Silver ion-selective electrodes based on bis(dialkyldithiocarbamates) as neutral ionophores

Sensors and Actuators B 122 (2007) 174–181

Silver ion-selective electrodes based on bis(dialkyldithiocarbamates) as neutral ionophores Zhenning Yan a,∗ , Yanqi Lu b , Xia Li a a b

Department of Chemistry, Zhengzhou University, Zhengzhou, Henan 450052, China Zhengzhou Railway Profession Technology College, Zhengzhou, Henan 450052, China

Received 11 January 2006; received in revised form 19 May 2006; accepted 19 May 2006 Available online 23 June 2006

Abstract Silver ion-selective electrodes (ISEs) have been prepared by incorporating six new bis(dialkyldithiocarbamates) as the neutral ionophores into the plasticized PVC membranes. The construction, response characteristic and application of the silver ISEs are investigated. The better results have been obtained with membranes containing ligands L3–L6 with dibutyl phthalate (DBP) as a plasticizer. These electrodes work well over a wide range of concentration (1.0 × 10−6 to 1.0 × 10−3 M) with Nernstian slopes. The present silver ISEs display very good selectivity for Ag+ ions against an interferent, Hg2+ ion, and the values are around −3.0. The silver ISEs have been used as indicator electrodes for potentiometric titration of Cl− ions in Vitamin B1 tablets using a standard solution of AgNO3 . The proposed electrodes have also been used for the direct determination of silver ions in water samples. © 2006 Elsevier B.V. All rights reserved. Keywords: Silver ion; Sensor; Bis(dialkyldithiocarbamates)

1. Introduction The multi-purpose nature of silver makes its analysis in and recovery from waste material, drink water, and other samples of importance [1]. The crystal membrane silver ISEs made from Ag2 S have been used in the determination of silver. But, Hg2+ ions gave a serious interference [2]. In recent years, the design of carrier-based Ag+ -ISEs has gained importance, mainly due to the neutral ionophore-based Ag+ -ISEs with better selectivity than the standard Ag2 S-based solid-state electrodes for Hg2+ . Because of the ability of forming complexes with metal ions of compatible dimensions, many podands, crown ethers and calixarene derivatives have been exploited in polymeric sensors as ionophores for the determination of Ag+ ions [3–14]. In addition, polythiamacromolecules [15], bis-pyridine tetramide macrocycle [16], bis(dialkyldithiophosphate) derivative [17] and o,o,o-trialkyl phosphorothioates [18] have also been reported as effective ionophores to construct Ag+ -selective electrodes.

∗

Corresponding author. E-mail address: [email protected] (Z. Yan).

0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.05.020

It is well known that the soft heavy metal ions, Ag+ , Pb2+ , and Hg2+ display great affinity for soft coordination centers like sulfur and nitrogen. By using ionophores containing sulfur and nitrogen atoms in ion-selective electrodes, it is expected that the electrodes are sensitive to soft heavy metal ions and the selectivity for soft heavy metal ions against alkali metal ions is significantly increased [19]. In this respect, dithiocarbamates with sulfur donor atoms have attracted much interest owing to the unique properties of the compounds. The dithiocarbamate derivatives are highly versatile chelating agents for metal cations. This type of compounds form selective complexes with metal cations due to their flexible structure, appropriate positions of two dithiocarbamate radicals, and their nonmacrocyclic cavities with four donor sulfur atoms. Some dithiocarbamate derivatives have been used as ionophores in Cu(II), Pb(II), Ag(I) and Hg(II) selective membrane electrodes [20–24]. Taking this advantage into consideration, we designed and synthesized a series of new ligands bis(dialkyldithiocarbamates) as neutral ionophores in Ag(I) selective membrane electrodes. These ligands, which are easily synthesized and have different C-shaped cavities with four donor sulfur atoms, are expected to form selective complexes with transition-metal ions and to give an improved selectivity for the silver ions. The effect of several

Z. Yan et al. / Sensors and Actuators B 122 (2007) 174–181

parameters such as the ionophore concentration and pH was investigated and the optimized membranes were used to obtain the response characteristics and selectivities. In addition, the electrodes were used in titration experiments and for the determination of silver or chloride ion concentrations in real samples. 2. Experimental 2.1. Reagents All reagents were of analytical reagent grade. 2-Nitrophenyloctyl ether (o-NPOE) and potassium tetrakis(4chlorophenyl)borate (KTpClPB) were purchased from Fluka. Dibutyl phthalate (DBP), bis(2-ethylhexyl) sebacate (DOS), tetrahydrofuran (THF) (dried by sodium and distilled prior to use), poly(vinylchoride) (PVC) of high molecular mass were obtained from Shanghai Chemical Company. All aqueous solutions were prepared with doubly deionized, distilled water. 2.2. Ionophore synthesis 2.2.1. General remarks The synthetic routes are shown in Scheme 1. IR spectra were recorded on a Perkin-Elmer FTIR1750 spectrophotometer. 1 H NMR spectra were recorded on a BrukeDPX400 spectrometer, using CDCl3 as the solvent and tetramethylsilane as an internal standard. Elemental analyses were determined with a Carlo Erba 1106 elemental analyzer. All chemical reagents were used as commercial grade. 2.2.2. Synthesis of bis(dialkyldithiocarbamates) To a mixture of 21.5 g (0.2 mol) of N-methylaniline and 8 g (0.2 mol) of sodium hydroxide in 50 ml of ethanol was added 16 g (0.21 mol) of carbon disulfide at room temperature. The mixture was stirred and refluxed for about 30 min, and then evaporated to dryness. The residue solid was recrystallized from ethyl acetate, giving 32.4 g (79% yield) white solid (sodium Nmethyl-N-phenyldithiocarbamate) with mp >300 ◦ C. Sodium N-ethyl-N-phenyldithiocarbamate was similarly prepared; yield 73%, white solid, and mp 98–101 ◦ C. To a solution of 6.15 g (0.03 mol) of the above-prepared sodium N-methyl-N-phenyldithiocarbamate and 0.1 g of potassium iodide in 100 ml anhydrous acetone was added 2.82 g (0.015 mol) of 1,2-dibromoethane, and the mixture was refluxed

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for 8 h. White precipitate appeared during refluxing. After cooling and filtration, the solid was washed thoroughly with water and recrystallized from chloroform to give 4.0 g white crystals (L1). The synthetic procedures of L2–L6 were similar to that for L1. For dimethylene bis(N-methyl-N-phenyldithiocarbamate) L1 (C18 H20 N2 S4 ): yield: 68%, mp 241–243 ◦ C. Anal. found (%): C, 55.02; H, 5.12; N, 6.97. Calc. for C18 H20 N2 S4 : C, 55.10; H, 5.10; N, 7.14. IR (KBr, cm−1 ): 1589(m), 1490(s), 1446(s), 1366(s), 1257(vs), 1089(vs), 946(s), 771(s), 696(s), 631(s), 560(s). 1 H NMR: δ (ppm) 3.45(s, 4H, CH2 ), 3.74 (s, 6H, CH3 ), 7.21–7.49 (m, 10H, Ar–H). For dimethylene bis(N-ethyl-N-phenyldithiocarbamate) L2 (C20 H24 N2 S4 ): yield: 64%, mp 207–209 ◦ C. Anal. found (%): C, 57.41; H, 5.78; N, 6.40. Calc. for C20 H24 N2 S4 : C, 57.14; H, 5.71; N, 6.67. IR (KBr, cm−1 ): 1589(w), 1489(s), 1447(m), 1402(vs), 1276(vs), 1099(s), 994(m), 898(m), 763(s), 693(s), 552(s). 1 H NMR: δ(ppm) 1.24 (t, 6H, CH3 ), 3.43 (s, 4H, CH2 S), 4.32 (q, 4H, CH2 N), 7.17–7.49 (m, 10H, Ar–H). For trimethylene bis(N-methyl-N-phenyldithiocarbamate) L3 (C19 H22 N2 S4 ): yield: 60%, mp 170–173 ◦ C. Anal. found (%): C, 56.11; H, 5.42; N, 6.67. Calc. for C19 H22 N2 S4 : C, 56.16; H, 5.42; N, 6.90. IR (KBr, cm−1 ): 1590(m), 1489(s), 1449(s), 1366(vs), 1257(s), 1097(vs), 994(m), 946(s), 771(s), 698(s), 559(s). 1 H NMR: δ(ppm) 1.97 (m, 2H, CH2 ), 3.20 (t, 4H, CH2 S), 3.74 (s, 6H, CH3 N), 7.18–7.49 (m, 10H, Ar–H). For trimethylene bis(N-ethyl-N-phenyldithiocarbamate) L4 (C21 H26 N2 S4 ): yield: 61%, mp 88–90 ◦ C. Anal. found (%): C, 58.27; H, 6.10; N, 6.25. Calc. for C21 H26 N2 S4 : C, 58.06; H, 6.00; N, 6.45. IR (KBr, cm−1 ): 1588(m), 1485(s), 1449(s), 1405(vs), 1271(vs), 1098(s), 989(s), 896(m), 765(s), 700(s), 555(m). 1 H NMR: δ(ppm) 1.24 (t, 6H, CH3 ), 1.96 (m, 2H, CH2 ), 3.19 (t, 4H, CH2 S), 4.32 (q, 4H, CH2 N), 7.16–7.48 (m, 10H, Ar–H). For tetramethylene bis(N-methyl-N-phenyldithiocarbamate) L5 (C20 H24 N2 S4 ): yield: 58%, mp 146–149 ◦ C. Anal. found (%): C, 57.41; H, 5.78; N, 6.40. Calc. for C20 H24 N2 S4 : C, 57.14; H, 5.71; N, 6.67. IR (KBr, cm−1 ): 1590(m), 1489(s), 1449(s), 1361(vs), 1258(s), 1095(vs), 954(s), 804(s), 773(m), 696(s), 557(s). 1 H NMR: δ(ppm) 1.66 (bs, 4H, CH2 ), 3.16 (bs, 4H, CH2 S), 3.74 (s, 6H, NCH3 ), 7.20–7.50 (m, 10H, Ar–H). For tetramethylene bis(N-ethyl-N-phenyldithiocarbamate) L6 (C22 H28 N2 S4 ): yield: 75%, mp 146–148 ◦ C. Anal. found (%): C, 59.12; H, 5.96; N, 5.95. Calc. for C22 H28 N2 S4 : C, 58.93; H, 6.25; N, 6.25. IR (KBr, cm−1 ): 1588(w), 1488(m), 1445(s),

Scheme 1. The synthetic routes of the ligands.

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1398(vs), 1267(s), 1093(m), 993(m), 896(m), 763(m), 699(s), 556(m). 1 H NMR: δ(ppm) 1.25 (t, 6H, CH3 ), 1.66 (bs, 4H, CH2 ), 3.15 (bs, 4H, CH2 S), 4.32 (q, 4H, CH2 N), 7.17–7.49 (m, 10H, Ar–H). 2.3. Preparation of electrodes and potentiometric measurements The mixture of 1 wt.% ionophore, 33 wt.% PVC and 66 wt.% plasticizer with total weight of 281 mg was dissolved in 5 ml of THF. The solution was poured into an 18 mm diameter glass ring and the solvent was allowed to evaporate at room temperature for 2 days. The resulting membrane was cut into small diameter disks and was sealed onto the end of the Ag/AgCl electrode barrel with a 5% THF solution of PVC (wt.%). For all electrodes, an internal reference solution of 0.01 M AgNO3 (saturated by AgCl) was employed. Prior to potentiometric measurements, the electrodes were conditioned in a 0.01 M Ag+ solution for 12 h. Potentials and pH values were measured by using an ion meter of model pXS-215 (Leici Instruments Corporation, Shanghai) and a pH meter of model 6071 (JENCO Electronics, Ltd, Shanghai). All potentials were measured relative to an Hg/Hg2 Cl2 double junction reference electrode with 0.1 M lithium acetate in the outer compartment and saturated potassium chloride in the inner compartment. The potentiometric measurements were made with the following electrochemical cell: Hg/Hg2 Cl2 /KCl(saturated)/0.1 M LiAc/sample solution//PVC membrane//0.01 M AgNO3 /Ag/AgCl. The performance of each electrode was investigated by measuring its potential in silver nitrate solutions prepared in the range of 1.0 × 10−1 to 1.0 × 10−8 M by serial dilution of the 0.1 M stock solution. The solutions were stirred and potential readings were recorded when they reached steady state values. The data were plotted as observed potential versus the logarithm of the Ag+ activity. The activities were calculated from the modified form of the Debye–H¨uckel equation [20]. The detection limit was determined according to IUPAC recommendations.

3. Results and discussion In preliminary experiments, six ligands were used as potential neutral ionophores for the preparation of membranes of electrodes for a variety of different metal ions. Among different cations examined, Ag+ with the most sensitive response seems to be suitably determined with the membrane electrodes based on the ligands L3–L6 expect ligands L1 and L2. The electrodes based on ligands L1 and L2 have a narrow response range, worse response to Ag+ and almost no response to other metal ions. This is likely due to the smaller size of the C-shaped cavity of the ligands L1 and L2, which are unfit for the Ag+ ions. The potential responses of Ag+ –PVC membrane electrodes based on ligands L1–L6 are illustrated in Fig. 1. In order to obtain a clue about the affinity of ligands L1–L6 towards Ag+ ions, their complex formation constants Kf with many metal ions have been calculated according to the segmented sandwich membrane method proposed by Mi and Bakker [25]. In the experiments, ion-selective electrode membranes were cast by dissolving the ionophores (2.9 mg), DBP (185 mg), PVC (93 mg) and KTpClPB (0.47 mg) in 5 ml THF. The blank membranes (without ionophores) were also prepared containing the same amounts of DBP, PVC and KTpClPB. The sandwich membrane was made by pressing two individual membranes (ordinarily one without ionophores and one with the same components and an additional ionophore) together immediately after blotting them individually dry with tissue paper. The combined segmented membrane was then rapidly mounted onto the electrode body and immediately measured. Membrane potential values were determined by subtracting the cell potential for a membrane without ionophores from that of the sandwich membrane. Based on the procedure proposed by Bakker, the membrane potential values were determined and the complex formation constants were calculated and listed in Table 1. As can be seen from Table 1, these ligands have high binding selectivity towards Ag+ . The complex formation constants (log Kf ) of L3–L6 with Ag+ are higher than those of L1 and L2. Maybe this is one of the reasons that the electrodes

2.4. Content assay of Vitamin B1 in Vitamin B1 tablets Vitamin B1 tablets were finely powdered. A portion of the powder about 2.0 g was weighed accurately and dissolved in 100 ml of distilled water. The resulting solution of 20 ml was transferred to a 50 ml beaker by a volumetric pipette and the electrode in conjunction with the reference electrode was immersed in it. The solution was titrated with a 5.00 × 10−2 M AgNO3 solution. The end-point of the titration was determined here potentiometrically. 2.5. Content assay of silver(I) in water sample The applicability of the sensors was illustrated by measuring the silver(I) ion potentiometrically in double distilled deionized water spiked with silver(I) nitrate and possessing 50 and 20 ppm silver(I). The Ag+ concentration was measured by a standard curve method and compared with those determined by atomic absorption spectrometric (AAS) analysis.

Fig. 1. EMF response of electrodes based on neutral ionphores L1–L6 towards Ag+ concentration.

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Table 1 Complex formation constants (log Kf ) of ionophores (L1–L6) with metal ions Ionophore

Ag+

Hg2+

Cu2+

Pb2+

Cd2+

Ni2+

Zn2+

Ca2+

Mg2+

L1 L2 L3 L4 L5 L6

5.01 5.92 7.91 7.63 7.30 7.33

4.01 4.52 5.27 4.95 4.91 5.18

2.76 2.68 2.82 2.56 2.50 2.51

2.47 2.65 2.71 2.65 2.43 2.50

2.71 2.52 2.28 2.34 2.27 2.25

2.98 2.40 2.27 2.39 2.30 2.36

2.75 2.33 2.25 2.21 2.21 2.16

2.48 2.46 2.17 2.18 2.11 2.23

2.40 2.44 2.12 2.06 2.08 2.15

based on ligands L1 and L2 have worse response to Ag+ . In addition, the performance characteristics of the electrodes summarized in Table 2 showed that the electrode based on tetramethylene bis(dialkyldithiocarbamate) had higher sensitivity than that based on trimethylene bis(dialkyldithiocarbamate). The response slope and detection limit of the electrodes based on L4 and L6 were somewhat superior to those of L3 and L5. On the other hand, the esters with an N-ethyl group showed higher sensitivity than the esters with an N-methyl group. These results are probably due to the increased lipophilicity of the corresponding ligands. The sufficient lipophilicity can prevent leaching of the ligands into the solutions surrounding the membrane electrodes. From Table 2, it is found that the upper detection limit is 1.0 × 10−3 M. This could be due to the consequence of a coextraction process of Ag+ and interfering anions (NO3 − ) from the sample into the ion-selective membrane. With increasing the sample concentration, sample anions will be extracted along with Ag+ that are complexed with the ionophores. According to Bakker et al. [26], all free carriers are used up eventually and the membrane contains primary cation–carrier complexes, and extracted sample anions. Therefore, an anion response of the electrode is expected. From Fig. 1, this response is observed for the electrodes based on ionophores L3–L6 in a high concentration range of 10−3 to 10−2 M. 3.1. Influence of membrane composition In general, the sensitivity, selectivity, working range, and stability of an ion-selective electrode depend not only on the nature of the ionophore, but are also strongly influenced by the nature and amount of the plasticizer and lipophilic additives. Table 3 shows that the nature of the plasticizer strongly affects the response characteristics of the electrodes due to its influence on the dielectric constants (εr ) of the membrane phase, the mobility of the ionophore molecules and the state of ligands [26]. As it

is obvious from Tables 2 and 3, among the three different plasticizers, DBP (εr = 6.4) showed better sensitivity to Ag+ than DOS (εr = 4.8) and o-NPOE (εr = 24.0). It is surprising that the membrane electrodes with DOS as a plasticizer presented a superNernstian slope. A reasonable explanation cannot be given. The influence of the amount of the plasticizer was investigated by using DBP as a plasticizer. The plasticizer percent changed from 64% (183 mg) to 69% (200 mg). As seen from Table 3, it is clear that the response slopes of the electrodes for Ag+ ions have improved with increasing the amount of DBP from 64% to 66%, but with increasing further the activities of the plasticizer beyond 66% (185 mg), the slope has decreased. This is consistent with literature in the point that in preparation of many PVC membrane electrodes, a plasticizer/PVC ratio (m/m) of nearly 2 has resulted in very suitable performance characteristics [27,28]. The effect of the amount of ionophores incorporated in the membrane on the electrode characteristics was also investigated. The amount of ionophores was changed while maintaining the same amounts of PVC and plasticizer (DBP) in the membranes. The concentration of ionophores was varied from 0.7% (2.0 mg) to 1.3% (3.8 mg) (Table 4). The optimum ionophore concentration for the electrodes based on L3–L6 was found to be 1.0% (2.9 mg). Many studies have shown that the ionic additives can improve the electrode response and for monovalent cations the optimal number of anionic sites is usually 50 mol% relative to the ionophore. In order to investigate the effect of anionic sites in the membranes containing L3–L6 and DBP, we incorporated 10 mol% KTpClPB relative to the ionophores in the membrane systems. A decrease in response slope of the calibration graph has been found. For further study, the influence of the amount of KTpClPB, i.e. 50 and 100 mol% relative to the ionophores L4 and L6, which had the best performance characteristics of electrodes, were examined. Table 5 gives the influence of the

Table 2 Response characteristics of the electrodes based on L1–L6 to Ag+ ions Electrode number

1 2 3 4 5 6

Membrane composition (mg) Ionophore

Plasticizer

PVC

L1, 2.9 L2, 2.9 L3, 2.9 L4, 2.9 L5, 2.9 L6, 2.9

DBP, 185 DBP, 185 DBP, 185 DBP, 185 DBP, 185 DBP, 185

93 93 93 93 93 93

Slope (mV decade−1 )

Linear range (M)

Detection limit (M)

43.8 45.6 53.0 56.0 53.3 60.2

1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3

1.4 × 10−6 1.4 × 10−6 3.3 × 10−7 3.7 × 10−7 3.8 × 10−7 2.3 × 10−7

to 1.0 × 10−5 to 1.0 × 10−5 to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−6

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Table 3 Influence of the nature of plasticizers and the amount of DBP on the characteristics of Ag+ selective electrodes Electrode number

3.1 3.2 3.3 3.4 4.1 4.2 4.3 4.4 5.1 5.2 5.3 5.4 6.1 6.2 6.3 6.4

Membrane composition (mg) Ionophore

Plasticizer

PVC

L3, 2.9 L3, 2.9 L3, 2.9 L3, 2.9 L4, 2.9 L4, 2.9 L4, 2.9 L4, 2.9 L5, 2.9 L5, 2.9 L5, 2.9 L5, 2.9 L6, 2.9 L6, 2.9 L6, 2.9 L6, 2.9

DOS, 185 NPOE, 185 DBP, 183 DBP, 200 DOS, 185 NPOE, 185 DBP, 183 DBP, 200 DOS, 185 NPOE, 185 DBP, 183 DBP, 200 DOS, 185 NPOE, 185 DBP, 183 DBP, 200

93 93 104 87 93 93 104 87 93 93 104 87 93 93 104 87

Slope (mV decade−1 )

Linear range (M)

Detection limit (M)

77.0 51.3 48.4 51.3 62.6 45.3 49.1 54.0 78.7 50.9 49.0 51.2 82.9 54.2 54.6 55.5

1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3

1.0 × 10−5 1.0 × 10−6 5.2 × 10−7 6.8 × 10−7 3.8 × 10−6 2.9 × 10−6 4.9 × 10−7 6.9 × 10−7 3.4 × 10−6 4.7 × 10−7 4.1 × 10−7 4.0 × 10−7 6.6 × 10−6 2.3 × 10−6 6.5 × 10−7 9.2 × 10−7

to 1.0 × 10−5 to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−5 to 1.0 × 10−5 to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−5 to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−5 to 1.0 × 10−5 to 1.0 × 10−6 to 1.0 × 10−6

Table 4 Influence of ionophere concentration on the characteristics of electrodes to Ag+ ions Electrode number

3.5 3.6 4.5 4.6 5.5 5.6 6.5 6.6

Membrane composition (mg) Ionophore

Plasticizer

PVC

L3, 2.0 L3, 3.8 L4, 2.0 L4, 3.8 L5, 2.0 L5, 3.8 L6, 2.0 L6, 3.8

DBP, 185 DBP, 185 DBP, 185 DBP, 185 DBP, 185 DBP, 185 DBP, 185 DBP, 185

93 93 93 93 93 93 93 93

Slope (mV decade−1 )

Linear range (M)

Detection limit (M)

50.0 48.8 51.5 52.7 50.5 48.7 57.9 53.4

1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3

6.3 × 10−7 6.2 × 10−7 9.8 × 10−7 8.3 × 10−7 3.4 × 10−7 2.7 × 10−7 8.0 × 10−7 7.3 × 10−7

amount of KTpClPB on the performance of electrodes. From Table 5, it is found that there is not a large difference between the response slopes of the electrodes containing KTpClPB as anionic sites. However, compared with the slopes of the electrodes without KTpClPB, the addition of KTpClPB caused a decrease of the response slopes of the silver(I) ion-selective electrodes based on L4 and L6. This could be due to the formation of AgL+ TpClPB ion pairs in the membrane, which leads to an increase in the mobility of anions or a decrease in the mobility of Ag+ ions, and eventually a reduction in the response slope. Thus, the membranes with an optimized ingredient composition of ionophore:DBP:PVC = 1:66:33 with L3–L6 as ionophores

to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−6

were selected to prepare the PVC membrane electrodes that were sensitive to Ag+ ions. 3.2. Response characteristics and selectivity of the electrodes The response of the optimized membranes based on ligands L3–L6 towards solution pH was studied in the range from 1 to 9 using 1.0 × 10−4 and 1.0 × 10−5 M Ag+ concentrations. All pH values of the tested solutions were adjusted with HNO3 and NaOH solutions. It was observed from Fig. 2 that no obvious pH dependence over the range of 2–6, 2–7, 4–8, 4–6 for the

Table 5 Influence of the amount of KTpClPB on the characteristics of electrodes to Ag+ ions Electrodenumber

4.7 4.8 4.9 6.7 6.8 6.9

Membrane composition (mg) Ionophore

Plasticizer

PVC

KTpClPB (mol%)

L4, 2.9 L4, 2.9 L4, 2.9 L6, 2.9 L6, 2.9 L6, 2.9

DBP, 185 DBP, 185 DBP, 185 DBP, 185 DBP, 185 DBP, 185

93 93 93 93 93 93

0.3 (10%) 1.6 (50%) 3.2 (100%) 0.3 (10%) 1.6 (50%) 3.2 (100%)

Slope (mV decade−1 )

Linear range (M)

Detection limit (M)

54.0 53.9 55.7 51.6 51.9 52.0

1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3 1.0 × 10−3

9.7 × 10−7 8.3 × 10−7 5.1 × 10−7 8.5 × 10−7 9.6 × 10−7 7.0 × 10−7

to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−6 to 1.0 × 10−6

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Fig. 2. Plots showing the variation of membrane potentials with pH for electrodes 3–6 at: (a) 1.0 × 10−4 and (b) 1.0 × 10−5 M silver ion concentration.

1.0 × 10−5 M Ag+ solutions and 2–4, 2–7, 2–5, 2–8 for the 1.0 × 10−4 M Ag+ solutions for the electrodes based on ligands L3–L6, respectively. The response time (t95% ) of the electrodes was tested by measuring the time required to achieve a 95% steady potential. The resulting potential–time responses of the electrodes based on ligands L3–L6 were obtained upon changing the Ag+ concentration from 10−6 to 10−5 , 10−5 to 10−4 , 10−4 to 10−3 M (by fast injection of a microliter-amount of a concentrated solution). The response time of the electrodes based on ligands L3–L6 was fast (<20 s). The relative lifetime of the electrodes were studied by periodically recalibrating in standard Ag+ solutions and calculating the response slope over the range of 1.0 × 10−7 to 1.0 × 10−2 M AgNO3 . During this period, the electrodes were used weekly. When the electrodes based on ligands L3–L6 were repeatedly calibrated four times during a period of 1 month, no significant change was observed in the performance of the electrodes. The potentiometric selectivities of the optimized electrodes for Ag+ against other interfering ions were determined by a mixed solution method, where the concentration of the silver ion was varied while those of the interference ions were 1.0 × 10−3 M except Hg2+ , which was 1.0 × 10−4 M. The values of selectivity coefficients are summarized in Table 6. As evident from Table 6, the alkali metal, alkaline earth metal and some common transition metal ions gave no interference in the performance of the membrane electrodes. As for the strongest interferent Hg2+ , which had the similar characteristics to Ag+ , the proposed sensors showed better results than pot the Ag2 S electrode (log KAg·Hg = −2.1) and other electrodes previously reported for silver ions [8,11,14,16,18]. The selectivity of the Ag+ electrodes against other transition metal ions

Fig. 3. Plot showing the potentiometric titration of 2.5 × 10−3 M NaCl with 1.0 × 10−3 M AgNO3 solution.

is similar to the results of complex formation constants. Hg2+ (except Ag+ ) has the biggest complex formation constant with pot ionophores L3–L6. The selectivity coefficients KAg·Hg are of bigger values. This result agreed well with the view that soft coordination sites of sulfur and nitrogen offered great affinity towards Ag+ and Hg2+ [19]. 3.3. Analytical applications 3.3.1. Pharmaceutical assays The Ag+ ion-selective electrodes based on ligands L3–L6 were successfully used as the indicator electrode in the titration

Table 6 pot Selectivity coefficients (log KAg,M ) of the electrodes based on ionophores L3–L6 Interfering cations

Electrode 3 Electrode 4 Electrode 5 Electrode 6

Hg2+

Cu2+

Pb2+

Cd2+

Ni2+

Zn2+

Ca2+

Mg2+

K+

Na+

NH4 +

−3.04 −3.10 −2.98 −2.73

−3.70 −4.53 −3.80 −4.07

−3.62 −4.30 −3.92 −4.02

−3.54 −4.81 −4.02 −4.46

−4.11 −4.59 −4.03 −5.12

−4.02 −5.08 −4.16 −5.37

−4.17 −4.64 −4.51 −4.62

−4.04 −5.35 −4.20 −4.94

−2.94 −3.74 −3.19 −3.85

−2.81 −3.41 −2.73 −3.07

−2.78 −3.14 −2.95 −3.07

180

Z. Yan et al. / Sensors and Actuators B 122 (2007) 174–181

3.3.2. Water sample assays The results obtained were compared with those obtained by atomic absorption spectrometric (AAS) analysis (Table 8) and were found in good agreement each other. 4. Conclusions

Fig. 4. Plot showing the potentiometric titration of Vitamin B1 with 0.05 M AgNO3 solution. Table 7 Content of Vitamin B1 in Vitamin B1 tablets

PVC membrane electrodes incorporating bis(dialkyldithocarbamates) (ionophores L3–L6) showed high selectivity and sensitivity towards silver ions. The best results were obtained at the ionophore:DBP:PVC weight ratio of 1.0:66:33. The present silver ion-selective electrodes displayed very good selectivity for Ag+ with respect to alkali, alkaline earth, and common transition metal ions. In particular, the present silver ion-selective electrodes exhibited very low response towards Hg2+ , which was a major interfering ion for the determination of silver ions using the conventional silver ion-selective electrodes based on the neutral ionophores containing soft donor atoms such as N and S atoms. The silver ion-selective electrodes can be employed as an indicator electrode in potentiometric titration and determination of silver ions in water samples.

Electrode

Ag+ -ISE(%)/S.D. (n = 3)

Conventional titration (%)/S.D. (n = 3)

Acknowledgment

3 4 5 6

16.18/0.02 16.12/0.03 16.26/0.02 16.18/0.04

16.30/0.02 16.30/0.02 16.30/0.02 16.30/0.02

This work was supported by the Natural Science Foundation of the Education Commission of Henan Province (200510459014). References

of NaCl (Fig. 3). The endpoint of titration could be explicitly defined from the titration curve by using AgNO3 as the titrant. Vitamin B1 is a chloride salt, which has two Cl− ions in a Vitamin B1 molecule. The content of Vitamin B1 in Vitamin B1 tablets can be determined by a potentiometric titration method. The typical titration curves for the drug in aqueous solutions are shown in Fig. 4. The equivalence point was determined graphically. The significance tests were carried out by using the conventional titration method, in which the standard AgNO3 solution was the titrant and K2 CrO4 was an indicator. Both results from two methods are shown in Table 7. It shows that there are no significant differences between results from the electrode method and from the titration. Table 8 Analysis of water samples spiked with silver(I) Electrode

Sample no.

Ag+ -ISE (ppm)/S.D. (n = 3)

AAS (ppm)/S.D. (n = 3)

3

1 2

19.1/0.3 48.5/0.2

19.8/0.3 48.5/0.3

4

1 2

18.4/0.2 50.0/0.3

19.8/0.2 49.6/0.2

5

1 2

19.3/0.4 51.0/0.2

19.8/0.3 49.6/0.2

6

1 2

21.0/0.2 48.0/0.2

19.8/0.3 48.5/0.2

[1] R.M. Izatt, G.C. Lindh, R.L. Bruening, P. Huszthy, C.W. McDaniel, J.S. Bradshaw, J.J. Christensen, Separation of silver from other metal cations using pyridone and triazole macrocycles in liquid membrane systems, Anal. Chem. 60 (1988) 1694–1699. [2] Y. Umezawa (Ed.), Handbook of Ion-selective Electrodes: Selectivity Coefficients, CRC Press, Boca Raton, FL, 1990. [3] S. Chung, W. Kim, S.B. Lee, D.D. Sung, Sensor molecules for silver(I)selective membranes based on mono-toquadridentate podands, Chem. Commun. (1997) 965–966. [4] S.S. Lee, M.K. Ahn, S.B. Park, Silver(I)-selective membrane electrodes based on mono-toquadridentate podands, Analyst 123 (1998) 383–386. [5] M. Oue, K. Akama, K. Kimura, M. Tanaka, T. Shono, Lipophilic thiacrown ether derivatives as neutral silver-ion selective carriers, J. Chem. Soc. Perkin Trans. (1989) 1675–1678. [6] M. Shamsipur, M. Javanbakht, V. Lippolis, A. Garau, G. De Filippo, M.R. Ganjali, A. Yar, Novel Ag+ ion-selective electrodes based on two new mixed azathioether crowns containing a 1,10-phenanthroline sub-unit, Anal. Chim. Acta 462 (2002) 225–234. [7] E. Malinowska, Z. Brzozka, K. Kasiura, R.J.M. Egberink, D.N. Reinhoudt, Silver selective electrodes based on thioether functionalized calix[4]arenas as ionophores, Anal. Chim. Acta 298 (1994) 245–251. [8] L. Chen, X. Zeng, H. Ju, X. He, Z. Zhang, Calixarene derivatives as the sensory molecules for silver ion-selective electrode, Microchem. J. 65 (2000) 129–135. [9] Y. Liu, B. Zhao, L. Chen, X. He, Liquid membrane transport and silver selective electrode based on novel bis(3-pyridinecarboxylate) calix[4]arene as ionophore, Microchem. J. 65 (2000) 75–79. [10] L. Chen, X. He, B. Zhao, Y. Liu, Calixarene derivatives as neutral carrier the silver ion-selective electrode and liquid membrane transport, Anal. Chim. Acta 417 (2000) 51–56. [11] R.K. Mahajian, M. Kumar, V. Sharma, I. Kaur, Silver(I) ion-selective electrode membrane based on Schiff base-p-tert-butylcalix[4]arene, Analyst 126 (2001) 505–507.

Z. Yan et al. / Sensors and Actuators B 122 (2007) 174–181 [12] L. Chen, H. Ju, X. Zeng, X. He, Z. Zhang, Silver ion-selective electrodes based on novel containing benzothiazolyl calix[4]arene, Anal. Chim. Acta 437 (2001) 191–197. [13] S.M. Lim, H.J. Chung, K. Paeng, C. Lee, H. Choi, W. Lee, Calix[2]furano[2]pyrrole and related compounds as the neutral carrier in silver ion-selective electrode, Anal. Chim. Acta 453 (2002) 81–88. [14] R.K. Mahajan, I. Kaur, M. Kumar, Silver ion-selective electrodes employing Schiff base p-tert-butyl calix[4]arene derivatives as neutral carries, Sens. Actuators B 91 (2003) 26–31. [15] J. Casabo, L. Mestres, L. Escriche, F. Tesidor, C.P. Jimenez, Silver(I) ion-selective electrode based on polythiamacromolecules, J. Chem. Soc., Dalton Trans. (1991) 1969–1971. [16] R.K. Mahajan, O. Parkash, Silver(I) ion selective PVC membrane based on bis-pyridine tetramide macrocycle, Talanta 52 (2000) 691–693. [17] D. Liu, J. Liu, D. Tian, W. Hong, X. Zhou, J.C. Yu, Polymeric membrane silver-ion selective electrodes based on bis(dialkyldithiophosphates), Anal. Chim. Acta 416 (2000) 139–144. [18] D. Xu, T. Katsu, o,o,o-Trialkyl phosphorothioates as simple and effective ionophores for silver ion-selective membrane electrodes, Anal. Chim. Acta 443 (2001) 235–240. [19] R.G. Pearson, Hard and soft acids and bases, J. Am. Chem. Soc. 85 (1963) 3533–3539. [20] S. Kamata, H. Murata, Y. Kubo, A. Bhale, Copper(II)-selective membrane electrodes based on o-xylene-bis(dithiocarbamates) as neutral carriers, Analyst 114 (1989) 1029–1031. [21] S. Kamata, A. Bhale, Y. Fukunage, H. Murata, Copper(II)-selective electrode using thiuram disulfide neutral carriers, Anal. Chem. 60 (1988) 2064–2067. [22] E. Linder, K. Toth, E. Pungor, F. Behm, P. Oggenfuss, D.H. Welti, D. Ammann, W.E. Morf, E. Pretsch, W. Simon, Lead-selective neutral carrier based liquid membrane electrode, Anal. Chem. 56 (1984) 1127–1131. [23] S. Kamata, K. Onoyama, Lead-selective using methylene bis(diisobutyldithiocarbamate) neutral carrier, Anal. Chem. 63 (1991) 1295–1298. [24] M. Lerchi, E. Reltter, W. Simon, E. Pretsch, D.A. Chowdhury, S. Kamata, Bulk optodes based on neutral dithiocarbamate ionophore with high selec-

[25]

[26]

[27]

[28]

181

tivity and sensitivity for silver and mercury cations, Anal. Chem. 66 (1994) 1713–1717. Y. Mi, E. Bakker, Determination of complex formation constants of lipophilic neutral ionophores in solvent polymeric membranes with segmented sandwich membranes, Anal. Chem. 71 (1999) 5279–5287. E. Bakker, P. B¨uhlmann, E. Pretsch, Carrier-based ion-selective electrodes and bulk optodes. 1. General characteristics, Chem. Rev. 97 (1997) 3087–3132. C. Hongbo, E.H. Hansen, J. Ruzicka, Evaluation of critical parameters for measurement of pH by flow injection analysis, Anal. Chim. Acta 169 (1985) 209–220. S. Erden, A. Demirel, S. Memon, M. Yılmaz, E. Canel, E. Kılıc, Using of hydrogen ion-selective poly(vinyl chloride) membrane electrode based on calix[4]arene as thiocyanate ion-selective electrode, Sens. Actuators B 113 (2006) 290–296.

Biographies Zhenning Yan received her PhD degree from Tianjin University, Department of Chemical Engineering in 1999. She moved to Zhengzhou University for postdoctoral research work on ion-selective electrodes and their applications in 2000. Now she is an Associate Professor of Department of Chemistry at Zhengzhou University. Her main research interests are ion-selective electrodes, electroanalytical chemistry, and the equilibrium constants of some organic ligands. Yanqi Lu has graduated from Henan Normal University, Department of Chemistry Education in 1992. She received her MS degree in the field of ion-selective electrodes in 2002 in Zhengzhou University, Department of Chemistry. She is now an Assistant Professor of Analytical Chemistry in Zhengzhou Railway Profession Technology College. Her research interests have been in the areas of ion-selective electrodes and their applications. Xia Li has graduated from Changde Normal College, Department of Chemistry Education in 2003. Her MS started in 2003 in Zhengzhou University and still going on. She is investigating the electrochemical properties of some calixarenes and their analytical applications.