The discharge of organic and metal pollutants into the environment is a serious problem facing numerous industries. Because heavy metals are not biodegradable under natural conditions,they tend to accumulate in living organisms causing various diseases and disorders . The toxicity,bio-availability and accumulation of particulate matter and sediment of heavy metal ions are well known to be strongly dependent on their activity rather than their total concentration. Thus,the activity measurements of these metal cations in aqueous systems are of vital importance from an environmental point of view .
The silver content of environmental samples increases with increasing use of silver compounds and silver-containing preparations in industry and in medicine [3, 4]. Silver can enter the environment via industrial waste water because it is often an impurity in copper,zinc,arsenic and antimony ores . Owing to the use of soluble silver compounds to disinfect water for drinking and recreation purposes,low-level exposure to silver compounds is widespread. It is suggested that silver might pose a potential risk as water pollutant because of the lack of recycling of mixed silver . Therefore,the selective and sensitive determination of Ag+ ion is of increasing interest.
Potentiometric detection based on ion-selective membrane electrodes (ISMEs),as a simple method,offers several advantages, such as fast response,easy preparation,simple instrumentation, widelineardynamic range,relatively lowdetectionlimit,reasonable selectivity,application in colored and turbid solutions and lowcost.
As a result of extensive research in this area,a number of ion selective electrodes (ISEs),mainly for alkali,alkaline earth and some heavy and transition metal ions,are now commercially available. Since the introduction of the first silver cation-ISE using silver sulfide at the end of 1960s ,and after the description of the first Ag+-selective ionophore in 1986 ,great attention for potentiometric sensing of Ag(Ⅰ) cations has been observed during the last three decades. Based on the chemical recognition principle, several types of carriers of suitable size and specific metal ioncarrier interaction have been successfully utilized for the construction of Ag+-ISEs [9, 10].
Asignificantnumber ofmacrocyclic ligands,have been used in the construction of polyvinyl chloride (PVC) basedmembrane electrodes fordeterminationof alkaliandalkaline earthmetal cations in solution . Much interest has been paid to the use of ionophore ligands as sensing materials for selective electrodes due to their unique properties . The selectivity of these electrodes is based on the high bonding affinity of the ionophores containing oxygen or nitrogen atoms for metal cations. Some of the acyclic and cyclic polyethers containing nitrogen or sulfur atoms,such as thia crown ethersandcryptands,inwhichallor someof theoxygenatoms inpoly ether bridge substituted by sulfur or nitrogen atoms have been used for construction of Ag(Ⅰ) ion-selective electrode [13, 14].
The ionophores having nitrogen atoms as the coordination sites tend to suffer from undesirable pH effects due to the protonation of the nitrogen atom. Also the ligands having sulfur atom as coordination sites,experience the lowering of Ag(Ⅰ) ion selectivity against the interfering ions due to the strong affinity of sulfur for the soft metal ions. However,few attempts based on other binding sites,such as oxygen atoms,have been reported [15, 16]. In most cases,these ionophores have showed less discrimination for transition and alkali metal ions when compared with the sulfurcontaining ionophores.
In our previous research,the selective transport experiments for the heavy metal cations involving Ag(Ⅰ),Ni(Ⅱ),Co(Ⅱ),Zn(Ⅱ), Cd(Ⅱ),Cr(Ⅲ) and Pb(Ⅱ) from an aqueous source phase through some organic membranes into an aqueous receiving phase were carried out using 15-crown-5 as an ion carrier in the membrane phase. The results showed that this macrocyclic ligand is a highly selective ionophore for Ag(Ⅰ) ion . For example,the results which were obtained using various organic membrane phases are shown graphically in Fig. 1. As is evident in Fig. 1,15-crown-5 shows a high selectivity for Ag(Ⅰ) cations in the presence of the other heavy metal cations.
|Fig. 1.Comparison of the results for competitive metal ion transport (water/organic solvent/water) studies for 15-crown-5. Source phase: pH 5.0 (CH3COOH/ CH3COONa) containing metal ions at 0.01 mol/L (10 mL). Membrane phase: contains 15-crown-5 (0.001 mol/L) (50 mL). Receiving phase: pH 3.0 (NaOH/ HCOOH) (30 mL) stirred for 24 h.|
In the present work,we describe the construction of a PVC based electrochemical sensor for Ag(Ⅰ) cations using 15-crown-5 as a suitable ionophore. The fabricated Ag(Ⅰ) ion selective electrode showed a high sensitivity,good stability,fast response and a high selectivity for Ag(Ⅰ) ions over many common metal ions in solution. It was used as an indicator electrode in potentiometric titration of Ag(Ⅰ) ion with I- and Cl- anions and it was also successfully applied to the determination of Ag(Ⅰ) ion in real samples. 2. Experimental 2.1. Reagents
Reagent grade 15-crown-5,o-nitrophenyloctylether (NPOE), dibutyl phthalate (DBP),dioctyl phthalate (DOP),oleic acid (OA), diethyl sebacate (DES),bis (2-ethylhexyl) sebacate (BEHS),sodium tetraphenyl borate (NaTPB),tetrahydrofuran (THF),nitrobenzene (NB) and polyvinyl chloride (PVC) with a high relative molecular weight,were purchased from Merck Chemical Company. The nitrate salts of all metal cations (all from Merck) were of the highest purity and used without further purification. Doubly distilled deionized water was used throughout the study. 2.2. Preparation of the electrodes
The general procedure for the preparation of the polymeric membrane electrode was as follows. The mixture of ionophore (15- crown-5,5.6 mg),polyvinyl chloride (PVC,30 mg),plasticizer (NPOE,60.5 mg) and additive (NaTPB,3.9 mg) with total weight of 100 mg was dissolved in 2 mL of THF. The solution was poured into a 2 cm diameter glass dish and stirred vigorously for 30 min. Then, the solvent was evaporated until an oily concentrated mixture was obtained. A poly-ethylene tube (2-3mm i.d.) was dipped into the mixture for about 4 s so that amembrane of about 0.3mmthickness was formed at the end of the tube. Then,the tube was withdrawn from the mixture and kept at room temperature for about 24 h. Then the tube was filled with the internal filling solution (1.0 × 10-3 mol/L AgNO3). The electrode was finally conditioned for 6 h by soaking in a 1.0 × 10-3 mol/L solution of Ag(Ⅰ) nitrate. 2.3. Potentiometric measurements
The potentiometric measurements were carried out by the following electrochemical cell: Ag/AgCl(s),KCl(satd.)/internal solution (1.0 × 10-3 mol/L AgNO3)/PVC membrane/test solution// KCl(satd.),AgCl(s)/Ag. The potentials were measured relative to an Ag,AgCl double junction reference electrode with an outer chamber filled with 0.01 mol/L KNO3 solution. The performance of each electrode was investigated by measuring its potential in silver nitrate solutions prepared in the range 1.0 × 10-1-1.0 × 10-9 mol/L by serial dilution of the 0.1 mol/L stock solution. The solutions were stirred and the potential readings were recorded when they reached steady state values. The data were plotted as observed potentials versus the logarithm of the Ag(Ⅰ) ion activity. The activities were calculated from the modified Debye-Hu¨ ckel equation. The ratio of the various ingredients of the membrane was optimized to provide a membrane which results in a Nernstian response with high sensitivity,good selectivity and reproducible and stable potentials under experimental conditions. 3. Results and discussion 3.1. Influence of membrane composition
It is well known that the electrode response for a specific metal ion,depends on the amount and the nature of the components of the electrode . Besides the critical role of the nature of the ionophore,the performance characteristics of an electrochemical sensor depend on the nature of the plasticizer  and ionic additive  which are used in the preparation of the membrane. Therefore,we investigated the effect of the membrane composition on the potential responses of the Ag(Ⅰ) electrochemical sensor. The nature of plasticizer influences the dielectric constant of the membrane phase,the mobility of the ionophore molecules, selectivity of the electrochemical sensor and also the state of the ligand in the membrane [21, 22]. Thus,to find the best plasticizer,different membranes with several plasticizers,such as DBP,DOP,DES,o-NPOE,NB,and BEHS with 15-crown-5 as an ionophore were prepared. The results are summarized in Table 1.
The plasticizers,which are used in the construction of ion selective membranes,should exhibit high lipophilicity,high molecular weight,low tendency for exudation from the polymer matrix,low vapor pressure and high capacity to dissolve the substrate and other additives present in the membrane and possess an adequate dielectric constant . Among the six different plasticizers used in this study,we determined that NPOE is a more effective solvent mediator in preparing the Ag(Ⅰ) selective membrane electrode.
It is known that the presence of lipophilic anionic additives in cation selective membrane electrodes is necessary to introduce perm selectivity,so that without such additives,the electrodes may fail to respond properly . The original motive for adding a tetraphenyl borate salt to the membrane of a cation-selective electrode is to reduce the anionic interference observed in the presence of lipophilic anions . At the same time,the electrical resistance of the membrane is lowered,which is especially important with microelectrodes . Therefore,as further improvement in the performance of the electrochemical sensor, we added appropriate amounts of four anionic additives to the membranes to reduce the interference from sample anions and the bulk membrane impedance . 3.2. Effect of pH
The effect of pH on the potentials of the proposed sensor was studied over the pH range 2.0-9.0 at 1.0 × 10-4 and 1.0 × 10-3 mol/L Ag+ ion concentration after adjusting the pH of the solutions with HNO3 and NaOH. The results are shown in Fig. 2. As is evident in Fig. 2,the potential remained constant in the pH range 3-8,which can be taken as the working pH range of the proposed sensor. The sharp change in potential above pH 8 may be due to the precipitation of Ag(Ⅰ) cations by OH- ions in solution. On the other hand,the change in potential below pH 3 may be attributed to the response of the electrode to H3O+ cation.
|Fig. 2.Effect of pH of test solution on the response of the Ag(Ⅰ) ion-selective electrode,[Ag+] = 1.0 × 10-3 mol/L (a) and 1.0 × 10-4 mol/L (b).|
The response characteristics of the Ag(Ⅰ) electrochemical sensor were assessed according to IUPAC recommendation . The emf response of the membrane at varying Ag(Ⅰ) concentrations showed a linear range from 1.0 × 10-1 to 1.0 × 10-7 mol/L with a Nerstian slope of 58.9 ± 0.5 mV/decade (Fig. 3). The detection limit was 8.09 × 10-8 mol/L,as determined from the intersection of two extrapolated segments of the calibration plot.
|Fig. 3.Calibration curve for Ag(Ⅰ) ion-selective electrode based on 15-crown-5 as an ionophore.|
Probably one of the most important characteristics of an ISE,is its relative response to the other ions present in solution,which expressed in terms of selectivity coefficients. There are different methods for determination of selectivity coefficients and in this present study,the selectivity coefficients were determined using two methods: the mixed solution method (MSM)  and matched potential method (MPM) . The MSM based on the semi empirical Nicolsky-Eisenman equation is of questionable validity when ions of different charge are present; therefore,the MPM was applied for determination of selectivity coefficients of the interfering ions. In MSM,the potentiometric selectivity coefficients (KPot Ag) can be evaluated from the potential measurements on solutions containing a fixed concentration of Ag(Ⅰ) cation (1 × 10-4 mol/L) and varying amounts of the interfering ions (Mn+) according to the following equation:
The other or matched potential method has two advantages . The first one is that when the ions of unequal charges are involved,theMPMis recommended,as it gives practical KAB values. Secondly,when interfering ions and/or the primary ion do not satisfy the Nernstian condition,the MPM is also recommended, even if the charges of the primary and interfering ions are equal. According to this method,the potentiometric selectivity coefficient is defined as the activity ratio of the primary ion (A) and the interfering ion (B) that gives the same potential change in a reference solution . Thus,first the change in potential upon changing the primary ion activity is measured and then,the interfering ion would be added to an identical reference solution until the same potential change is obtained. The selectivity coefficient,KMPM AB is determined by the following equation:
For any ISE,the response time is one of the most important factors. In this study,the practical response time of the electrochemical sensor was recorded by changing the Ag(Ⅰ) cation concentration in solution from 1.0 × 1010-4 to 1.0 × 10-3 mol/L. The potentials versus time traces are shown in Fig. 4. As can be seen from Fig. 4,over the entire concentration range,the plasticized membrane electrode reaches its equilibrium responses in a very short time (<10 s). The life time of the sensor was investigated by measuring the potentials over a period of 8 weeks. During this period,the sensor was used daily over an extended period (1 h per day). The performances of the electrode with respect to the Nernstian slope and detection limit were measured. It is important to emphasize that the electrodes were stored in 0.001 mol/L Ag(Ⅰ) cation solution when not in use.
|Fig. 4.The response time curve of the Ag(Ⅰ) selective membrane electrode (the concentration of Ag(Ⅰ) ion: 1.0 × 10-3 mol/L and 1.0 × 10-4 mol/L).|
It is known that the dynamic response time of a sensor is also an important factor in its evaluation. To measure the dynamic response time of the proposed sensor,the concentration of the test solution was successively changed from 1.0 × 10-3 mol/L to 1.0 × 1010-4 mol/L. The resulting data,which are depicted in Fig. 5,show that the time needed to reach a potential within ± 1 mV of the final equilibrium value after successive immersion of the electrode to a series of Ag+ ion solutions,each having a tenfold difference in concentration,is 10 s.
|Fig. 5.Dynamic response time of the sensor for reversibility with step changes in concentration of Ag(Ⅰ) (1.0 × 10-4-1.0 × 10-3 mol/L).|
The response characteristics of the proposed PVC membrane electrode are compared with those of the best Ag(Ⅰ) ion-selective electrodes reported earlier in Table 3. As is evident in Table 3,the proposed electrode is superior to the existing electrodes with regard to the working concentration range,pH range and detection limit.
The polymeric membrane Ag(Ⅰ) cation selective electrode has potential application in a variety of fields. We successfully applied the constructed Ag+ ISE as an indicator electrode in potentiometric titration of 20.0 mL of Ag(Ⅰ) cation solution (1.0 × 10-3 mol/L) with 1.0 × 10-2 mol/L solutions of Cl- and I- anions. The resulting titration curves are shown in Fig. 6. As is evident in Fig. 6,the exact amount of the Ag(Ⅰ) cation in solution can be accurately determined with this electrochemical sensor.
|Fig. 6.Potentiometric titration curves of 20 mL 1.0 × 10-3 mol/L Ag(Ⅰ) solution with 1.0 × 10-2 mol/L Cl- and I- solutions.|
The proposed sensor was also successfully used for direct monitoring of Ag(Ⅰ) ion in radiology wastes. The Ag(Ⅰ) cation concentration of the sample was determined using the proposed electrode by the standard addition method. The content of Ag(Ⅰ) ion in radiology waste solution by using the proposed electrode and atomic absorption spectrometry (AAS) was 2.77 ± 0.04 and 2.65 ± 0.02 mmol/L respectively. Comparison of the results obtained in this study to those obtained by AAS shows a good agreement. It seems that the proposed ion-selective electrode is a fast and simple analytical tool for the determination of Ag(Ⅰ) ion in real samples. 4. Conclusion
The macrocyclic ligand,15-crown-5,was used as a suitable neutral ionophore for construction of a PVC-based membrane selective electrode for the direct determination of Ag(Ⅰ) ion in solutions. It is well-known that the sensitivity and selectivity of the ISEs depend not only on the nature of ionophores used,but also significantly on the membrane composition and the properties of the plasticizer. The best composition of the proposed electrode was found to be ionophore/TPB/NPOE/PVC = 5.6/3.9/60.5/30 (w/w/w/ w),respectively. The proposed electrode has good potentiometric figures of merit for the determination of Ag(Ⅰ) cation,including high sensitivity,low detection limit (8.09 × 10-8 mol/L),fast response time (<10 s),good Nernstian slope (58.9 ± 0.5 mV/ decade) and a wide dynamic linear range (1.0 × 10-7- 1.0 × 10-1 mol/L) with respect to Ag(Ⅰ) cation concentration. This electrochemical sensor works well in a pH range 3-8 and exhibits a good reproducibility. The proposed electrode has an excellent selectivity for Ag(Ⅰ) ion over a number of mono-,bi- and trivalent cations,which generally show interference in some of the other reported Ag(Ⅰ) ion selective electrodes. It is possible to determine the concentration of Ag(Ⅰ) cation in real samples with this ISE and it can also be used as an indicator electrode in potentiometric titration of different anions with Ag(Ⅰ) cation in solutions.Acknowledgments
The authors gratefully acknowledge the support of this research work by Ferdowsi University of Mashhad,Mashhad,Iran.
|||C.D. Klaassen, Casarett & Doull's Toxicology: Basic Science of Poisons, McGraw- Hill, New York, 1996.|
|||R. De Marco, Response of copper (Ⅱ) ion-selective electrodes in sea water, Anal. Chem. 66 (1994) 3202-3207.|
|||I.M. Kolthoff, P.J. Elving, Treatise on Analytical Chemistry. Part II, vol. 4, Interscience, New York, 1966.|
|||H. Rener, 4th ed., Ulmanns Encyclopadie der Tehnischen Chemie, vol. 21, VerlagChemie, Weinheim, 1982.|
|||S.R. Oager, Metallic Raw Materials Dictionary, Bank Tobel, Zürich, 1984.|
|||A.T. Wan, R.A.J. Conyers, C.J. Coombs, J.P. Masterton, Determination of silver in blood, urine, and tissues of volunteers and burn patients, Clin. Chem. 37 (1991) 1683-1687.|
|||Ł. Tymecki, E. Zwierkowska, S. Głab, R. Koncki, Strip thick-film silver ion-selective electrodes, Sens. Actuators B 96 (2003) 482-488.|
|||M.T. Lai, J.S. Shih, Mercury (Ⅱ) and silver (I) ion-selective electrodes based on dithia crown ethers, Analyst 111 (1986) 891-895.|
|||S. Yajima, N. Yoshioka, M. Tanaka, K. Kimura, Soft metal ion-selective electrodes based on p-coordinate calixarene derivatives, Electroanalysis 15 (2003) 1319- 1326.|
|||Z. Szigeti, A. Malon, T. Vigassy, et al., Novel potentiometric and optical silver ionselective sensors with subnanomolar detection limits, Anal. Chim. Acta 572 (2006) 1-10.|
|||K. Kimura, T. Shono, Y. Inoue, G.W. Gokel, Cation Binding by Macrocycles, Marcel Dekker, New York, 1990.|
|||H. Hirata, K. Higashiyama, Ion selective chalcogenide electrodes for a number of cations, Talanta 19 (1972) 391-398.|
|||S.S. Park, S.O. Jung, S.M. Kim, J.S. Kim, Lipophilic pyrrole-based tetra aza crown ether as neutral carrier for silver ion-selective electrode, Bull. Korean Chem. Soc. 17 (1996) 405-407.|
|||J. Casabó, L. Mestres, L. Escriche, F. Teixidor, C. Pérez-Jiménez, Silver(I) ionselective electrodes based on polythiamacrocycles, J. Chem. Soc. Dalton Trans. (1991) 1969-1971.|
|||X.B. Zhang, Z.X. Hana, Z.H. Fang, G.L. Shen, R.Q. Yu, 5,10,15-Tris (pentafluorophenyl) corrole as highly selective neutral carrier for a silver ion-sensitive electrode, Anal. Chim. Acta 562 (2006) 210-215.|
|||M.J. Goldcamp, K. Ashley, S.E. Edison, J. Pretty, J. Shumaker, A bis-oxime derivative of diaza-18-crown-6 as an ionophore for silver ion, Electroanalysis 17 (2005) 1015-1018.|
|||F. Karimian, G.H. Rounaghi, M.H. Arbab-Zavar, Highly efficient and selective membrane transport of silver(I) using 15-crown-5 as a selective ion carrier, Russ. J. Appl. Chem. 86 (2013) 1670-1675.|
|||E. Bakker, P. Bühlmann, E. Pretsch, Carrier-based ion-selective electrodes and bulk optodes. 1. General characteristics, Chem. Rev. 97 (1997) 3083-3132.|
|||S. Amarchand, S.K.Menon, Y.K. Agarwal, Rare-earth hydroxamate complexes as sensor materials for ion-selective electrodes, Electroanalysis 12 (2000) 522-526.|
|||M.R. Ganjali, A. Daftari, M. Rezapour, T. Poursaberi, S. Haghgoo, Gliclazide as novel carrier in construction of PVC-based La(Ⅲ)-selective membrane sensor, Talanta 59 (2003) 613-619.|
|||X.H. Yang, N. Kumar, H. Chi, D.D. Hibbert, P.N.W. Alxeander, Lead-selective membrane electrodes based on dithiophenediazacrown ether derivatives, Electroanalysis 9 (1997) 549-553.|
|||R. Eugster, T. Rrosatzin, B. Rusterholz, et al., Plasticizers for liquid polymeric membranes of ion-selective chemical sensors, Anal. Chim. Acta 289 (1994) 1-13.|
|||M. Delosa, A. Perez, L.P.Martin, J.C. Quintana,M. Yazdani-Pedram, Influence of different plasticizers on the response of chemical sensors based on polymeric membranes for nitrate ion determination, Sens. Actuators B 89 (2003) 262- 268.|
|||W.E. Morf, D. Ammann, W. Simon, Elimination of the anion interference in neutral carrier cation-selective membrane electrodes, Chimia 28 (1974) 65-67.|
|||D. Ammann, Ion-Selective Microelectrodes, Springer-Verlag, Berlin, 1986.|
|||P.M. Gehring, W.E. Morf, M. Welti, E. Pretsch, W. Simon, Catalysis of ion transfer by tetraphenylborates in neutral carrier-based ion-selective electrodes, Helv. Chim. Acta 73 (1990) 203-212.|
|||G.G. Guilbault, R.A. Durst, M.S. Frant, et al., Recommendations for nomenclature of ion-selective electrodes, Pure Appl. Chem. 48 (1976) 127-132.|
|||K. Srinivasan, G.A. Rechnitz, Selectivity studies on liquid membrane, ion-selective electrodes, Anal. Chem. 41 (1969) 1203-1208.|
|||Y. Umezava, K. Umezawa, H. Sato, Selectivity coefficients for ion selective electrodes: recommended methods for reporting KA,B Pot values (IUPAC AC technical report), Pure Appl. Chem. 67 (1995) 507-518.|
|||E. Bakker, Selectivity of liquid membrane ion-selective electrodes, Electroanalysis 9 (1997) 7-12.|
|||R.K. Mahajan, I. Kaur, M. Kumar, Silver ion-selective electrodes employing Schiff base p-tert-butyl calix  arene derivatives as neutral carriers, Sens. Actuators B 91 (2003) 26-31.|
|||M.H. Mashhadizadeh, M. Shamsipur, Silver(I)-selective membrane electrode based on hexathia-18-crown-6, Anal. Chim. Acta 381 (1999) 111-116.|
|||R.K. Mahajan, O. Parkash, Silver(I) ion selective PVC membrane based on bispyridine tetramide macrocycle, Talanta 52 (2000) 691-693.|
|||S.M. Lim, H.J. Chung, K.J. Paeng, et al., Calixfuranopyrrole and related compounds as the neutral carrier in silver ion-selective electrode, Anal. Chim. Acta 453 (2002) 81-88.|
|||A. Demirel, A. Doğan, G. Akkuş, M. Yılmaz, E. Kılıç, Silver(I)-selective PVC membrane potentiometric sensor based on a recently synthesized calixarene, Electroanalysis 18 (2006) 1019-1027.|