A new cerium (III)-selective membrane electrode based on 2-aminobenzothiazole

Sensors and Actuators B 99 (2004) 410–415

A new cerium (III)-selective membrane electrode based on 2-aminobenzothiazole Morteza Akhond∗ , Mohammad Bagher Najafi, Javad Tashkhourian Department of Chemistry, College of Science, Shiraz University, 71454, Shiraz, Iran Received 18 August 2003; received in revised form 10 November 2003; accepted 6 December 2003

Abstract A new poly(vinyl chloride) (PVC) membrane electrode based on 2-aminobenzothiazole and oleic acid (OA) as a good lipophilic additive for highly selective determination of Ce3+ ion has been developed. The electrode exhibits a Nernstian slope of 19.6±1.0 mV per decade over wide Ce3+ ion concentration range from 2.0×10−6 to 2.0×10−2 M and low detection limit 1.8×10−6 M. The electrode possesses a fast response time of ∼13 s, relatively long lifetime (at least 3 month). The proposed electrode revealed excellent selectivity for Ce3+ over a wide variety of alkali, alkaline earth, some transitions, and heavy metal ions. The electrode could be used in a pH range of 4.1–7.3. The practical utility of the electrode has been demonstrated by its usage as an indicator electrode in potentiometric titration of oxalate (C2 O4 2− ) and fluoride (F− ) ions with Ce3+ solution. The proposed electrode was also successfully applied to the determination of F− ion in mouthwash solution. © 2003 Elsevier B.V. All rights reserved. Keywords: Cerium (III) ion-selective electrode; 2-Aminobenzothiazole; PVC membrane; Potentiometry

1. Introduction Potentiometric ionophore-based membrane sensors are preferred measuring tools for a variety of applications. In fact, ion-selective electrodes (ISEs) for as many as 60 analytes have been described so far [1]. A large number of novel and analytically useful ionophores have been discovered only in the past few years, indicating that this field is steadily moving forward [1,2]. The advantages of ISEs over many other methods for cation and anion detections are their easy handling, non-destructive analysis and inexpensive sample preparation. Traditional ISE detection limits have been reasonably improved and measurements down to picomolar range are now possible [3]. A significant number of ionophores including crown ethers, cryptands, aza-crowns, thiocrowns and thiocompounds have already been exploited for fabrication of poly(vinyl chloride) (PVC) membrane electrodes for series of alkali, alkaline earth, transition and heavy metal ions [4–14]. Cerium is a member of the lanthanum group of elements and the most abundant of them. It is found in monazite, ceric bastnaesite, and silicate rocks and is widely used in production of ductile iron, cast steel, and some stainless ∗ Corresponding author. Tel.: +98-711-2284822; fax: +98-711-2286008. E-mail address: [email protected] (M. Akhond).

0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2003.12.008

steels [15]. Thus, the determination of Ce(III) in different samples is of special interest. The classical methods for the determination of Ce(III) are multiple steps, time-consuming and low precise [15]. Thus, the development of convenient direct methods for the assay of cerium ion in different samples is of urgent need. Determination of Ce(III) by ISEs could be fast, simple, cheap, and precise, and to the best of our knowledge there are only limited reports on Ce3+ ion-selective electrode [16–18]. In this paper, we reported an ISE for determination of Ce3+ ion based on 2-aminobenzothiazole (I) (Fig. 1) as a neutral carrier (ionophore) to construct a PVC-based membrane electrode.

2. Experimental 2.1. Reagents 2-aminobenzothizole (I), sodium tetrephenylborate (NaTPB), high relative molecular weight poly(vinyl chloride) (PVC), tetrahydrofurane (THF), o-nitrophenyloctyl ether (NPOE), dibutyl phthalate (DBP), benzyl acetate (BA), dimethyl sebacate (DMS), oleic acid (OA), were purchased from Merck and used as received. The nitrate salts of all cations (all from Merck) were used. Triply distilled water was used throughout.

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411

Fig. 1. Chemical structure of 2-aminobenzothiazole (I).

2.2. Preparation of PVC membrane The general procedure for preparing the PVC membrane was to mix 24 mg powdered PVC, 3.3 mg ionophore and 13 mg OA with 59.8 mg o-NPOE as the solvent mediator. The mixture was then thoroughly dissolved in 2 ml of THF. The resulting mixture was transferred into a glass dish with a diameter of 2 cm. The solvent was slowly evaporated until an oily concentrated mixture was obtained. A Pyrex tube of 5 mm i.d. was dipped into the mixture for about 5 s so that a non-transparent membrane of about 0.3 mm thickness was formed. After removing the tube from the mixture, it was kept at room temperature for about 3 h. Then it was filled with internal filling solution 1.0 × 10−3 M Ce(NO3 )3 . The electrode was finally conditioned for 20 h by soaking in a 1.0 × 10−3 M cerium (III) nitrate. A silver/silver chloride coated wire was used as an internal reference electrode. 2.3. Apparatus A HIOKI Digital Hitester (model 3256-01) was used for potential measurement at 25 ◦ C. A double-junction saturated Calomel electrode (SCE, Philips) was used as the reference electrode. A Corning 130-pH meter was used for pH measurement at 25 ◦ C. 2.4. Electrode potential measurement The PVC based electrode containing 2-amonobenzothiazole was used as the measuring electrode in conjunction with a double junction SCE. The electrochemical system for this electrode can be represented as follows: Ag–AgCl|KCl (0.1 M), internal filling solution (1.0 × 10−3 M Ce(NO3 )3 )|PVC membrane| test solution|Hg–Hg2 Cl2 , KCl (satd.) The performance of the electrode was investigated by measuring its potential in cerium nitrate solutions prepared in the concentration range 10−2 to 10−6 M by serial dilution. All solutions were freshly prepared by dilution from stock standard solution, 0.1 M, with triply distillated water.

3. Results and discussion Ionophores for use in sensors should have rapid exchange kinetics and adequate complex formation constants in the

Fig. 2. The potential responses of various ISEs based on 2-aminobenzothiazole.

membrane. Also, they should be well soluble in the membrane matrix and have a sufficient lipophilicity to prevent leaching from the membrane into the sample solution. In addition, the selectivity of the neutral carrier-based ISEs is known to be governing by stability constant of the neutral carrier-ion complex and its partition constant between the membrane and sample solution [19] (Fig. 1). To investigate the suitability of 2-aminobenzothiazole as an ion carrier in PVC membranes, the potential responses of various ISEs based on 2-aminobenzothiazole (with the same composition) for each ion were obtained separately. The results are shown in Fig. 2. As seen, among different cations tested, Ce3+ with the most sensitive response seems to be more sensitively determined with the constructed electrode. This is probably due to both the selective behavior of the ionophore against Ce3+ , in comparison to other metal ions, and the rapid exchange kinetics of the resulting 2-aminobenzothiazole–Ce3+ complex [19]. It is well known that, not only the nature of ionophore, but also the membrane composition and the properties of plasticizer affect on the sensitivity, selectivity and linearity of ISE [19]. Since the nature of plasticizer influences the dielectric constant of the membrane phase both the mobility of the ionophore molecule and the state of the ligands [19,20] it was expected to play a key role in the characteristics of the ion selective electrode. Thus, several plasticizers including BA, o-NPOE, DBP, and DMS, which are often used with PVC-membrane electrodes, were evaluated and the results are summarized in the Table 1. As it is seen from Table 1, among four different plasticizers used, o-NPOE resulted in a best Nernstian slope. In other hand the linear range for the electrode constructed with o-NPOE was better than to the other plasticizers. It is also known that the presence of lipophilic anions in cation-selective membrane electrodes not only diminishes the ohmic resistance [21] and enhances the response behavior, selectivity, and sensitivity of the membrane electrodes [22,23], but also may catalyze the exchange kinetics at the

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Table 1 Optimization of membrane ingredients No.

PVC (mg)

Plasticizer (mg)

Ionophore (mg)

Additive (mg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

28.2 27.6 27.3 25.9 25.4 24.8 24.0 24.0 24.0 24.0 27.9 33.5 23.1 25.4 24.8 24.0 24.9

70.3, 69.1, 68.2, 68.8, 63.3, 62.0, 59.8, 59.8, 59.8, 59.8, 55.8, 50.2, 57.6, 63.3, 62.0, 59.8, 62.2,

1.5 3.3 4.5 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3

– – – 6.0, OA 8.0, OA 10.0, OA 13.0, OA 13.0, OA 13.0, OA 13.0, OA 13.0, OA 13.0, OA 16.0, OA 8.0, NaTPB 10.0, NaTPB 10.0, OA+3.0 NaTPB 13.0, OA

NPOE NPOE NPOE NPOE NPOE NPOE DBP DMS BA NPOE NPOE NPOE NPOE NPOE NPOE NPOE NPOE

sample-membrane interface [24]. Without such additives, many electrodes do not respond properly toward a cation. However, the response of the proposed cerium (III)-selective electrode in the absence of an additional lipophilic cation exchanger may be caused by possible trace anionic impurities within the PVC used [20,25]. From the data given in Table 1, it is immediately obvious that the nature and amount of additive influences the performance characteristics of the membrane sensor significantly. The use of ionic additives such as different tetraphenyl borate salts and its more lipophilic derivative, tetrakis(p-chlorophenyl)borate(K-TCPB) also fatty acids such as oleic acid as lipophilic additives is widely reported in the preparation of different ion selective electrodes [26–29]. Their main role is attributed to the inducing permselectivity to some PVC membrane selective electrodes [28,29]. Addition of 13 mg OA as a lipophilic additive showed better Nernstian response and linear range (no. 10). Sodium tetraphenylborate (NaTPB) was also investigated as additive salt. The data given in Table 1 shows that OA is more suitable additive than sodium tetraphenyl borate salt. However, increasing the amount of OA shows no beneficial influence on the membrane electrode response, further more the observed potential was seen to be unstable. Moreover 3.3 mg of ionophore was chosen as the optimum amount of ionophore in the PVC membrane. Further addition of ionophore, however resulted in some decreases in the response of the electrode, most probably due to some inhomogeneities and possible saturation of the membrane [20]. The high amount of the ionophore may also induce strong interactions between polymeric chains and ionophore-preventing mobility of the segments as explained by Hall considering experimental observations of Reinhoudt et al. [30]. In general, the thickness and hardness of the membrane depend upon the amount of PVC used. At higher PVC content, the membrane becomes too dense, which makes the transport of cations into the membrane more difficult

Slope (mV/decade) 9.0 11.0 10.0 13.0 16.5 17.8 14.8 17.6 15.7 19.6 17.0 15.3 Unstable 12.0 7.0 13.0 ∼0

and results in the increased resistance. At lower PVC content, the membrane becomes mechanically weak and swells up easily in aqueous solution. The o-NPOE/PVC ratios of 1.5–2.5 were examined. The membrane prepared with a o-NPOE/PVC ratio of ∼2.5 was found to have the highest sensitivity and the widest linear range. In order to investigate the influence of internal solution concentration on potential response of the Ce3+ -selective electrode, the Ce(NO3 )3 concentration was changed from 1.0 × 10−3 to 1.0 × 10−1 M and the potential responses of the Ce3+ -selective electrode were measured. It was found that the variation of the concentration of the internal solution did not cause any significant difference in the potential response, except for an expected change in the intercept of the resulting Nernstian plots. A 1.0 × 10−3 M of the reference Ce(NO3 )3 solution is quite appropriate for smooth functioning of the electrode system. The optimum conditioning time for the membrane electrode was obtained 20 h. Afterward it then generates stable potentials when placed in contact with Ce3+ solutions. The response time of the electrode was tested by measuring the time required to achieve a 90% of the steady potential when the concentration of Ce(NO3 )3 solution was rapidly increased by one decade from 1.0 × 10−4 to 1.0 × 10−3 M. The electrode has a fast response time of ∼13 s. Reproducibility of electrode was examined by using six similar constructed electrodes under the optimum conditions. The results showed good reproducibility for proposed electrode. For instance, the slopes observed were 19.7 ± 1.2 mV per decade. The long-term stability of the electrode was studied by periodically re-calibrating in standard solutions and calculating the response slope. The slope of the electrode responses was reproducible over a period of at least 3 months. Therefore the proposed electrode can be used for 3 months without a considerable change in its response characteristics towards Ce3+ .

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413

Table 2 Selectivity coefficients log KCe(III),M of various interfering ions for the proposed Ce3+ -selective electrode Cations

Na+

Fig. 3. The effect of pH on the response of Ce(III) membrane electrode.

The pH dependence of the electrode potential was investigated over the pH range of 2–9 for 1.0 × 10−4 M of Ce3+ ions. As shown in Fig. 3, the potential was independent of pH in the range of 4.1–7.3. The diminished potential of the electrode at pH < 4 could be due to the high proton affinities of OA [31] and ionophore. On the other hand the decreasing of the potential at higher pH values can be attributed to the formation of some hydroxyl complexes of Ce3+ ion in solution. The potential response of the optimized electrode, to varying concentration of Ce3+ ions, was examined. The calibration plot is shown in Fig. 4, which indicates a linear range of 2.0 × 10−6 to 2.0 × 10−2 M with a Nernstian slope of 19.6 ± 1.0 mV per decade of Ce3+ activity. The practical limit of detection was 1.8 × 10−6 M as determined from the intersection of the two extrapolated segments of the calibration graph based on recommended procedure by IUPAC [32]. The influence of interfering ions on the response behavior of the ion-selective membrane electrode is usually Pot . To indescribed in terms of selectivity coefficients, KA,B vestigate selectivity of the membrane electrode proposed, its potential response was investigated in the presence of a wide variety of various foreign cations using the matched

Ag+ Mg2+ Ca2+ Ba2+ Co2+ Cu2+ Zn2+ Pb2+ Cd2+ Hg2+ La3+ Fe2+ Al3+ Cr3+ Cs+ Rb+ NH4 + Sr2+

Present study

−6.00 −2.85 −6.00 −6.00 −6.00 −4.55 −2.39 −6.00 −2.59 −3.68 −2.77 −2.52 −2.33 −2.49 −3.92 −6.00 −6.00 −6.00 −6.00

Reference [6]

[7]

[8]

−2.62 −1.49 −2.51 −1.66 −1.92 −1.49 −1.42 −1.20 −1.96 −1.15 −4.00 −1.30 – – – – – – –

– −3.96 – – – −4.29 −4.22 −4.16 −2.08 −3.10 −4.09 −2.40 – – – – – – –

−3.85 −4.00 −2.40 −2.30 −2.51 −2.28 −2.28 −2.28 −2.27 −2.64 −2.26 −1.38 −2.00 −1.68 −1.70 −1.49 – – –

potential method (MPM) [33,34]. This is a recommended procedure that gets rid of the limitations of the corresponding methods based on the Nicolski–Eisenmann equation for the determination of potentiometric selectivity coefficient [35]. These limitations include non-Nernstian behavior interfering ions, inequality of charges of any primary interPot . According to fering ions, and activity dependence of KA,B the MPM, the selectivity coefficient is defined as the activity ratio of the primary ion and the interfering ion that gives the same potential change in a reference solution [33,34]. Thus, one should measure the change in potential upon changing the primary ion activity. Then the interfering ion would be added to an identical reference solution until the Pot same potential change is obtained. The resulting log KA,B 3+ values for the proposed Ce ion-selective electrode are summarized in Table 2. The selectivity coefficients for the previously reported cerium-selective electrodes are also included in Table 2. As seen, the alkali, alkaline earth, and transition metal ions used as interfering ions will not disturb the functioning of the Ce3+ ion-selective membrane electrode significantly. Moreover a comparison between the selectivity coefficients of the proposed electrode with those previously reported for the cerium reveals that the proposed electrode show somewhat similar, in some cases, and superior, in most cases selectivity behavior to foreign ions.

4. Analytical applications

Fig. 4. Calibration plot for the proposed Ce(III)-selective electrode.

The proposed Ce3+ ion-selective electrode was found to work well under laboratory conditions. It was used as an indicator electrode in potentiometeric titration of 20.0 ml ox-

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oride mouth wash solution (Shahre Daru Co., Tehran, Iran) was chosen as a pharmaceutical sample. An appropriate volume of the sodium fluoride mouthwash solutions were chosen (20.0 ml of the 0.2% solution) and the pH was adjusted to 5.0 (using a concentrated NaOH solution). The resulting solution was then titrated with a 0.1 M of cerium nitrate solution. The titration curve with its first derivative was shown in Fig. 6. The results show that there is a satisfactory agreement between the declared fluoride content (0.2%) and the determined value (0.2 ± 0.01).

5. Conclusions

Fig. 5. Potentiometric titration curves (a) 20 ml 1.0 × 10−3 M F− with 0.1 M Ce3+ and (b) 20 ml 1.0 × 10−4 M C2 O4 2− with 0.01 M Ce3+ using Ce3+ ISE as an indicator electrode.

alate ion solution (1.0 × 10−4 M) and also 20 ml fluoride ion solution (1.0 × 10−3 M) with a 0.01 and 0.1 M Ce3+ solution, respectively. The resulting titration curves are shown in Fig. 5.The exact amount of oxalate and fluoride ions were then evaluated from the sharp inflection points of the resulting titration curve. Since the stoichiometry ratio between Ce3+ ion and oxalate (C2 O4 2− ) is 2/3 and between Ce3+ ion and fluoride ion (F− ) is 1/3, therefore, for titration of oxalate and fluoride ion solutions the inflection point appear about at 134 and 66.7 ␮l respectively. It is seen that the amount of anions in solution can be accurately determined with the proposed electrode. The electrode was also successfully applied to the determination of F− ion in pharmaceutical sample. Sodium flu-

On the results discussed in this paper, 2-aminobenzothiazole can be considered as a suitable neutral ionophore for construction of a PVC-based membrane selective electrode for the direct determination of cerium (III) ion in solution. The results showed that oleic acid(OA) is a good lipophilic additive for electrode construction. The proposed electrode shows high selectivity and sensitivity to Ce3+ ion, wide dynamic range, low detection limit and fast response time. The proposed electrode revealed excellent selectivity for Ce3+ over a wide variety of alkali, alkaline earth, some transitions, and heavy metal ions. The proposed Ce3+ ion-selective electrode was found to work well under laboratory conditions. It was used as an indicator electrode in potentiometeric titration of oxalate (C2 O4 2− ) and fluoride (F− ) ions with Ce3+ solution. The proposed electrode was also successfully applied to the determination of F− ion in mouthwash solution.

Acknowledgements The support of this work by the Shiraz University Research Council is gratefully acknowledged. We thank Dr. G. Absalan for carefully reading the manuscript and for his helpful suggestions. References

Fig. 6. Potentiometric titration curve of 20 ml sodium fluoride mouthwash solution with 0.1 M Ce3+ using Ce3+ ISE as an indicator electrode.

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Biographies Morteza Akhond received his bachelor degree in chemistry from Ahwaz University in 1978. He, later, obtained his M.Sc. (1986) and Ph.D. (1995) in analytical chemistry from the University of Shiraz, Iran. He joined the faculty of Shiraz University in 1995. The overall objective of his research is to investigate and apply new techniques for transport through liquid membranes, design and construction of sensors and optodes and chemometrics. M.B. Najafi received his bachelor degree in chemistry from Isfahan University in 1999 and M.Sc. degree in analytical chemistry from Shiraz University in 2002. His field of interest is development of new chemical sensors for cations and anions species. J. Tashkhourian studied chemistry at the Shiraz University. He received his B.Sc. in chemistry (1997) and M.Sc. in analytical chemistry (2000). He is currently studying for the degree of Ph.D. at the chemistry department of Shiraz University. His research interests are on design and construction of chemical sensors and application of chemometrics in multi-elemental and trace analysis.