Analytica Chimica Acta 569 (2006) 35–41 Highly selective and sensitive chromium(III) membrane sensors based on a new tridentate Schiff’s base Mohammad Reza Ganjali ∗ , P. Norouzi, F. Faridbod, M. Ghorbani, M. Adib Department of Analytical Chemistry, Tehran University, P.O. Box 14155-6455, Tehran 14155 6455, Iran Received 23 November 2005; received in revised form 25 March 2006; accepted 27 March 2006 Available online 6 April 2006 Abstract In order to get a view about the tendency of N-(1-thien-2-ylethylidene)benzene-1,2-diamine (SNS), towards chromium and some other metal ions, theoretical calculations and conductance studies were carried out. Then, a highly selective and sensitive plasticized membrane sensor for chromium(III) ions, based on SNS as membrane carrier, was prepared. The sensor shows a linear dynamic range of 1.0 × 10−6 to 1.0 × 10−1 M with a Nernstian slope of 19.9 ± 0.3 mV decade−1 and a detection limit of 7.0 × 10−7 M (∼40 ppb). It has a fast response time of <12 s and can be used for at least eight weeks without any considerable divergences in its potentials. The proposed sensor revealed very good selectivities with respect to most of the common metal ions including Li, Na, K, Rb, Cs, Be, Mg, Ca, Cu, Co, Ni, Zn, Pb, Hg, Fe, La, Ce and Eu ions. The proposed sensor could be used in a pH range of 3.0–6.6. It was also used in the determination of Cr(III) in wastewater of chromium electroplating and leather industries, showing satisfactory results. The sensor was also applied for monitoring the chromium ion level in wastewater of chromate industries. © 2006 Elsevier B.V. All rights reserved. Keywords: S-N Schiff’s base; Potentiometry; Chromium(III) sensor; PVC membrane 1. Introduction In the recent decades, many intensive studies have been introduced on the design and synthesis of highly selective ion-carrier as sensory molecules in the fabrication of ion-selective electrodes. There are many examples about the selective affinity of Schiff’s bases towards metal ions, and hence their application in the construction of ISEs [1–14]. Regarding this and taking into account the charge density, and size of chromium ion and also the concept of soft–hard acid–base, we decided to design an ionophore that contained suitable intermediate or soft donor atoms. Such an ionophore had to have a semi-cavity of proper size, and be able to form wrap-around complexes with chromium ions. All this, as it is obvious, takes place under the optimum free energy regime. With regard to what is said above, the primary tests were theoretical calculations and conductance studies (for complexation). The results revealed the tendency of SNS to selectively complex with Cr3+ . This can be justified, taking into ∗ Corresponding author. Tel.: +98 2161112788; fax: +98 2166495291. E-mail address: ganjali@khayam.ut.ac.ir (M.R. Ganjali). 0003-2670/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2006.03.105 account the intermediate nature of the chromium ion, caused by its free d-orbital, which makes it suitable to complex with SNS, that contains intermediate donor atoms (two N and one S atoms). In spite of the successful progress in the design of highly selective ion-carrier for various organic and inorganic ions, there is only a limited number of reports on the development of selective ion-carrier for Cr(III) [1–14]. Potentiometer monitoring, based on the ion-selective membrane sensor as a simple method, offers several advantages such as speed and ease of preparation and procedure, simple instrumentation, relatively fast response, wide dynamic range, reasonable selectivity and low cost. These characteristics have inevitably led to sensors for several ionic species and the list of available electrodes has grown substantially over the last few years [15]. Due to the vital importance of Cr(III) in many complex biological systems and industrial samples [16–18], the search for new selective and sensitive PVC membrane electrodes for its quick measurement is a challenging goal [16,19]. The Schiff’s bases are known to form very stable complexes with transition metal ions [20,21]. The resulting 1:1 complexes have been frequently used as catalysts in such diverse processes as oxygen- and atom-transfer [22]. However, despite the 36 M.R. Ganjali et al. / Analytica Chimica Acta 569 (2006) 35–41 extensive scientific reports on the synthesis, characterization and crystalline structure of the transition of the metal–Schiff’s bases complexes, there are only a few reports on the use of Schiff’s bases molecules as ionophore in the construction of ion-selective electrode studies [23–25]. In this work, we wish to report the fabrication of a highly selective and sensitive Cr(III) membrane sensor based on a new Schiff’s base for the monitoring of Cr(III) ion concentration in water and wastewater samples. 2. Experimental 2.1. Reagents Reagent grade o-nitrophenyloctyl ether (NPOE), dibutyl phthalate (DBP), benzyl acetate (BA), acetophenone (AP), potassium tetrakis(p-chlorophenyl) borate (KTpClPB), oleic acid (OA), tetrahydrofuran (THF), 1,2-phenylenediamine, 2acetylthiophene, acetic acid and high relative molecular weight PVC were purchased from Merck and Aldrich and used as received. The nitrate and chloride salts of all cations used (all from Merck and Aldrich) were of the highest available purity and used without any further purification, except for vacuum drying over P2 O5 . Triply distilled de-ionized water was used throughout. 2.2. Ionophore synthesis The procedure for the preparation of N-(1-thien-2ylethylidene)benzene-1,2-diamine (SNS; Fig. 1) is as follows: a mixture of 2-acetylthiophene (0.01 mol, 1.26 g), 1,2phenylenediamine (0.01 mol, 1.08 g) and catalytic amount of acetic acid was refluxed in absolute ethanol (20 ml) for 1 h. After cooling to room temperature, the pure product was obtained in the form of yellow crystals, mp 124–127 ◦ C, 1.9 g, yield 90%; IR (KBr) (υmax (cm−1 )): 3310 and 3261 (NH), 1598, 1466, 1310, 1230, 1009, 881, 804, 765; MS, m/z (%): 226 (M+ , 39). Anal. Calcd for C12 H12 N2 S (216.31): C, 66.63; H, 5.59; N, 12.95. Found: C, 66.4; H, 5.8; N, 12.9%. 1 H NMR (250.1 MHz, CDCl solution): δ 2.33 (3H, s, CH ), 3 3 3.72 (2H, br., NH2 ), 6.64 (1H, d, J = 7.2 Hz, CH), 6.70–6.78 (2H, m, 2 CH), 6.95 (1H, t, J = 7.6 Hz, CH), 7.05 (1H, dd, J = 3.5 and 1.6 Hz, CH), 7.43–7.48 (2H, m, 2 CH). 13 C NMR (62.5 MHz, CDCl 3 solution): 16.68 (CH3 ), 113.66, 120.31, 121.54, 122.69, 126.84, 127.89 and 135.20 (7 CH), 140.12, 141.71 and 144.00 (3 C), 157.59 (C N). 2.3. Electrode preparation The general procedure to prepare the PVC membrane involved the thorough mixing of 30 mg of powdered PVC and 60 mg of NPOE in 5 ml of fresh THF. To this mixture were added 3 mg KTpClPB and 7 mg SNS. Afterwards, the solution was mixed well and the resulting mixture was transferred into a glass dish of 2 cm in diameter. The THF content of the mixture was evaporated slowly, until an oily concentrated mixture was obtained. A Pyrex tube (3–5 mm o.d.) was dipped into the mixture for about 10 s, so that a transparent membrane of about 0.3 mm thickness was formed [4,10,15–17,19]. The tube was then pulled out of the solution and kept at room temperature for 10 h. The tube was then filled with an internal filling solution (1.0 × 10−3 M CrCl3 ). The electrode was finally conditioned for 24 h by soaking in a 1.0 × 10−3 M solution of CrCl3 . A silver/silver chloride coated wire was used as an internal reference electrode. 2.4. EMF measurements Potential measurements were carried out by means of a Corning ion analyzer 250 pH/mVmeter at room temperature. A cell assembly comprises: Ag–AgCl|1.0 × 10−3 M CrCl3 |PVC membrane|test solution|Hg–Hg2 Cl2 , KCl (saturated). 3. Results and discussion 3.1. Preliminary theoretical calculations To get a view about the tendency of SNS towards chromium and some other metal ions, ab initio quantum-mechanical calculations were carried out. The pair-wise interaction energy E(A–B) between two molecules A and B is estimated as the difference between the energy of the complex E(A–B) and those of isolated constituents: E(A–B) = E(A–B) − E(A) − E(B) (1) The calculations were performed according to the secondorder Møller–Plesset (MP2) perturbation theory [26,27], which takes into account the electron correlation energy in addition to the Hartree–Fock (HF) energy. The use of such calculations is fully justified by the fact that the description of base stacking requires calculations with explicit inclusion of the electron correlation [28]. The interaction energy, at a given order of the Møller–Plesset (MP) perturbation expansion, is calculated as: EMPn = EHF + Fig. 1. Optimal conformation of ligand after complexation with Cr3+ . n
i=2 ECorr (MPi) (2) 37 M.R. Ganjali et al. / Analytica Chimica Acta 569 (2006) 35–41 Table 1 Interaction energy between metal ions and SNS Table 2 The formation constants of SNS–Mn+ complexes Compounds Interaction energy (kcal/mol) Cation −603.174 −229.430 −145.224 −45.131 Cr3+ Cr3+ –ligand La3+ –ligand Pb2+ –ligand K+ –ligand where EHF is the HF energy and ECorr (MPi) is the ith order perturbative correction to the correlation energy. During our calculations, only the valence electrons were explicitly correlated, which correspond to the usual frozen core approximation. We also limited the perturbation expansion (2) to the second order, which is expected to take into account the major contributions to the van der Waals energies (electrostatic, polarization, dispersion, electron transfer and exchange contributions) [29]. The lanl2mb basis-set was used for all atoms to optimize molecules. All the calculations were performed using the Gaussian 98 package [30]. Interaction energies for ionophore and some metal ions were calculated from Eq. (1) and are listed in Table 1. From the data given in Table 1, it is obvious that the calculated interaction energies decreased in the order of: Cr3+ ≫ Co2+ > La3+ > Pb2+ > K3+ . Thus, based on the above ab initio calculation results, the ligand could be used as a suitable ionophore in the preparation of a chromium ion-selective membrane electrode. The optimized structures of the ligand and its Cr3+ complex are shown in Fig. 2. 3.2. Conductance study of SNS complexation with metal ions in acetonitrile solution In the SNS structure, the existence of three donating nitrogen and sulfur atoms was expected to increase both the stability and selectivity of its complexes with transition and heavy metal ions, more than other metal ions [19,31–34]. Thus, at the first experiment, the SNS complexation with a number of transition and heavy metal ions was investigated conductometrically in acetonitrile solutions (1.0 × 10−4 M of cation solution and 1.0 × 10−2 M of ligand) at 25 ± 0.1 ◦ C [35]. Twenty-five milliliters of Cr3+ solution was titrated with 0.01 M of SNS solution in order to obtain a clue about the stability and selec- Fig. 2. Structure of SNS. La3+ Ce3+ Ni2+ Co2+ Na+ log Kf Cation log Kf 5.07 ± 0.15 2.77 ± 0.13 2.86 ± 0.09 2.43 ± 0.10 2.35 ± 0.17 <2.0 Cu2+ 2.89 ± 0.12 2.38 ± 0.09 2.44 ± 0.18 2.23 ± 0.11 2.09 ± 0.15 <2.0 Zn2+ Pb2+ Cd2+ Hg2+ K+ tivity of the resulting complexes. The formation constants (Kf ) of the resulting 1:1 complexes are given in Table 2. As it can be seen, the SNS with the most stable complex with Cr(III) ion is expected to act as a suitable ion-carrier for the fabrication of a Cr(III) ion-selective membrane sensor. Therefore, in the next step, SNS was used as a potentially suitable neutral carrier in the fabrication of a number of PVC membrane ion-selective electrodes for Cr(III) ion and common mono-, di- and trivalent metal ions. The potential responses for these metal ions are depicted in Fig. 3. As it can be concluded from Fig. 3, among the examined metal ions, only the resulting Cr(III)-selective sensor possesses a Nernstian behavior over a very wide concentration range. 3.3. Membrane composition effect on the potential response of the Cr(III) sensor based on SNS The sensitivity and selectivity obtained for a given ion-carrier depend significantly on the membrane ingredients, the nature of the solvent mediator and the used additive [36–46]. Consequently, the influences of the membrane composition on the potential responses of the Cr(III) sensor were studied. These results are summarized in Table 3. As it is seen, the best sensitivity is achieved when the amount of ion-carrier increases up to a value of 7% in the presence of 3% KTpClPB and 60% of polar solvent (NPOE). Among the four used solvent mediators, Table 3 illustrates that the membrane, containing NPOE (membrane F) and featuring higher polarity than the AP, the DBP and the BA, exhibits a Nernstian response. This is caused by the ability of NPOE to Fig. 3. Potential responses of various ion-selective electrodes based on SNS. 38 M.R. Ganjali et al. / Analytica Chimica Acta 569 (2006) 35–41 Table 3 The optimization of the membrane ingredients Membrane A B C D E F G H I J L Composition (%) Slope (mV/decade) PVC Plasticizer Ionophore Additive 30 30 30 30 30 30 30 30 30 30 30 67 NPOE 65 NPOE 63 NPOE 62 NPOE 61 NPOE 60 NPOE 60 AP 60 DBP 60 BA 55 NPOE 51 NPOE 3 5 7 8 7 7 7 7 7 7 7 – – – – 2 KTpClPB 3 KTpClPB 3 KTpClPB 3 KTpClPB 3 KTpClPB 8 OA 12 OA extract Cr(III) ions with high charge density from the aqueous phase to the organic membrane phase. Table 3 reveals that the slope of the Cr(III) membrane sensor, in the absence of KTpClPB or OA as suitable additives, is about two-third of the expected Nernstian value (membrane C). However, addition of 3% KTpClPB will increase the sensitivity of the electrode response considerably so that the membrane sensor (membrane F) displays a nice Nernstian behavior. It is well known that the presence of lipophilic anions in cation-selective membranes, based on neutral carrier, not only diminishes the ohmic resistance and enhances the response behavior and selectivity but also, in cases where the extraction capability is poor, increases the sensitivity of the membrane electrodes [45–47]. Nonetheless, the membrane with the composition of 30% PVC, 7% SNS, 3% KTpClPB and 60% NPOE displays a Nernstian behavior. 10.9 11.7 13.2 12.7 18.7 19.9 18.3 17.4 17.3 15.6 17.1 ± ± ± ± ± ± ± ± ± ± ± 0.3 0.6 0.2 0.3 0.5 0.3 0.2 0.3 0.5 0.4 0.3 ion, obtained when the linear regions of the calibration graph are extrapolated to the base line potential, is 7.0 × 10−7 M (∼40 ppb). The standard deviation of 12 replicate potential measurements for the proposed sensor was maximum, ±0.3 mV. 3.5. pH effect The pH dependence of the Cr(III) membrane sensor was tested over a pH range of 1.5–8.5 in a 1.0 × 10−4 M Cr(III) solution and the corresponding results are illustrated in Fig. 5. From this figure it is obvious that the potential remains fairly constant in a pH range of 3.0–6.6. Beyond this range, a gradual change in the potential was detected. The observed potential drift at higher pH values could be attributed to the formation of some soluble and insoluble hydroxy complexes of Cr(III) in the solution. At lower pH values, the potentials increased, indicating that the membrane sensor responds to proton ions, too. 3.4. Calibration graph and statistical data The plot of EMF versus pCr(III) obtained under optimal membrane ingredients for the sensor (Fig. 4) indicates that it has a Nernstian behavior over a very wide concentration range of Cr(III) ion (1.0 × 10−6 to 1.0 × 10−1 M). The slope and the linear range of the resulting calibration graph were 19.9 ± 0.3 mV per decade and 1.0 × 10−6 to 1.0 × 10−1 M, respectively. The limit of detection (LOD), defined as the concentration of Cr(III) Fig. 4. Calibration graph of the Cr(III) membrane sensor based on SNS (with membrane F). 3.6. Dynamic response time For analytical purposes, response time is one of the most important factors that are taken into account. In this research, the practical response time was recorded by immediate and successive change of the Cr(III) concentration from 1.0 × 10−6 to 10 × 10−1 M. The results are shown in Fig. 6. As it can be seen, in the whole concentration range, the electrode reaches to its equilibrium response in a short time (<12 s). Fig. 5. The effect of the pH of the test solution (1.0 × 10−3 M) on the potential response of the Cr(III) membrane sensor (with membrane F). 39 M.R. Ganjali et al. / Analytica Chimica Acta 569 (2006) 35–41 Table 5 Selectivity coefficients of various interfering cations for the membranes Ion Li+ Fig. 6. Dynamic response time of the Cr(III) sensor for step changes in the concentration of Cr3+ : (A) 1.0 × 10−6 M, (B) 1.0 × 10−5 M, (C) 1.0 × 10−4 M, (D) 1.0 × 10−3 M, (E) 1.0 × 10−2 M and (F) 1.0 × 10−1 M. 3.7. Life time study For the investigation of the stability and the lifetime of the Cr(III) membrane sensor, three sensors were tested over a period of 10 weeks. During this period, the sensors were used daily over an extended time period (1 h per day). Their slopes and detection limits were measured. The results are summarized in Table 4, where it is concluded that, after 10 weeks, slight changes were observed in the slopes and detection limits (from 19.9 and 7.0 × 10−7 to 18.6 mV per decade and 3.0 × 10−6 M, respectively). 3.8. Selectivity coefficients evaluation The potentiometric selectivity coefficients, which reflected the relative response of the membrane sensor towards the primary ion, over other ions present in solution, is perhaps one of the most important characteristics of an ion-selective electrode. In this paper, the potential responses of the proposed Cr(III) membrane sensor to a wide variety of cations were investigated through the matched potential method (MPM) [48]. The resulting selectivity coefficient values, obtained for the Cr(III) membrane electrode, are summarized in Table 5. Evidently, the selectivity coefficients for the tested alkali metal ions (Li+ , Na+ , K+ , Rb+ and Cs+ ) are in the range of 2.0 × 10−4 to 3.7 × 10−4 . The selectivity coefficients of the tested divalent cations (Zn2+ , Table 4 The lifetime behavior of the Cr(III) membrane sensor Period (weeks) Slope (mV/decade) Detection limit 1 2 3 4 5 6 7 8 9 10 19.9 19.9 19.7 19.8 19.4 19.1 19.0 18.9 18.8 18.6 7.0 × 10−7 7.0 × 10−7 8.0 × 10−7 1.0 × 10−6 1.0 × 10−6 1.5 × 10−6 2.5 × 10−6 2.5 × 10−6 3.0 × 10−6 3.0 × 10−6 ± ± ± ± ± ± ± ± ± ± 0.2 0.3 0.3 0.2 0.4 0.3 0.2 0.1 0.3 0.4 Na+ K+ Rb+ Hg2+ Cs+ Be2+ Mg2+ Ca2+ Cu2+ Zn2+ Co2+ Ni2+ Pb2+ Cd2+ Fe3+ Ce3+ La3+ Eu3+ Ksel , MPM F E G H −4.50 −5.56 −5.54 −4.64 −4.78 −5.30 −4.95 −3.32 −4.49 −4.86 −4.71 −4.64 −4.65 −4.60 −4.27 −2.64 −4.76 −4.88 −3.53 −4.55 −4.50 −4.52 −4.88 −3.22 −4.88 −5.00 −3.50 −3.52 −3.15 −3.35 −3.40 −3.49 −3.72 −3.65 −2.67 −3.22 −3.14 −3.69 −4.90 −4.67 −4.88 −4.90 −3.29 −4.95 −4.72 −3.11 −3.50 −3.13 −3.28 −3.35 −3.34 −3.56 −3.27 −2.63 −3.23 −3.11 −3.53 −4.43 −4.40 −4.28 −4.35 −3.04 −4.37 −4.82 −3.50 −3.50 −3.10 −3.28 −3.35 −3.34 −3.56 −3.72 −2.63 −3.23 −3.11 −3.53 Conditions: concentration of reference solution (1.0 × 10−6 M of CrCl3 ), primary ions (1.0 × 10−5 M of CrCl3 ) and interfering ions (1.0 × 10−1 to 1.0 × 10−2 M). Ni2+ , Ca2+ , Co2+ , Cd2+ , Pb2+ , Hg2+ , Be2+ , Mg2+ and Cu2+ ) are also small, in the range 1.1 × 10−4 to 7.7 × 10−4 . In the case of trivalent cations (Fe3+ , La3+ , Ce3+ and Eu3+ ), the selectivity coefficients are relatively small (in the range 2.3 × 10−3 to 7.7 × 10−4 ). The obtained selectivity coefficients indicate that the disturbance, produced by these cations in the function of the proposed Cr(III) membrane sensor, is negligible. Table 6 compares the selectivity coefficients of the proposed Cr(III) sensor, based on the SNS, with those of the best previously Cr(III) electrodes reported in the literature [2–5,9,11,13]. Table 6 Comparison of the selectivity coefficients of different Cr(III) sensors Interfering ions (B) Hg2+ Co2+ Ni2+ Cd2+ Zn2+ Pb2+ Cu2+ La3+ Ce3+ Al3+ Na+ K+ Cs+ Ca2+ Sr2+ Ba2+ MPM log KCr 3+ ,B Ref. [4], MPM Ref. [5], MPM Ref. [9], MPM This work, MPM −2.80 – – – −3.10 −3.00 −2.85 −3.41 – – −3.70 −3.20 – – – – – −2.16 −2.92 – −2.16 −2.22 −2.02 – −2.62 −2.96 −2.48 −2.62 – −2.44 −2.38 – −2.15 – −2.49 −2.17 −0.65 −2.85 −1.57 −4.57 – −3.15 −0.76 – – −1.85 −1.12 −1.05 −4.78 −4.64 −4.65 −4.27 −4.71 −4.43 −4.86 −4.88 −4.76 – −5.56 −5.54 −5.30 −4.49 – – 40 M.R. Ganjali et al. / Analytica Chimica Acta 569 (2006) 35–41 Table 7 Determination of Cr(III) in four samples of wastewater electroplating industry (A) and in three leather industrial wastewater samples (B) using AAS and proposed Cr(III) sensor based on SNS Sample no. A1 A2 A3 A4 B1 B2 B3 a ISE (ppm) 1.27 1.03 5.15 2.77 13.2 10.9 19.1 ± ± ± ± ± ± ± 0.03a 0.04 0.12 0.10 0.1a 0.2 0.1 Acknowledgement The authors express their appreciation to the Tehran University Research Council for the financial support of this study. AAS (ppm) 1.25 1.00 5.12 2.70 12.9 10.6 18.5 ± ± ± ± ± ± ± 0.04a 0.03 0.11 0.08 0.1a 0.1 0.1 Results are based on five measurements. From the data given in Table 6, it is immediately obvious that the selectivity coefficients of the sensor are superior to those reported by other researchers. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] 3.9. Analytical application [11] The proposed sensor was used for the direct Cr(III) monitoring in wastewater of four chromium electroplating industrial samples. 10.0 ml of each sample (containing about 1.2 g Cr6+ , 0.03 g H2 SO4 and 0–0.012 g) was taken and diluted with 10.0 ml of buffer acetic acid/sodium acetate (pH 8.0) and distilled water in a 100.0 ml flask. Then, the potential of the resulting solutions was measured and with the help of the calibration curve, the Cr(III) concentration was determined. The results were compared with the data obtained from the atomic absorption spectrometry (AAS) (Table 7). As it can be seen from Table 7, the obtained results present a satisfactory agreement with those obtained by AAS. The similar procedure was also applied to the Cr(III) concentration monitoring of the leather industries wastewater (Charmshahr, Tehran, Iran). Because these samples contain high interfering ions concentration, the ionic strength of the sample solutions and the standard Cr(III) solutions must be maintained at 0.1–0.01 M using NaCl. The corresponding results deriving from the proposed sensor and the AAS are depicted in Table 7. Obviously, the Cr(III) sensor can be used for the Cr(III) ions direct monitoring in leather industries wastewater with acceptable accuracy. [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] 4. Conclusion The use of the new S-N Schiff’s Base SNS with the NPOE, being the solvent mediator, results in the best response characteristics with Nernstian behavior over a wide concentration range of 1.0 × 10−6 to 1.0 × 10−1 M Cr3+ and a fast response time of <12 s. 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