A sensitive and environmentally friendly method for determination of chemical oxygen demand using NiCu alloy electrode

Article abstract of DOI:10.1016/j.electacta.2012.04.048

A simple, sensitive and environmentally friendly method was developed for determination of chemical oxygen demand (COD) by cyclic voltammetry using nickel-copper (NiCu) alloy electrode. The structure and the electrochemical behavior of NiCu alloy electrode were investigated by atomic force microscope, energy dispersive X-ray spectrometer, and cyclic voltammetry, respectively. The results indicated that NiCu alloy film with high quality was stably modified on the surface of glass carbon (GC) electrode, which could effectively oxidize a wide range of organic compounds. Subsequently, the parameters affecting the analytical performance were investigated, including pH, dissolved oxygen and concentration of chloride ion. Under optimized conditions, the linear range was 10-1533 mg L^-1 and the detection limit was 1.0 mg L^-1. The results obtained from the proposed method were linearly correlated to those by the classic dichromate method (r = 0.9978, p < 0.01, n = 13). Finally, the validated method was used to determine the COD values of surface water, reclaimed water and wastewater. It was shown that the proposed method had an excellent practical perspective on determination of COD in water quality control and pollution evaluation.

Full text of DOI:10.1016/j.electacta.2012.04.048

Electrochimica Acta 74 (2012) 165–170
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
A sensitive and environmentally friendly method for determination of chemical
oxygen demand using NiCu alloy electrode
∗
∗∗
Yusun Zhou, Tao Jing , Qiaolin Hao, Yikai Zhou, Surong Mei
MOE Key Laboratory of Environment & Health, Institute of Environmental Medicine, School of Public Health, Tongji Medical College, Huazhong University of Science and Technology,
13 Hangkong Road, Wuhan, Hubei 430030, 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:
A simple, sensitive and environmentally friendly method was developed for determination of chemical
oxygen demand (COD) by cyclic voltammetry using nickel–copper (NiCu) alloy electrode. The structure
and the electrochemical behavior of NiCu alloy electrode were investigated by atomic force microscope,
energy dispersive X-ray spectrometer, and cyclic voltammetry, respectively. The results indicated that
NiCu alloy film with high quality was stably modified on the surface of glass carbon (GC) electrode, which
could effectively oxidize a wide range of organic compounds. Subsequently, the parameters affecting the
analytical performance were investigated, including pH, dissolved oxygen and concentration of chlo-
Received 16 November 2011
Received in revised form 12 April 2012
Accepted 14 April 2012
Available online 21 April 2012
Keywords:
Chemical oxygen demand (COD)
NiCu alloy electrode
Cyclic voltammogram
Water samples
−1
ride ion. Under optimized conditions, the linear range was 10–1533 mg L and the detection limit was
−1
1
.0 mg L . The results obtained from the proposed method were linearly correlated to those by the classic
dichromate method (r = 0.9978, p < 0.01, n = 13). Finally, the validated method was used to determine the
COD values of surface water, reclaimed water and wastewater. It was shown that the proposed method
had an excellent practical perspective on determination of COD in water quality control and pollution
evaluation.
©
2012 Elsevier Ltd. All rights reserved.
1
. Introduction
In recent years, developing of rapid and environmentally
friendly methods for the determination of COD has attracted more
attention. Most of these new methods were based on electrocataly-
sis (EC) using AgO/CuO [2], PbO [3,4], Rh O /Ti [5], Cu/CuO [6], and
boron-doped diamond (BDD) [7] electrodes. Although these meth-
ods have many advantages over the traditional COD method such as
the rapid analysis, directness of acquisition of analytical signals and
possibility of incorporation into online monitoring, they are still far
from perfect. The main reason is its incapacity for effectively oxi-
dizing a wide spectrum of organic compounds. Furthermore, the
risk of releasing toxic heavy metal (Pb) is unavoidable and the high
preparing cost of BDD is not overlooked.
Nickel (Ni) is a versatile and environmentally friendly electrode
material, which has been widely studied in the fields of electro-
chemical water treatment and electrochemical analysis. In alkaline
media, it can act as the electrocatalyst for oxidization of organic
compounds, such as drug [8], amino acids [9], urea [10], cyclohex-
anol [11], methanol [12], ethanol [13], methane [14] and glycine
Chemical oxygen demand (COD) is defined as the number of
oxygen equivalents consumed in the oxidation of organic com-
pounds using strong oxidizing agents [1]. COD is mainly used to
assess organic pollution in aqueous systems and is one of the most
important parameters in water monitoring. In many countries, such
as China and Japan, it has been accepted as a national standard for
organic pollution evaluation. The traditional method estimates COD
via the oxidative degradation of organic matter present in water
and wastewater with strong oxidizing agents such as dichromate
or permanganate. Unfortunately, this method involves several dis-
advantages, such as a large sample volume, a high consumption of
expensive (e.g. Ag SO ), corrosive (concentrated H SO ) and toxic
2
2
3
2
4
2
4
(
HgSO4 and Cr(VI)) chemicals and a time-consuming reflux pro-
cess (2–4 h). Additionally, high probability of errors is unavoidable
due to the tedious experimental procedures and low sensitivity.
Thus, numerous efforts have been made to overcome these disad-
vantages, and to develop simple and rapid analytical method for
determination of COD.
[15] on the basis of a Ni(OH) /NiOOH redox couple. Furthermore,
2
Ni and copper (Cu) metals have the same face centered cubic struc-
ture with similar lattice parameters, which makes it possible to
have a wide range of compositions for NiCu alloys [16]. Compared
with single metal and mixed-metal film electrode, alloy electrode
has the advantage of high electrocatalytic activity, easy preparation
and long-term stability [17]. Furthermore, the addition of Cu to the
Ni(OH)2/NiOOH redox couple represents a very efficient strategy
∗
Corresponding author. Tel.: +86 27 83657849.
Corresponding author. Tel.: +86 27 83657849; fax: +86 27 836577.
E-mail addresses: jingtao1982@126.com (T. Jing), meisurong@yahoo.com.cn
∗
∗
(
S. Mei).
0
013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2012.04.048

1
66
Y. Zhou et al. / Electrochimica Acta 74 (2012) 165–170
for suppressing the formation of ␥-NiOOH phase and promoting
the formation of ␤-NiOOH phase. ␤-NiOOH was expected to be
an excellent electroactive material for oxidization of organic com-
pounds in alkaline solution [16–18]. Thus, NiCu alloy has great
promise as an ideal electrode material to determine COD values
in real water samples.
All experiments of COD determination were performed at room
temperature.
3. Results and discussion
3.1. Preparation of NiCu alloy electrode
Although several literatures had demonstrated that NiCu alloy
was an excellent electrode materials for the glucose oxidation
Based on the literature [21], the nano-Ni sensing film was elec-
trodeposited on the surface of GC electrode under potentiostatic
conditions in mixed solution of NiSO ·6H O and NiCl ·6H O. In this
[
16,19], to the best of our knowledge, this study was the first
attempt for COD detection using NiCu alloys electrode. Subse-
quently, a simple, sensitive and environmentally friendly method
was developed for determination of COD values in surface water,
reclaimed water and wastewater.
4
2
2
2
study, CuSO ·5H O was added to prepare the NiCu alloy electrode
4
2
and the amount was then investigated. The results indicated that
NiCu alloy film with the high electrocatalytic activity was received
−
1
when 0.5 mol L CuSO ·5H O was added. Increasing the amount
4
2
2
. Experimental
of CuSO ·5H O would not enhance the electrocatalytic activity of
4
2
NiCu alloy film. Therefore, the optimized amount of Cu was fixed
−
1
2.1. Materials and sample preparation
at 0.5 mol L
.
The surface morphology of the electrodeposited Ni and NiCu
alloy film was investigated by atomic force microscope as displayed
in Fig. 1A and B. The NiCu alloy film appeared to be continuous
and uniform over the entire substrate surface without any notice-
able cracks, which could ensure high electrochemical stability of
alloy electrode. The average diameter of NiCu alloy particles was
about 50.1 nm, which was larger than that of Ni particles (22.4 nm),
and the average thickness of NiCu alloy film was about 4.2 nm. The
results of chemical composition analysis obtained by EDX method
revealed that the composite contains of Ni (69 wt%) and Cu (31 wt%,
Fig. 1C). It was demonstrated that NiCu alloy film with high quality
was electrodeposited on the surface of GC electrode.
Eight organic compounds were used to prepare standard sam-
ples with known COD values and synthetic samples with unknown
COD values: phenol, lactose, citric acid, aniline, ethanol, glucose,
glycine and pyrrole. Potassium hydrogen phthalate (KHP) as cal-
ibration standard sample was used for classical COD analysis. All
these reagents, nickelous sulfate (NiSO ·6H O), nickelous chlo-
4
2
ride (NiCl ·6H O), copper sulfate (CuSO ·5H O) and boric acid
2
2
4
2
(
H BO ) were purchased from Shanghai Chemical Reagent Com-
3 3
pany (Shanghai, China) and used without further purification.
Ultrapure water was obtained from a Milli-R04 purification system
Millipore, Germany).
(
2.2. Preparation of NiCu alloy electrode and Ni electrode
3.2. Electrochemical behavior of NiCu alloy electrode
The glassy carbon (GC) electrode (3.0 mm diameter) was further
To investigate the electrochemical performance of the NiCu
polished with 0.05-mm alumina powder on a polishing microcloth
and rinsed thoroughly with doubly distilled water prior to modifi-
cation. The NiCu alloy film was electrodeposited on the surface of
alloy electrode, cyclic voltammetry was employed over a poten-
tial range from +0.1 to +0.6 V. Fig. 2 shows the consecutive cyclic
voltammograms of the bare GC electrode and the NiCu alloy elec-
−
1
GC electrode under −0.9 V for 25 min in mixed solution of 1 mol L
−1
trode in 0.1 mol L
NaOH solution. There was no reaction peak
−
1
−1
NiSO ·6H O, 0.2 mol L NiCl ·6H O, 0.5 mol L CuSO ·5H O and
4
2
1
2
2
4
2
current observed for the bare GC electrode at the electrochemical
window. However, in the first sweep, a pair of well-defined redox
peaks at 0.46 and 0.35 V vs. SCE were observed for the NiCu alloy
electrode, which were assigned to the Ni(II)/Ni(III) redox couple
according to:
−
.5 mol L H BO , the pH value of which was adjusted to 3 with
3 3
mol L H SO . Subsequently, the resulting NiCu alloy modified
0
1
−
1
2
4
GC electrode was rinsed with re-distilled water to remove any
adsorbed species. Ni modified electrode was prepared under identi-
cal conditions except that there was no CuSO ·5H O present during
4
2
Ni + 2OH− − 2e− → Ni(OH)2
(A.1)
(A.2)
deposition. The surface morphology of the deposit was evaluated
by atomic force microscopy analyses (DI NanoScope IV AFM, Veeco
Co. Ltd., USA). The chemical composition of the deposit was evalu-
ated by the energy dispersive X-ray spectrometer (EDX; FEI Sirion
Ni(OH) + OH− − e− ⇔ NiOOH + H O
2
2
In the subsequent sweep, the peak current increased with the
number of potential scans, indicating the progressive enrichment of
the accessible electroactive species on or near the electrode surface.
After prolonged cycling, the redox potentials were stabilized at 0.44
and 0.33 V vs. SCE. Furthermore, the rapid formation of Ni(OH)2 at
low potential leaded to a Cu-rich metal surface, which was oxi-
dized to Cu O and further to Cu(OH) . Therefore, the surface layer
2
00, Holland).
2.3. Analysis of real water samples
The real samples were collected from the East Lake, the Yangtze
River, the Hanjiang River, and various industrial sites in Wuhan,
China. All samples were simultaneously analyzed by both the pro-
posed method and the classical COD method (GB11914-89 National
Standard of China) [20]. In this study, COD measurement was
performed using an electrochemical workstation (CH Instruments
2
2
was subsequently transformed to a mixture of NiOOH and Cu(OH)2
[16,18].
Fig. 3 shows the cyclic voltammograms of the NiCu alloy elec-
−
1
trode recorded in 0.1 mol L
NaOH solution at different scan
6
60C, Shanghai Chenhua Co. Ltd., China) and a standard three-
rates. It was shown that both anodic and cathodic peak cur-
rents increased clearly with increasing potential scan rates. In the
electrode system with NiCu alloy electrode (or Ni electrode) as
the working electrode, a platinum wire as the counter electrode
and a saturated calomel electrode (SCE) as the reference electrode.
The electrolyte was prepared by mixing 10 mL real water sample
with 40 mg NaOH in the beaker and homogenizing with a magnetic
stirrer. NaOH solution (0.1 mol L ) was used to obtain the base
current. The cyclic voltammograms were recorded by cycling the
potential between 0.1 and 0.6 V vs. SCE at a scan rate of 0.2 V s
−
1
range of 0.02–0.2 V s , both the anodic and cathodic peak cur-
rents were linearly dependent on the scan rate (Fig. 3B), suggesting
that this system was a mass-diffusion-controlled mechanism. This
behavior indicated that the counter-ions were mobile enough to
maintain the electroneutrality at the electrode surface during the
redox process. Furthermore, the stability of the NiCu alloy elec-
trode was also investigated by recording its cyclic voltammograms
−1
−
1
.

Y. Zhou et al. / Electrochimica Acta 74 (2012) 165–170
167
Fig. 2. Consecutive cyclic voltammograms of NiCu alloy electrode (blank line) and
−
1
−1
.
bare GC electrode (gray line) in 0.1 mol L NaOH solution at the scan rate of 0.2 V s
Potential sweep numbers 1–20.
gradually decreased (Fig. 4A). It was because Ni(III) species on the
electrode surface rapidly oxidized organic compound, producing
Ni(II) species and sacrificing Ni(III) ones, which was as follows:
−
−
Ni(OH) + OH → NiOOH + H O + e
(B.1)
2
2
NiOOH + organics(reduzate) + H O → Ni(OH)2
2
+
organics(oxid) + OH−
(B.2)
Furthermore, the slope of the linear regression (0.13) obtained
on NiCu alloy electrode was higher than that (0.05) on Ni electrode,
implying that the electrocatalytic activity of NiCu alloy electrode
was higher that that of Ni electrode (Fig. 4B). Thus, the precision
and accuracy of COD measurement by NiCu alloy electrode was bet-
ter than that by Ni electrode. A possible explanation was that the
addition of Cu(OH)2 to the NiOOH species represented a very effi-
cient strategy of suppressing the formation of ␥-NiOOH phase and
promoting the formation of ␤-NiOOH phase [16]. The CV responses
of glucose at different scan rates on NiCu alloy electrode are also
investigated. Both the anodic and cathodic peak currents of glucose
−
1
were linearly dependent on the scan rate from 0.02 to 0.2 V s
suggesting an adsorption-controlled process.
,
Fig. 1. Morphology of electrode surface after electrodeposition of (A) Ni film and
(
B) NiCu alloy film. EDX results of the (C) NiCu alloy film.
−
1
(
20 consecutive scans) in 0.1 mol L NaOH solution at a scan rate
−
1
of 0.2 V s . No significant changes in the peak current were found
during these curves, implying that the NiCu alloy film was stably
modified on the surface of GC electrode.
In order to further investigate the electrocatalytic activity of
the NiCu alloy electrode, cyclic voltammograms were obtained in
synthetic samples with known COD values, which are shown in
Fig. 4. The synthetic samples were prepared by dissolving glu-
cose as standard COD test reagent with 0.1 mol L NaOH solutions.
The results indicated that anodic peak currents increased clearly
with increasing COD values, meanwhile the cathodic peak currents
Fig. 3. (A) Cyclic voltammograms of NiCu alloy electrode in 0.1 mol L 1 NaOH solu-
−
−
1
tion at various potential sweep rates of 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18
and 0.2 V s , respectively. (B) Correlation between peak currents and the square
−1
−
1
roots of sweep rates (0.02–0.2 V s ).

1
68
Y. Zhou et al. / Electrochimica Acta 74 (2012) 165–170
Fig. 4. (A) Cyclic voltammograms of NiCu alloy electrode in synthetic samples with
−
1
−1
−1
−1
−1
−1
COD values of 0 mg L , 10.7 mg L , 107 mg L , 321 mg L , 535 mg L , 749 mg L
−
1
−1
and 1070 mg L , respectively. Potential sweep rate was 0.2 V s . (B) Correlation
Fig. 5. Correlation between net increase in anodic peak current (ꢀI) and theoretical
COD values of standard samples in electrochemical cell containing a corresponding
amount of phenol, lactose, citric acid, aniline, ethanol, glucose, glycine and pyrrole.
between net increase in anodic peak current (ꢀI) and COD values.
−
1
3.3. Effect of pH
Supporting electrolyte was 10 mL of 0.1 mol L NaOH solution.
An alkaline medium was required to enhance the electrocat-
measurement, which is given in Fig. 5. The results showed that all
organic compounds investigated were fitted into a linear equation
alytic activity of several transition metals. Thus, the effect of pH
value on electrocatalytic performance of NiCu alloy electrode was
investigated. With the concentration of NaOH increasing from 0 to
ꢀI = 6.83 + 0.14 COD, R = 0.9980 (n = 54). It could be known from the
slope of regression line that a high response current to the unit
−
1
0
.1 mol L , both the anodic and cathodic peak currents increased,
−1
theoretical COD values, 6.69 ␮A/1 mg L COD, was obtained. Fur-
because organics were easily oxidized at high pH. At the same time,
the over potential for oxygen evolution would be lowered, and then
the oxygen evolution gradually turned into the main reaction. Thus,
thermore, cyclic voltammograms were obtained in eight kinds of
standard samples with the same COD value, respectively and the
relative standard deviation (RSD) was 3.1% (n = 5). It was shown that
the COD measurements by the proposed method were almost inde-
pendent of the type of organic species, which was the quantitative
foundation for the COD determination. Concerning on COD mea-
surements using Ni electrode, the slope of regression line (0.06)
was significantly lower than that (0.14) of the proposed method
using NiCu alloy electrode. It was confirmed that the electrocat-
alytic activity of the NiCu alloy electrode was higher than that
of Ni electrode, and the proposed method was more efficient for
COD measurement. In addition, the detection limit of the proposed
−
1
when the concentration of NaOH increased above 0.1 mol L , a
high background noise was obtained and lots of gas bubbles were
observed on the surface of the NiCu alloy electrode. The results
were consistent with the previous experimental phenomena that
an increased background current was observed at a high NaOH con-
−1
centration [22]. In our experiment, 0.1 mol L NaOH solution was
chosen as the optimal medium.
3.4. Influence of dissolved oxygen and chloride ion
−
1
method was around 1.0 mg L
.
As reported, Ni sites were considered to be responsible for the
The reproducibility and stability of the proposed method were
evaluated using a synthetic sample prepared with glucose as stan-
dard COD test reagent. Cyclic voltammograms were obtained using
a series of NiCu alloy electrode prepared in the same way and
relative standard deviation (RSD) was 4.2% (n = 10). After analyz-
ing synthetic sample ten times using an electrode, RSD of current
changes was 3.8%, which exhibited a good reproducibility of the
proposed method. Furthermore, the results of operational stabil-
ity evaluation revealed that no significant performance change
(RSD = 4.7%) was observed after the electrode performed 3 assays
every day for one week. During this time, the electrode was
remained in the 10 mL beaker filled with Milli-Q water at room
temperature when it was not being used.
Fig. 6 shows the correlation between the proposed method and
the classic dichromate method in terms of the COD test with syn-
thetic samples. These synthetic samples were prepared by mixing
all eight chemicals with different concentrations of COD. A highly
significant correlation (r = 0.9978, p < 0.01, n = 13) between the two
methods indicated that the two methods had a good consistency.
More importantly, this almost unity slope value (1.03) suggested
that both methods were accurately measuring the same COD value.
oxygen evolution reaction [23]. However, there was no signifi-
cant difference between the COD values obtained under the N2
and air saturation in this study. The results demonstrated that the
dissolved oxygen had no intensive interference with the determi-
nation of COD. The facts were probably because low ␣/␥ redox
contribution and good stabilization of ␤/␤ nickel oxyhydroxide on
the surface of NiCu alloy electrode could markedly diminish the
oxygen evolution reaction [16].
Furthermore, many electrodes based on metal and alloy usu-
ally lost their activity because of being poisoned by chloride ions
24]. Therefore, in order to evaluate the antifouling ability of the
[
NiCu alloy electrode, cyclic voltammograms were obtained in syn-
thetic samples (glucose) with and without high concentration of
−1
chloride ions (1 mol L NaCl). The result indicated that the cur-
rent response of the NiCu alloy electrode was no difference toward
organic compound oxidation. It was shown that the proposed
electrode exhibited good resistance to surface fouling and would
become a desirable tool to determine COD values in real water
samples.
3.5. Validation of analytical principle
3.6. Real samples analysis
To evaluate the linear range and the detection limit of the
proposed method, eight organics with known theoretical COD val-
ues were selected as standard samples for calibrating the COD
The analysis of real samples (surface water, reclaimed water and
wastewater) was carried out by both the proposed method and the

Y. Zhou et al. / Electrochimica Acta 74 (2012) 165–170
169
Table 1
Correlation between the proposed method and the classic dichromate method in terms of the COD test with real water samples.
Samples
CODNiCua (mg L−1
)
RSD (%) (n = 5)
CODCrb (mg L−1
)
Relative error (%)
The East Lake
Sample 1
Sample 2
Sample 3
The Hanjiang River
Sample 1
65.8
72.1
69.7
4.4
4.5
3.1
65.3
73.8
72.4
0.8
−2.3
−3.7
61.2
67.4
71.7
2.2
5.3
4.5
59.5
66.3
70.7
2.9
1.7
1.4
Sample 2
Sample 3
The Yangzte River
Sample 1
Sample 2
60.4
75.6
67.9
160.2
278.7
2.3
1.5
3.2
3.5
5.8
57.2
77.2
70.3
153.8
261.6
5.6
−2.1
−3.4
4.2
Sample 3
Reclaimed water
Domestic wastewater
Chemical wastewater
Sample 1
Sample 2
Sample 3
6.5
784.4
1248.1
1404.1
6.7
7.3
5.5
842.7
1181.3
1466.8
−6.9
5.7
−3.8
a
COD values were determined by the proposed method using NiCu alloy electrode.
COD values were determined by the classic dichromate method.
b
Table 2
Comparison of the reported electrochemical sensors to the one described in this article.
Linearity (mg L−1)
LOD (mg L
−1
)
The detection method
Kinds of standard COD test reagent
Li et al. [4]
F-PbO2 electrode
Nano-Cu electrode
Cu/CuO electrode
Ti/TiO2 electrode
Mixed-phase TiO2 electrode
Glucose
Glycine
Glucose
Potassium hydrogen phthalate
Six kinds of organic compounds (glutaric acid, succinic acid
malonic acid, etc.)
100–1200
4.8–600
53–2801
50–2000
0–120
15
3.6
20.3
16
0.2
Yang et a. [6]
Silva et al. [24]
Li et al. [25]
Zhang et a. [26]
Yu et al. [27]
This article
Boron-doped diamond electrode
NiCu alloy electrode
Nine kinds of organic compounds (4-hydroxybenzoic acid,
glutamic acid, acetic acid, etc.)
Eight kinds of organic compounds (phenol, lactose, citric acid, etc.)
2–175
1.0
1.0
10–1533
classic dichromate method. The results are shown in Table 1. It was
shown that the proposed method was satisfactory to determine
the COD values in real water samples and the relative errors of the
COD determination were in the range −6.9% to 6.5%. Furthermore,
the correlation assay was performed to confirm the reliability and
practicality of the protocol, based on the COD values obtained by
the proposed method and the classic dichromate method. The lin-
ear equation was CODcr = 1.01 CODNiCu − 2.45 (r = 0.9967, p < 0.01,
n = 14). The excellent agreements indicated an excellent practical
perspective of the proposed method on COD measure. In a word,
this work could provide a promising tool for determination of COD
in water quality control and pollution evaluation.
3.7. Comparison of the proposed method with existing
electrochemical sensors
Several electrochemical sensors for the determination of COD
have been published (Table 2). Each method has its advantages
and limitations in terms of linearity, sensitivity and practical appli-
cation. Li et al. [4], Silva et al. [24] and Li et al. [25] applied
F-PbO , Cu/CuO, and Ti/TiO2 electrode for determination of COD,
2
respectively. Although excellent linear ranges were received, the
−
1
detection limits (15–20.3 mg L ) were higher than that of the
−
1
proposed method (1 mg L ) and single compound was used for
calibrating the COD values in water samples, which could be eas-
ily attributed to the unavoidable experimental errors. Yang et al.
[
6] developed a sensitive electrochemical sensor for COD mea-
sure using nano-Cu electrode. Similar detection limit was obtained,
−
1
however, the linear range (4.8–600 mg L ) was narrower than that
−1
of the proposed method (10–1533 mg L ). Furthermore, Zhang
et al. [26] and Yu et al. [27] reported electrochemical sensors using
TiO2 electrode and BDD electrode with high catalytic activity and
−
1
sensitivity, respectively, but the linear ranges (about 0–175 mg L
)
were clearly narrower than that of the proposed method, which
limited the practical application of the electrochemical sensors.
Compared with the existing methods, it was confirmed that the
proposed method was simple and sensitive for determination of
COD in real water samples and had wide practical perspective on
screening of organic pollutants.
4. Conclusions
A simple, sensitive and environmentally friendly method was
developed for determination of COD by cyclic voltammetry using
NiCu alloy as the sensing film. This alloy electrode exhibited higher
Fig. 6. Correlation between the COD values of synthetic samples determined by the
proposed method and the classic dichromate method.

1
70
Y. Zhou et al. / Electrochimica Acta 74 (2012) 165–170
electrocatalytic activity than Ni electrode and GC electrode and
then greatly increased the sensitivity of COD detection. Because
of the wide linear range, excellent reproducibility and satisfactory
correlation between the proposed method and the classic dichro-
mate method, this method could be used for routine analysis of
COD in surface water, reclaimed water and wastewater. Therefore,
a large number of real water samples were analyzed to confirm the
reliability and practicality of the proposed method, and the results
revealed a good practical perspective on assessing water quality
and pollution control.
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