Separation/preconcentration of trace copper by extraction into [C 4mim] [PF6] RITL containing p-tert-butyl-calix[4]arene functionalized with o-phenanthroline prior to flame absorption atomic spectrometry

Article abstract of DOI:10.1016/j.molliq.2012.11.013

A liquid-liquid extraction (LLE)-flame atomic absorption spectrometry (FAAS) for the separation/analysis of copper has been developed. The method was based on the fact that the inclusion complex of copper(II) with 25, 27-di (5-phenanthrolinylaminocarbonylmethoxy)-26,28-dihydroxy-p-tert-butylcalix[4] arene (TBCP) could be extracted into room temperature ionic liquids (RTILs, 1-butyl-3-methylimidazolium hexafluoro-phosphate [C₄mim] [PF ₆] as the green extractant, TBCP as the complexlant), and the copper(II) was subsequently determined by FAAS. The LLE was optimized in detail. The selectivity and mechanism of LLE were approached. Under optimized experimental conditions, the linear range of calibration curve for the determination of copper(II) was 0.020-10.0 μg/mL. The detection limit estimated (S/N = 3) was 3.59 ng/mL with 3.0% RSD. It has been applied for the determination of copper(II) in water samples with satisfactory results.

Full text of DOI:10.1016/j.molliq.2012.11.013

Journal of Molecular Liquids 178 (2013) 20–24
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Separation/preconcentration of trace copper by extraction into [C4mim] [PF6] RITL
containing p-tert-butyl-calix[4]arene functionalized with o-phenanthroline prior to
flame absorption atomic spectrometry
⁎
Lina Ma, Xiashi Zhu , Wenjun Wang
College of Chemistry & Chemical Engineering, Yangzhou University, Yangzhou 225009, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A liquid–liquid extraction (LLE)-flame atomic absorption spectrometry (FAAS) for the separation/analysis of cop-
per has been developed. The method was based on the fact that the inclusion complex of copper(II) with 25,
27-di (5-phenanthrolinylaminocarbonylmethoxy)-26,28-dihydroxy-p-tert-butylcalix[4]arene (TBCP) could be
extracted into room temperature ionic liquids (RTILs, 1-butyl-3-methylimidazolium hexafluoro-phosphate
[C4mim] [PF6] as the green extractant, TBCP as the complexlant), and the copper(II) was subsequently deter-
mined by FAAS. The LLE was optimized in detail. The selectivity and mechanism of LLE were approached. Under
optimized experimental conditions, the linear range of calibration curve for the determination of copper(II) was
0.020–10.0 μg/mL. The detection limit estimated (S/N=3) was 3.59 ng/mL with 3.0% RSD. It has been applied
for the determination of copper(II) in water samples with satisfactory results.
Received 23 September 2011
Received in revised form 15 November 2012
Accepted 15 November 2012
Available online 29 November 2012
Keywords:
Copper(II)
Room temperature ionic liquid
p-tert-butyl-calix[4]arene
Liquid–liquid extraction
Flame atomic absorption spectrometry
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
preconcentration and matrix isolation techniques in analytical chemis-
try. However, there were some disadvantages for LLE, such as the emul-
Copper is one of the essential micronutrients for human bodies,
animals and plants. Lack of copper could cause coronary heart dis-
ease, vitiligo, blindness or anemia [1]. But excessive copper is toxic
and the free cupric ion is highly toxic for marine organisms and its de-
termination is an important analytical task. The upper control limit of
the copper in drinking water was 1000 ng/mL in China [2]. Therefore,
it is necessary to develop sensitive and economical methods for the
determination of trace amount of copper [3].
Useful analytical methods for Cu(II) include spectrophotometry
[4], atomic absorption spectrometry (AAS) [5,6], atomic emission
spectrometry (AES) [7], chromatography [8], mass-spectrography
[9], potentiometric stripping out analysis [10] and so on. FAAS is
the most common determination method in this group due to its
low cost, friendly operation and high sample throughput, but it suf-
fers from poor sensitivity and/or matrix interferences in the analysis
of heavy elements in environmental samples like natural water and
other real samples. This drawback could be overcome by the combi-
nation of a suitable preconcentration technique with subsequent
AAS [11,12].
sion formation, the use of large sample volumes and toxic organic
solvents and the generation of large amounts of pollutants [3]. For this
reason, the green of the liquid–liquid extraction attempts to eliminate
or minimize these drawbacks.
Room temperature ionic liquids (RTILs) were an important green sol-
vent, which have attracted considerable attention in the fields of extrac-
tion separation due to its low melting point, tunable acidity and good
solubility characteristics [16–18]. Rogers et al. firstly reported that ionic
liquids ([C4mim] [PF6]) were used for the metal ion liquid–liquid extrac-
tion system and the extraction distribution coefficient was 0.05 [19].
The extraction distribution coefficient could be over 1 when PAN or
TAN is used as chelating agent in the above LLE and it could be more
than 100 or even higher when crown ethers will serve as chelating
agents [16,20,21]. Therefore, the properties (hydrophobic property,
selectivity) of the chelating agent were significant for the LLE system.
Calixarenes, appeared after crown ethers and cyclodextrins, as the
third generation of inclusion compounds, are largely easily modifiable
ring receptors. There are hydrophobic cavities or many regular phe-
nolic hydroxyl groups at the top or bottom of cup-like structure of
calixarenes, which could develop the inclusion complexes with neu-
tral molecules or cations [22]. The special modified functional group
(1,10-phenanthroline) at the bottom of the derivatives of calixarene
is a highly selective host molecule which has a highly selective recog-
nition function and has been applied in metal ions analysis [23–25].
So calixarenes which have a more complicated structure than crown
ethers as chelating agents must be bound to get higher extraction
Several methods have been reported for the separation and
preconcentration of Cu(II), such as liquid–liquid extraction (LLE) [13],
solid phase extraction (SPE) [14], cloud point extraction (CPE) [15]
and so on. Liquid–liquid extraction (LLE) was among the oldest of the
⁎
Corresponding author. Tel./fax: +86 514 7975244.
E-mail addresses: xszhu@yzu.edu.cn, zhuxiashi@sina.com (X. Zhu).
0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molliq.2012.11.013

L. Ma et al. / Journal of Molecular Liquids 178 (2013) 20–24
21
distribution coefficient [26]. At present, the combination of [C4mim]
C(CH3)3 C(CH3)3
[PF6] ionic liquid as the green extractant and calyx[4]arenes chelating
agent for developing an extraction/preconcentration analytical meth-
odology has not been established.
2
In this paper, copper(II) was chelated with TBCP, then the chelate
was extracted into the RTILs ([C4mim] [PF6]), and analyzed by FAAS.
The selectivity and mechanism of LLE were approached with the in-
clusion interaction of TBCP and transition metal ions. The method
was applied for the separation/analysis of copper(II) in Slender
West Lake water sample with satisfactory results.
OCH2CONH
OH
N
N
2. Experimental
Fig. 1. The molecular formula of TBCP.
2.1. Apparatus and chemicals
The absorbance measurements were performed with AAS ZEEnit700
atomic absorption spectroscopy (Manufacturer Analytik Jena AG), the
task selection of WinAAS was application/cookbook (Table 1).
F-4500 fluorescence spectrophotometer (Hitachi, Japan); JJ-1 precise
intensifier electric mixer (Changzhou Guohua Electric Appliances Co.,
Ltd); pHS-25 type pH meter (Shanghai Precision kore magnetic Factory);
TDL80-2B Feige high-speed centrifuge (Shanghai Anting Scientific
Instruments Plant); Electronic balance (Beijing).
N-methylimidazole (Shanghai Dagro Fine Chemical Co., Ltd);
potassium hexafluorophosphate (Shanghai Bangcheng Chemical
Co., Ltd); N-butyl-bromide, diethyl ether, ammonium acetate,
acetic acid, ammonium chloride, stronger ammonia water, N,
N-dimethylformamide (Sinopharm Chemical Rwagent Co., Ltd).
25,27-di(5-phenanthrolinylaminocarbonylmethoxy)-26,28-
dihydroxy-p-tert-butylcalix[4]arene (TBCP, Fig. 1, 99.9% purity) was
prepared by the Organic Chemistry Lab in the Chemistry College of
Yangzhou University. A stock solution of TBCP (6.0×10−4 mol/L) was
prepared in N,N-dimethylformamide (DMF).
2.2.2. Ionic liquid extraction procedure
In a 5.0 mL scale centrifugal tube 0.4 mL Cu2+ (100.0 μg/mL), 0.5 mL
CH3COONH4 (pH=7.0) buffer solution, 1.0 mL TBCP (6.0×10−4 mol/L)
and 1.5 mL [C4mim] [PF6] were added. The mixed solution was diluted
to a final volume with distilled water, shaken thoroughly. Phase separa-
tion was achieved by centrifugation at 3500 rpm for 4 min, and then
1.5 mL of the organic phase was visibly observed at the bottom of the
centrifuge tube. The aqueous phase (3.5 mL) on the upper of the centri-
fuge tube was easily removed with a pipette to a 5 mL centrifuge tube
and diluted to 5 mL with distilled water, shaken thoroughly, and then
determined by FAAS (AW). Following the same method, a solution with-
out [C4mim] [PF6] was prepared as reference solution (A0). The
extraction ratio could be calculated by Eq. (1), where A0 was the absor-
bance before extraction, and AW was the absorbance of the bulk aqueous
phase after extraction [31].
Eð%Þ ¼ 100ðA0−AwÞ=A0
ð1Þ
A working standard solution of Cu2+ (100.0 μg/mL) was prepared
from stock solution of Cu2+ (1.000 mg/mL) by dilution with distilled
water.
The different pH values of CH3COOH–CH3COONH4 buffer solution
(c: 1.2 mol/L) and NH3–NH4Cl (c: 1.8 mol/L) buffer solution were
employed.
2.2.3. The Benesi–Hildebrand method
In this experiment, the Benesi–Hildebrand method [32,33] (double
reciprocal plot) was used for calculating the inclusion constant (K) of
M-TBCP assuming a 1:1 inclusion model. And the expression was given
by Eq. (2), where [TBCP]0 was the total concentration of TBCP, [M] was
the concentration of metal ions, ΔF was the fluorescence quenching
value and α was constant. Thus, the inclusion constant (K) of the 1:1
complex, which had been calculated by dividing the intercept with the
slope of the double reciprocal plot.
All chemicals were of analytical grade.
2.2. Procedure
2.2.1. Synthesis and spectroscopic characterization of ionic liquid [27,28]
Briefly, 1-butyl-3-methylimidazolium bromide[C4mim][Br] was syn-
thesized by adding equal amount (0.2 mol) of n-methylimidazole
(16.4 g) and n-butyl-bromide (27.4 g) to a 250 mL round bottom flask
fitted with reflux condenser. The flask and its content were stirred and
heated at 75 °C for 2 h until a golden viscous liquid was formed.
Then, 100 mL potassium hexafluorophosphate (36.8 g, 0.2 mol) solu-
tion was added slowly and the mixture solution was stirred at room
temperature for 5 h. Subsequently, the product was washed three
times with 10 mL water and 10 mL diethyl ether in a separation funnel,
respectively. Finally, the collected ionic liquid was heated at 80 °C under
vacuum for 24 h to remove the solvent.
1
1
1
1
¼
ꢁ
þ
ð2Þ
ΔF K⋅α⋅½TBCPꢀ0 ½Mꢀ α⋅½TBCPꢀ0
3. Results and discussion
3.1. Optimization of extraction
3.1.1. Effect of pH
The influence of pH on the extraction ratio (E%) was investigated.
As could be seen in Fig. 2 that (1) E% was gradually enhanced with the
The final RTILs were also identified and approved by IR (Table 2) and
NMR (TMS as the reference for NMR) (Table 3), and the data and results
were in good accordance with that of the literatures [29,30].
Table 2
IR data of RTIL.
Table 1
υmax (cm−1
)
Spectral bands belongs
C–H of imidazole ring
Operational parameter of FAAS.
3171, 3125
2967, 2877
1572, 1466
1169
ν
Instrument parameter
Atomizer
νCH –, ν–CH – of imidazole side chain
3
2
Sensitive line
Slit width
HCL current
213.9 nm
0.5 nm
4.0 mA
C/O-stoichiometric (C2H2/air)
Fuel flow
Usable burner height
0.075
50 NL/h
6 mm
the skeleton vibration of imidazole ring
C–H of imidazole ring
νP–F
δ
881

22
L. Ma et al. / Journal of Molecular Liquids 178 (2013) 20–24
Table 3
1H NMR data of RTIL.
100
δ×10−6
[Bmim] [PF6]
90
1H,s,NCHN
9.06
1H,m,CH3NCHCHN
1H,m,CH3NCHCHN
2H,t,NCH2(CH2)2CH3
3H,s,NCH3
2H,m,NCH2(CH2)2CH3
2H,m,NCH2(CH2)2CH3
3H,t,NCH2(CH2)2CH3
7.73
7.66
4.15
3.84
1.77
1.26
0.90
80
70
60
50
40
increase of pH and reached the maximum when pH=6.0–8.0; and
(2) E% was diminished when pH>8.0. This was because that the ap-
propriate value of pH was subject to the formation of hydrophobic in-
clusion complex of TBCP-Cu(II), which was easily extracted into RTIL
phase. E% was drastically reduced caused by Cu(OH)2 precipitation
(Ksp =2.2×10−20) when pH>8.9. Thus, CH3COONH4 buffer solution
of pH=7.0 was chosen for the determination, and 0.5 mL of the buff-
er solution was selected as suitable for the optimized method.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
V (mL)
Fig. 3. Effect of the amount of TBCP on extraction ratio. The apparent concentration:
(1) Cu=8.0 μg/mL.
very short. E% was maintained a higher value(90%) and kept a steady
trend when t≥4 min. So the process of centrifugation was taken on
the heels of shaking thoroughly and the centrifugation time of
4 min at 1000 rpm was sufficient for the separation process.
3.1.2. Effect of TBCP amount
The effect of the amount of TBCP was tested. The results were
shown in Fig. 3. E% was gradually increased with the amount of
TBCP due to the formation of hydrophobic inclusion complex of
TBCP-Cu(II) extracted into RTIL phase. When the amount of TBCP
(6.0×10−4 mol/L) was 0.8 mL, E% reached the maximum and tended
to be steady. Thus, 1.0 mL TBCP (6.0×10−4 mol/L) was chosen for
the assay.
3.1.5. Effect of equilibrium temperature
The effect of equilibrium temperature on E% was tested. It was
found that E% was steady ranging from 5 to 45 °C, in which extraction
efficiency keep higher. Therefore, the suitable equilibrium tempera-
ture of 25 °C was recommended for the work.
3.1.3. Effect of RTIL amount
The effect of the amount of RTIL was investigated. As was shown
in Fig. 4, E% was remarkably raised with the increase of RTIL amount.
Meanwhile, the quantitative extraction ratio (90%) was observed for a
minimal RTIL amount of 1.0 mL. No significant change was observed
on the extraction efficiency with the higher RTIL amounts. Thus,
1.5 mL RTILs were chosen for the following experiments.
3.2. Effect of foreign substances
The effects of the different foreign substrates were discussed in
the determination of the 8.0 μg/mL of Cu2+. The results are shown
in Table 4. It was observed that most of the common metal ions did
not influence the determination of Cu2+
.
3.3. Analytical performance
3.1.4. Effect of equilibrium time and centrifugation time
In this experiment, the effect of equilibrium time and centrifuga-
tion time was studied. The system was quickly separated into two
phases after the mixture solution was diluted to a final volume and
shaken thoroughly, which illustrated that the equilibrium time was
Under optimum conditions, the linear regression equation was
determined to be: A=0.00456+0.0489c (μg/mL), R=0.9977. A linear
relationship was observed over the range of 0.020 μg/mL–10.0 μg/mL.
100
90
80
70
60
50
40
100
90
80
70
60
50
40
0.0
0.5
1.0
1.5
2.0
2.5
3.0
4
5
6
7
pH
8
9
10
V (mL)
Fig. 4. Effect of the amount of RTIL on extraction ratio. The apparent concentration:
(1) Cu=8.0 μg/mL; (2) TBCP=1.2×10−4 mol/L.
Fig. 2. Effect of pH on extraction ratio. The apparent concentration: (1) Cu=8.0 μg/mL;
(2) TBCP=1.2×10−4 mol/L.

L. Ma et al. / Journal of Molecular Liquids 178 (2013) 20–24
23
1.1x10-2
1.0x10-2
Table 4
Tolerance of interfering ions.
9.0x10-3
Foreign ions
Foreign/Cu(II) (w/w)
Foreign ions
Foreign/Cu(II) (w/w)
Na+
750
750
285
143
38
38
38
12
Fe3+
Al3+
Cd2+
NO3−
Cl−
18
9
8.0x10-3
K+
7.0x10-3
6.0x10-3
5.0x10-3
4.0x10-3
3.0x10-3
2.0x10-3
Ca2+
Mg2+
Mn2+
Cr(VI)
Mo(VI)
Zn2+
3
750
750
750
285
38
Br−
CO32−
SO42−
Table 5
Determination results of standard sample (n=3).
1.0x10-3
0.0
Standard sample
Certified (μg/mL)
Measured (μg/mL)
10.4 0.040
0.0
0.2
0.4
0.6
0.8
1/C
1.0
1.2
1.4
1.6
GBW(E) 080397a
10.0 0.020
a
The standard sample was diluted by 10 times.
Fig. 5. Double reciprocal plot of transition metal ion in TBCP. Mn+: μg/mL.
The detection limit estimated (S/N=3) was 3.59 ng/mL, and RSD
was 3.0%.
3.5.2. The inclusion constant and the extraction efficiency
The hydrophobicity of the extracted object directly affects itself
into the extraction phase. The hydrophilic transition metal ions
were included into the hydrophobic cavities of TBCP to form a stable
hydrophobic inclusion complex, which was beneficial for hydropho-
bic RTIL to extract the inclusion complex of TBCP-metal ions. Thus it
was assumed that the inclusion constant K of TBCP-metal ion
(M-TBCP) influences the extraction efficiency of RTIL. Under opti-
mum experimental conditions, the extraction ratio of transition
metal ions (Cu2+, Zn2+, Fe3+, Cd2+) was determined in Table 7.
3.4. Sample analysis
The proposed method was satisfactorily applied to the determina-
tion of Cu2+ in standard sample (Table 5). The result agreed well with
the certified values.
The standard addition method was used for the determination of
trace amount of Cu2+ in Slender West Lake water sample (in Table 6).
The recovery ratio ranged from 96.7% to 103.5%.
The trend of the extraction ratio E% (E%Cu(II)-TBCP >E%Zn(II)-TBCP
>
E%Fe(III)-TBCP >E%Cd(II)-TBCP) was consistent with the inclusion con-
stant K (KCu(II) >KZn(II) >KFe(II) >KCd(II)). So TBCP had a stronger rec-
ognition ability to the special transition metal ion who had the larger
value of inclusion constant K. In other words, the larger the K, the
higher the E%. Hence, TBCP had very good selectivity extraction for
Cu(II).
3.5. Discussion of extraction mechanism
3.5.1. The inclusion interaction of TBCP and transition metal ions
TBCP was an easy-to-selective modified calix[4]arene with
1,10-phenanthroline at the bottom of calix[4]arene. 1,10-Phenanthroline
is rigid and contains two nearby nitrogen atoms, which acted as π-
electron acceptors and have good coordination capacity with the
transition metal ions.
4. Conclusion
The inclusion constant K is an important parameter for characteriz-
ing the inclusion interaction of host molecule with guest molecule or
ions [33]. The value of the inclusion constant K reflects the intensity of
the binding force of host molecule with metal ions. In this paper, the in-
clusion constants of host molecule (TBCP) with transition metal ions
(Cu2+, Zn2+, Fe3+, Cd2+) were determined and shown in Fig. 5.
According to the Benesi–Hildebrand method, it was found that the
double reciprocal plot had good linear relationships (RCu(II)=0.9981,
In this paper, hydrophobic ionic liquid 1-butyl-3-methylimidazolium
hexafluorophosphate (RTIL, [C4mim] [PF6]) as the green extractant was
researched to extract inclusion complex of TBCP-Cu(II) at pH=7.0.
Based on this, a novel LLE-FAAS for the separation/analysis of Cu(II)
has been developed. The method has been applied for the determination
of Cu(II) in Slender West Lake water sample with satisfactory results.
RZn(II) =0.9956, RFe(III)=0.9952, RCd(II)=0.9976), which could support
Acknowledgments
the formation of a 1:1complex. And the values of inclusion constant
were shown in Table 7 (KCu(II)≫KZn(II)>KFe(III)>KCd(II)). The larger
the value of K, the more steady the inclusion complex. As a result, the
inclusion complex of TBCP-Cu(II) was the most steady complex in this
experiment.
The authors acknowledge the financial support from the National
Natural Science Foundation of China (20875082, 21155001), Jiangsu
Key Laboratory of Environmental Material and Environmental
Engineering and the Foundation of Excellence Science and Technology
Invention Team in Yangzhou University.
Table 6
Determination results of Slender West Lake water.
Sample
Concentration
Concentration of recovery
Recovery
ratio (%)
Table 7
of Cu2+ (μg/mL)
Cu2+ (μg/mL)
Determination results of inclusion constant and extraction ratio.
Slender West
Lake water
0
ND
Transition metal ion
Inclusion constant (K)
Extraction ratio (E)
2.0
4.0
6.0
8.0
2.16
4.62
5.80
7.80
103.0%
103.5%
96.7%
Cu2+
Zn2+
Fe3+
Cd2+
3.79×105 L/mol
1.42×104 L/mol
6.57×103 L/mol
1.72×103 L/mol
91.4%
77.0%
74.0%
71.5%
97.5%
ND: non detected.

24
L. Ma et al. / Journal of Molecular Liquids 178 (2013) 20–24
[17] S. Dai, Y.H. Ju, C.E. Barnes, Journal of the Chemical Society Dalton Transactions
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