# The Study of a New Method to Determine Copper Ion by Square-Wave Voltammetry-Extraction Iodometry at the Liquid/Liquid Interfaces

Authors: Changyan Shi; Shaoai Xie; Jinping Jia
Journal: Journal of Automated Methods and Management in Chemistry
 (2008)

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## Abstract

A new method of indirect determination ofCu2+ was developed based on square-wave voltammetry by the oxidation of iodide in organic solvent at the liquid/liquid (L/L) interface. The limit of detection for the determination of Cu2+ in this method was found to be 5×10−4 mol/L, and the concentration ranged up to 1×10−2 mol/L gave a linear limiting current versus concentration response. For the same simulated wastewater, this method showed high accuracy compared with the result tested by sodium diethyldithiocarbamate extraction spectrophotometry. This approach could be applied to the indirect determination of the oxidative heavy metals in the industrial wastewater.


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## Body

## 1. Introduction

Heavy metals such as cadmium, lead, zinc, nickel, copper, and
chromium (VI) or their compounds have been used extensively by various metal finishing,
mining, and chemical industries. This has led to a sharp increasing in the
contamination of water. Due to their toxicity, the presence of these metals in
excessive quantities will interfere with many beneficial uses of the water. For
these reasons, it is necessary to explore a simple, sensitive and accurate
detection method.Many
analytical methods have been developed for determination of heavy metals in water,
such as colorimetric analysis [1], UV-VIS spectroscopy
[2–4], and
either flame or graphite furnace atomic absorption spectrometry (AAS) [5–8].In addition to these techniques, methods for multielemental
determination have been developed, such as ion chromatography (IC) [9–11] and
inductively coupled plasma combined with either atomic emission spectrometry (ICP-AES) [12] or
mass spectrometry (ICP-MS) [13, 14].Although
these methodologies are rapid and sensitive for the determination of trace
amounts of metals, they require complicated instrumentation with high capital
and operational costs. Furthermore, these methods are not easily implemented
into fully portable analytical tools for screening, detecting, identifying, and
quantifying metal ions.Different
from these methods, electrochemical detection (ED), such as polarography, offers
high sensitivity and selectivity for metals that are readily oxidized or
reduced with simple instrumentation and low capital and operational costs. Therefore,
it is suitable for online monitoring. Now, the application of polarography has
been limited for the toxicity of mercury.Electrochemistry at liquid/liquid (L/L) interfaces is mostly
studying charge (ion and charge) transfer across an interface between two
immiscible electrolyte solutions and the associated chemical reactions. Based
on the special property of the interface between two immiscible electrolyte
solutions (ITIESs), using appropriate organic solvents with low toxicity to
substitute toxic mercury is a new area for monitoring heavy metals.Square-wave voltammetry is a large-amplitude differential technique in
which a waveform composed of a symmetrical square wave,  superimposed on a base
staircase potential, is applied to the working electrode. Now,  square-wave
voltammetry has been applied in the field of ITIES. Mirceski and  Scholz [15] studied the electrochemistry of iodine/iodide
redox couple at the three-phase arrangement organic liquid (aqueous solution) graphite electrode of a reversible electrochemical
process using square-wave voltammetry as the electro chemical  method.In this paper, a simple method of monitoring copper ions indirectly was
designed by investigating the electrochemical reactions of iodine  at the L/L
interface with square-wave voltammetry. Different from the study of Mirceski and  Scholz, a new composite
electrode was chosen as the working electrode, which was prepared by inserting a Pt wire electrode into a
micropipette. An  organic solvent containing extracted iodine was injected into
the pipette, thus the
liquid/liquid interface was supported by setting the pipette just on the surface
of aqueous solution
below. Xie et al. [16]
have studied the electrochemical behavior of iodine using such kind of
electrode,  and analyzed it by cyclic
voltammetry. This paper improved their study from two aspects. First,
by using square-wave voltammetry to substitute cyclic voltammetry,
the analysis time was drastically reduced and its keen-edged peak
shape made the curve more easily to scale the peak current and realize
quantitative detection compared with cyclic voltammetry. Second, in the process of extraction, the volume of
organic solvent was smaller than the volume of the stock aqueous solution which
contained triiodide, rather than equal. As the concentration of extracted iodine in the organic
solvent showed to be dramatically high in the smaller volume of organic solvent
under the premise that the total amount of extracted iodine did not change, the
peak current could be higher.


## 2. Experimental Part

### 2.1. Instruments and Chemicals

All the salts used as the supporting electrolytes were used
as purchased and all the chemicals used here were of analytical grade. The aqueous
solutions were prepared in deionized water. Organic solvents were saturated
with deionized water prior to experiments. A stock aqueous solution of triiodide
was determined by iodometry. Some solutions of iodine in organic solvent
(methyl isobutyl ketone (MIBK)) were prepared by the extraction of iodine from a
triiodide stock solution in water saturated MIBK.All electrochemical experiments were performed at room
temperature(25±1)C°  with an electrochemical
analyzer (CHI832b, Chenhua, Shanghai, China); UV-VIS spectra were measured with a
UV-VIS (UNICO UV-2102,  UNICO (Shanghai) Instrument
Co. Ltd, Shanghai, China).


### 2.2. Experimental Procedures

A conventional three-electrode cell was used, in this work by
using saturated calomel electrode (SCE) as the reference electrode (E  = 0.241 V versus
standard hydrogen electrode) and a platinum electrode as the auxiliary
electrode. The working electrode was a platinum wire (d  =  0.1 mm) sealed in a classical capillary (d = 1.0 mm). Prior to all experiments, the
platinum wire was immerged into 1 mol/L   H2SO4 to be activated
during cyclic voltammetric experiments. After activation, it was polished with
sheepskin, cleaned successively with alcohol and acetone, and then dried with a
hair drier. After that, 20~40μL of MIBK
containing iodine was injected into a pipette using a microsyringe, and the
platinum wire electrode was immersed into it. Then the combined electrode was
immediately set just on the surface of the aqueous solution. The schematic
diagram of the experimental assembly is shown in Figure 1. The liquid/liquid
interface between aqueous phase and organic phase was supported as shown in part
5 of the Figure 1 (the cell employed and an interfacial
area of 0.8 mm2). During
square-wave voltammetry experiments,  neither a significant distortion nor a
change of the color of the organic phase had been observed. The interfacial
area influences only the measured currents,  where both the position of the
response (peak potential) and the shapes were not affected.Figure 1
Schematic diagrams of the electrochemical
cell (1) WE (Pt
wire), (2) RE(SCE), (3) CE(Pt), (4) 0.1 mm Pt wire, (5) L/L interface, (6) aqueous solution, (7) pipette, (8) organic phase, and (9) Lid.


## 2.1. Instruments and Chemicals

All the salts used as the supporting electrolytes were used
as purchased and all the chemicals used here were of analytical grade. The aqueous
solutions were prepared in deionized water. Organic solvents were saturated
with deionized water prior to experiments. A stock aqueous solution of triiodide
was determined by iodometry. Some solutions of iodine in organic solvent
(methyl isobutyl ketone (MIBK)) were prepared by the extraction of iodine from a
triiodide stock solution in water saturated MIBK.All electrochemical experiments were performed at room
temperature(25±1)C°  with an electrochemical
analyzer (CHI832b, Chenhua, Shanghai, China); UV-VIS spectra were measured with a
UV-VIS (UNICO UV-2102,  UNICO (Shanghai) Instrument
Co. Ltd, Shanghai, China).


## 2.2. Experimental Procedures

A conventional three-electrode cell was used, in this work by
using saturated calomel electrode (SCE) as the reference electrode (E  = 0.241 V versus
standard hydrogen electrode) and a platinum electrode as the auxiliary
electrode. The working electrode was a platinum wire (d  =  0.1 mm) sealed in a classical capillary (d = 1.0 mm). Prior to all experiments, the
platinum wire was immerged into 1 mol/L   H2SO4 to be activated
during cyclic voltammetric experiments. After activation, it was polished with
sheepskin, cleaned successively with alcohol and acetone, and then dried with a
hair drier. After that, 20~40μL of MIBK
containing iodine was injected into a pipette using a microsyringe, and the
platinum wire electrode was immersed into it. Then the combined electrode was
immediately set just on the surface of the aqueous solution. The schematic
diagram of the experimental assembly is shown in Figure 1. The liquid/liquid
interface between aqueous phase and organic phase was supported as shown in part
5 of the Figure 1 (the cell employed and an interfacial
area of 0.8 mm2). During
square-wave voltammetry experiments,  neither a significant distortion nor a
change of the color of the organic phase had been observed. The interfacial
area influences only the measured currents,  where both the position of the
response (peak potential) and the shapes were not affected.Figure 1
Schematic diagrams of the electrochemical
cell (1) WE (Pt
wire), (2) RE(SCE), (3) CE(Pt), (4) 0.1 mm Pt wire, (5) L/L interface, (6) aqueous solution, (7) pipette, (8) organic phase, and (9) Lid.


## 3. Results and Discussion

### 3.1. The Effect of Extractive Volume Ratio on Peak Current

The effect of volume ratio between organic solvent (MIBK) and
triiodide stock solution on peak current is shown in Figure2. Different volumes
of MIBK, that is, 10 mL, 8 mL, 6 mL, 4 mL, 2 mL were used to extract iodine
from 10 mL aqueous phase. As shown in Figure 2, the peak currents varied when
using different volumes of extractant and appeared significantly high when volume
ratio was 0.2. This is in agreement with the principle that the concentration
of iodine in MIBK increases
along with the decrease of extractant volume under the premise that the total
amount of extracted iodine did not change.Figure 2
Relationship between volume ratio and
the peak current.


### 3.2. Quantitative Determination of Iodine

The triiodide stock aqueous solution was prepared with I2 and KI, and after investigation the authors came to the conclusion that the
peak current does not change when using different mole ratio of  I2 and KI. So a solution in which the mole ratio between I2 and KI was 1:8 was chosen and diluted into different concentrations.The square-wave voltammograms (SWVs) of various
concentrations of iodine in the triiodide stock aqueous solution chosen above
are shown in Figure3(a). The mechanism of iodine oxidation at the electrode
surface can be explained as  in
literature 16. The peak current was directly  proportional to I2 concentration as shown in Figure 3(b). The proportionality coefficient was
0.9970. So it is reasonable to suggest that the indirectly quantitative
determination of Cu2+ is feasible.(a) Square-wave voltammograms of various
iodine concentrations in MIBK: amplitude 0.15 V and frequency 10 Hz, (b) the peak current versus concentration
of iodine in solution: the iodine concentrations (in mol/L): 0.0049, 0.0040,
0.0030, 0.0020, 0.0012, 0.00062, 0.00040, 0.00031.
(a)(b)


### 3.3. Quantitative Determination of Cu2+

As it is well known, an oxidizing analyte is added to excess
I− to produce iodine and then the iodine is titrated with standard
thiosulfate solution [17]. In this method, an indicator and standard
solution are prepared. Here, the method of measuring the content of iodine in
solution was improved. Iodine was prepared according to the chemical reactions
as follows: (1)2Cu2++5I−⟺2CuI(s)+I3−. Then various  concentrations of iodine were extracted into
MIBK. Solvent extraction is a well established separation technique based on
the principle that a solute can distribute itself in a certain ratio between
two immiscible solvents, one of which is usually water and the other organic
solvent. This method is widely used because of its simplicity, speed, and
applicability to low analyte concentrations.The square-wave voltammograms of the
iodine prepared by reaction of 0.02 mol/L Cu2+ and various
concentrations of I− in a neutral electrolyte were obtained and the
relationship between the concentrations of I− and the peak current
is shown in Figure 4. The peak current increased along with the increasing of I−’s
amount when the concentrations of I− were less than 0.8 mol/L  and stayed
almost invariable when the  concentrations of I− were higher than 0.8 mol/L. Then the conclusion was made that the peak currents are invariable when
the amount of I− used is excess enough. So this study chose 1 mol/L  KI
as a reducer to produce iodine with different concentrations of Cu2+.Figure 4
Dependence of peak current on the
concentration of KI.The square-wave voltammograms of the iodine prepared by the reaction
of various concentrations of Cu2+ and 1 mol/L  I− in a
neutral electrolyte are shown in Figure 5(a). The peak current was directly
proportional to Cu2+ concentration as shown in Figure 5(b). The
proportionality coefficient was 0.9983. The limit of detection for the determination
of Cu2+ indirectly was found to be 5  ×  10−4 mol/L, and the
concentration ranged up to 1 × 10−2 mol/L  gave a linear limiting
current versus concentration response.(a) Square-wave voltammograms of various
iodine concentrations in MIBK: amplitude 0.15 V and frequency 10 Hz, (b) the peak current versus
concentration of copper ion in solution; the Cu2+ concentration (in mol/L):
0.01, 0.0025, 0.002, 0.0015, 0.001, 0.0005.
(a)(b)


### 3.4. Detection for Actual Samples

A simulated wastewater containing Cu2+ was
prepared and the peak current was determined through the same procedure as
mentioned before. Then the concentration of Cu2+ in the sample was
determined by the regression equation of  Figure 6(b) (Y = 4.952 × 10−7+ 2.587 × 10−4X) based on the peak current (1.731 × 10−6  A). The result is 4.777  ×  10−3 mol/L.Figure 6
The absorbency degree versus
concentration of Cu2+ in solution.Besides  the method studied in this paper, another classical method, spectrophotometry
was also used as comparative study to determine the same sample. The absorbency
degree of various concentrations of Cu2+ colored by sodium diethyldithiocarbamate
(DDTC) was detected by the UV-VIS at the wavelength of 440 nm and directly
proportional to Cu2+ concentration as shown
in Figure 6. The proportionality coefficient was 0.9952. After diluting 1000
times, the concentration of Cu2+ in the sample was determined by the
regression equation of Figure 6(b) (Y = −0.045+116848.0315 X) based on the absorbency degree (0.446). So the Cu2+ concentration
in this sample is 4.202 × 10−3 mol/L.The two results of the same sample showed that the method
studied in this paper gives
good result with high accuracy in agreement with the result of spectrophotometry.The interference experiment also showed that the peak current
would not be interfered when the concentration of coexisted reductive metals
(such as Zn2+, Ni2+)
was ten times as the target metal. When existing two or more than two kinds of
oxidative metals, the method could determine the total amount.


## 3.1. The Effect of Extractive Volume Ratio on Peak Current

The effect of volume ratio between organic solvent (MIBK) and
triiodide stock solution on peak current is shown in Figure2. Different volumes
of MIBK, that is, 10 mL, 8 mL, 6 mL, 4 mL, 2 mL were used to extract iodine
from 10 mL aqueous phase. As shown in Figure 2, the peak currents varied when
using different volumes of extractant and appeared significantly high when volume
ratio was 0.2. This is in agreement with the principle that the concentration
of iodine in MIBK increases
along with the decrease of extractant volume under the premise that the total
amount of extracted iodine did not change.Figure 2
Relationship between volume ratio and
the peak current.


## 3.2. Quantitative Determination of Iodine

The triiodide stock aqueous solution was prepared with I2 and KI, and after investigation the authors came to the conclusion that the
peak current does not change when using different mole ratio of  I2 and KI. So a solution in which the mole ratio between I2 and KI was 1:8 was chosen and diluted into different concentrations.The square-wave voltammograms (SWVs) of various
concentrations of iodine in the triiodide stock aqueous solution chosen above
are shown in Figure3(a). The mechanism of iodine oxidation at the electrode
surface can be explained as  in
literature 16. The peak current was directly  proportional to I2 concentration as shown in Figure 3(b). The proportionality coefficient was
0.9970. So it is reasonable to suggest that the indirectly quantitative
determination of Cu2+ is feasible.(a) Square-wave voltammograms of various
iodine concentrations in MIBK: amplitude 0.15 V and frequency 10 Hz, (b) the peak current versus concentration
of iodine in solution: the iodine concentrations (in mol/L): 0.0049, 0.0040,
0.0030, 0.0020, 0.0012, 0.00062, 0.00040, 0.00031.
(a)(b)


## 3.3. Quantitative Determination of Cu2+

As it is well known, an oxidizing analyte is added to excess
I− to produce iodine and then the iodine is titrated with standard
thiosulfate solution [17]. In this method, an indicator and standard
solution are prepared. Here, the method of measuring the content of iodine in
solution was improved. Iodine was prepared according to the chemical reactions
as follows: (1)2Cu2++5I−⟺2CuI(s)+I3−. Then various  concentrations of iodine were extracted into
MIBK. Solvent extraction is a well established separation technique based on
the principle that a solute can distribute itself in a certain ratio between
two immiscible solvents, one of which is usually water and the other organic
solvent. This method is widely used because of its simplicity, speed, and
applicability to low analyte concentrations.The square-wave voltammograms of the
iodine prepared by reaction of 0.02 mol/L Cu2+ and various
concentrations of I− in a neutral electrolyte were obtained and the
relationship between the concentrations of I− and the peak current
is shown in Figure 4. The peak current increased along with the increasing of I−’s
amount when the concentrations of I− were less than 0.8 mol/L  and stayed
almost invariable when the  concentrations of I− were higher than 0.8 mol/L. Then the conclusion was made that the peak currents are invariable when
the amount of I− used is excess enough. So this study chose 1 mol/L  KI
as a reducer to produce iodine with different concentrations of Cu2+.Figure 4
Dependence of peak current on the
concentration of KI.The square-wave voltammograms of the iodine prepared by the reaction
of various concentrations of Cu2+ and 1 mol/L  I− in a
neutral electrolyte are shown in Figure 5(a). The peak current was directly
proportional to Cu2+ concentration as shown in Figure 5(b). The
proportionality coefficient was 0.9983. The limit of detection for the determination
of Cu2+ indirectly was found to be 5  ×  10−4 mol/L, and the
concentration ranged up to 1 × 10−2 mol/L  gave a linear limiting
current versus concentration response.(a) Square-wave voltammograms of various
iodine concentrations in MIBK: amplitude 0.15 V and frequency 10 Hz, (b) the peak current versus
concentration of copper ion in solution; the Cu2+ concentration (in mol/L):
0.01, 0.0025, 0.002, 0.0015, 0.001, 0.0005.
(a)(b)


## 3.4. Detection for Actual Samples

A simulated wastewater containing Cu2+ was
prepared and the peak current was determined through the same procedure as
mentioned before. Then the concentration of Cu2+ in the sample was
determined by the regression equation of  Figure 6(b) (Y = 4.952 × 10−7+ 2.587 × 10−4X) based on the peak current (1.731 × 10−6  A). The result is 4.777  ×  10−3 mol/L.Figure 6
The absorbency degree versus
concentration of Cu2+ in solution.Besides  the method studied in this paper, another classical method, spectrophotometry
was also used as comparative study to determine the same sample. The absorbency
degree of various concentrations of Cu2+ colored by sodium diethyldithiocarbamate
(DDTC) was detected by the UV-VIS at the wavelength of 440 nm and directly
proportional to Cu2+ concentration as shown
in Figure 6. The proportionality coefficient was 0.9952. After diluting 1000
times, the concentration of Cu2+ in the sample was determined by the
regression equation of Figure 6(b) (Y = −0.045+116848.0315 X) based on the absorbency degree (0.446). So the Cu2+ concentration
in this sample is 4.202 × 10−3 mol/L.The two results of the same sample showed that the method
studied in this paper gives
good result with high accuracy in agreement with the result of spectrophotometry.The interference experiment also showed that the peak current
would not be interfered when the concentration of coexisted reductive metals
(such as Zn2+, Ni2+)
was ten times as the target metal. When existing two or more than two kinds of
oxidative metals, the method could determine the total amount.


## 4. Conclusions

A new method of indirect determination of Cu2+ has
been characterized by the oxidation of iodide in organic solvent at the L/L
interface, in which the ion transfer from aqueous solution to organic phase
proceeds. Considering the strong reductive properties of iodide in aqueous
solution, iodine can be obtained by chemical reactions between excess iodide
and heavy metal ions
(e.g., Cu2+) in solution, then the content of heavy metal ions can
be determined indirectly using the present method. The limit of detection for
the determination of Cu2+ indirectly was found to be 5 × 10−4 mol/L, and the concentration ranged up to 1 × 10−2 mol/L   gave a linear
limiting current versus concentration response. Compared the result with sodium
diethyldithiocarbamate extraction spectrophotometry, this method shows high
accuracy. The advantage of this method is that it can be operated much more
easier than sodium diethyldithiocarbamate extraction spectrophotometry.The limit of detection was not low enough because the peak position
appeared at 0.8 V, away from its normal position when the concentration of Cu2+ was lower than 5 × 10−4 mol/L. The reason for the potential shift is
yet to be studied.This approach could be applied to the indirect determination
of the oxidative heavy metals, such as Cu2+,  Cr (VI)  in the industrial
wastewater.



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*Source: 453429-2008-12-14.xml*