the re-action conditions. If the acidity of samples is high andlead concentration is low, neutralization with ammo-nia up to pH∼ 2–3 is necessary.

3.3. Separation of Pb and Bi

To assess the accuracy and precision of the analysis,an electrochemical separation followed by lead deter-mination using ICP-AES, F-AAS, ET-AAS, 10 mg ofdifferent SRMs was performed.

3.3.1. Results of analysis of SRMThe results of the analysis of SRMs consisting of

unalloyed copper (NIST–SRM-398, NIST–SRM-399and NIST–SRM-400) and bronze SRM (BAM–SRM-211, BAM–SRM-227 and BAM–SRM-228), using theoutlined procedure, are presented inTable 2. The val-ues are in good agreement with the certified values for

Table 2Results of Pb and Bi separation from copper and copper alloy

Reference Element ICP-AES F-AAS ETAAS Certified value,×10−6 g/g (%)×10−6 g/g (%) SRe % SRe ×10−6 g/g SRe

NIST–SRM-398a Pb – – – – 9.5± 0.3 10.10 9.90± 0.06Bi – – – – ND – 2.00± 0.01

NIST–SRM-399a Pb 113.3± 1.7 3.51 – – 112± 4 7.72 114.0± 3.4Bi – – – – ND – 10.500± 0.025

NIST–SRM-400a Pb 128± 6 9.53 – – 124± 4 8.91 128.0± 3.8Bi – – – – 22.1± 0.7 15.51 24.500± 0.037

BAM–SRM-211b Pb (0.76± 0.03) 10.41 0.73± 0.04 6.76 – – (0.74± 0.02)Bi – – – – 18.5± 0.6 13.5 20± 2

BAM–SRM-227c Pb (4.3± 0.1) 10.68 4.10± 0.21 10.7 – – (4.120± 0.002)Bi – – – – 89.3± 2.7 7.61 88± 2

BAM–SRM-228d Pb (1.34± 0.05) 15.72 1.22± 0.06 11.29 – – (1.2400± 0.0004)Bi – – – – 83.1± 2.5 9.18 86± 3

a Unalloyed copper.b Cu–Sn alloy.c Cu–Sn–Pb alloy.d Cu–Sn–Zn alloy.e SR is evaluated according[15], SR= [(|Cexp − Cst| + 2U)/Cst] × 100; Cexp: experimental value;Cst: certificate value;U: standard

deviation; SR< 25%: excellent agreement; 25%< SR< 50%: acceptable agreement; SR> 50%: unacceptable agreement.

Pb and Bi, which is also confirmed by the standardratio (SR) criterion (SR< 25%).

3.3.2. Separation of Pb and Bi from silver and silveralloy

The results of lead determination in silver and sil-ver alloy samples are given inTable 3. They showedthat the separation is quantitative for the studiedquantity in Ag and Ag–Cu matrix. After the leadseparation, all of the synthetic solutions were ana-lyzed by ICP-AES. In the range of 0.01–0.03 mg, noloss of Fe, Ni, Co, Zn and Cd was observed, andafter electrolysis the solution could be used for thedetermination of these elements with ICP-AES orAAS.

3.3.3. Determination of lead isotopic ratiosThe lead isotopic ratios of the used SRM were

determined. The results concerning208Pb/206Pb,207Pb/206Pb and206Pb/204Pb ratios are represented onTable 4. As a referent material for lead isotopic ratiosthe SRM–NIST-981 was used (for comparison, dataon the ratio values according to Todt et al.[16] areapplied).

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Table 3Results of Pb and Bi separation from silver and silver–copper alloy

Matrix QPb (mg) Element Concentration SRa

Added (10−6 g/g) Found (10−6 g/g)

10 mg Ag 0.025 Pb 32.5± 0.9 33.5± 1.0 9.2Bi 9.5 ± 0.5 8.4± 0.4 12.4

10 mg Ag 0.1 Pb 105.6± 3.2 103.2± 3.1 8.1Bi 12.7 ± 0.6 12.9± 0.6 2.5

10 mg Ag 1 Pb 1050± 30 1074± 30 8.0Bi 10.9 ± 0.6 10.5± 0.5 4.6

5 mg Ag + 5 mg Cu 0.025 Pb 27.8± 0.8 28.1± 0.84 3.2Bi 10.4 ± 0.5 9.2± 0.5 12.5

5 mg Ag + 5 mg Cu 0.1 Pb 97.6± 2.9 98.1± 2.9 6.5Bi 12.1 ± 0.7 11.9± 0.6 2.6

5 mg Ag + 5 mg Cu 1 Pb 1015± 20 1021± 20 4.5Bi 11.5 ± 0.6 12.05± 0.6 5.8

a See explanation underTable 2.

Table 4Results of Pb isotope ratios determination

Sample 208Pb/206Pb ∆ (%)a 207Pb/206Pb ∆ (%)a 206Pb/204Pb ∆ (%)a

NIST–SRM-398 2.08892± 0.00006 – 0.85717± 0.00002 – 18.215± 0.001 –NIST–SRM-399 1.97637± 0.00005 – 0.79696± 0.00002 – 19.779± 0.001 –NIST–SRM-400 1.95514± 0.00006 – 0.78481± 0.00002 – 20.130± 0.002 –BAM–SRM-211 2.10408± 0.00008 – 0.86233± 0.00002 – 18.105± 0.003 –BAM–SRM-227 2.12333± 0.00006 – 0.87915± 0.00002 – 17.723± 0.001 –BAM–SRM-228 2.09663± 0.00007 – 0.86228± 0.00002 – 18.096± 0.001 –BAM–SRM-228, 80◦C, 1 min

(∼30% isolated lead)2.09601± 0.00006 −0.03 0.86220± 0.00002 −0.01 18.095± 0.001 −0.01

BAM–SRM-228, 80◦C, 5 min(∼80% isolated lead)

2.09610± 0.00006 −0.03 0.86221± 0.00002 −0.01 18.093± 0.001 −0.01

BAM–SRM-228, 80◦C, 10 min(∼95% isolated lead)

2.09626± 0.00005 −0.02 0.86226± 0.00002 −0.002 18.097± 0.001 0.01

BAM–SRM-228, 80◦C, 25 min(100% isolated lead)

2.09587± 0.00006 −0.04 0.86203± 0.00002 −0.03 18.101± 0.001 0.03

NIST-981 2.16635± 0.00004 0.03b 0.91460± 0.00002 0.002b 16.942± 0.002 0.04b

NIST-981 2.16603± 0.00006 0.05b 0.91454± 0.00002 0.01b 16.943± 0.001 0.04b

NIST-981 2.16680± 0.00006 0.01b 0.91467± 0.00003 0.01b 16.944± 0.002 0.04b

NIST-981 [16] 2.16701± 0.00013 – 0.914585± 0.00004 – 16.936± 0.0007 –

a ∆ is evaluated according∆ (%) = [(Ci − C30)/C30] × 100; Ci: experimental value; C30: 30 min (BAM–SRM-228) value.b SR value;Cst from [16].

The results on lead isotopic ratios for SRM–BAM-228 (lead is isolated electrochemically with differentduration of the process—1, 5, 10 and 25 min corre-sponding to 30, 80, 95 and 100% yield, respectively)are also represented inTable 4. The values indicatethat during the process of electrochemical oxidation ofPb2+ to PbO2, isotopic fractioning was not detected.

4. Conclusions

A method for expressing electrochemical isolationof lead from archaeological copper, copper alloy, silverand silver alloy artifacts to determinate lead concentra-tion and isotopic ratios was proposed. Results from de-termination of lead concentration in SRM demonstrate

I. Iliev et al. / Analytica Chimica Acta 497 (2003) 227–233 233

good accuracy and reproducibility. When only the iso-topic ratios are under determination, the dissolvingprocedures, the operating conditions and the durationof electrochemical oxidation could be considerablyshortened and simplified, which makes the method anexcellent alternative of ion-exchange chromatography.

Acknowledgements

One of the authors (I.I.) is grateful to the Volkswa-gen Foundation for the financial support during hisdoctoral work.

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