Interaction of Pb and Cd during anodic stripping voltammetric analysis at boron-doped diamond electrodes

Article abstract of DOI:10.1016/j.electacta.2004.03.004

Highly boron-doped diamond (BDD) films were utilized for simultaneous electrochemical measurement of micromolar-level concentrations of Pb and Cd, and for the examination of their interactions. Differential pulse anodic stripping voltammetry (DPASV) was used for this detection. This approach can help to understand the possible detection of trace metals at BDD electrodes without the aid of mercury. These metals were found to strip at their characteristic potentials, in solutions containing Cd or Pb alone, and in those containing these metals together. The mixed solutions (concentration range: 1-5μM) yielded well-separated stripping peaks for Pb and Cd and the differential stripping peak currents for the respective metals increased linearly with increasing metal concentration. There were mutual interferences due to Pb-Cd interactions, but these can be taken into account with the aid of three-dimensional calibration plots. A model has been developed to help explain the Pb-Cd interactions.

Full text of DOI:10.1016/j.electacta.2004.03.004

Electrochimica Acta 49 (2004) 3313–3318
Interaction of Pb and Cd during anodic stripping voltammetric
analysis at boron-doped diamond electrodes
a,∗
b
c
d
A. Manivannan , R. Kawasaki , D.A. Tryk , A. Fujishima
a
Physics Department, West Virginia University, Morgantown, WV 26506, USA
b
Department of Applied Chemistry, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
c
Department of Chemistry, University of Puerto Rico, San Juan, PR 00931-3346, USA
d
Kanagawa Academy of Science & Technology, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi 213-0012, Japan
Received 6 September 2003; received in revised form 29 February 2004; accepted 5 March 2004
Available online 6 May 2004
Abstract
Highly boron-doped diamond (BDD) films were utilized for simultaneous electrochemical measurement of micromolar-level concentrations
ofPbandCd, andfortheexaminationoftheirinteractions. Differentialpulseanodicstrippingvoltammetry(DPASV)wasusedforthisdetection.
This approach can help to understand the possible detection of trace metals at BDD electrodes without the aid of mercury. These metals were
found to strip at their characteristic potentials, in solutions containing Cd or Pb alone, and in those containing these metals together. The mixed
solutions (concentration range: 1–5 ␮M) yielded well-separated stripping peaks for Pb and Cd and the differential stripping peak currents for
the respective metals increased linearly with increasing metal concentration. There were mutual interferences due to Pb–Cd interactions, but
these can be taken into account with the aid of three-dimensional calibration plots. A model has been developed to help explain the Pb–Cd
interactions.
©
2004 Elsevier Ltd. All rights reserved.
Keywords: Boron-doped diamond; Differential pulse anodic stripping voltametry; Pd–Cd interactions
1
. Introduction
for electroanalytical applications has been recognized due to
their durability, electrical conductivity, and corrosion resis-
tance [13–18]. The excellent chemical inertness, combined
with low background current, and the large potential win-
dow between oxygen and hydrogen evolution favor the use
of diamond for the detection of a variety of analytes, includ-
ing trace metals [9,19–36]. In this report, we demonstrate
the principle of applying diamond film electrodes and try
to understand the behavior of Pb and Cd in mixed solutions
with DPASV, even though there are mutual interferences.
Recently, we reported the use of BDD electrodes in the
detection of Pb using DPASV [9]. A concentration as low as
Detection of toxic trace metals in the environment is a
challenging analytical problem. There are various available
techniques, including atomic absorption, atomic emission,
and electrochemical techniques. Electroanalytical tech-
niques have attractive features, including sensitivity, easy
operational procedures, and portability. The conventional
electroanalytical determination of metals usually involves
the use of mercury-based electrodes, e.g., hanging mer-
cury drop electrodes [1] or mercury film-coated electrodes,
including substrates such as glassy carbon [2] or, most re-
cently, iridium [3]. Mercury is considered to be the electrode
of choice for metal stripping analysis, but disposal problems
and high cost are major drawbacks. Most importantly, how-
ever, mercury toxicity has been a concern and has motivated
the effort to develop mercury-free electrodes [4–12]. Re-
cently, the utility of boron-doped diamond (BDD) electrodes
−
9
10
M was detected at bare diamond. Those results indi-
cated that small amounts of Pb can be quantitatively stripped.
Thus, the use of diamond film electrodes, particularly in
combination with anodic stripping voltammetry, appears to
be well-suited for the detection of trace metals, as confirmed
by other workers [11,37], as well as by our recent work, in
which mercury itself is analyzed [35].
∗
It has long been recognized that electrochemical strip-
ping analysis is a powerful technique for measuring trace
Corresponding author. Tel.: +1-304-2933422; fax: +1-304-2935732.
E-mail address: amanivan@wvu.edu (A. Manivannan).
0
013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.03.004

3
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A. Manivannan et al. / Electrochimica Acta 49 (2004) 3313–3318
metals [8,38–40]. Its remarkable sensitivity is due in part
to the deposition step, during which the analyte metals are
accumulated on the working electrode. This method is rela-
tively inexpensive when compared to the spectroscopic tech-
niques, and the feasibility of compact portable instruments
makes it attractive for field on-line monitoring of trace met-
als [5,6,10,41]. Recently, Hocevar et al. [12] have reported
the simultaneous analysis of Zn, Cd, and Pb at a bismuth
film electrode, without the use of Hg. That result, together
with results for the BDD electrode, including that of Prado
et al. [37] on the analysis of Pb and Cu, and the present
work on Pb and Cd, should help to accelerate the develop-
ment of environmental trace-metal-monitoring instruments
that are themselves environmentally friendly.
Fig. 1. DPASV curves for the stripping of Cd deposited from solutions
containing 1–5 ␮M Cd(NO3)2 in 0.2 M acetate buffer (pH = 5.0); the
deposition time was 2 min at −1.0 V vs. SCE.
2
. Experimental
Highly BDD films were deposited on conductive silicon
observed in both cases. The corresponding uncertainties for
the slopes and intercepts of the straight lines are given in
Table 1 [42]. For the Pb-stripping peak (5 ␮M Pb), slight
increases in the peak current were observed with increasing
Cd concentration (curve c).
Fig. 4 shows the DPASV results for various concentra-
tions of Pb alone. Excellent linear dependence was observed.
This is similar to our previous observation in 0.1 M KCl (pH
substrates by use of microwave-assisted plasma-enhanced
chemical vapor deposition [19]. The microcrystallites in
these polycrystalline films were of high structural quality,
and the sizes were in the 5–10 ␮m range. A specially de-
signed O-ring-type three-electrode electrochemical cell was
used. The metal solutions were prepared with analytical
grade reagents. A saturated calomel electrode (SCE) was
used as the reference electrode, and a platinum wire was used
as a counter electrode. A Hokuto-Denko Model HZ-3000
potentiostat was used for all of the electrochemical measure-
ments. The supporting electrolyte used was 0.2 M acetate
buffer (pH 5). The deposition time was 2 min. It was found
not to be necessary to remove oxygen from the solutions,
due to the high overpotential for oxygen reduction [19,20].
1) electrolyte [9]. Fig. 5 shows the results for Pb stripping in
acetate buffer in the presence of 5 ␮M Cd(NO3)2. During the
first incremental addition of Pb (1 ␮M), the differential strip-
ping peak current for 5 ␮M Cd decreased markedly, drop-
ping from 10.8 to 6.3 ␮A. Further additions of Pb caused
only slight additional decreases in the peak currents for Cd
stripping. In the presence of 5 ␮M Cd2 (Fig. 6, curve b),
+
the peak currents for Pb were ca. 40% larger compared to
those for Pb2 in the absence of Cd (curve a). Again, this
+
2+
3
. Results and discussion
is mainly due to the interaction between Pb and Cd. How-
In this report, we describe principally the characteri-
zation and electrochemical determination of trace Pb and
Cd separately as well as in mixtures. Fig. 1 shows the
DPASV-stripping peaks for Cd deposited from solutions
with various concentrations of Cd in acetate buffer (pH
5
), with a peak potential of ca. −0.85 V versus SCE. A
linear increase in the differential current with concentra-
tion is apparent. Fig. 2 shows the stripping behavior of
Cd deposited from various solution concentrations of Cd
in the presence of 5 ␮M of Pb(NO3)2. In addition to the
Cd-stripping peak at −0.85 V, the Pb-stripping peak can be
seen at −0.65 V versus SCE. In the presence of 5 ␮M Pb,
the peak currents for Cd (Fig. 2) were consistently ca. 55%
smaller than those obtained in pure Cd solutions (Fig. 1).
The decreases in the peak currents for Cd can be explained
by an interaction between Pb and Cd, as discussed later.
Fig. 3 depicts the calibration curves for Cd in pure solution
2
+
Fig. 2. DPASV curves for the stripping of Cd and Pb from solutions
containing Cd(NO3)2 in 0.2 M acetate buffer with 5 ␮M Pb(NO3)2; the
deposition time was 2 min at −1.0 V vs. SCE.
(
curve a) as well as in the presence of Pb (curve b) based
on the DPV results in Figs. 1 and 2. Linear behavior was

A. Manivannan et al. / Electrochimica Acta 49 (2004) 3313–3318
3315
Fig. 3. Calibration curves for the differential stripping peak current for
Fig. 5. DPASV curves for the stripping of Cd and Pb from solutions
containing Pb(NO3)2 in 0.2 M acetate buffer with 5 ␮M Cd(NO3)2; the
deposition time was 2 min at −1.0 V vs. SCE.
2
+
Cd, from solutions containing 0 M to 5 ␮M Cd in 0.2 M acetate buffer:
2
+
(
5
curve a) in the absence of added Pb ; and (curve b) in the presence of
2
+
␮M Pb . In addition, the differential peak currents for Pb stripping are
shown (curve c) for the same solutions; the deposition time was 2 min at
1.0 V vs. SCE.
ever, the linearity of the two curves was excellent, with the
corresponding uncertainties in slopes and intercepts given
in Table 1 [42]. The slight decreases in the Cd peak cur-
rents with further additions of Pb are also evidence for the
interaction of the two metals.
It is interesting that the differential peak current for strip-
ping from 5 ␮M Pb solution increased with the addition of
1 ␮M of Cd, although the increase was quite small. In con-
trast, there was a decrease in the differential current for
stripping from 5 ␮M Cd solution with the addition of 1 ␮M
of Pb, and this decrease was quite large. Both of these ob-
servations can be explained by the deposition of a portion
of the Cd on Pb nanoparticles, as discussed later.
−
Table 1
Analytical parameters for the ASVDPV analysis of lead and cadmium,
both alone and in the presence of each other
Slope (␮A ␮M 1)
−
Intercept (␮A)
Detection
limit (␮M)
(
[
A) Cd analysis (based on results in Fig. 3)
Pb ] (␮M)
2
+
0
5
2.2 ± 0.1
−0.1 ± 0.2
−0.2 ± 0.2
0.25
0.45
0.96 ± 0.04
(
[
B) Pb analysis (based on results in Fig. 6)
Cd ] (␮M)
2
+
0
5
2.58 ± 0.04
−0.1 ± 0.1
+0.3 ± 0.4
0.31
0.35
In order to examine the feasibility of determining both
Pb and Cd quantitatively in solution, the concentrations of
3.6 ± 0.1
Note: uncertainties are given as one standard deviation. The detection
limits are given in terms of two standard deviations.
Fig. 6. Calibration curves for the differential stripping peak current for
2
+
Pb, from solutions containing 0 to 5 ␮M Pb in 0.2 M acetate buffer:
2
+
(
curve a) in the absence of added Cd ; and (curve b) in the presence of
2
+
Fig. 4. DPASV curves for the stripping of Pb from solutions containing
Pb(NO3)2 in 0.2 M acetate buffer (pH 5.0); the deposition time was 2 min
at −1.0 V vs. SCE.
5 ␮M Cd . In addition, the differential peak currents for Cd stripping
are shown (curve c) for the same solutions; the deposition time was 2 min
at −1.0 V vs. SCE.

3
316
A. Manivannan et al. / Electrochimica Acta 49 (2004) 3313–3318
to oxidize, i.e., have more negative standard potentials, tend
to deposit on metals that are more difficult to oxidize, i.e.,
have less negative standard potentials. This tendency can
also be related to the correlation between the tendency of a
given metal to undergo underpotential deposition (UPD) on
a second metal and the difference in the work functions of
the two metals [45]. On the basis of the respective standard
potentials, it would be expected that Cd would tend to de-
posit on Pb. Moreover, Pb tends to deposit more efficiently
than Cd, as evidenced by the larger stripping peaks for the
same solution concentrations.
Therefore, the proposed sequence of events occurring dur-
ing the deposition and stripping from a solution contain-
ing both Pb and Cd is as follows. The Pb tends to de-
posit preferentially during the accumulation stage of the
ASV experiment (see Fig. 8A). Such differences in deposi-
tion efficiency have been reported for many years but have
not been explained in detail [46]. Cd then deposits at a
mass-transport-controlled rate more or less evenly across the
surface, directly on both the BDD surface and Pb nanoparti-
cles that are already present on the surface (Fig. 8B). The Cd
that deposits on Pb most likely is at the monolayer level, as
argued below. During the stripping potential sweep, the Cd
that was deposited directly on the BDD surface is stripped at
the potential expected for bulk Cd, ca. −0.85 V versus SCE
(Fig. 8C). Finally, at a more positive potential, the Cd that
Fig. 7. Three-dimensional calibration curves for the differential pulse
anodic stripping currents for (A) Cd and (B) Pb in acetate buffer solutions
containing both metals.
remains on Pb as a monolayer is stripped along with the Pb
itself (Fig. 8D). The shift in the stripping potential for Cd
should be an indication of its interaction energy with Pb.
The diagrams on the left side of Fig. 8 illustrate the situ-
ation for a lower Cd solution concentration, and the ones on
the right side illustrate that for a higher concentration, both
with the same Pb solution concentration. This comparison
is meant to show why the calibration plot for Cd remains
both metals were allowed to vary over the range from 1 to
5
␮M, i.e., a matrix of 25 different combinations or, includ-
ing the pure solutions, 36 different combinations. The re-
sults are shown in Fig. 7 as three-dimensional plots, with the
Cd-stripping peak heights shown in part A and the Pb peak
heights in part B. The plots already shown in Figs. 3 and 6
are consistent with these results and show clearly that: (i) the
calibration curves for both Pb and Cd are quite linear, but
with slopes that depend upon the fixed concentration of the
second metal; (ii) increasing Cd concentration leads to in-
creases in the Pb peak height; and (3) the presence of even a
small amount of Pb affects the Cd peak height. The specific
procedure for obtaining the Pb2 and Cd concentrations
for unknown solutions from this plot will be discussed later.
The interaction between Pb and Cd probably does not in-
volve the formation of an alloy. The phase diagram for the
Pb–Cd alloy system (not shown) indicates that there is es-
sentially zero solubility of Cd in Pb at room temperature
+
2+
Fig. 8. Diagrammatic representation of the events occurring during depo-
sition and stripping from a solution containing both a smaller (left) and
[43]. However, there can still be a significant interaction be-
2+
a larger (right) concentration of Cd ions in the presence of a constant
tween the two metals, which would most likely involve the
deposition of a portion of the Cd, initially as a monolayer,
on Pb nanoparticles that are already deposited on the dia-
mond surface. It is well known from the literature on cat-
alytic intermetallic clusters that metals can interact in this
way at the nanoparticle level, even though they do not form
alloys [44]. It is also well known that metals that are easier
2+
Pb concentration. (A) At the deposition potential, Pb deposits prefer-
entially; (B) Cd then deposits on both the Pb deposits and bare diamond;
(C) during the initial stages of stripping, Cd nanoparticles deposited on
bare diamond are removed; and (D) at more positive potentials, Cd that is
deposited as a monolayer on Pb strips along with Pb itself. It should be
noted that the Cd that deposits on bare diamond probably forms nanopar-
ticles rather than a monolayer, as in the case of Pb, due to differences in
the interaction energies.

A. Manivannan et al. / Electrochimica Acta 49 (2004) 3313–3318
3317
linear in the presence of Pb (Fig. 3, curve b). This is under-
standable if we assume that Pb deposits significantly faster
than Cd, so that the Cd ions are always presented with a
similar topography of Pb nanoparticles and bare diamond
surface for a given Pb solution concentration. Thus, a con-
stant fraction of the Cd tends to be deposited on Pb and
then stripped along with the Pb. It is important to note that
the constant fraction should only be obtained if we assume
that the Cd depositing on Pb is at the monolayer level. If
multilayers deposit, then the amounts of Cd stripping at the
standard potential for bulk Cd would tend to increase with
increasing Cd concentration.
This model also explains the fact that even a relatively
small Pb concentration has a large effect on the Cd strip-
ping. The higher deposition efficiency of Pb means that it
will be able to provide an adequate substrate for the sub-
sequent deposition of Cd. Further increases in Pb solution
concentration would be expected to produce a smaller effect.
The model can also be used to explain why the calibration
curve for Pb remains linear in the presence of Cd (Fig. 6,
curve b). In this case, we must assume that not only the Cd
deposits on Pb as a monolayer but also the relative surface
area taken up by the Pb on diamond tends to increase lin-
early with the Pb solution concentration, so that it captures
a constant fraction of the subsequently depositing Cd. Of
course, these assumptions should be examined in greater de-
tail in future work, for example, over wider ranges of solu-
tion concentration, but they appear to be valid for the ranges
used in the present work.
give a range of possible Cd concentrations. This range would
then be used to narrow down the possible range of Pb con-
centrations that would be obtained from the horizontal cut
on the 3D surface of Fig. 7B, obtained from the Pb-stripping
peak height. The resulting range of Pb concentrations would
then be used to narrow down the range of possible Cd con-
centrations. Working back and forth between the two 3D
plots, a good approximation could be made for both con-
centrations. Of course, this procedure can be implemented
mathematically in a straightforward manner.
Our objective in the present work was to examine specif-
ically the interaction between Pb and Cd and not to try to
achieve low detection limits for Pb and Cd in water. Nev-
ertheless, the uncertainties calculated for the least squares
fitting can be used to obtain a very approximate estimate
of the detection limits for Cd in the presence and absence
of Pb (see Table 1). The main point of this exercise is to
show that, due to the relatively mild nature of the mutual
interference, the detection limit for Cd in the presence of
Pb is only slightly worse than that in its absence. However,
both values are approximately one order of magnitude above
−
1
those mandated by the EPA for drinking water (5 ␮g l or
−
8
4.4 × 10 M) [47], so that it is clear that further optimiza-
tion of the present methods is required. In other reports on
Cd–Pb analysis with ASV, the detection limits for Cd were
−
1
−9
0.2 ␮g l (1.8 × 10 M) for the bismuth film electrode
[12], and 0.5 ␮g l (4.4 × 10−9 M) for the Hg film on an
−
1
Ir substrate [3].
The uncertainties were also used to estimate the detection
limits for Pb in the presence and absence of Cd. In this case,
the presence of Cd did not degrade the detection limit for
Pb at all (see Table 1). As with Cd, these values are approx-
imately one order of magnitude above the EPA-mandated
A final set of observations that can be explained with this
model has to do with the slopes of the calibration curves in
Figs. 3 and 6, for Cd and Pb, respectively. The slope for Cd
concentration decreases by a factor of approximately two in
the presence of 5 ␮M Pb2 (Fig. 3). This is explained by
noting that the Cd stripping peak at ca. −0.85 V is proposed
to be due to stripping from Cd that is deposited directly on
the diamond surface. When there is Pb already deposited on
the diamond surface, the Cd that deposits on Pb would no
longer tend to be stripped at this potential but would strip at
+
−1
−8
levels for drinking water (15 ␮g l or 7.3×10 M). How-
ever, other work from our laboratory has already shown that
it is possible to determine Pb concentrations at levels down to
−
1
−9
0.2 ␮g l (1 × 10 M) under slightly different conditions,
i.e., acidified KCl solution [9]. We also found it possible to
−1
−9
measure Pb concentrations down to 0.41 ␮g l (2×10 M)
with linear sweep voltammetry at BDD electrodes, even in
the presence of Cu, in neutral and acidified KCl solution
[48]. In the other reports on Cd–Pb analysis with ASV, the
−
0.6 V, along with Pb itself. Thus, the more Pb is deposited,
the lower the slope of the Cd calibration curve would be
expected to be, as shown below, although the dependence is
fairly weak after the initial addition of Pb2 . Regarding the
slopes of the Pb calibration curves, a contrasting situation
+
−1
−9
detection limits for Pb were 0.8 ␮g l (3.9×10 M) for the
−1
−9
bismuth film electrode [12], and 0.5 ␮g l (4.4 × 10 M)
exists, i.e., in the presence of Cd2 in solution, a certain
fraction of the Cd deposits on top of the Pb and tends to
strip along with Pb itself, effectively increasing the height of
the Pb-stripping peak. The increase of slope for Pb stripping
+
for the Hg film on an Ir substrate [3]. In related work,
−1
Saterlay et al. [49] reported a detection limit of 0.33 ␮g l
−
9
(1.6 × 10 M) for Pb in a Pb–Cu aqueous solution, using
an ultrasound-enhanced cathodic stripping technique on a
BDD electrode.
was ca. 30% for a Cd2 concentration of 5 ␮M (Fig. 6).
With the 3D plots, it is now possible to carry out a quan-
titative analysis for a Pb–Cd mixed solution, even though
there is a significant cross-interference. This can be done by
means of successive approximations in the following man-
ner. Taking the Cd-stripping height peak for an unknown
sample containing Pb and Cd, a horizontal cut would be
made on the 3D surface of the plot in Fig. 7A. This would
+
4. Concluding remarks
In general, the scope of stripping analysis has progressed
dramatically during the past decade. With diamond elec-
trodes, it is likely to expand further, because of the wider

3
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A. Manivannan et al. / Electrochimica Acta 49 (2004) 3313–3318
[16] R. Tenne, C. Levy-Clement, Isr. J. Chem. 38 (1998) 57.
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ularly in the positive potential direction, so that noble metals
such as Pt and Au can also be detected. In the present work,
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Cd at diamond electrodes without benefit of mercury. Al-
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not preclude quantitative analysis. Three-dimensional cur-
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ASV, the linearity of the calibration curves for both Pb and
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Acknowledgements
This research work was supported by the Japan Soci-
ety for the Promotion of Science (JSPS) Research for the
Future Program (Exploratory Research on Novel Materials
and Substances for Next Generation Industries), the U.S.
Department of Energy (DE-FC26-01NT41091), and EPA
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