Modulated Cr(III) oxidation in KOH solutions at a gold electrode: Competition between disproportionation and stepwise electron transfer

Article abstract of DOI:10.1016/j.electacta.2011.06.110

The electrochemical oxidation of aqueous Cr(III) was examined using cyclic voltammetry with a polycrystalline Au electrode in KOH solutions of varying pH and Cr(III) concentration. The mechanism and kinetics for the oxidation of Cr(III) is a quasi-reversible diffusion-controlled reaction and is largely dependent on the solution pH. The reaction mechanism is initiated by an irreversible electrochemical electron transfer to form Cr(IV) which is the rate-determining step (RDS). Following the RDS, subsequent oxidation of Cr to its hexavalent state occurs by the disproportionation of Cr(IV) at low KOH concentrations and electron transfer at high KOH concentrations due to the involvement of OH^- in the disproportionation reaction. As the solution pH increases, the Cr(III) oxidation peak potential shifts negatively owing to the involvement of OH^- in the RDS. The competitive adsorption of OH^- and CrO₂^- on the electrode surface also plays an important role in the oxidation behavior.

Full text of DOI:10.1016/j.electacta.2011.06.110

Electrochimica Acta 56 (2011) 8311–8318
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Modulated Cr(III) oxidation in KOH solutions at a gold electrode: Competition
between disproportionation and stepwise electron transfer
Wei Jina,b,c, Michael S. Moatsb, Shili Zhenga, Hao Dua,∗, Yi Zhanga, Jan D. Millerb,∗∗
a National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Zhongguancun Ber-2-Tiao
1, Beijing 100190, People’s Republic of China
b Department of Metallurgical Engineering, University of Utah, 135 S 1460 E, Rm 412, Salt Lake City, UT 84112, USA
c Graduate School, Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
The electrochemical oxidation of aqueous Cr(III) was examined using cyclic voltammetry with a polycrys-
talline Au electrode in KOH solutions of varying pH and Cr(III) concentration. The mechanism and kinetics
for the oxidation of Cr(III) is a quasi-reversible diffusion-controlled reaction and is largely dependent on
the solution pH. The reaction mechanism is initiated by an irreversible electrochemical electron transfer
to form Cr(IV) which is the rate-determining step (RDS). Following the RDS, subsequent oxidation of Cr
to its hexavalent state occurs by the disproportionation of Cr(IV) at low KOH concentrations and electron
transfer at high KOH concentrations due to the involvement of OH− in the disproportionation reaction. As
the solution pH increases, the Cr(III) oxidation peak potential shifts negatively owing to the involvement
Received 23 March 2011
Received in revised form 24 May 2011
Accepted 30 June 2011
Available online 7 July 2011
Keywords:
Cr(III) oxidation
KOH
Mechanism transition
pH dependent
Competitive adsorption
of OH− in the RDS. The competitive adsorption of OH− and CrO2 on the electrode surface also plays an
−
important role in the oxidation behavior.
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
The oxidation of Cr(III) compounds is strongly influenced by the
reaction media, especially pH [14]. In acidic media, and based on
Due to the wide application in geochemistry [1], environmen-
tal treatments [2] and industrial processes [3], considerable efforts
have been conducted on the redox cycling of chromium compounds
in soils and aquatic environments over recent years. Chromium is
mainly present in two stable oxidation states, trivalent and hex-
avalent [4]. Trivalent chromium is considered relatively harmless
and even a trace nutrient for mammals [5], but the low solubility of
Cr(III) compounds, especially oxides and hydroxides, are likely to be
the major limitation to their mobility in aquatic environments [6].
In contrast, hexavalent chromium compounds, which are very sol-
uble and consequently mobile in groundwater [3], are considered
to be extremely toxic and carcinogenic [7]. Most previous studies
have been focused on the reduction of Cr(VI) to Cr(III) owing to
its significant environmental concern [8–10]. Recently, due to the
importance in mineral processing, solid waste treatment and phar-
maceutical analysis, mechanistic investigations for the oxidation of
Cr(III) have gained particular impetus [11–13].
XPS analysis, Banerjee and Nesbitt [15] proposed the consecutive
three step mechanisms involving Cr(IV) and Cr(V) as intermedi-
ates during the oxidation of aqueous Cr3+ at bimessite (MnOOH)
surfaces. Silvester et al. [16] postulated that the origin of Cr(IV)
and Cr(V) compounds in the Cr(III)–MnO1.75 system was not only
controlled by reaction at the mineral surface, but also the dispro-
portionation of these intermediates in the aqueous phase. However,
under alkaline conditions, the mechanism for oxidation of Cr(III)
compounds appears to be significantly different. Zhao et al. [17]
investigated the aqueous Cr(III) oxidation by the hydroxyl radical
in NaOH solution and found that the formation of Cr(VI) is from the
disproportionation of a Cr(IV)-bearing dimer. Friese [18] discovered
that there is a mechanism transition for the Cr(III) oxidation by per-
oxydisulfate due to the media alkalinity, and the Cr(V) intermediate
could not be detected, possibly due to its very short lifetime. Obvi-
ously, the Cr(III) oxidation mechanism is substantially affected by
solution pH, and the reaction usually includes the intermediates
of Cr(IV) and Cr(V) in acidic solution, while in alkaline solution it
appears that only Cr(IV) is involved.
Besides, the pH-dependent Cr(III) speciation is closely related
to its oxidation behavior [19,20]. Although the chromium–water
system is very complex and some discrepancy still exists on the
Cr speciation, it is generally accepted that the transition between
different Cr species takes place in a narrow pH range [20]. Cr(III)
∗
Corresponding author. Tel.: +86 10 62520910; fax: +86 10 62520910.
Corresponding author. Tel.: +1 801 581 5160; fax: +1 801 581 4937.
E-mail addresses: duhao121@hotmail.com (H. Du), Jan.Miller@utah.edu
∗∗
(J.D. Miller).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.06.110

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W. Jin et al. / Electrochimica Acta 56 (2011) 8311–8318
and Cr(VI) are present as different forms depending on pH, and
these species possess different redox stabilities, leading to the
different oxidation nature of Cr(III) [20]. In addition, the Cr(III)
mobility is significantly affected by the environment, especially
the pH condition in both solid phase and aqueous solution, and
consequently the Cr(III) oxidation mechanisms vary [19,21,22].
Chinthamreddy and Reddy [23] attributed the low extent of Cr(III)
oxidation in manganese-enriched glacial till to the initial precipita-
tion of Cr(OH)3 due to the high pH of the soil. Skoluda [24] studied
the electrochemical oxidation of Cr(III) at different single crystal
gold electrodes, and identified that the electrode surface structure
effect originates from the competitive adsorption between OH− and
200
100
0
KOH
KOH+ Trivalent Cr
-100
-200
−
CrO2 at the electrode surfaces.
It is obvious that Cr(III) oxidation can be manipulated by adjust-
ing the pH of the reaction media. Welch et al. [25] investigated the
electrochemical oxidation of Cr(III) compounds (0.1–1 mM) in KOH
solution (0.1 and 1.0 M) using a polycrystalline Au electrode and
discovered the involvement of OH− ions in this multi-step reaction.
It is proposed that the pathway for the electrochemical oxidation
of Cr(III) is composed of the first rate-determining electron trans-
fer and subsequent diproportionation or further two fast electron
transfer reactions as follows:
-1.5
-1.0
-0.5
0.0
0.5
Potential/V vs. SCE
Fig. 1. Cyclic voltammograms of gold in the presence (red) and absence (black) of
0.5 mM Cr(III) in 0.1 M KOH solution at a gold electrode, scan rate = 200 mV/s. (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
3. Results and discussion
CrO2− + OH− → e− + H+ + CrO3
(1)
(2)
(3)
2−
3.1. Identification of the chemical step in the electro-oxidation of
Cr(III)
CrO32− → e− + CrO3
−
CrO3− + OH− → e− + H+ + CrO4
2−
The electrochemical oxidation of Cr(III) was investigated at a
polycrystalline gold electrode in KOH solutions in an electrochem-
ically active region (pH > 12) [25]. The trivalent Cr occurs as CrO2
However, due to the complexity of this system, the mechanism
and kinetics of Cr(III) electro-oxidation in alkaline solution are still
not fully understood. Therefore, in order to obtain further infor-
mation regarding the oxidation behavior and elucidate the effect
of media pH on the Cr(III) oxidation mechanism, the electrochemi-
cal oxidation behavior of Cr(III) compounds in KOH solutions with
concentration ranging from 0.1 M to 3.0 M was investigated in this
study by means of cyclic voltammetry.
−
in this pH region [26]. Fig. 1 shows the cycle voltammetry (CV)
response of 0.50 mM Cr(III) and its corresponding blank curve in
0.1 M KOH solution at 200 mV/s. As can be seen in the blank solu-
tion curve, the formation of gold(I) oxide and gold(III) oxide are
observed at −0.08 V and 0.35 V, respectively, and their correspond-
ing reduction peaks emerge at −0.18 V and 0.06 V, respectively [27].
Moreover, formation of the Au(OH)− species, which is the surface
active species for Cr(III) oxidation, occurs over a wide potential
range via different pathways as follows [25]:
2. Experimental
2.1. Chemicals and solutions
−0.44 V < E < −0.04 V, Au + OH− → Au(OH)a−ds
E > −0.04 V, Au + OH− − e− ↔ Au(OH)a−ds
(4)
(5)
KOH pellets (p.a. grade, Merck) and Cr(NO3)3 (99.99%,
Sigma–Aldrich) were used without further purification. All solu-
tions were prepared from Millipore Milli-Q water (18 Mꢀ cm).
Before each experiment, the test solution was degassed with ultra-
pure nitrogen and then maintained under a nitrogen atmosphere.
Clearly, the later reaction via electron transfer occurs around the
same position as formation of the gold(III) oxide reaction and thus
experimentally appears as a pair of single peaks. In the presence
of trivalent chromium, the oxidation peak at 0.08 V is attributed
to the Cr(III) oxidation, and there is no corresponding reduction
peak, indicating a pseudo-irreversible reaction. It should be noted
that the increase in the oxidation peak at 0.35 V stems from the
“tail” effect of the Cr(III) oxidation peak and not increased gold
oxide formation. The peak current for the gold surface reactions
decreases due to the passivation effect of chromium [28]. The pas-
sivating effect is observed by the decreased reduction peak current
in the presence of chromium at 0.06 V.
The effect of the Cr(III) concentration on oxidation behavior was
measured in a wide range of 0.15–3.00 mM and illustrated in Fig. 2a.
The increase of peak current is linearly proportional to the Cr(III)
concentration in the range of 0.15–1.10 mM as shown in Fig. 2b and
the peak potential of Cr(III) remains constant at 0.08 V in this region,
indicating first order kinetics which is consistent with results from
the previous study [25] for the region between 0.1 mM and 1 mM.
However, at Cr(III) concentrations above 1.80 mM, the peak poten-
tial (Epa) for Cr(III) oxidation monotonically shifts positively and the
corresponding peak current appears below the first order relation-
ship with respect to the Cr(III) concentration as shown in Fig. 2b.
2.2. Instrumentation
Experiments were conducted in a standard 150 mL three-
electrode electrochemical cell at 25 0.2 ◦C with an electro-
chemical workstation (PCI4G750, Gamry) by employing cyclic
voltammetry (CV). A gold (99.99% purity, polycrystalline, 0.23 cm2
effective area) cylinder electrode was used as the working elec-
trode. A large-area Pt foil served as the counter electrode. All
potentials were measured and reported vs. a saturated calomel
electrode (SCE).
Prior to each measurement, the working electrode was pol-
ished with decreasing grades of alumina (1.0–0.3 ␮m) to achieve
a mirror-like surface, then it was sonicated and washed in purified
water to remove traces of alumina. After that, the working electrode
was electrochemically pretreated by cycling between the onset
potential of H2 and O2 evolution reactions in the corresponding
KOH solution until reproducible and well-defined cyclic voltam-
metry curves were obtained and the final potential was stopped at
the onset of H2 evolution.

W. Jin et al. / Electrochimica Acta 56 (2011) 8311–8318
8313
a
0.50 mM
1.10 mM
2.00 mM
3.00 mM
600
400
200
0
-200
-1.5
-1.0
-0.5
0.0
0.5
Potential/V vs. SCE
b
c
0.07
800
600
400
200
0.06
0.05
0.04
0.03
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
-0.2
0.0
0.2
0.4
C(Trivalent Cr)/mM
log(Cr/mM)
Fig. 2. The effect of Cr(III) concentration on the oxidation behavior in 0.1 M KOH solution at a gold electrode: (a) comparison of the CV curves at 0.50, 1.10, 2.00 and
3.00 mM Cr(III) concentration, (b) a plot of the oxidation peak current vs. Cr(III) concentration and (c) variation of the peak current ratio (ipc/ipa) as a function of log(Cr). Scan
rate = 200 mV/s.
As the Cr(III) concentration increases, the peak current for gold
surface reactions decreases due to passivation. Interestingly, as the
Cr(III) concentration increases to 1.10 mM, the reduction peak at
0.06 V breaks into two peaks at 0.05 V and 0.12 V, respectively.
With a further increase in Cr(III), the reduction peak at 0.05 V
decreases more significantly than the reduction peak at 0.12 V. It
is believed this phenomenon−stems from the competitive adsorp-
at different scan rates. These are illustrated in Fig. 3a. The peak
current was found to be linear with the square root of scan rate
(25–400 mV/s) as shown in Fig. 3b, suggesting kinetics are diffusion
controlled. This behavior is also observed in other KOH solutions in
this study.
The initial part of the Cr(III) oxidation curve at a scan rate of
200 mV/s is presented in Fig. 3c. The oxidation exhibits a good Tafel
behavior and as such it can be used to gain information about the
rate-determining step (RDS) by calculating the Tafel slope. A slope
of 153 mV decade−1 is observed (Ea = 0.153 log(ia) + 0.109, R2 = 0.99,
27 points).
tion between OH− and CrO2 [24]. As CrO2 increases, the OH−
−
bearing surface reactions are greatly inhibited and consequently
the previous one reduction peak breaks into two peaks. The peak
at 0.05 V is for the OH− desorption reaction and the other peak at
−
0.12 V corresponds to the gold(III) oxide reduction. The OHads is
From the Tafel slope, the transfer coefficient can be calculated
from:
an important reaction intermediate for Cr(III) oxidation [25], thus
with the further increase of CrO2−, its blocking effect leads to the
shifts noted in peak potential (Epa) and peak current (ipa) for Cr(III)
oxidation.
2.303 RT
(1 − ˛)F
b =
(6)
Further, as the Cr(III) concentration increases, another new
reduction peak emerges at −0.75 V; this peak is attributed to the
corresponding Cr(VI) reduction [29]. As shown in Fig. 2c, there is
a linear relationship between the peak current ratio (ipc/ipa) for
the Cr(III)/(VI) couple and the log(Cr). This Cr(III) concentration-
dependent irreversibility suggests the involvement of a chemical
step. Moreover, the existence of the Cr(VI) reduction peak indicates
that the chemical reaction is following the irreversible electron
transfer mentioned above, which will be further discussed in the
following sections.
where b is the slope of the potential against log(i), ˛ the transfer
coefficient, R is the gas constant, T is temperature and F is Fara-
day’s constant. From the slope, a transfer coefficient of 0.62 was
calculated. This value suggests the rate-determining step is the
first electrochemically irreversible electron transfer rather than the
coupled chemical step mentioned above, which is consistent with
a previous study [25].
For further mechanistic insight, the oxidation peak potential
was plotted vs. the log of the scan rate in Fig. 3d. According to
the multistep electron transfer theory [30], the slope from such
a plot can be used to determine the reaction mechanism. A reac-
tion involving no chemical reaction and where the RDS is the
In order to gain more insight on the mechanism and kinetics,
CV curves of 1.5 mM Cr(III) in 0.1 M KOH solution were determined

8314
W. Jin et al. / Electrochimica Acta 56 (2011) 8311–8318
a
b
50 mV/s
100 mV/s
200 mV/s
300 mV/s
400
200
0
600
450
300
150
0
-200
0.15
0.30
0.45
0.60
-1.5
-1.0
-0.5
0.0
0.5
v1/2/ (V s-1)1/2
Potential/V vs. SCE
c
d
0.084
0.04
0.02
0.00
0.078
0.072
0.066
0.060
-0.7
-0.6
log (ia/ mA)
-0.5
-0.4
-1.5
-1.2
-0.9
-0.6
-0.3
log (v/ V s-1)
Fig. 3. Cr(III) oxidation behavior as a function of the scan rate in 1.5 mM Cr(III), 0.1 M KOH solution at a gold electrode: (a) cyclic voltammograms with increasing scan rates
(50–300 mV/s), (b) variation of the Cr(III) oxidation peak current as a function of the square root of the scan rate (25–400 mV/s), (c) Tafel slope of the Cr(III) oxidation at a
scan rate of 200 mV/s and (d) a plot of anodic peak potential Epa vs. log v (25–400 mV/s).
first electron transfer, the Ep would positively shift (for an oxi-
dation process) by 30/␣ mV decade−1 of scan rate. While slopes
of 60/n or 30/n mV decade−1 of scan rate (n is the number of
the electron transfer in successive steps) are expected for charge
transfer followed by a reversible or irreversible chemical reac-
tion, respectively. Experimentally, a slope of 19 mV was obtained
(Epa = 0.019 log(v) + 0.09, R2 = 0.98, 7 points) which is very close to
the 20 mV expected for a reaction involving charge transfer fol-
lowed by a reversible chemical reaction for this three-electron
process.
3.2. The effect of pH on the electro-oxidation behavior
In the previous study [25], the oxidation behavior was investi-
gated in 0.1 M and 1.0 M KOH solutions to determine the number
of protons in the rate-determining step as illustrated in Eq. (1).
In order to obtain the effect of pH on the electro-oxidation mech-
anisms and kinetics of Cr(III) in KOH solutions, the CV curves in
different concentrations of KOH (0.1–3.0 M) were recorded. The
Cr(III) concentration was set to 0.5 mM to eliminate the influence of
competitive adsorption mention previously. As can be seen in Fig. 4,
with increasing KOH concentration, the Cr(III) oxidation gradually
Consequently, based upon the identification of reaction steps in
this and previous studies [16,25], the Cr(III) oxidation mechanism
in 0.1 M KOH solution is as follows:
0.1 M
1.0 M
3.0 M
CrO2− + OH− → e− + H+ + CrO3
2CrIV ↔ CrIII + CrV
(3ꢀ)
(7)
(8)
2−
200
100
0
CrIV + CrV ↔ CrIII + CrVI
from which Eqs. (7) and (8) are generally taken to give an overall
reaction as follows:
-100
-200
3CrIV ↔ 2CrIII + CrVI
(9)
The electro-oxidation is initiated by an electrochemically irre-
versible reaction which is the rate-determining step with first order
kinetics, followed by the relatively fast disproportionation reaction
which leads to the formation of Cr(VI). Its corresponding reduction,
including the influence of pH, will be discussed in the following
section.
-1.5
-1.0
-0.5
0.0
0.5
1.0
Potential/V vs. SCE
Fig. 4. The effect of KOH concentration (0.1, 1.0 and 3.0 M) on cyclic voltam-
mograms measured in 0.5 mM Cr(III) using a polycrystalline gold electrode. Scan
rate = 200 mV/s.

W. Jin et al. / Electrochimica Acta 56 (2011) 8311–8318
8315
transfers from a pseudo-irreversible reaction to a quasi-reversible
reaction with a corresponding reduction peak, although the large
peak separation suggests a relatively slow electron transfer. Obvi-
ously, the Cr(III) oxidation behavior is strongly affected by pH, and
the disproportionation step may be replaced by further electron
transfer.
than ones at low pH, indicating the pH effect on both the reaction
thermodynamics and kinetics.
It is worth noting that with increasing alkalinity, the diffusion
properties of electroactive species are suppressed due to increas-
ing viscosity, and thus the diffusion controlled peak current should
be reduced [32]. Therefore, the increases in reversibility and peak
current suggest a mechanism transition probably has occurred due
to increasing pH.
The dependence of diagnostic parameters on pH are summa-
rized in Table 1 and illustrated in Fig. 5. The pH values are obtained
by employing the activity coefficient data of KOH solutions [31].
It should be noted that the difference of peak currents in different
KOH solutions are relatively small, thus the scan rate was set at
200 mV/s to better examine the experimental data. It can be seen
that with increasing pH, the Cr(III) oxidation peak current increases
and the peak separation decreases. This indicates the facilitating
effect of pH on reaction kinetics possibly due to a transition in the
reduction mechanism. More importantly, as the pH increases, the
peak potential for Cr(III) oxidation monotonically shifts negatively
and there is a linear relationship between the Epa and calculated pH
(Epa = − 0.083 × pHcal + 1.207, R2 = 0.99, 10 points), suggesting the
involvement of hydroxide ion in the reaction. The number of OH−
ions participating in the RDS can be determined using the following
equation:
To examine the influence of pH on the chemical step, the
dependence of the peak current ratio on the Cr(III) concentration
is presented for 0.1 M, 2.0 M and 3.0 M KOH solutions as shown
in Fig. 6c. The ipc/ipa at different KOH solutions exhibit a linear
relationship with the log(Cr) for 0.1 M and 2.0 M KOH solutions,
suggesting the existence of a disproportionation reaction. However,
the peak current ratio for the 2.0 M KOH solution is much higher
than the corresponding ratio for the 0.1 M solution, and the slope of
ipc/ipa against log(Cr) for the 2.0 M solution is lower. These results
indicate the inhibition of disproportionation at high pH. With a fur-
ther increase in the pH, the value of ipc/ipa increases to 0.144 in 3.0 M
KOH and is independent of Cr(III) concentration, suggesting the
elimination of disproportionation as part of the electro-oxidation
mechanism, which will be further discussed in the following sec-
tion.
∂Epa
2.303 mRT
According to previous studies [33–36], as the pH increases, the
disproportionation of Cr(IV) is hindered, and the Cr(V) intermediate
becomes increasingly difficult to be detected. Therefore, the dispro-
portionation reactions involved in the electro-oxidation of Cr(III) in
KOH solutions can be expressed as follows:
=
(10)
˛F
∂pHcal
where m is the number of OH− ions to be estimated. It is worth
noting that it is nearly constant at 0.62 in a wide pH region, but as
the pH increases above 1 M KOH, ˛ significantly increases to 0.75,
suggesting a change in reaction mechanism probably has occurred.
From this analysis, the number of OH− ions involved in the RDS was
determined to be 1, which agrees well with the proposed scheme
shown in Eq. (1).
2CrO32− + H2O ↔ CrO2− + CrO3− + 2OH−
(11)
(12)
2−
CrO32− + CrO3− ↔ CrO2− + CrO4
which can be given in an overall reaction as follows:
The peak current ratio (ipc/ipa) is seen to increase with an
increase in pH, suggesting the chemical step is dependent on the
media pH, which will be further discussed in the following section.
Consequently, there appears to be a pH-dependent mechanism
transition, which leads to an increase in reversibility.
3CrO32− + H2O ↔ 2CrO2− + CrO42− + 2OH−
(13)
With an increase in the solution pH, the Cr(IV) disproportion-
ation is suppressed due to the involvement of OH−, and the Cr(V)
disproportionation is relatively fast, leading to a short lifetime. Con-
sequently, the mechanism of Cr(III) electro-oxidation ranging from
0.1 M to 2.0 M KOH solutions is an electrochemically irreversible
electron transfer followed by the pH-dependent Cr(IV) dispropor-
tionation as shown in Eqs. (1) and (13).
3.3. Inhibition of the disproportionation step
The Cr(III) concentration-dependent oxidation behavior in 2.0 M
KOH solution was measured and is presented in Fig. 6 to elucidate
the pH effect on the disproportionation step. As compared to the CV
scans in 0.1 M KOH, the OH− desorption peak at 0.02 V appears at
a higher Cr(III) concentration in Fig. 6a, further confirming that the
appearance of this reduction peak reveals the competitive adsorp-
tion between OH− and CrO2−. Consequently, the reaction exhibits
first order kinetics in a relatively wider Cr(III) concentration region
as shown in Fig. 6b, than at the lower KOH concentration. Well-
defined Cr(VI) reduction peaks are also observed. There is still a
large peak separation and the peak current for oxidation is larger
3.4. Diagnosis of the oxidation mechanism in concentrated KOH
solutions
As mentioned previously, at high pH, the disproportionation
step is hindered and the transfer coefficient significantly increases.
This indicates a transition in the reaction mechanism at very high
pH. Consequently, the mechanistic parameters for the Cr(III) elec-
trochemical oxidation in 3.0 M KOH solution as a function of Cr(III)
concentration and scan rate were determined. The peak current
Table 1
The pH-dependent diagnostic parameters of 0.5 mM Cr(III) electro-oxidation in KOH solution at a gold electrode, scan rate = 200 mV/s.
a
CKOH/M
pHcal
Epa/V
ipa/␮A
Epc/V
ipc/␮A
ꢁEp/V
ipc/ipa
˛
0.1
0.2
0.3
0.5
0.8
1.0
1.5
2.0
2.5
3.0
12.90
13.18
13.34
13.56
13.77
13.87
14.09
14.25
14.39
14.51
0.085
0.060
0.051
0.031
0.010
157
163
170
177
186
195
204
207
210
212
–
–
–
–
–
–
–
–
–
–
–
–
–
0.62
0.63
0.61
0.62
0.63
0.63
0.66
0.68
0.72
0.75
Broad
Broad
−0.600
−0.602
−0.610
−0.612
−0.614
−0.616
–
19
21
23
25
29
31
0.610
0.607
0.594
0.584
0.575
0.570
0.105
0.110
0.115
0.120
0.138
0.144
0.005
−0.016
−0.028
−0.039
−0.046
a
The pH values were calculated using the activity coefficient data from Robinson [31].

8316
W. Jin et al. / Electrochimica Acta 56 (2011) 8311–8318
220
200
180
160
b
a
0.09
0.06
0.03
0.00
-0.03
-0.06
12.8
13.2
13.6
14.0
14.4
14.8
0.0 0.5 1.0 1.5 2.0 2.5 3.0
C(KOH)/ (mol L-1)
calculated pH
0.15
c
0.14
0.13
0.12
0.11
0.10
13.8
14.0
14.2
14.4
14.6
calculated pH
Fig. 5. The dependence of (a) Cr(III) oxidation peak current on KOH concentration, (b) oxidation peak potential and (c) peak current ratio on the pH during the 0.5 mM Cr(III)
electro-oxidation in KOH solutions at a gold electrode, scan rate = 200 mV/s.
a
800
600
400
200
0
0.50 mM
1.10 mM
2.00 mM
3.00 mM
-200
-1.5
-1.0
-0.5
0.0
0.5
Potential/V vs. SCE
0.1 M
2.0 M
3.0 M
c
b
800
600
400
200
0
0.16
0.12
0.08
0.04
0.0 0.5 1.0 1.5 2.0 2.5 3.0
C(Trivalent Cr)/ mM
-0.9
-0.6
-0.3
0.0
0.3
0.6
log(Cr/ mM)
Fig. 6. Electrochemical behavior of Cr as a function of Cr(III) concentration in 2.0 M KOH solutions at a gold electrode: (a) comparison of CV curves at 0.50, 1.10, 2.00 and
3.00 mM Cr(III), (b) variation of oxidation peak current as a function of Cr(III) concentration and (c) variation of peak current ratio (ipc/ipa) as a function of log (Cr) in 0.1 M,
2.0 M and 3.0 M KOH solutions. Scan rate = 200 mV/s.

W. Jin et al. / Electrochimica Acta 56 (2011) 8311–8318
8317
1000
900
800
700
600
500
transfer is the rate-determining step. This is consistent with the
previous calculated results as shown in expressions (1)–(3) [25].
Finally, the disproportionation step for Cr(III) electro-oxidation
in 3.0 M KOH solution was eliminated due to the high pH. This pro-
cess is determined to be a stepwise three-electron-transfer process
in which the first electron transfer is the RDS and accounts for the
pH-dependent peak current increase and the favored reversibility
mentioned above.
4. Conclusions
In this paper, the oxidation of Cr(III)at a polycrystallinegold elec-
trode has been investigated over a wide range of KOH (0.1–3.0 M)
and Cr(III) (0.5–3.0 mM) concentrations using cyclic voltammetry.
It is observed that the oxidation behavior is substantially affected
by the media pH and there is a pH-modulated mechanism transi-
tion from an EC (electron transfer – disproportionation) reaction
at low KOH concentrations to the stepwise three-electron-transfer
process at high KOH concentrations.
0.0
0.5
1.0
1.5
2.0
v/ V s-1
Fig. 7. The dependence of peak current function on the scan rate (25 mV/s to 2 V/s)
of 1.5 mM Cr(III) electro-oxidation in 3.0 M KOH solutions at a gold electrode.
The electro-oxidation of Cr(III) is attributed to a quasi-reversible
diffusion-controlled reaction. It is initiated by an electrochemi-
cal irreversible electron transfer which is the rate-determining
step, and it is followed by the reversible disproportionation of
Cr(IV) in dilute KOH solution. However, due to the involvement
of OH− in the disproportionation reaction, the mechanism trans-
fers to a stepwise three-electron-transfer process in which the
first irreversible electron transfer is followed by fast two-electron
transfers in concentrated KOH solution. This change in mechanism
favors the oxidation thermodynamics and kinetics. As the solution
pH increases, the Cr(III) oxidation peak potential shifts negatively
owing to the involvement of OH−− in the RDS. The competitive
adsorption between OH− and CrO2 on the electrode surface also
plays an important role in the oxidation behavior.
-0.03
-0.04
-0.05
-0.06
-0.07
-0.08
-1.5
-1.2
-0.9
-0.6
-0.3
log (v/ V s-1)
In summary, it is concluded that the Cr(III) electrochemical oxi-
dation is significantly affected by KOH concentration. This pH effect,
which influences the oxidation mechanism and kinetics, arises
from the diffusion properties of Cr(III) as affected by pH and the
involvement of the OH− ion in the reaction steps.
Fig. 8. The plot of anodic peak potential against log scan rate (25–400 mV/s) for the
1.5 mM Cr(III) electro-oxidation in 3.0 M KOH solutions at a gold electrode.
ratio of ipc/ipa (scan rate = 200 mV/s) is constant at 0.144 ranging
from 0.1 mM to 3.0 mM Cr(III), suggesting the elimination of the dis-
proportionation reaction in Fig. 6c. Characterization of the chemical
reaction coupled to charge transfers [37] can be determined from
the dependence of ip/(v1/2) on v which is illustrated in Fig. 7. Clearly,
over the entire range of scan rates, the quantity of peak current
function is a constant value, indicating the system at 3.0 M KOH
involves only multistep charge transfers.
Obviously, the trivalent Cr electro-oxidation at this pH (3.0 M
KOH) involves a multi-electron-transfer reaction. In considering
the reduction sequence for this oxidation, we assume that the three
electrons are transferred in sequential one-electron steps (an EEE
mechanism) rather than in a simultaneous multi-electron step,
since the former nature is generally assumed [38].
Acknowledgements
Financial support from the National Basic Research Devel-
opment Program of China (973 Program) under Grant No.
2007CB613501, National 863 Project of China under Grant No.
2009AA064003, National Natural Science Foundation of China
under Grant No. 51090382 are gratefully acknowledged. This
international collaboration was also carried out through financial
support of Dr. Wei Jin from the China Scholarship Council.
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