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Chinese Chemical Letters 22 (2011) 1087–1090 www.elsevier.com/locate/cclet
Iodide-selective polymeric membrane electrode based on copper(II) bis(N-2-bromophenylsalicyldenaminato) complex Ali Benvidi *, M.T. Ghanbarzadeh, M. Mazloum-Ardakani, R. Vafazadeh Department of Chemistry, Yazd University, Yazd, Iran Received 18 November 2010
Abstract A PVC membrane electrode based on copper(II) bis(N-2-bromophenylsalicyldenaminato) as ionophor was prepared. The ion selective electrode was tested by inorganic anions and showed a good selectivity for iodide ion. This sensor exhibited Nernstian behavior with a slope of 57.8 mV per decade at 25 8C. The proposed electrode showed a linear range from 1.0 105 to 1.0 101 mol/L with a detection limit of 5.0 106 mol/L. The electrode response was independent of pH in the range of 3.0– 10.0. The proposed sensor was applied to determine the iodide in water and antiseptic samples. # 2011 Ali Benvidi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Iodide-selective electrode; Polymeric membrane; Potentimetry
Iodine plays an important role in many biological activities such as cerebration, cell growth, neurological activities and metabolism. Although iodine is an essential trace element for life, but it is toxic and its vapor irritates the eyes and lungs [1]. Numerous analytical methods have been reported for the determination of iodide at low concentration levels. These methods have been explained in literature [2]. On the other hand, potentiometric methods based on ion selective electrodes (ISEs) offer great advantages [2]. They have been widely used for analysis of ions in biological, environmental and industrial samples. In view of such advantages, efforts have been made to make iodide selective electrode. ISEs for anions based on ion exchangers such as lipophilic quaternary ammonium or phosphonium salts display classical Hofmeister selectivity behavior in which membrane selectivity is controlled by the free energy of hydration of ions involved [3]. The antiHofmeister anion selectivity is obtained in the case of membrane electrodes incorporated with an organometallic complex [4], metalloporphyrines [5], metallophthalocyanines [6] and Schiff base metallic complexes [7]. These deviation results from the direct interaction between the central cation of ionophore and the analytical anion and steric effect resulted by the structure of the ligand. Therefore, the focus in this research is on the anion sensitive materials with antiHofmeister behavior. Several iodide ISEs based on variety of ion carriers have also been reported in the literature [2,8–13]. Some sensors have narrow linear range [9], narrow pH range [10–12], high detection limit [10], long response time [2], and some suffer serious interference from lipophilic anions like SCN [9,12,13] and Sal [8,9]. In this study we have synthesized copper(II) bis(N-2-bromophenylsalicyldenaminato) complex (Cu(II)BBPSD) as ionophore for the preparation of polymeric membrane sensor for determination of iodide ion. * Corresponding author. E-mail address:
[email protected] (A. Benvidi). 1001-8417/$ – see front matter # 2011 Ali Benvidi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2011.03.008
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Fig. 1. Structure of copper(II)-bis-N-2-bromophenilsalicylidenaminato complex.
1. Experimental Reagent grade sodium and potassium salts of all anions, dibutyl phthalate (DBP), hexadecyltrimethylammonium bromide (HTAB), tetrahydrofuran (THF) and PVC were purchased from Merck. The Cu(II)BBPSD (Fig. 1) was synthesized by authors using the reported procedure [14]. The general procedure to prepare the PVC membrane was to mix thoroughly 33 mg of powdered PVC, 63 mg of plasticizer DBP, 2 mg of ionophore and 2 mg of cationic additive HTAB. The mixture was dissolved in 3 mL of THF and was stirred to homogeneous solution, and then a Pyrex tube (3 mm i.d.) was dipped into the mixture for 5 s, so that a nontransparent membrane 0.3 mm thick was formed. The tube was then pulled out from the mixture and kept at room temperature for 12 h. The tube was then filled with an internal solution (1.0 103 mol/L KI). The electrode was finally conditioned for 24 h by soaking it in a 1.0 102 mol/L KI solution. Potentiometric measurements were accomplished by use of a Metrohm (model 691) digital pH/mV meter at 25 1 8C using the following cell: Ag j AgCl j KCl(satd.) j test solution jj PVCmembrane jj 0.001 mol/L KI j KCl(satd.) jAgCl j Ag. Activities were calculated according to the Debye–Huckle procedure [15], also, for the calibration curve, concentration was used instead of using the activity. 2. Results and discussion In preliminary experiments, various PVC-membrane ISEs with Cu(II)BBPSD were prepared and tested for different anions. The potential responses of various ion selective electrodes based on ionophore were examined in the activity range of 1.0 105 to 1.0 101 mol/L solutions containing a single type of anions. The membrane showed selectivity for iodide ion relative to most common inorganic anions. For investigation of membrane composition several membranes were prepared with different compositions and the best amounts of components were obtained using the slope of potentiometric response versus log[I]. The best response was observed with the membrane composed of DBP:PVC:complex:HTAB percent ratio of 63:33:2:2, and this was selected for the preparation of polymeric membrane electrode for iodide ion. The incorporation of cationic or anionic additives into the membrane not only reduces the membrane resistance but also enhances the response behavior and selectivity and reduces interference from sample. The electrodes lacking ionic sites respond to cations since the commercially available PVC contains anionic impurities that function as anionic sites [16]. The effect of HTAB concentration in the membrane was investigated at several carrier/additive weight ratios and the best weight percent for HTAB was found to be 2%. The influence of pH on the response of electrode was examined using 1.0 103 mol/L potassium iodide solutions over the pH range 2.0–12.0. To adjust the pH, very small volumes of concentrated HNO3 or NaOH solutions were used. The potential response remains constant over the pH range 3.0–10.0. In acidic media (pH < 3.0), the drift in the potential may be owing to the instability of the ionophore (Cu(II) complex) due to the nitrogen sites protonation. Since the competition of OH ion with I ion for the ionophore in membrane electrode, the potential decreased sharply in highly alkaline media [13]. The emf response of the calibration graph shows a Nernstian slope of 58.7 0.4 mV decade1 over the activity range of 1.0 105 to 1 1.01 mol/L with a detection limit (extrapolation of linear graph) of 5.0 106 mol/L
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Fig. 2. Calibration graph of iodide ion-selective electrode with an optimized membrane composition (membrane 5).
(Fig. 2). The stability and reproducibility of the electrode was also tested. The standard deviation of 20 replicate measurements at three concentrations of 1.0 102, 1.0 103 and 1.0 104 mol/L were 0.4, 0.5 and 0.8 mV, respectively. 2.1. Dynamic response time, lifetime and selectivity In this study, the practical response time of the introduced sensor was recorded by changing the iodide ion concentration in a series of solutions (1.0 105–1.0 101 mol/L). It was found that the electrode reaches the equilibrium response in a short time of about 10 s. This is most probably due to the fast exchange kinetics of association–dissociation of I ion with the central metal of ionophore at the test solution–membrane interface. To obtain the lifetime of the best membrane, slope of calibration curve was checked weekly. The slope remained reasonably constant over a period of 8 weeks. Cu(II) complex has very low solubility in aqueous solution, which causes leaching from the membrane and contamination of the sample solutions. To prevent the ionophore solubility, the electrode after being used, was washed with water, dried and kept aside for the next day’s use. The most important characteristic of any ISE is its response to the primary ion in the presence of other ions in solution, which is usually expressed in terms of potentiometric selectivity coefficient. The logarithm of selectivity coefficients ðlog K I ;An Þ for anions were evaluated by the fixed interference method (FIM) [17]. The results show that the electrode based on copper(II) complex as ionophor displays highly selective response to iodide and the selectivity sequence was found to be as this: I > Sal ¼ 2:20 > SCN ¼ 2:30 > ClO4 ¼ NO3 ¼ 2:40 > CN ¼ Br ¼ 2:63 > Cl ¼ 2:83 > CrO4 2 ¼ 3:13 > IO3 ¼ 3:21 > F ¼ 3:22 > NO2 ¼ 3:24 > SO3 2 ¼ 3:30 > BrO3 ¼ 3:42 > SO4 2 ¼ 3:51 > CO3 2 ¼ 4:00 It has been shown that the anion selectivity behavior in ISE exhibits anti-Hofmeister potentiometric pattern (i.e. selectivity based solely on lipophilicity of anions) [18]. The high potentiometric selectivity for iodide ion must be related to the unique interaction between Cu(II) complex and iodide ion. In Table 1 the performance characteristics of the proposed electrode are compared with those of the iodide PVC membrane electrodes previously reported [2,8–13]. As can be seen, pH range for proposed electrode is wider than other iodide-selective electrodes and the selectivity coefficients for some anions such as SCN and Sal obtained by proposed electrode are smaller than most of reported iodide-ISEs listed in Table 1. 2.2. Analytical performance In order to test the analytical utility of sensor it was used to determine iodide in antiseptic and water samples. In order to eliminate matrix effects, standard addition method was used. To determine [I] in antiseptic sample, 25 mL of POVIDONE (Donyae Behdasht pharmaceutical laboratories) sample was oxidized with 4 mL of 30% H2O2 and 8.0 mL of NaOH. The mixture was heated, neutralized with H2SO4 to pH 7.1 and diluted with water. The obtained sample solution was analyzed by proposed membrane sensor (3.87(0.08) 104 mol/L). The water sample also was analyzed by spectrophotometric method (3.93(0.07) 104 mol/L) [19]. Based on the t-test [20], the texp was calculated to be 1.05. Since texp is less than tcrit. (tcrit. = 2.78) therefore, there is no evidence for systematic error.
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Table 1 Comparison of the potentiometric parameters of the proposed iodide-sensor with the other iodide sensors. Ref. no. [8] [9] [10] [11] [12] [2] [13] [This work]
Linear range (mol/L) 6
2
1.0 10 –1.0 10 1.0 105–1.0 102 1.0 105–1.0 101 5.0 107–5.0 104 8.2 107–1.0 101 1.0 105–1.0 101 5.0 106–2.0 101 1.0 105–1.0 101
Slope
Detection limit
Response time
pH range
log K I ;An
59.4 57.7 58.6 58 58.8 58.6 60.2 58.7
N.R. 2.3 106 1.6 105 N.R. 5.3 107 3.0 106 1.0 106 5.0 106
8 10 3 N.R. 3 9–25 10 10
2.0–8.0 0.8–7.0 At pH 7.0 8.0–10.0 2.0–5.0 3.0–10.0 3.0–9.0 3.0–10.0
Sal (1.05), SCN Sal (1.05), SCN N.R. SCN (1.9) SCN (0.73) SCN (3.58), Sal SCN (1.00) SCN (2.20), Sal
(1.95) (1.05)
(1.9) (2.20)
Table 2 Analytical results for iodide in water samples and by proposed sensor. Sample
Added iodide (mol/L)
Found (mol/L) 6
Recovery %
Well water
0 2.00 105 4.00 105
6.31 10 2.57 105 4.47 105
– 96.9 96.0
Drinking water
0 2.00 105 4.00 105
– 2.03 105 4.11 105
– 101.5 100.7
In addition the new iodide-ISE was satisfactorily applied to determine iodide ion in water samples. The results are summarized in Table 2. 3. Conclusions This work demonstrated that ISE based on Cu(II)BBPSD as modifier exhibits iodide selectivity, with particularly low interferences. The proposed electrode has been shown to have good operating characteristics (sensitivity, stability, response time, detection limit, and a wide linear range). It is easy to prepare and use. The electrode was also successfully applied for monitoring concentration of iodide in real samples. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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