Electrochemical Studies of Polyether N,N-Di

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Considering Eq. 4 and 5, it is possible to obtain the following equation26 ..... H Matsuda, Y. Yamaha, K. Kanamori, Y Kudo, and Y. Takeda, Bull. Chem. ... R. C. Weast and D. R. Lide, CRC Handbook of Chemistry and Physics, 70th ed., p. F187 ...
Journal of The Electrochemical Society, 157 共10兲 F144-F148 共2010兲

F144

0013-4651/2010/157共10兲/F144/5/$28.00 © The Electrochemical Society

Electrochemical Studies of Polyether N,N-Di(methylenecarboxyethoxy) 4,10-diaza-2,3,11,12-dibenzo18-crown-6 at the Waterⱍ1,2-Dichloroethane Interface G. Guerrero-Trejo,a J. C. Aguilar,b J. Amador-Hernández,a,c and M. Velázquez-Manzanaresa,c,z a

Instituto de Ecología, Universidad del Mar, CP 70902 Oaxaca, Mexico Departamento de Química Analitica, Facultad de Química, Universidad Nacional Autónoma de México, CP 04510 México D.F., Mexico

b

The electrochemical behavior of the new ionophore N,N-di共methylenecarboxyethoxy兲 4,10-diaza-2,3,11,12-dibenzo-18-crown-6 共DIAZ18C6兲 at the water兩1,2-dichloroethane 共1,2-dichloroethane = 1,2-DCE兲 interface was studied by means of cyclic and square wave voltammetry techniques. DIAZ18C6 was able to transfer Pb2+, Zn2+, and Cd2+ across the interface and stoichiometries of 1:1 共Pb2+,Zn2兲 and 1:2 共Cd2+兲 were found. DIAZ18C6 is highly soluble in 1,2-DCE; this mean that an interfacial complex formation takes place at the interface. These studies showed that a facilitated transfer of Pb2+ requires less energy to be transferred from water to the organic phase than the facilitated transfer of Zn2+ and Cd2+ across the water 兩 1,2-DCE interface. As a consequence, this ligand showed selectivity for Pb2+ over the other studied cations. The present work could contribute to a better understanding of the transfer of heavy metal in a biological cell or of selective metal extraction. © 2010 The Electrochemical Society. 关DOI: 10.1149/1.3467803兴 All rights reserved. Manuscript submitted January 18, 2010; revised manuscript received June 7, 2010. Published August 9, 2010.

Heavy metals such as lead and cadmium are toxic for living systems; they are eliminated by the industry and can contaminate oceans, rivers, and soil. In the biological cell, these kinds of metals have a high ability to react with several classes of organic molecules, for example, citrate or fulvic acid.1 In particular, the transfer of these metals across biological membranes implies absorption and distribution in the body.2 Metals such as zinc at lower concentrations are also a key for the economy of a biological system but can be toxic at higher concentrations.1 Cadmium and lead even at lower concentrations can be powerfully dangerous for living systems. For this reason, great effort has been made to understand the transport mechanism and bioaccumulation of heavy metals in the biological cell.2 The interface of two immiscible electrolyte solutions 共ITIES兲 has been an interesting tool in studying the partition of ionic species across hydrophobic barriers. For a recent review, see Ref. 3. The ITIES can be useful in simplifying the ion transfer mechanism across biological membranes. Professor Koryta was the first to postulate that the ITIES could behave as a metal electrode兩electrolyte solution interface,4 so that traditional electrochemical techniques such as cyclic voltammetry and ac impedance spectroscopy can be applied to the study charge transfer across the ITIES.5 In the literature, there are numerous works related to the study of the facilitated transfer process of metal ions across the ITIES in which organic ionophores of different chemical natures have been tested, such as crown ethers,6 antibiotic polyether carboxylic,7 diazadibenzo crown ethers,8 thioethers,9 10 9-ethyl-3-carbazolecarboxaldehyde-thiosemicarbazone, ETH 1062,11 and the use of poly共ethylene glycol兲 as a new method for lead determination proposed by Sun and Vanysek.12 In general, these ionophores have shown selectivity for specific metal ions, which have been evaluated in the functions for energy requirements, ionic radius, and oxidation state. The study of metal transfer at the ITIES can help to mimic the behavior of the natural transporters that are found in biological membranes2 by mean of an analysis of the metal partition across the water 兩 oil interface.13 For example, the facilitated transfer of Cu2+ by a tetradentate ligand that contains phosphorus and nitrogen in its structure has been studied at the ITIES in a rare sequence of donor atoms “PNNP,” giving an interesting chemistry of coordination re-

c

Present address: Instituto de Biotecnología, Universidad del Papaloapan, Circuito Central, no. 200, Col. Parque Industrial, C. P. 68301, Tuxtepec, Oaxaca, Mexico. z E-mail: [email protected]

sults. This chemical characteristic enhances the selectivity of the ionophore for a facilitated transfer of the transition metal ions at the water兩organic solvent interface.14 Diazadibenzo crown ethers have donor atoms 共nitrogen and oxygen兲, which are able to form a complex with divalent metal cations with a large selectivity over monovalent cations.15 This variety of ligands can transport divalent metal cation across the plasticized cellulose triacetate membranes. Especially the new N,N-di共methylenecarboxyethoxy兲 4,10-diaza-2,3,11,12-dibenzo-18crown-6 共DIAZ18C6兲 is highly soluble in organic solvent with a partition coefficient 共log P = 2.55兲 and two acid dissociation constants 共pKa = 1.68 and 0.99兲 determined in water and chloroform.15 The electroassisted transfer of heavy metals using DIAZ18C6 has not yet been reported. The purpose of the present work was to investigate the electrochemical properties of the new ionophore DIAZ18C6 to assisted transfer of Pb2+, Cd2+, and Zn2+ across the water兩1,2-DCE interface. Experimental The cyclic voltammetry and square wave voltammetry studies were carried out in a glass cell with a four-electrode configuration with a contact area between the two liquids of 0.2 cm2. The cell design consisted of four compartments: two compartments for reference electrodes 共one in each phase兲 and two counterelectrodes, one in the organic phase and the other in the aqueous phase. More details on the cell design were showed in Ref. 16. The interfacial potential was controlled with a potentiostat/ galvanostat 共Autolab PGSTAT30, Eco Chemie兲. Thus the measured potential corresponds to the following cell SCE1兩TPAsCl共w兲 10 mM



TPAsTPBCl共o兲 10 mM

ionophore x mM



␴兩MCl2共w兲兩SCE2 y mM

where x mM is the ionophore concentration in the organic phase, y mM is the metal concentration in the aqueous phase, ␴ represents the interface of study, and SCE1 and SCE2 are saturate calomel electrodes. Tetraphenylarsonium chloride 共Fluka兲, 1,2-dichloroethane 共1,2DCE, Gold Label, Aldrich兲 and potassiumtetrakis共4chlorophenyl兲borate 共Fluka purum兲 were used as received. Tetraphenylarsonium tetrakis共4-chlorophenyl兲borate 共TPAsTPBCl兲 was precipitated from the corresponding chemicals and recrystallized twice from acetone 共BDH, AnalaR兲. The ligand DIAZ18C6 共Fig. 1兲 was prepared at the Universidad Nacional Autonoma de Mexico.

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Journal of The Electrochemical Society, 157 共10兲 F144-F148 共2010兲

O

O

O

O

O N

N

O

O O

Figure 1. Chemical structure of DIAZ18C6.

Details on its preparation and characterization can be found in Ref. 15. The metal salts employed were PbCl2, ZnNO3, ZnCl2, and CdCl2 共Sigma-Aldrich兲. The aqueous solutions were prepared in ultrapure water 共Easypure UV, Barnstead兲. All the experiments were carried out at 25 ⫾ 1°C. During measurements, the electrochemical cell was placed inside a Faraday cage. The potentials were reported on the Galvani potential scale and calculated according to17 w Ecell = ⌬w o ␾ − ⌬o ␾TPAs+

⌬w o␾

关1兴 ⌬w o ␾TPAs+

is the Galvani interfacial potential and is the where organic reference liquid junction potential.18 The standard transfer o 19 + potential of TPAs+, ⌬w o ␾TPAs+ for TPAs was taken as ⫺0.364 V.

possible to study the transfer of any “electroactive species.” At the edge of the potential window, in the cathodic direction, the aqueous phase is more negative than in the organic phase; thus, TPAs+ is transferred to the aqueous phase and the anion 共Cl−兲 into the organic phase. The width of the window potential depends on the Gibbs transfer energy of the anions and cations present in the supporting electrolyte in both phases. The ionophore causes a reduction in the energy that metal cations require to be transferred into the organic phase by means of a hydrophobic complex formation 共ionophore-cation兲. DIAZ18C6 was added and kept for 20 min to get a homogeneous distribution of the ionophore in the organic phase after a cyclic voltammetry experiment was performed. The CV shows two separate current peaks for the facilitated transfer of Pb2+; these most likely correspond to the two transfer mechanisms as the metal is transferred into the organic phase 共Fig. 2a兲 with two energetic requirements. These two peaks are reversible because the peak-to-peak separation for this process was 60 mV, which agrees with the prediction of the Nernst equation for the transfer of one single charge20 rather than a double charge, as could be expected for the facilitated transfer of Pb2+. A likely explanation for this result involves a very stable monovalent complex because chloride anions can coordinate with Pb2+ to form PbCl+ 共log Kf共PbCl+兲兲 = 1.6.21 Due to the low solubility of other lead共II兲 compounds, it was difficult to study the effect of other anions such as SO2− 4 . Because the partition constant of DIAZ18C6 at the water and chloroform system is higher, it can be assumed that the facilitated transfer of Pb2+ takes place by interfacial complexation 共TIC兲 or it is a transfer by interfacial dissociation 共TID兲 reaction.22 Thus, the most likely chemical equilibrium for the facilitated transfer of Pb2+ across the water兩1,2-DCE interface for the first and second peaks can be written as follows

Results and Discussion Cyclic voltammetric studies.— Figure 2 shows the cyclic voltammogram 共CV兲 for the facilitated transfer of Pb2+ across the water兩1,2-DCE interface. The supporting electrolyte TPAsTPBCl in the organic phase and PbCl2 in the aqueous phase 共Fig. 2a兲 give a wide potential window. TPAsTPBCl in 1,2-DCE is highly soluble and dissociated in TPAs+ and TPBCl−. When the polarization is scanned in the anodic direction, the aqueous phase is made positive with respect to the organic phase. Thus, TPBCl− is transferred into the aqueous phase and the positive charge of the aqueous phase moves into the organic phase. In fact, the transfer of Pb2+ into the organic phase takes place at a large standard transfer interfacial Galo vani potential 共⌬w o ␾Pb2+兲 of 505 mV at the water兩1,2-DCE 9 interface. This value is outside the potential window given by the aforementioned supporting electrolyte; for this reason it is not possible to observe any current wave in the CV corresponding to the free transfer of Pb2+. When the direction of the scan potential is reversed, TPBCl− is transferred back into the organic phase; if the scan potential keeps going, an ideal polarization region appears 共no faradaic reaction takes place兲. This means that in this region it is

30

15

2+

Pb

2+

Pb

PbCl共+w兲 + L共o兲 ⇔ PbLCl共+o兲

Ip = 2.69 ⫻ 105An3/2D1/2␯1/2Co

-2

-30 -0.30

(a)

-0.15

0.00 w

∆ oφ/V

0.15

0.1 mM 0.2 mM 0.3 mM

Figure 2. 共a兲 CV for the facilitated transfer of Pb2+ by DIAZ18C6 across the water兩1,2-DCE interface. Supporting electrolytes: 0.01 M TPAsTPBCl in the organic phase and 0.01 M PbCl2 in the aqueous phase. Sweep rate: 50 mV/s. 共b兲 Square wave voltammetry for the facilitated transfer of Pb2+ by DIAZ18C6. Frequency 10 Hz and sweep rate 50 mV/s. Experimental conditions as in Fig. 2.

10

5

0.30

-0.15

(b)

关3兴

where Ip is the current peak, A is the interfacial area between the two immiscible liquids, D is the diffusion coefficient, ␯ is the sweep rate, n is the transferred ion charge, and Co is the concentration of the ionophore in the organic phase. The diffusion coefficient for this

j/μAcm

j/μAcm

-2

0.1mM 0.2mM 0.3mM Baseline

-15

关2兴

where L = ionophore. A half-wave interfacial Galvani potential 共⌬w o ␾1/2兲 of −148 ⫾ 4 and 72 ⫾ 3 mV was found. In the present studies, the experiments were carried out within pH 4.8–5.0 in the aqueous phase. The metal concentration was higher than the proton concentration 共关H+兴 ⬍ 关Pb2+兴兲, and due to the acid pKa of this compound, the proton did not interfere with the analysis of assisted transfer of Pb2+ from water to the organic phase. For a constant ionophore concentration in the organic phase, the current peaks are proportional to the square wave of the polarization sweep rate 共data not shown兲. This behavior can be analyzed with the Randles–Sevčik equation for a controlled diffusion process20

15 0

F145

0.00

0.15

w

∆ oφ/V

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Journal of The Electrochemical Society, 157 共10兲 F144-F148 共2010兲

F146

⌬w o ␾1/2 and the DIAZ16C6 concentration can be simplified if the association constant for each intermediate complex are smaller than ␤so

0.00

w

z+

∆ oφ1/2, LM / V



-0.06

o w 共s − j兲␤s−j cM init

-0.18

⌬w o ␾1/2

-4.62

-4.29

-3.96

o

log[cL /2] / M

w

process was DPb2+ = 1.86 ⫻ 10−6 and 3.2 ⫻ 10−6 cm2 s−1. These values are close to the previous values reported by other authors.23 To measure the ⌬w o ␾1/2 at the water兩1,2DCE interface with more accuracy, a square wave voltammetry technique was employed 共no IR compensation was used兲. Figure 3b shows the square wave voltammogram 共SWV兲 for the facilitated transfer of Pb2+ assisted by different concentrations of DIAZ18C6 across the liquid|liquid interface. The result shows the two separate peaks for the transfer of this metal into the organic phase; these peaks correspond to the ⌬w o ␾1/2 for the facilitated transfer of Pb2+ by the ionophore. These results are consistent with the cyclic voltammetric studies. The current peaks increase on the function of the ionophore concentrations; however, the ⌬w o ␾1/2 remains in the same potential. The two observed waves in the CV for the transfer of Pb2+ facilitated by DIAZA18C6 can be explained by the fact that DIAZ16C6 is quite hydrophobic being soluble only in organic phase and that PbLCl+ is not formed and not soluble in the water phase. The reversible half-wave potential for this process can assume that the following structure of the complex is 关PbLCl兴+; it can follow the discussion given in Ref. 24 and 25

w cM init

=

o⬘ ⌬w o ␾Mz+

冋兺 冉 冊 册

RT − ln zF

s

w j␤jocMz+

init

j=1

关4兴

2

cLo init

45

2+

Cd

cLo 2

共s−1兲

关5兴

-2

0 0.1 mM 0.2 mM 0.3 mM

-15

w ⬘ ⌬w o ␾1/2 = ⌬o ␾Mz+ + o

0.00

0.15 w

∆ oφ/V

0.30

w ⬘ DoL = DLMz+, then ⌬w o ␾1/2 = ⌬o ␾Mz+ . According to this assumption, the formal Gibbs energy for the transfer of the metal cation o

o

0.1mM 0.2mM 0,3mM

2+

30

-0.15

(b)

关7兴 o

15

0.45

DLo RT ln o zF DLMz+

where z is the charge of the transferred species and DLMz+ and DoL are the diffusion coefficients of the complex 共ionophore-metal兲 and the ionophore in the organic phase, respectively. Assuming that

-30 -0.15

关6兴

s

Cd

15

冉 冊

cLo init 共s − 1兲RT RT o w − ln共s␤s cMz+兲 − ln s 2 zF zF

concentration of the metal in the aqueous phase, coL is the concentration of the ligand in the organic phase, and F is Faraday’s constant. Thus, log ␤o1 = 13.03 and 9.32 for the two observed peak currents on the CV and SWV, whereas the stoichiometric coefficient for the facilitated transfer of the Pb2+ by the ligand was s = 1 for both peaks. The relative values for the facilitated transfer of Pb2+ agree with a prior work where the facilitated transfer of Pb2+ was tested using the same family of ionophores.8 The interaction of the heavy metal with the donor atoms in the ligand makes this complex quite stable, and the presence of the esters groups increases the ability of the ionophore to protect the cation from the hydrophobic environment. Thus, at the same time as it forms the complex in the interface water phase and 1,2-DCE phase, the ionophore replaces the water molecules that surround the metal ion, carrying the metal in the organic phase shortly afterward. The latter assumption can be explained as TIC-TID reactions,22,27 as it was mentioned before. This kind of reaction is quite similar to the electron transfer at a metallic electrode.6 In a general way, the following equation that describes the facilitated transfer of metal cations at the ITIES28 was considered

j/μAcm

-2

冉冊

⬘ where ⌬w o ␾Mz+ is the formal transfer interfacial Galvani potential for the metal-free cation 共including the activity coefficient of the metal in the aqueous phase and the association constant兲, s is the stoichiw ometric coefficient, ␤so is the stability of the complex, cMz+ is the

30

j/μAcm

j=1

w Ⰶ s␤socM init

j−1

cLo init

and are the initial concentrations of the metal ion in where the water phase and the ligand in the organic phase, respectively. In fact in Ref. 25, the authors assumed that the relationship between

(a)

共s−1兲−j s−1

o

Figure 3. Half-wave potential 共⌬ao␾1/2兲 in the function of the logarithms of the ionophore concentrations for the facilitated transfer of Pb2+ at the water|1,2-DCE interface.

⌬w o ␾1/2

=

o⬘ ⌬w o ␾Mz+

-3.63

init

-0.30

cLo 2

The variation in the ⌬w o ␾1/2 was plotted as a function of log关DIAZ18C6兴 and a linear relationship was observed; this can suggest that equilibrium effectively displays a multiligand complex. Considering Eq. 4 and 5, it is possible to obtain the following equation26 共Fig. 3兲

-0.12

45

冉冊 册

0.00

0.15

Figure 4. 共a兲 CV for the facilitated transfer of Cd2+ by DIAZ18C6 across the water兩1,2-DCE interface. Supporting electrolytes: 0.01 M TPAsTPBCl in the organic phase and 0.01 M CdCl2 in the aqueous phase. Sweep rate: 50 mV/s. 共b兲 SWV for the facilitated transfer of Cd2+ by DIAZ18C6. Frequency 10 Hz and sweep rate 50 mV/s. Experimental conditions as in Fig. 4a.

0.30

w

∆ oφ/V

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Journal of The Electrochemical Society, 157 共10兲 F144-F148 共2010兲

2+

2+

Zn

Zn

15

j/μAcm-2

-2

j/μAcm

0.1mM 0.2mM 0.3mM

8

0

4

0.1mM 0.2mM 0.3mM

-15

-0.30

-0.15

0.00

(a)

0.15 ∆ oφ/V w

0.30

0

0.45

-0.15

0.00

(b)

o⬘,w→o 共⌬w 兲 from water to the organic phase can be estimated o Gt from29 o⬘,w→o 共␮Mz+ − ␮Mz+兲 ⌬w o Gt = zF zF 0,o

⬘ ⌬w o ␾Mz+ = o

0,o

F147

0,w

关8兴

0,w

where ␮Mz+ and ␮Mz+ are the standard chemical potentials for metal ion 共M z+兲 in the organic and aqueous phases, respectively. The

o⬘,w→o for the facilitated transfer of Pb2+ by the DIAZ18C6 was ⌬w o Gt ⫺14.78 and 6.95 kJ/mol for the two observed peaks in the CV and the SWV, respectively. These values are smaller when compared to o⬘,w→o = 97 kJ mol−1 9 for the transfer of free metal. This the ⌬w o Gt confirms that DIAZ18C6 is able to diminish the energy needed for the transfer of metal to the organic phase independently of the mechanism of the metal transfer.

Facilitated transfer of Cd2⫹ and Zn2⫹.— Figure 4a shows the CV for the facilitated transter of Cd2+ at the water兩1,2-DCE interface. When DIAZ18C6 is added and the polarization is swept in the positive potential direction, one not well-defined positive current was observed in comparison with the two current peaks observed on the CV for the facilitated transfer of Pb2+. For the facilitated transfer of Cd2+, it was difficult to establish the amount of transferred charge across the interface because it is hard to evaluate the peak-to-peak separation in the CV. However, the current peaks can be attributed to the facilitated transfer of Cd2+ across the interface. When the direction of the scan potential is reversed, two negative current peaks are observed at lower DIAZ18C6 concentration. If the concentration of DIAZ18C6 increases, the negative current peaks “disappear;” actually, the more positive peak is hidden due to the enhanced signal and is not easy to evaluate; thus, apparently, this has simply become one “quasi-reversible wave.” It is highly probable that TPBCl− from the supporting electrolyte can participate on the Cd–ionophore complex when this is found in the organic phase; this probably explains the negative current on the CV when the metal is released into the aqueous phase when the potential goes back to the positive potential direction. The stoichiometry of the transfer process cannot be evaluated using Homolka criteria because the peak-to-peak separation was 0.128 V at higher ionophore concentration; this is different from the proposed value of 0.112 V for an ion-to-ligand stoichiometry 1:3.27 Instead, the stoichiometry was evaluated using Eq. 6 and assuming one transferring charge 共discussed below兲. The potential is more positive than the lead transfer facilitated by the ionophore, and a plausible explanation is that cadmium共II兲 has a stronger water shell due to its smaller ionic radius 共Cd2+ = 0.097 nm兲 compared to Pb2+ 共0.120 nm兲.30 This characteristic increase the charge density and the water shell on Cd2+ became to be stronger compared to the Pb2+ cation; as a consequence, thus, this makes it difficult for the ionophore to replace the water molecules that surround the metal ion and guess the cation. From this point of view, DIAZA18C6 is not able to extract the cation efficiently. This may be responsible for the

0.15 w ∆ oφ/V

Figure 5. 共a兲 CV for the facilitated transfer of Zn2+ by DIAZ18C6 across the water兩1,2-DCE interface. Supporting electrolytes: 0.01 M TPAsTPBCl in the organic phase and 0.01 M ZnCl2 in the aqueous phase. Sweep rate: 50 mV/s. 共b兲 SWV for the facitated transfer of Zn2+ by DIAZ18C6. Frequency 10 Hz and sweep rate 50 mV/s. Experimental conditions as in Fig. 5a.

0.30

quasi-reversible transfer behavior; however, due to the large log P of DIAZA18C6 and the excess of metal concentration, the mechanism is still TIC. However, the diffusion coefficient could be obtained from the w positive current and a value of DCd2+ = 2.14 ⫻ 10−6 cm2 s−1 共at 0.3 ionophore concentrations兲 was found. SWV studies were performed to evaluate the ⌬w o ␾1/2. Figure 4b shows the SWV for the facilitated transfer of Cd2+ by means of different DIAZ18C6 concentrations in the organic phase. It is possible to observe that there is one main current peak rather than the two peaks observed on the CV studies, in such case the supporting electrolyte signal is subtracted from the facilitated transfer of Cd2+ by DIAZ18C6; thus, the pure signal for the facilitated metal transfer is obtained. The SWV proved that the small observed peak current on the CV can be attributable to the supporting electrolyte contribution maybe as a part of the complex metal formation or ion paring formation. The SWV makes it easier to evaluate the ⌬w o ␾1/2 = 51.8 ⫾ 5 mV. The observed current peak increases as a function of the concentration of the ionophore; however, the peak current shifts lightly to more negative potential values. Taking into account the same assumption as for the previous metal transfer 共Pb2+兲 and using one charge transfer across the inter-

o⬘,w→o = 4.9 kJ mol−1 and s face and Eq. 6 and 8, values of ⌬w o Gt = 2 are obtained and log ␤o1 = 9.85. This means that for the transfer of this metal, two ionophores per one metal 共2:1兲 are required. The o⬘,w→o is higher compared to the second peak of the value of ⌬w o Gt facilitated transfer of Pb2+. The explanation for this discrepancy is that DIAZ18C6 has more amity with Pb2+ than with Cd2+. In fact,

⬘ w o⬘,w→o values of ⌬w = 103 kJ mol−1 for a o ␾Mz+ = 535 mV and ⌬o Gt 2+ free Cd transfer at the ITIES were observed.9 This difference can be explained by the ionic radius and the hydrated cation radius 共Cd2+ = 0.275 nm and Pb2+ = 0.261 nm兲.30 Figure 5a shows the CV for the facilitated transfer of Zn2+. o

⬘ The free Zn2+ transfer requires a large amount of energy 共⌬w o ␾Mz+ o

o⬘,w→o = 565 mV and ⌬w = 109 kJ mol−1兲 and has been o Gt 9 evaluated at the ITIES. The observed positive current peak on the CV shows a low current and it is not a well-defined peak. The signal lightly increases when the ionophore concentration increases. The transferred charge across the interface was not quantified due to the peak-to-peak separation, which made it impossible to evaluate. At the main positive current peak 共at 0.3 mM of the ionophore兲 and a metal constant concentration, the diffusion coefficient could be obw tained and a value of DZn2+ = 2.1 ⫻ 10−7 cm2 s−1 was found for this process. The lower current observed on the CV experiments is probably due to DIAZ18C6 not being able to form a strong complex with Zn2+ due to its large hydration radius 共0.295 nm兲 and the ionic radius 共0.074兲.30 It is therefore probable that the ionophore cannot efficiently replace the water molecule around the metal cation as it does with the Pb2+ cation, but it is still a better ionophore for the

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F148

Journal of The Electrochemical Society, 157 共10兲 F144-F148 共2010兲

facilitated transfer of Zn2+ compared to the facilitated transfer of Cd2+. Similar experiments of SWV were carried out to evaluate the ⌬w o ␾1/2 and other thermodynamic parameters. Values of o⬘,w→o ␾ = −7 ⫾ 0.08 mV and ⌬w = −0.675 kJ mol−1 and ⌬w 1/2 o o Gt o using Eq. 6, log ␾1 = 11.54 and s = 1 were found. The latter shows that this ionophore can form a metal complex with Zn2+ in a o⬘,w→o is spontaneous way similar to Pb2+, but its value of ⌬w o Gt 2+ larger when compared with the second peak of the Pb transfer 共−14.78 kJ mol−1兲 at the ITIES. However, if we observe the current peak values on CV and SWV for the facilitated transfer of Pb2+, these are larger compared to the facilitated transfer of Zn2+; thus, the extraction seems to be much more efficient for Pb2+ than Zn2+. The smallest extra observed peak current on the CV and SWV for the facilitated transfer of Zn2+ is quite difficult to evaluate because we require a further experimental setup such as ac impedance spectroscopy applied to liquid|liquid interface to show if this ionophore is absorbed at the interface 共when Zn2+ is present兲 on the function of the Galvani applied potential or if it is part of the facilitated transfer of Zn2+ mechanism across the interface. Conclusions These studies showed the ability of the ionophore DIAZ18C6 to assist Pb2+, Zn2+, and Cd2+ across the water兩1,2-DCE interface. The results showed that DIAZ18C6 has more affinity with Pb2+ than with Cd2+ or Zn2+ to be transferred into the organic phase. The profile of this ionophore was Pb2+ ⬎ Zn2+ ⬎ Cd2+ across the water 兩 1,2-DCE interface. The latter depends on the physicochemical characteristics of each cation, such as the ionic radius and the hydrated ionic radius. Acknowledgments The support 共project UMAR-CUP 2IE0505兲 is gratefully acknowledged and G.G.T. thanks CONACyT for granting this scholarship. Universidad del Papaloapan assisted in meeting the publication costs of this article.

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