An EQCM-D study of the influence of chloride on the lead anodic oxidation

Article abstract of DOI:10.1016/j.electacta.2012.06.016

The influence of chloride on the electrodeposition of lead films and their dissolution in anodic stripping voltammetric experiments was examined. Gold substrates were plated with lead films, and mass changes were monitored by using the electrochemical quartz crystal microbalance with dissipation factor (EQCM-D). The results showed that the amount of electrodeposited lead is slightly dependent on the chloride concentration. The charge/mass ratio data indicated the presence of Pb(I) and Pb(II) as a result of film dissolution, and the precipitation and deposition of PbCl₂ onto the electrode surface. Scanning electron microscopy images revealed that the morphology of the lead film was strongly influenced by chloride present in the plating solution and that much rougher films were obtained in comparison with those obtained in the absence of chloride. The rate of the anodic dissolution was higher for lead films with higher surface areas, which lead to an increase in their stripping voltammetric currents.

Full text of DOI:10.1016/j.electacta.2012.06.016

Electrochimica Acta 78 (2012) 347–352
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
An EQCM-D study of the influence of chloride on the lead anodic oxidation
∗
Maiara O. Salles, Vitor L. Martins, Roberto M. Torresi, Mauro Bertotti
Instituto de Química – Universidade de São Paulo, São Paulo, SP 05508-900, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
The influence of chloride on the electrodeposition of lead films and their dissolution in anodic strip-
ping voltammetric experiments was examined. Gold substrates were plated with lead films, and mass
changes were monitored by using the electrochemical quartz crystal microbalance with dissipation factor
Received 21 March 2012
Received in revised form 4 June 2012
Accepted 5 June 2012
(EQCM-D). The results showed that the amount of electrodeposited lead is slightly dependent on the chlo-
Available online 15 June 2012
ride concentration. The charge/mass ratio data indicated the presence of Pb(I) and Pb(II) as a result of film
dissolution, and the precipitation and deposition of PbCl2 onto the electrode surface. Scanning electron
microscopy images revealed that the morphology of the lead film was strongly influenced by chloride
present in the plating solution and that much rougher films were obtained in comparison with those
obtained in the absence of chloride. The rate of the anodic dissolution was higher for lead films with
higher surface areas, which lead to an increase in their stripping voltammetric currents.
© 2012 Elsevier Ltd. All rights reserved.
Keywords:
Lead electrodeposition
Chloride effect
Quartz crystal microbalance
Stripping analysis
Scanning electron microscopy
1
. Introduction
Electroanalytical methods for metal determination often
[17,19–21] and the effect of other substances, such as oxygen, sul-
fur, bismuth, and copper on the film deposition [16,22–24].
The determination of lead by stripping analysis using gold as a
substrate has been reported in the literature [14,25–31]. In some
reports, the influence of chloride on the lead electrodeposition
has been described, [14,31,32] but to the best of our knowledge,
this influence has not been extensively investigated and character-
ized. The use of EQCM-D and scanning electron microscopy (SEM)
facilitated a better understanding of this phenomenon by show-
ing clearly the influence of chloride anions on the deposition and
dissolution processes. This understanding is very important in eval-
uating the influence of sample composition on the electroanalytical
determination of Pb(II) and in ensuring reliable analysis.
require a preconcentration step in order to detect the analyte at
very low concentrations. Mercury is the most commonly used elec-
trode for such analyses in stripping voltammetry techniques [1–3],
mainly due to the high hydrogen overvoltage and the ability of
mercury to form amalgams with several metals. However, since
mercury is a toxic metal, other materials such as bismuth and anti-
mony have been used in stripping analyses [4–9]. The accumulation
of the analyte can also be done directly at bare surfaces and the
literature reports the use of carbon [10–12] and gold electrodes
[
13,14].
The electrochemical quartz crystal microbalance (EQCM) tech-
nique is widely used for measuring the change in the mass of an
electrode while a film is deposited on or stripped off from the elec-
trode surface. When an EQCM with dissipation (EQCM-D) is used,
the recording of resonance frequencies of the electrode and the
dissipation of a film during its electrodeposition or dissolution can
provide certain information, e.g., about the mechanism by which
the deposition or the dissolution occurs, deposition/dissolution
efficiency (based on charge/mass ratio), and viscoelastic properties
2
. Experimental
2.1. Reagents
All reagents were of analytical grade and used without any fur-
ther purification. Pb(NO ) , KCl, and K [Fe(CN) ] solutions were
prepared by dissolving the reagents in deionized water processed
through a water purification system (18.0 M ꢀ cm , Nanopure
Infinity, Barnstead, Iowa, USA). The nitric acid solution was pre-
pared by diluting the stock solution, as necessary. All reagents were
obtained from Merck (Darmstadt, Germany).
3
2
3
6
−1
[
15–18].
Several studies for analyzing the electrodeposition of lead onto
different substrates by using the EQCM or EQCM-D have been per-
formed by focusing, for instance, on the underpotential deposition
2.2. Square wave voltammetry experiments
∗ Corresponding author. Tel.: +55 11 30912694.
E-mail address: mbertott@iq.usp.br (M. Bertotti).
An Autolab PGSTAT 30 (Eco Chemie) was used for experiments
with square wave voltammetry (SWV), a technique that has been
0
013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2012.06.016

3
48
M.O. Salles et al. / Electrochimica Acta 78 (2012) 347–352
largely used to improve the sensitivity of analytical determina-
tions [33]. A gold disc microelectrode was used as the working
electrode for the stripping analysis. A homemade Ag/AgCl(sat. KCl)
electrode, inserted in a separate compartment to avoid chloride
contamination, and a platinum wire were used as reference and
counter electrodes, respectively. Before each experiment, a positive
potential (Ecleaning = 0.70 V) was applied to the electrode for 2 min
(
tcleaning = 120 s). Lead was then deposited at the microelectrode
surface (Edeposition = −0.70 V) in solutions containing 0.1% (m/v)
−
1
2+
HNO , 0.1 ␮mol L Pb with different chloride concentrations (in
3
−
1
the range 0–50 mmol L ). Lead was stripped off by SWV at the
same solution employed in the deposition step. The potential was
scanned from −0.70 to 0.10 V and the potential step (Estep), poten-
tial amplitude (E
) and frequency parameters were 5 mV,
amplitude
2
5 mV and 300 Hz, respectively.
The gold microelectrode was fabricated as follows: a gold fiber
diameter = 25 ␮m, Goodfellow Cambridge Ltd., Cambridge, Eng-
land) was connected to a Ni/Cr wire with silver ink conductive paint
Joint Metal Comércio Ltda, São Paulo, Brazil), and then inserted into
−
Fig. 1. Stripping peak current/charge deposition as a function of Cl concentration.
(
Lead analysis performed using square wave voltammetry on a gold microelec-
−
1
2+
trode (r = 12.5 ␮m) with solutions containing 0.1% (m/v) HNO3, 0.1 ␮mol L Pb
−
−1
and varying Cl concentrations (12.5, 25 and 50 mmol L ) in a quiescent solution.
Parameters used: Ecleaning: 0.70 V, tcleaning: 120 s, Edeposition: −0.70 V, tdeposition: 480 s,
Efinal: 0.10 V, Frequency: 300 Hz, Estep: 5 mV, Eamplitude: 25 mV, tequilibrium: 10 s.
(
a glass capillary and flame-fuse sealed in the tip of the glass cap-
illary. The radius of the working microelectrode was determined
by measuring the steady-state current in a K [Fe(CN) ] solution of
3
6
1
00 mV s 1). Experiments involving film dissolution were per-
−
known concentration, and the value was found to be 12.5 ␮m.
formed using solutions containing 0.1% (m/v) HNO3 and different
concentrations of chloride and Pb2+. As in the deposition step, chlo-
2.3. Scanning electron microscopy experiments
−
1
2+
ride concentrations were in the range of 0–75 mmol L . Pb was
−
1
mainly at a concentration equal to 5 mmol L unless in one of the
experiments, where a free lead solution was used. At this experi-
ment, a peristaltic pump (Gilson MINIPULS Evolution, France) was
used to slowly exchange the deposition solution from the dissolu-
tion solution.
A scanning electron microscope (FESEM JSM-7401F, JEOL) using
a LEI detector, 1.0 kV accelerating voltage and 8 mm working
distance was used for image acquisition of different lead films elec-
trodeposited onto gold microfibers (r = 63 ␮m). Electrodeposition
for 480 s was accomplished at −0.70 V using solutions containing
−
1
2+
0
.1% (m/v) HNO and 0.1 ␮mol L
Pb , with different chloride
3
−1
concentrations (in the range 0–75 mmol L ).
3. Results and discussion
3.1. Square wave voltammetry experiments
2.4. Electrochemical quartz crystal microbalance with dissipation
experiments
Preliminary experiments were carried out to confirm the influ-
ence of chloride on the lead stripping voltammetric oxidation peak.
Data were obtained by square wave voltammetry, as described in
the experimental section, using a gold microelectrode in solutions
The EQCM-D measurements were carried out with a QCM-D sys-
tem from Q-Sense (Göteborg, Sweden) and an Autolab PGSTAT 30
Eco Chemie, The Netherlands), by simultaneously monitoring the
changes in resonance frequency (ꢁf), dissipation (ꢁD) and current.
The change in ꢁf can be related to the change in mass (ꢁm) in the
case of a rigid film by using the Sauerbrey equation (1) [34]:
(
−
1
2+
containing 0.1% (m/v) HNO3 and 0.1 ␮mol L Pb in the presence
−
1
of different chloride concentrations (range of 0–50 mmol L ). Fig. 1
shows the ratio of the peak current (Epeak = 0.10 V) recorded during
the stripping step to the charge associated with the lead electrode-
position process, under each experimental condition. Since the
SWV current was normalized by the charge (typical values ranged
between −20 and −30 ␮C), the increase in this ratio as a function
of the chloride concentration confirms the significant influence of
the halide on the stripping response, as already reported in the
literature [14,31,32].
ꢀ
ꢁ
C
n
−ꢁfn =
ꢁm
(1)
−
2
−1
where the mass sensitivity C was 17.7 ng cm Hz and n is the
overtone (1, 3, 5, 7, 9, 11, and 13). A gold electrode (Q-Sense)
was used as the working electrode after cleaning it with an alka-
◦
line piranha solution (1:1:5 H O /NH OH/H O mixture at 70 C)
2
2
4
2
for 5 min. The electrode was then rinsed with deionized water,
sonicated for 5 min in deionized water and ethanol, dried with a
nitrogen stream, and then kept in a UV/Ozone cleaner (ProCleaner
Plus BioForce) for 15 min prior to use. A Pt spiral wire and a home-
made Ag/AgCl(sat. KCl), which was inserted in another compartment
to avoid chloride contamination, were employed as counter and
reference electrodes, respectively.
3.2. Effect of chloride on film deposition
The influence of chloride on the electrodeposited lead film was
firstly studied by scanning electronic microscopy (SEM). Images of
the gold surface were taken after lead electrodeposition performed
−
1
in the absence and presence of 25, 50, and 75 mmol L
chlo-
The studies regarding the film deposition (at −0.70 V)
ride under potentiostatic conditions (E = −0.70 V). In the absence
of chloride, a flat surface was obtained (Fig. 2(A)), whereas much
rougher surfaces were seen when the lead was plated in solutions
containing chloride (Fig. 2 (B–D)). These results clearly demonstrate
the influence of chloride on the morphology of the electrodeposited
lead film. Jones et al. reported that differences in stripping voltam-
metry can be explained by the differences in the orientation and
morphology of the deposited film, since the surface structure can
affect the stripping process [35].
were performed in solutions containing 0.1% (m/v) HNO and
5
3
−
1
2+
mmol L
Pb with different chloride concentrations (in the
−1
range 0–75 mmol L ). In order to examine the film dissolution,
the electrodeposition was carried out at potentiostatic conditions
−
1
2+
(
−0.70 V) in solutions containing 0.1% (m/v) HNO , 5 mmol L Pb
3
and different chloride concentrations (in the range 0–75 mmol L−1).
Then, the film was stripped off using linear sweep voltammetry
LSV) from −0.70 to 0.70 V at different scan rates (from 1 to
(

M.O. Salles et al. / Electrochimica Acta 78 (2012) 347–352
349
Fig. 2. Scanning electron micrograph of a lead film electrodeposited on a gold fiber (r = 63 ␮m) under potentiostatic conditions (Edep = −0.70 V, tdep = 480 s) using a solution
−
1
2+
−1
−
containing 0.1% (m/v) HNO3 and 5 mmol L Pb in the absence (A) and presence of 25 (B), 50 (C), and 75 (D) mmol L Cl .
In addition to the effect on the film morphology, chloride could
also have an influence on the amount of electrodeposited lead. In
order to examine this question, lead film formation on the gold
electrode (crystal quartz) was examined by EQCM-D in the absence
and presence of chloride at various concentrations (25, 50, and
The current dependence on time during electrodeposition in all
the experiments described above (Pb2+ deposited onto a crystal
quartz gold electrode in solutions containing different chloride con-
−
1
centrations (0, 25, 50 and 75 mmol L ) at −0.70 V) and the mass
change values (determined from the frequency changes for the 5th
overtone, chosen arbitrarily) were investigated. A plot of charge as a
function of the mass change in the deposition process (Fig. 3(II)) was
obtained by calculating the charge by integrating the current/time
profile. A linear relationship between charge and mass was found
for all supporting electrolyte solutions used in the experiments.
Since the slope values (charge/mass) of the straight lines presented
no significant variation, chloride does not play an important role in
the charge/mass ratio during the electrodeposition process.
−1
7
5 mmol L ). During the experiments, the changes in frequency
(
(
ꢁf) and film dissipation (ꢁD) were recorded for all overtones
n = 1, 3, 5, 7, 9, 11, and 13). Fig. 3(Ia) shows a typical profile of ꢁf
and ꢁD vs. time obtained during the lead film deposition and sub-
sequent dissolution in the absence of chloride. Fig. 3(Ib) shows the
applied potential/time program used to obtain the aforementioned
EQCM-D experiment. At the beginning of the experiment (t ), no
1
potential was applied to the electrode and it remained at open cir-
−
1
cuit. The decrease in ꢁf at t is a consequence of the increase in the
The mean slope value was (815 ± 55)C g , which is lower than
2
2+
mass due to the beginning of lead electrodeposition (E = −0.70 V).
expected when taking into account the electroreduction from Pb
−
1
At t , a potential sweep was imposed on the working electrode from
to Pb (931.3C g ). However, this can be explained by considering
the possible confinement of water molecules into inhomogeneities
in the surface, as proposed by Schumacher et al. [36]. As shown by
the SEM images, the presence of chloride in the deposition solution
results in films with different roughness and the presence of water
inside the inhomogeneities of the films can increase the measured
mass value without decreasing proportionally the charge, resulting
in a decrease of the charge/mass ratio.
3
−
1
−
0.70 V to 0.70 V at 10 mV s . The starting potential was selected
as −0.70 V to avoid the chemical dissolution of the lead film due
to dissolved oxygen. Lead continued to be electrodeposited up to
−
0.40 V. At this potential (t ), ꢁf started to increase, indicating
4
the beginning of metallic lead dissolution. ꢁf values approached
Hz at t5, indicating that lead stripping was complete. ꢁf and
D continued to be measured at open circuit for approximately
min. Neither the frequency nor the dissipation varied significantly
0
ꢁ
1
among the overtones; hence, the formed film can be considered to
be acoustically rigid. In addition, the change in the film dissipation
is almost insignificant when compared to the frequency variation,
confirming the rigid nature of the lead film formed on the electrode
surface. In the case of an acoustically rigid film, the Sauerbrey equa-
tion can be used to obtain information on the mass change from
the frequency change (Eq. (1)). A similar ꢁf and ꢁD profile was
observed when the deposition was performed with solutions con-
3.3. Effect of chloride on film dissolution
Lead is oxidized during the voltammetric stripping step, and the
influence of chloride on the rate of this process was investigated.
To this end, the mass change was recorded during the dissolution
process at different scan rates in a preliminary experiment carried
out in the absence of chloride. The amount of lead deposited on
the electrode surface in the absence of chloride was maintained
constant in all experiments by controlling the charge deposition
(−23 mC). Fig. 4 shows that the mass returns to its initial value
and that this is independent of the scan rate, which was varied
−1
taining chloride (from 25 to 75 mmol L ), therefore those films
have also a rigid nature and the Sauerbey equation can be applied
to the results obtained at these experimental conditions.

3
50
M.O. Salles et al. / Electrochimica Acta 78 (2012) 347–352
−
1
Fig. 5. Linear sweep voltammograms recorded at 10 mV s in the presence (ꢁ and
−
1
)
and absence ( and
) of 50 mmol L chloride after lead deposition per-
formed in a resonant crystal quartz under potentiostatic conditions (Edep = −0.70 V)
−
1
in the presence (ꢁ and
) and absence ( and
) of 50 mmol L chloride with
−
2
mass cut off ca. 124 ␮g cm . Note: current and charge were normalized by total
deposited mass (mdep) after the complete exchange of deposition solution (0.1%
−
1
2+
−1
2+
(
m/v) HNO3 and 5 mmol L Pb
(
and ) or 0.1% (m/v) HNO3, 5 mmol L Pb
)) by dissolution solution (0.1% (m/v) HNO3 (
−
1
and 50 mmol L chloride (ꢁ and
and ) or 0.1% (m/v) HNO3 and 50 mmol L chloride (ꢁ and
−
1
)).
from 1 to 100 mV s 1. Further, one can confirm that during this
time window the film was completely stripped off and there was
no kinetic problem associated with the film dissolution.
−
Subsequently, in order to evaluate the influence of chloride on
anodic lead dissolution, four lead deposits were prepared: two in
the absence ( and
) and two in the presence of chloride (ꢁ
); the films were then stripped off in the absence ( and
and presence of chloride (ꢁ and ). The deposition was per-
and
)
formed under potentiostatic conditions (−0.70 V), and when the
−
2
Fig. 3. (Ia) Dependence of frequency (ꢁfn) and dissipation (ꢁDn) at different over-
tones (n = 1, 3, 5, 7, 9, 11 and 13) as functions of time for lead films electrodeposited in
amount of mass deposited reached ca. 24 ␮g cm , the electrolyte
was replaced with a new Pb2+-free one, while maintaining the
potential at −0.70 V. During the solution exchange, no mass varia-
tion was observed and the cathodic current decreased to values less
−
1
2+
a resonant crystal quartz in a 0.1% (m/v) HNO3, 5 mmol L Pb solution in absence
−
of Cl according to the potential program showed in Fig. 3 (Ib): indicated by: (t1)
−
1
open circuit (0.40 V); (t2) −0.70 V; (t3) beginning of the potential sweep at 10 mV s
;
−
2
than 1 ␮A cm . Then, linear sweep voltammograms (LSV) exper-
iments were carried out at −0.70 V to 0.70 V in the absence and
(
(
(
t4) initial of lead film dissolution (−0.40 V); (t5) end of the potential sweep (0.70 V).
II) Relationship between deposition charge and mass deposited in solutions with 0
−1
−
−1
ꢀ), 25 ( ), 50 ( ), and 75 ( ) mmol L Cl . Lead films were electrodeposited in
presence of chloride (50 mmol L ). Fig. 5 shows the voltammo-
the same conditions as Fig. 3(Ia).
grams obtained (current was normalized by total deposited mass,
mdep) after dissolution of each deposit. From the obtained results,
two main conclusions can be drawn. At the same conditions of
deposition (absence of chloride, ( and ) and presence of chloride
(
ꢁ and )), the film oxidation takes place at less positive potential
values when chloride is present in the solution during the dissolu-
tion step (curves marked with black square (ꢁ) and up blue triangle
(
)), as a consequence of the formation of stable complexes with
+
1.59
chloride, i.e., PbCl (stability constant = 10
) [37]. Moreover, the
peak current in the presence of chloride (curves marked with black
square (ꢁ) and up blue triangle ( )) was much higher when com-
pared to the one in the chloride-free electrolyte (curves marked
with red circle ( ) and down green triangle ( )). This indicates
that the halide plays an important role in the stripping off of the
lead film, regardless of whether the deposition was performed in
the presence or absence of chloride. In addition, considering the
same dissolution solution (presence of chloride – curves marked
with black square (ꢁ) and up blue triangle ( ) and absence of chlo-
ride – curves marked with red circle ( ) and down green triangle
(
)), higher peak current values were always observed when the
deposition was performed in the presence of chloride (ꢁ and ),
as a result of the increase in the surface roughness.
Fig. 4. Mass change as a function of time after lead film electrodeposited under
potentiostatic conditions (Edep = −0.70 V) from a solution containing 0.1% (m/v)
The correlation between the anodic current during the disso-
lution of the lead film and dm/dt (i.e., the derivative of mass m
with respect to time t) was also examined. This was accomplished
−1
2+
HNO3 and 5 mmol L Pb in a resonant crystal quartz. The value of Qdep is −23 mC
−
1
(
A) and film dissolution was performed at 1 (B1), 20 (B2) and 100 (B3) mV s
.

M.O. Salles et al. / Electrochimica Acta 78 (2012) 347–352
351
was 133 ␮g cm 2, the charge expected for the dissolution process
−
−
2
should be 123.9 mC cm , assuming a two-electron process. How-
ever, in all cases, the total charge value obtained experimentally
was approximately 28% lower than expected. Gioda et al. have pro-
posed the formation of adsorbed Pb+ in HClO4 solutions and El Aal
et al. have also proposed the presence of the same ad-ion in HCl
medium [38,39]. Hence, the lower charge value obtained could be
attributed to the partial formation of Pb+ during the anodic disso-
lution of the film.
In addition, from Fig. 6(II), two linear regions with different
slopes are obtained in all the experiments, in the absence or pres-
−
2
ence of chloride. The first region (until approximately 100 ␮g cm
)
is related to the first voltammetric peak observed in Fig. 6(Ia),
and the charge/mass ratio values were found to be 646.2, 705.6,
−
1
6
15.6, and 708.7C g
for experiments performed in solutions
−
1
−
containing 0, 25, 50, and 75 mmol L Cl , respectively. The the-
2+
−1
oretical charge/mass ratio for the Pb/Pb couple is 931.3C g ,
and for the Pb/Pb+ couple this value is 465.6C g . However, all
charge/mass ratio values experimentally obtained were found to
be between these limits. Hence, at this first region, which cor-
−1
+
responds to the first oxidation peak, both reactions (Pb/Pb and
Pb/Pb2+) are likely to occur, independent of the concentration of
chloride. The second region is related to the second voltammetric
peak and the charge/mass ratio values were 845.6, 874.5, 997.0,
−
1
and 1117C g for solutions with different chloride concentrations
(
0, 25, 50, and 75 mmol L 1 respectively). Charge/mass values that
−
were higher than that expected for a two-electron process were
obtained in solutions containing greater chloride concentrations
−
1
(
50–75 mmol L ). A possible explanation involves the deposition
−
1
Fig. 6. (Ia) Linear sweep voltammograms (LSVs) recorded at 10 mV s in solutions
−
1
2+
of PbCl2 onto the electrode surface [39–41], as the concentration of
Pb(II) at the electrode surface is higher at more positive potentials.
Thus, the mass decrease due to lead oxidation is partially compen-
sated by the deposition of PbCl , resulting in the observed increase
in the charge/mass ratio.
containing 0.1% (m/v) HNO3 and 5 mmol L Pb and having chloride concentrations
of 0 (ꢀ), 25 ( ), 50 ( ), or 75 ( ) mmol L 1. Lead film deposition was performed
in a resonant crystal quartz under potentiostatic conditions (Edep = −0.70 V) with a
−
−2
mass cut off of 133 ␮g cm using the same solution that was used to obtain the
LSVs. (Ib) Derivative of mass as a function of potential, which was calculated from
the LSV. (II) Relationship between dissolution charge and mass loss in the presence
2
−
1
−
.
of 0 (ꢀ), 25 ( ), 50 ( ), or 75 ( ) mmol L Cl
by electrodeposition of lead films and subsequent linear sweep
voltammetry (LSV) performed in the same solution. Four solutions
were used in this experiment, each of them containing different
chloride concentrations (0, 25, 50, and 75 mmol L ). The deposi-
tion was carried out under potentiostatic conditions (−0.70 V) with
4. Conclusions
Lead stripping current increases with increasing chloride con-
centration of the electrodeposition solution, and this effect was
examined by employing voltammetry, SEM and EQCM-D. The pres-
ence of chloride in the electrodeposition solution contributes to
the increase in roughness of the lead film, which was confirmed
by SEM images. As a consequence, stripping current peaks change
significantly for films that are prepared with the same amount of
lead but that have different morphologies, as shown by the electro-
chemical crystal microbalance experiments. EQCM-D experimental
results showed that the amount of lead deposited on the gold elec-
trode depends slightly on the concentration of chloride. Similarly,
the electrodeposition efficiency (charge/mass ratio) is the same for
all tested chloride concentrations; however, the value obtained is
smaller than the theoretically calculated one, indicating that water
may be trapped in the pores of these rough lead films. The analy-
sis of current, mass changes, and charge dissolution during linear
sweep voltammetry experiments in the absence and presence of
chloride confirmed the influence of the halide on the dissolution
process. The increase in the halide concentration of the supporting
electrolyte solution results in a pronounced potential shift towards
less positive values, as a consequence of the thermodynamically
favored formation of complex species of lead ions and chloride.
Peak current values were much higher in the presence of chloride,
charge/mass ratio values indicate the presence of Pb(I), in addition
to Pb(II), during the anodic dissolution and the rate of lead disso-
lution is enhanced in the case of films with increased surface area.
The results shown in this work confirm that plating lead films
under differing experimental conditions, especially in the presence
−
1
−
−
2
controlled mass deposition (ca. 124 ␮g cm ) and the LSV was per-
1
formed from −0.70 V to 0.70 V at 10 mV s . During the LSV, an
increase in the mass due to the deposition of the lead film was
observed until the potential reached −0.40 V, and this resulted in
−
2
a total deposited mass of ca. 133 ␮g cm . Fig. 6(Ia) shows that the
anodic oxidation peaks changed with increasing chloride concen-
tration, despite the fact that the amount of electrodeposited lead
−
2
was always the same (ca. 133 ␮g cm ). In all cases, two anodic
peaks were noticed and these shifted to less positive potentials as
the concentration of chloride increased, as was observed in Fig. 5.
To confirm that both anodic peaks are associated with the oxida-
tion of lead, dm/dt values were calculated. Since m = q(M/nF) (where
q is the charge, M is the molar mass, n is the number of electrons
involved in the reaction, and F is the Faraday constant), the deriva-
tive of mass with respect to time is proportional to the current. The
profile of the dm/dt vs. E plots is similar to those of the voltammo-
grams, as seen in Fig. 6(Ib), so it can be concluded that both anodic
processes are related to the lead stripping. By analyzing changes
in dm/dt values, especially in the potential region from −0.70 to
−
0.20 V, an interesting observation is made: the dissolution rate
increases with the increase in the chloride concentration.
From this same set of experiments, it is also possible to analyze
the relationship between the anodic charge (obtained by integrat-
ing the current/time profile shown in Fig. 6(Ia)) and mass loss; this
relationship is shown in Fig. 6(II). Since the total deposited mass

3
52
M.O. Salles et al. / Electrochimica Acta 78 (2012) 347–352
of halides, results in the formation of films with different rough-
ness, and this, in turn, can result in significantly different stripping
currents. In such conditions (e.g., lead determination in seawater),
the use of the standard addition method is strongly recommended,
since the added spikes behave in a manner identical to that of the
sample analyte, thereby compensating for the influence of chloride.
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