A new cerium (III) ion selective electrode based on 2,9-dihydroxy-1,10-diphenoxy-4,7-dithia decane, a novel synthetic ligand

Electrochimica Acta 56 (2011) 9756–9761

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A new cerium (III) ion selective electrode based on 2,9-dihydroxy-1,10-diphenoxy-4,7-dithia decane, a novel synthetic ligand Gholamhossein Rounaghi ∗ , Roya Mohammad Zadeh Kakhki, Hamid Sadeghian Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran

a r t i c l e

i n f o

Article history: Received 12 May 2011 Received in revised form 30 July 2011 Accepted 3 August 2011 Available online 9 August 2011 Keywords: Cerium (III) cation 2,9-Dihydroxy-1,10-diphenoxy-4,7dithiadecane (DHDPDTD) Ion selective electrode Potentiometry

a b s t r a c t In the present study, a novel electrode based on 2,9-dihydroxy-1,10-diphenoxy-4,7-dithiadecane (DHDPDTD) that is selective to cerium (III) cations was evaluated electrochemically, and a Nerenstian slope (19.3 ± 1 mV decade−1 ) over a concentration range of 1.0 × 10−8 –1.0 × 10−1 M and a detection limit of 2.1 × 10−9 M were observed. The proposed electrochemical sensor displayed a rapid response time of 10 s, improved selectivity towards Ce (III) cations in the presence of alkali, alkaline earth, transition and heavy metal cations, and could be used in a pH range of 5.0–8.0. Additionally, the proposed sensor was used as an indicator in the potentiometric titration of fluoride and the determination of F− ions in real samples. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Potentiometric ion selective electrodes (ISEs) are one of the most important types of chemical sensors. A significant number of ionophores, including crown ethers, cryptands, aza-crowns, thiacrowns and thio compounds, have been exploited for the fabrication of polyvinyl chloride (PVC) membrane electrodes for the determination of alkali, alkaline earth, transition and heavy metal ions [1,2]. Ionophores used in sensors should have rapid exchange kinetics and adequate complex formation constants in the membrane. Moreover, ionophores should be soluble in the membrane matrix and have sufficient lipophilicity to prevent leaching from the membrane into the sample solution. In addition, the selectivity of neutral carrier-based ISEs is governed by the stability constant of the neutral carrier–ion complex and the partition constant between the membrane and sample solution [3]. Changes in the structure of the ionophore, such as replacing oxygen with nitrogen or sulfur and/or introducing other constituents into the macrocyclic and noncyclic core, may alter the ligand binding strength and selectivity of the sensor [4]. Sulfur ligands coordinate to transition metal and heavy metal cations as exclusive donor atoms. Thus, macrocyclic and noncyclic thio compounds have attracted widespread attention due to

∗ Corresponding author. Tel.: +98 511 8796416; fax: +98 511 8796416. E-mail address: [email protected] (G. Rounaghi). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.08.015

their unique properties [5,6]. However, little or no reaction occurs between sulfur-containing ligands and alkali or alkaline earth metal ions. In recent years, thia-substituted ligands have been used as neutral carriers in the construction of ISEs for the detection of heavy and transition metal ions [7–10]. Cerium (III) is traditionally referred to as a rare earth element. However, cerium is more plentiful in the earth’s crust than many other rare earth elements. For instance, cerium (III) is found in monazite, ceric bastnaesite, and silicate rocks and is widely used in the production of ductile iron, cast steel, stainless steel [11] and rareearth alloys. Thus, the determination of Ce (III) in samples is of significant interest. Analytical techniques such as ICP-AES [12], electrothermal atomic absorption, spectrofluorometry [13], ICP-AES/HPLC [14] and stripping voltammetry [15] have been used to determine cerium (III) cations; however, these methods are expensive and may be unavailable in some areas. Potentiometric electrodes possess several advantages, including the direct, simple, rapid, inexpensive and selective detection of ionic activity. The selectivity of these sensors stems from the highly selective interactions between the membrane material and the target species [16]. In spite of the significant progress in the design of highly selective ionophores for various metal ions, a number of reports on the development of selective ionophores for lanthanum and other lanthanide ions with relatively good selectivity and sensitivity have been presented [17–20]. Nevertheless, in the majority of these studies, disadvantages such as high detection limits, narrow dynamic range and serious interferences were observed.

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J 6.2 and 15.8, 2 × CH2 O), 4.12 (2H, m, 2 × CH), 4.15 (2H, dd, J 4.1 and13.7, 2 × CH2 O), 6.84–7.38 (10H, m, 2 × Ph); ıC 33.10 (2 × t), 35.80 (2 × t), 69.91 (2 × t), 70.92 (2 × d), 114.90 (2 × d), 121.09 (d), 120.74 (2 × d), 159.23 (d); m/z EI 394 (M+ ), 287 (85%) C13 H19 O3 S2 , 243 (86) C11 H15 O2 S2 , 211 (90) C11 H15 O2 S, 75 (100) C6 H5 [22]. Scheme 1. Structure of C20 H26 O4 S2 .

In the present study, a novel PVC-membrane sensor based on 2,9-dihydroxy-1,10-diphenoxy-4,7-dithiadecane (Scheme 1) as a neutral carrier for the selective and sensitive determination of Ce (III) cations in aqueous media was developed. 2. Experimental

2.3. Apparatus The potentials were measured with a PHM-632 (Metrohm, Swiss) potentiometer equipped with an Ag/AgCl reference electrode. The accuracy of the potentiometer was ±0.01 mV. The measurements were carried out at 25 ◦ C, and the pH of the sample solutions was monitored simultaneously with a conventional glass pH electrode.

2.1. Reagents and standard solutions

2.4. EMF measurement and calibration

Tetrahydrofuran (THF, Merck), ethyl acetate (EtOAc, Merck), cerium (III) nitrate (Merck), copper (II) nitrate (BDH), zinc (II) nitrate (Merck), silver nitrate (Merck), cadmium (II) nitrate (Riedel), lead (II) nitrate (BDH), chromium (III) nitrate (BDH), calcium nitrate (Merck), sodium nitrate (BDH), strontium nitrate (Riedel), magnesium nitrate (Merck), potassium nitrate (Merck), lithium nitrate (Riedel), nickel (II) nitrate (Riedel) and aluminum nitrate (Riedel) were used without further purification. Polyvinyl chloride (PVC) powder, dioctylphetalate (DOP), diobutylphetalate (DBP) and ortho-nitrophenyloctyl ether (oNPOE) were purchased from Fluka. All metal ion solutions were prepared in doubly distilled water by diluting 0.1 M stock solutions.

The following cell assembly was used for all of the measurements: Ag–AgCl/KCl (sat’d)/internal solution 1.0 × 10−3 M Ce(NO3 )3 /PVC membrane/test solution//Ag–AgCl/KCl (sat’d). The performance of the electrode was investigated by measuring the potential of solutions of Ce (III) ions with concentrations ranging from 1 × 10−9 to 1 × 10−1 M at a constant pH. The solutions were stirred, and the potential was recorded when a steady state value was attained. The observed potential versus the logarithm of the Ce (III) ion concentration was plotted. The characteristic properties of the optimized coated membrane were studied. The calibration curve of Ce (III) cations is shown in Fig. 1. Over a wide concentration range (1.0 × 10−8 –1.0 × 10−1 M) of Ce (III) ions, the electrode potential response was linearly related to the logarithm of the Ce (III) cation concentration, and the detection limit was equal to 2.1 × 10−9 M. The slope of the calibration curve was 19.3 ± 1.0 mV decade−1 . Potentiometric titrations of solutions of Ce (III) ions were carried out with a fluoride ion solution, and the PVC membrane electrode was used as an indicator electrode in conjunction with an Ag/AgCl electrode. The concentration of fluoride in tap water and toothpaste was determined, and the results were comparable to those obtained by a fluoride ion selective electrode. Potentiometric selectivity coefficients (KY,M ) were determined according to the separated solution method (SSM).

2.2. Synthesis of 2,9-dihydroxy-1,10-diphenoxy-4,7-dithiadecane The synthetic route for the preparation of the proposed acyclic polyether is described in Scheme 2. ␤,␤ -Dihydroxy-dithioether was prepared by combining two equivalents of epoxide with the deprotonated dimer of captoethane, which was formed by abstracting a proton with carbonate anion under reflux and vigorous stirring. The proposed method is efficient and environmentally friendly [21], and excellent yields and high regioselectivity were observed. In addition, organic solvents were not required. To synthesize 2,9-dihydroxy-1,10-diphenoxy-4,7-dithiadecane, dimercaptoethane (13.5 mL, 160 mmol) and the epoxide (300 mmol) were added to a solution of potassium carbonate (50 g, 350 mmol) in water (65 mL). The mixture was refluxed in an oil bath and was vigorously stirred. The reaction was determined to be complete by TLC (silica gel 60 F254, benzene–ethyl acetate 50:50). Upon completion, the reaction mixture was cooled, and the precipitated product was filtered, washed with water (3 × 50 mL) and dried in an oven at 50–55◦ for 4 h to obtain a white solid (Found: C, 61.13; H, 6.72; S, 16.07. C20 H26 O4 S2 requires C, 60.88; H, 6.64; S, 16.25%); mp = 79 ◦ C (from carbon tetrachloride); ıH 2.39 (2H, br, 2 × OH), 2.71 (2H, dd, J 7.2 and 15.5, 2 × CH2 S), 2.79 (2H, dd, J 4.2 and 17.8, 2 × CH2 S), 2.85 (4H, s, SCH2 CH2 S), 4.04 (2H, dd,

2.5. Electrode preparation To prepare the PVC membrane, 30 mg of powdered PVC, 61.5 mg of NPOE, 5.5 mg of PA, and 3 mg of ionophore were thoroughly mixed in a glass dish with a diameter of 2 cm. Subsequently, the mixture was completely dissolved in 3 mL of THF. The solvent was evaporated slowly until a concentrated mixture was obtained, and a Pyrex tube (2.5 mm o.d.) was dipped into the mixture for 10 s to produce a transparent membrane. Next, the tube was removed from the mixture, stored at room temperature for 24 h, and filled with

Scheme 2. Synthetic route for preparation of the ligand.

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Fig. 1. Calibration curve for cerium (III) ion selective electrode.

Fig. 2. Molar conductance–mole ratio plots for [Ce–DHDPDTD]3+ complex in EtOAc.

an internal filling solution (1.0 × 10−3 M Ce(NO3 )3 ). The electrode was conditioned for 6 h in a 1.0 × 10−3 M solution of Ce(NO3 )3 . The ratio of various membrane ingredients, contact time and concentration of equilibrating solution were optimized to obtain membranes that provided reproducible, stable and noiseless potentials.

tial was applied to the electrodes. The cell constant was equal to 0.73 cm−1 .

3. Results and discussion The ionophore (the membrane-active recognition element) is the most important component of the membrane. Due to the poor water solubility of 2,9-dihydroxy-1,10-diphenoxy-4,7dithiadecane and the observed similarities in the sulfur-based functional groups of the proposed ligand and those used in cerium (III) ion selective electrodes [23,24], the suitability of 2,9dihydroxy-1,10-diphenoxy-4,7-dithiadecaneas an ionophore in a cerium (III) ion selective electrode was examined. In our preliminary studies, to examine the interactions between Ce (III) cations and the ligand, complex formation between the ligand and cerium (III) cations was investigated. For this purpose, a conductometric titration was performed, and the change in molar conductivity (m ) versus the ligand to cation molar ratio ([L]t /[M]t ) was studied to determine the stability constant of the complexation of DHDPDTD with cerium (III) cations in EtOAc. As shown in Fig. 2, the stoichiometry of the complex formed between cerium (III) cation and DHDPDTD was 1:2. The following experimental procedure was employed: a solution of metal salt (1 × 10−4 M) was placed in a titration cell at 25 ◦ C, and the conductance of the solution was measured. Next, the amount of ligand solution (2 × 10−3 M in the same solvent) was increased incrementally by rapidly transferring the solution to the titration cell using a microburette, and the conductance of the solution in the cell was measured at 25 ◦ C after each transfer. The conductance measurements were performed using a digital AMEL conductivity apparatus (model 60) in a water bath thermostated at a constant temperature, which was maintained within ±0.03 ◦ C. The electrolytic conductance was measured using a cell consisting of two platinum electrodes, and an alternating poten-

3.1. Influence of the membrane composition The sensitivity and selectivity of a potentiometric sensor is related to the composition of the membrane, the nature of the plasticizer, the plasticizer/PVC ratio and the type of additive [25,26]. In the present study, the effect of the nature and amount of plasticizer and additive on the potential response of the proposed Ce (III) potentiometric sensor was investigated, and the results are shown in Table 1. Polar plasticizers lead to lower membrane resistances than apolar plasticizers, which contain functional groups with potential coordination sites that can compete with the carrier [27]. Thus, several solvents, such as DOP, NB, DBP, and o-NPOE, were tested in the proposed sensor. With palmitic acid as an ionic additive, DOP and DBP provided low linear ranges (10−5 –10−2 ), and NB produced a non-Nernstian slope of 14. The Ce (III) ion selective electrode based on o-NPOE provided superior results compared to the other mediators; therefore, o-NPOE was selected for further investigations. The potential response of the electrode was also investigated in the presence of ionic liquids and lipophilic surfactants such as oleic acid, tetraphenyl borate (TPB) and palmitic acid. The results showed that the slope and linear range improved in the presence of palmitic acid (a long-chain fatty acid). The nature and amount of lipophilic additives strongly influences the response of an ion selective electrode. In fact, the presence of negatively charged lipophilic additives improves the potentiometric behavior of certain cation selective electrodes by reducing the ohmic resistance and improving the response behavior and selectivity [28]. Moreover, in some cases, lipophilic additives may catalyze the exchange kinetics at the sample–membrane interface [29]. In the presence of ionophores with poor extraction capacities, negatively charged lipophilic additives may increase the sensitivity of membrane electrodes [30–32]. Palmitic acid was likely interposed between the matrix (61.5 wt% NPOE, 30 wt% PVC) and the ionophore, facilitating effective binding. Palmitic acid may also prevent the localization of the active

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Table 1 Optimization of membrane ingredients. Membrane

1 2 3 4 5 6 7 8 9 10 11 12 13 14 a

Composition (wt%)

PVC

Plasticizer

Ionophore

Additive

30 30 30 30 30 30 30 30 30 30 30 30 30 30

62 (DOP) 62 (DBP) 62 (NPOE) 61.5 (NPOE) 61.5 (NB) 62(NB) 61.5 (NPOE) 61.5 (NPOE) 62 (NPOE) 62 (DBP) 62 (NPOE) 61 (NPOE) 61.5 (NPOE) 62 (NPOE)

2 2 2 3 3 2 2.5 3 2 2 2 2 3 2

6 (PA) 6 (TPB) 6 (ionic liquida ) 5.5 (ionic liquida ) 5.5 (PA) 6 (PA) 6 (TPB) 5.5 (TPB) 6 (TPB) 6 (PA) 6 (OA) 7 (PA) 5.5 (PA) 6 (PA)

Slope (mV decade−1 )

Linear range (log cx )

17.2 22.5 12.5 10 14.25 14.23 9.2 9.21 7.5 18 16 21.3 19.3 21.7

−5 to −2 −6 to −2 −3 to −1 −3 to −1 −7 to −1 −7 to −2 −5 to −1 −8 to −2 −8 to −2 −5 to −2 −3 to −1 −6 to −1 −8 to −1 −8 to −1

1-Ethyl-3-methylimidazolium hexafluorophosphate.

site of the ionophore deep within the membrane. The addition of a lipophilic counter ion such as a fatty acid to the organic membrane phase can impart a greater degree of lipophilicity to the cerium (III) selective membrane. Thus, the addition of 5.5 wt% PA (Membrane No. 7) significantly increased the sensitivity of the response of the Ce3+ sensor.

and the change in the potential vs. time is shown in Fig. 4. As shown in the figure, throughout the entire concentration range, the electrode reached equilibrium in a short period of time (10 s) due to the fast exchange kinetics of the complexation–decomplexation of Ce (III) cations with the ion carrier at the test solution–membrane interface.

3.2. Effect of pH

3.4. Potentiometric selectivity

The relationship between the pH and the potentials of the proposed ion selective electrode was investigated by measuring the potential at a pH range of 1–11. Experimentally, the pH was adjusted with dilute solutions of HNO3 and NaOH. The effect of the pH on the proposed ion selective electrode is shown in Fig. 3. As shown in the figure, the potential remained constant over a pH range of 5.0–8.0. Therefore, the working pH range of the proposed electrode is 5.0–8.0. A significant change in the potential response was observed at pHs greater than 8.0, which may be due to the formation of hydroxyl complexes of Ce (III) cations, which reduce the free cation concentration in solution. The observed drift in the electrode potentials at pHs less than 5 may be due to the hydrolysis of Ce (III) cations in solution.

Selectivity is one of the most important characteristics of a chemical sensor. Thus, the selectivity of ISEs is measured in terms Pot ). In the present of the potentiometric selectivity coefficient (KY,M work, the separated solution method (SSM) was applied [33,34], and the concentration of interfering ions was set to 0.1 M. Selectivity parameter data for various ions are presented in Table 2. The selectivity coefficient of the proposed sensor was compared to those of other cerium (III) ion selective electrodes, and the results indicated that the present electrode has good selectivity for Ce (III) ions in the presence of other metal ions, including alkali, alkaline earth, transition and heavy metal ions.

3.3. Response time

The proposed ion selective electrode displayed excellent performance under laboratory conditions. A typical potentiometric titration curve for the titration of Ce (III) cations (20 ml of 1 × 10−3 M) with a solution of sodium fluoride (0.01 M) is shown in Fig. 5. The end point of the titration and the concentration of Ce (III) cations in solution can be determined potentiometrically using

The response time is an important factor for ion selective electrodes. In the present study, the practical response time was recorded over a Ce (III) cation concentration range of 10−4 –10−1 M,

Fig. 3. Effect of pH of the test solution on the potential response of the cerium (III) ISE.

3.5. Analytical applications

Fig. 4. The response time curve of the cerium (III) selective electrode for four different concentrations (mol dm−3 ).

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Table 2 Selectivity coefficient for various interfering ions for various Ce3+ ISEs. Mn+

Present study

[7]

Mg2+ Li+ K+ Ag+ Pb2+ Na+ Zn2+ Ca2+ Cu2+ Ni2+ Al3+ Sr2+

<−7 −2.35 −5.3 −4.95 −4.95 <−7 −6 <−7 −6.55 <−7 <−7 <−7

−2.5 −2.79 −2.2 −1.5 −1.9 −2.62 −1.2 −1.6 −1.4 −1.89 – –

[10]

[35]

[36]

[37]

[23]

–

−2.4

−6.0

−3.89 −4.0 −2.3 −3.85 −2.27 −2.3 −2.3 −2.29 −1.68 −2.28

– −2.8 −2.6 −6.00 −6.00 −6 −2.4

−0.4 – −0.7 −1.2 −0.7 − − −0.6 −0.6 – – –

−3.42, −3.65 −3.5, − 4.36 −3.68, −4.49 −2.11, −2.15 −2.52, −2.79 −3.6, −4.36 −2.32, −3 −3.66, −3.82 −2.14, −2.54 −2.19, −2.82

– −3.9 −2

– −4.2

−2.49 −6.00

[38] −1.39 −1.03

−1.19 −3.02 −1.33 −3.98 −1.20

−3.72, −3.85

[39]

[40]

−2.03 – – −3.38 −3.06 −3.54 −2.42 − −2.22 −2.21

−2.55

−2.10

−3.22

2.30 −2.32 −2.58 −3.00 −3.24 −2.52 −2.34

Table 3 Results of the determination of the fluoride in the different samples. Sample

Fluoride electrodea

Found ISEb

Rec %

Sodium fluoride toothpaste Sodium fluoride tap water

0.74 ± 0.02 (%) 0.12 ± 0.03 (mg dm−3 )

0.61 ± 0.03 (%) 0.12 ± 0.04 (mg dm−3 )

0.21 0.20

a b c

Solid state fluoride electrode. Proposed cerium sensor. Relative error.

Table 4 General performance characteristics of some cerium ion selective sensors. References [7] [10] [35] [36] [23] [37] [38] [39] [40] This work

Linearity/mol/dm3 −6

Detection limit/mol/dm3 −2

2.0 × 10 –2.0 × 10 4.7 × 10−4 –2.5 × 10−8 6.6 × 10–7 –6.2 × 10 2 2.0 × 10−6 –2.0 × 10−2 5.0 × 10−6 –5.0 × 10−2 , 1.0 × 10−7 –1.0 × 10−2 1 × 10−5 –1 × 10−1 10−1 –2.5 × 10−6 1.41 × 10−7 –1.0 × 10−2 1.0 × 10−5 –1.0 × 10−1 1.0 × 10−8 –1.0 × 10−1

−1

−5

1.0 × 10 –5.0 × 10 2.0 × 10−8 2.3 × 10−7 1.8 × 10−6 3.5 × 10−6 and 8.0 × 10−8 3 × 10−5 1.6 × 10−6 As 8.91 × 10−8 7.6 × 10−6 2.1 × 10−9

the proposed ion selective electrode. The present electrode was successfully used for the determination of fluoride ions in aqueous solutions of tap water and pharmaceutical preparations such as toothpaste. In each case, the ionic strength of the solutions was adjusted using a TISAB solution and the pH value was adjusted to 5.0 and the solution was successfully titrated. The fluoride ion concentration in a sample solution was also determined with a fluoride ion selective electrode as a reference, and the results are compared in Table 3. As shown in the table, the results obtained with both ion selective electrodes were in agreement.

Slope/mV decade−1

Response time/s

pH range

19.4 ± 0.4 19.2 ± 0.1 19.5 ± 0.3 19.6 ± 1.0 19.4 19.0 19.5 ± 0.2 20 19.4 ± 0.3 19.3 ± 0.43

<15 10 <10 13 15 20 <10 <10 15 10

5.0–8.0 5.0–8.0 4.5–8.5 4.1–7.3 5–8 3.5–8.0 4–8 3–8 3.5–10.0 5–8

4. Conclusions A PVC membrane electrochemical sensor incorporating 2,9dihydroxy-1,10-diphenoxy-4,7-dithiadecane as a novel ionophore was successfully used to determine cerium (III) cations over a concentration range of 1.0 × 10−8 –1.0 × 10−1 M. The sensor displayed a Nernstian slope of 19.3 ± 1 mV decade−1 and a response time of 10 s. The proposed sensor showed superior selectivity, stability, working concentration range and slope compared to other Ce (III) ion selective electrodes reported in the literature (Table 4). The Ce (III) selective membrane sensor can be used for the direct determination of Ce (III) cations in aqueous solutions and was successfully applied for the determination of F− in real samples. Acknowledgment The authors acknowledge the support by Ferdowsi University of Mashhad, Mashhad, Iran. References

Fig. 5. Potential titration curves for 20.0 mL of 1.0 × 10−3 mol dm−3 cerium (III) cation with 0.01 mol dm−3 sodium fluoride.

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