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Preparation and characterization of thiocyanate-selective electrodes based on new complexes of copper(II) as neutral carriers
Desalination 249 (2009) 139–142
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Preparation and characterization of thiocyanate-selective electrodes based on new complexes of copper(II) as neutral carriers Wen-Ju Xu, Yun Zhang, Ya-Qin Chai, Ruo Yuan ⁎ Chongqing Key Laboratory of Analytical Chemistry, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, PR China
a r t i c l e
i n f o
Article history: Accepted 28 June 2009 Available online 2 October 2009 Keywords: Hydroxycitronellal (o-aminobenzoic acid) copper(II) Salicylaldehyde (o-aminobenzoic acid) copper(II) Thiocyanate Ion-selective electrode Potentiometric response
a b s t r a c t The complexes of hydroxycitronellal (o-aminobenzoic acid) copper(II) (Cu(II)-HXAB) and salicylaldehyde (o-aminobenzoic acid) copper(II) (Cu(II)-SHAB) were used as neutral carriers in PVC-based membrane ionselective electrodes. The electrode based on Cu(II)-HXAB exhibited near-Nernstian potential response to thiocyanate (SCN−) in a linear range of 1.0 × 10− 6 to 1.0 × 10− 1 M with a detection limit of 8.5 × 10− 7 M and a slope of − 57.3 mV/decade in 0.01 M phosphate buffer solution (pH 5.0). The electrode exhibited high selectivity to SCN− over other tested anions with an anti-Hofmeister selectivity sequence. The selectivity behavior might be discussed in terms of UV–Vis spectrum and infrared spectrum. The transfer process of thiocyanate across the membrane interface was investigated by making use of the AC impedance technique. The electrode containing Cu(II)-HXAB could be applied to thiocyanate analysis in waste water with satisfactory results. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Thiocyanate is administered as a drug in the treatment of thyroid conditions [1]. However, high concentrations of thiocyanate in human body can lead to vertigo or unconsciousness [2]. Thiocyanate ion is derived endogenously as a detoxification production of the reaction between cyanide and thiocyanate in the liver [3]. It is abundantly present in blood and especially in saliva. A high saliva thiocyanate concentration can be quantitatively used as an indication for exposure to tobacco smoke. Therefore, an accurate, simple and rapid method in aspect of the determination of thiocyanate is significant in field of medicine and the life sciences [4]. Various methods, such as spectrophotometry [5], electrochemistry [6], ion chromatography [7], have been reported for the determination of various different anions. Unfortunately, most of them are rather sophisticated and need to make use of high-cost equipment. As an alternative, potentiometric method using ion-selective electrodes (ISEs) is simple and economical owing to the advantages such as fast response time, low detection limit, good selectivity and easy preparation. Anionsensitive membrane electrodes based on ion exchanges such as lipophilic quaternary ammonium or phosphonium salts display classical Hofmeister selectivity sequence, in which membrane selectivity is controlled by the free energy of hydration of ions which were involved [8,9]. Recently, some anion-selective membrane electrodes based on metalloporphyrins [10,11], metallophthalocyanines [12–14], Schiff base
metallic complexes [15–20], and other organometallic compounds [21,22] as carriers have been reported and the electrodes demonstrated potentiometric anion selectivity deviating from Hofmeister pattern. The deviation resulted from the direct interaction between the central metal of the sensing carrier and analytical anion and the steric effect associated with the structure of the carrier. Up to now, some thiocyanate ISEs based on different complexes as sensing carriers have been reported [12,14,19,20,23–29]. But there have only a few complexes of copper(II) as neutral carriers for preparing thiocyanate-selective electrodes. Studies in our laboratory showed that solvent polymeric membranes incorporating some Schiff base complexes of copper(II) as carriers exhibited considerable selectivity for thiocyanate [20,21]. In the present work, we report the preparation and characterization of thiocyanate ion-selective electrodes by using two Schiff base complexes of copper(II), hydroxycitronellal (o-aminobenzoic acid) copper(II) (Cu(II)-HXAB) and salicylaldehyde (o-aminobenzoic acid) copper(II) (Cu(II)-SHAB), as neutral carriers. The experimental observations indicated that the electrode based on Cu(II)-HXAB containing two electron-repelling methyl groups which can increase the combination between Cu(II)-HXAB and thiocyanate showed good near-Nernstian slope, a wide response range, a fast response time and high selectivity to thiocyanate. 2. Experimental 2.1. Apparatus and reagents
⁎ Corresponding author. Fax: +86 23 68252277. E-mail address: [email protected] (R. Yuan). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.06.014
Potentiometric and pH measurements were made with a model MP230 lonalyzer pH meter (Mettler-Toledo, Switzerland) or a pHS-3C
140
W.-J. Xu et al. / Desalination 249 (2009) 139–142
digital ion analyzer pH/mV meter (Leici Instruments, Shanghai, China). UV–Vis spectra were recorded on a Lambda 17 UV/Vis spectrophotometer (PE, USA) and a Beckman (USA) DU-7. IR spectra of the complexes were recorded with a Spectrum GX FTIR (USA). The AC impedance plot of an electrode membrane was recorded with an IM6e impedance measurement unit (Zahner Elektrick, Co., Germany) equipped with THALES software 6.88 (frequency, 106–10− 2 Hz; ac amplitude, 50.0 mV; at 25 ± 1 °C). The Cu(II)-HXAB (C16H25NO3Cu(II)) and Cu(II)-SHAB (C14H11NO4 Cu(II)) were synthesized by the reported procedures [30] (see Fig. 1). The elemental analysis (D-6450, Heracus Co., Germany) were as follows: calculated for Cu(II)-HXAB, C 56.04%, H 7.35%, N 4.09%, O 14.00%; found: C 55.25%, H 7.17%, N 4.26%, O 14.07%; for Cu(II)-SHAB, calculated: C 52.41%, H 3.46%, N 4.37%, O 19.95%; found: C 52.24%, H 3.27%, N 4.36%, O 19.78%. The structure was identified by infrared spectrum technique and thermogravimetric analysis (SDTQ600, TA Co., USA; 100 ml/ml flowing rate of nitrogen atmosphere, 10 °C/min. heating rate). 2-nitrophenyloctyl ether (o-NPOE) was prepared according to the literature method [31]. Dioctyl phthalate (DOP), dibutylphthalate (DBP), trioctylmethylammonium chloride (TOMAC), tetrahydrofuran (THF), and poly (Vinyl chloride) (PVC) of Chromatographic grade were purchased from Shanghai Chemical Co. (Shanghai, China). All aqueous solutions were prepared with deionized redistilled water. All chemicals used were of analytical-reagent grade. 2.2. Preparation of electrode PVC membrane electrodes based on Cu(II)-HXAB and Cu(II)-SHAB were fabricated and assembled according to the method of Moody et al. [32] and Graggs et al. [33]. The membrane composition was optimized by using an orthogonal experimental design with the electrode linear response range for the concentration of thiocyanate ion as the object function for optimization. The optimum composition (wt. ) of the membranes containing Cu(II)-HXAB and Cu(II)-SHAB was 3.5, 3.2 ionophore, 30.4, 30.2 PVC, 65.8, 66.3 o-NPOE and 0.3, 0.3 TOMAC, respectively. A solution of 0.01 M sodium thiocyanate was used as the internal filling solution. A saturated calomel electrode was employed as the reference electrode. Before use, the electrodes were conditioned by soaking in a 0.1 M sodium thiocyanate solution for 24 h, which was adjusted to the working pH value using H3PO4 and NaOH. Potentiometric measurements were performed with the following galvanic cell: Ag=AgCl ∣NaSCNð0:01MÞ ‖PVC membrane ‖test solution∣Hg=Hg2 Cl2 ∣KCl ðsat:Þ
3. Results and discussion
Table 1 Composition (wt. ) of SCN−-selective membrane based on Cu(II)-HXAB and Cu(II)SHAB. Component
Carrier PVC o-NPOE TOMAC
Membrane no. 1
2
3
4
– 33.1 66.9 –
Cu(II)-HXAB, 3.3 31.5 65.2 –
Cu(II)-HXAB, 3.5 30.4 65.8 0.3
Cu(II)-SHAB, 3.2 30.2 66.3 0.3
other two plasticizers, the carriers were more soluble in o-NPOE and the potentiometric response signal of the electrode plasticized with o-NPOE significantly increased. Thus, o-NPOE was selected as the plasticizer in the membranes. Several membranes plasticized with o-NPOE are shown in Table 1. Their potentiometric response characteristics to SCN− in pH 5.0 phosphate buffer solution are displayed in Fig. 2. One can see that both membrane 1 without carriers and Membrane 2 without cationic additive TOMAC displayed no Nernstian potentiometric response to SCN−. However, membrane 3 based on Cu(II)-HXAB displayed a nearNernstain response to SCN− ranging from 1.0×10− 6 to 1.0×10− 1 M with a detection limit of 8.5×10− 7 M and a slope of −57.3 mV/decade. For the membrane 4 based on Cu(II)-SHAB, the inferior potentiometric response characteristics were observed, involving a narrow concentration range from 4.0×10− 6 to 1.0×10− 1 M, a detection limit of 2.0×10− 6 M and a super-Nernstian slope of −69.4 mV/decade. It was suggested that both carrier and additive played an important role in improving potentiometric response performances of electrodes to thiocyanate. The response time (t95 ) of the electrode based on Cu(II)-HXAB was 15–20 s, implying the fast reversibility of ion exchange in membrane interface. The standard deviation (SD) of the proposed electrode potential reading over a period of 5 h in 1.0× 10− 4 M NaSCN solution (pH 5.0) was 0.97 mV (n = 20). The potential readings for the same electrode dipped alternately into solutions of 1.0 ×10− 3 and 1.0 × 10− 4 M NaSCN gave a SD of 0.65 mV over 3 h (n= 30). After being successively traced over 45 days, no substantial changes of the potential response characteristics of the proposed electrode were observed. The results are shown in Fig. 3. All above observations suggested that the present electrode based on Cu(II)-HXAB as sensing carrier displayed some better potentiometric response characteristics, such as linear range, detection limit, sensitivity, lifetime and etc., than other reported thiocyanate-selective electrodes [10,13,21,23,24,27,29]. 3.2. Effect of pH on the electrode The potentiometric response of the electrode was found to be sensitive to pH changes. Fig. 4 exhibits the results obtained by immersing the electrode into SCN− solutions with different pH values.
3.1. Potentiometric response characteristics of electrodes As well known, the compositions of the PVC membrane significantly influence the sensitivity and selectivity of ion-selective electrode. In this study, the complexes Cu(II)-HXAB and Cu(II)-SHAB were used as neutral carriers for the fabrication of ion-selective electrode using three different plasticizers including o-NPOE, DOP and DBP. As compared with
Fig.1. Structure of the complexes used as the carriers in this work (left, Cu(II)-HXAB; right, Cu(II)-SHAB).
Fig. 2. Potentiometric response characteristics of different membranes to SCN−.
W.-J. Xu et al. / Desalination 249 (2009) 139–142
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3.3. Selectivity
Fig. 3. The stability of the working electrode based on Cu(II)-HXAB in 45 days.
As we can see that the electrode gave the optimum potentiometric response to thiocyanate at pH 5.0. The effect of pH on the potentiometric response characteristics might be explained in according to the competitive coordination of SCN− and OH− ions with the central metal of Cu(II)-HXAB. At higher concentration of SCN− solutions, SCN− is the primary analyte ion. At lower concentrations of SCN−, OH− becomes the primary analyte ion with the increasing of pH; and the potentiometric response characteristics of the electrode to SCN− deteriorated.
The most important characteristic of any ion-sensitive is its relative response to primary ion over other ions, which is expressed in terms of − pot the potentiometric selectivity coefficients, lg KSCN−, jn . Selectivity coefficients, describing the electrode's preference for an interfering ion − jn relative to SCN−, were determined by the IUPAC-defined separate solution method [34]. The results of the electrode based on Cu(II)-HXAB were presented in Fig. 5, which are compared with those of the electrodes based on Cu(II)-SHAB and HTOAI [35]. It is clear to see that the Cu(II)-HXAB-based electrode exhibited a highly selective potentiometric response to thiocyanate and an anti-Hofmeister selectivity sequence in − − − − the following order: SCN− >Sal− > ClO− 4 > I > NO3 >HCO3 > H2PO4 > − − > Cl > F , and the electrode could be used to assay thiocyanate NO− 2 content in the presence of coexisting of at least 10-fold concentration of Sal− ion, which could somewhat lead to the accurate analysis of thiocyanate in real samples. From this point of view, the present electrode based on Cu(II)-HXAB exhibited lower selectivity toward some anions than some reported thiocyanate-selective electrodes [10,13,23,24,29]. 3.4. Mechanism to thiocyanate response The unique potentiometric selectivity of the electrode based on Cu (II)-HXAB towards thiocyanate might be related to the special interaction between carrier and SCN−. It is possible to distinguish the interaction with UV–Vis spectroscopy and infrared spectroscopy. UV– Vis absorption spectra of Cu(II)-HXAB in CHCl3 before and after treated with 10− 4 M NaSCN for 0.5 h were recorded and are shown in Fig. 6. The substantial increase in absorbance of 320 nm and a considerable red shift of about 60 nm after treated with NaSCN suggested the preferential recognition of sensing carrier (Cu(II)-HXAB) to targeted anion (SCN−). The preferential recognition may be originated from an axial coordination between the central metal Cu atom and SCN− (see Fig. 7). In order to further identify the potentiometric response mechanism of the electrode, the infrared spectra of the membrane containing Cu(II)HXAB was investigated. Fig. 8 shows the comparison of the infrared
Fig. 4. Effect of pH on the potentiometric response of the electrode based on Cu(II)HXAB.
Fig. 6. UV–Vis absorption spectra of CHCl3 solution of Cu(II)-HXAB before (a) and after (b) treated with 10− 4 M NaSCN solution for 0.5 h.
pot
−
Fig. 5. Potentiometric selectivity coefficients, lg KSCN−, jn of different electrodes (HTOAI cited from Ref. [35]).
Fig. 7. Suggested coordination scheme of Cu(II)-HXAB with thiocyanate.
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W.-J. Xu et al. / Desalination 249 (2009) 139–142 Table 2 Recovery of thiocyanate in waste water sample. Sample no.
Added (×10− 2 M)
Found (× 10− 2 M)
Recovery (%)
n
1 2 3 4
1.0 1.0 1.0 5.0
0.998 0.985 0.979 4.975
99.8 98.5 97.9 99.5
5 5 5 5
5. Conclusions
Fig. 8. Comparison of infrared spectra of the Cu(II)-HXAB membrane before (a) and after (b) treated with 0.1 M NaSCN for 24 h.
The results of this study showed that the PVC membrane electrode based on a complex of Cu(II)-HXAB as a novel neutral carrier demonstrated high selectivity and sensitivity towards thiocyanate with excellent potentiometric response characteristics such as a wide dynamic range, a low detection limit, fast response and easy preparation. The properties of the proposed electrode could lead its use to the thiocyanate analysis in actual samples. Acknowledgements This work was supported by the National Natural Science Foundation of China (20675064) and the Natural Science Foundation of Chongqing City, China (CSTC-2004BB4149, 2005BB4100). References
Fig. 9. Impedance plots of the Cu(II)-HXAB-based membrane immersed in 10− 5 M SCN− solution (pH 5.0).
spectra of the membrane before and after dipped into 0.1 M NaSCN for 24 h. One can see that a strong peak appearing at 2000–2500 cm− 1 (Fig. 8b) might be resulted from the complexing of Cu(II)-HXAB and thiocyanate. 3.5. AC impedance of the electrode The AC impedance plots of the proposed electrode were investigated by immersing the membrane based on Cu(II)-HXAB in 10− 5 M NaSCN solution and are shown in Fig. 9. A well-resolved bulk and surface impedance at high frequencies and a Warburg impedance at low frequency were observed, which might suggest that Cu(II)-HXAB could carry thiocyanate across the solvent PVC membrane and the transfer is controlled by diffusion process. 4. Preliminary applications In order to demonstrate the application of the SCN−-selective electrode doped with Cu(II)-HXAB, using standard addition method it was applied to the determination of the recovery of thiocyanate from waste water collected from laboratory. 25.0 ml of sample was adjusted to pH 5.0 with 0.1 M H3PO4 or NaOH and diluted to 50.0 ml with buffer solution for direct detection. Under stirring, 1.0 × 10− 2 M SCN− standard solution (pH 5.0) was added into the above solution, then the corresponding potential value was recorded. The results of thiocyanate recovery obtained are given in Table 2.
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Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Preparation and characterization of thiocyanate-selective electrodes based on new complexes of copper(II) as neutral carriers Wen-Ju Xu, Yun Zhang, Ya-Qin Chai, Ruo Yuan ⁎ Chongqing Key Laboratory of Analytical Chemistry, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, PR China
a r t i c l e
i n f o
Article history: Accepted 28 June 2009 Available online 2 October 2009 Keywords: Hydroxycitronellal (o-aminobenzoic acid) copper(II) Salicylaldehyde (o-aminobenzoic acid) copper(II) Thiocyanate Ion-selective electrode Potentiometric response
a b s t r a c t The complexes of hydroxycitronellal (o-aminobenzoic acid) copper(II) (Cu(II)-HXAB) and salicylaldehyde (o-aminobenzoic acid) copper(II) (Cu(II)-SHAB) were used as neutral carriers in PVC-based membrane ionselective electrodes. The electrode based on Cu(II)-HXAB exhibited near-Nernstian potential response to thiocyanate (SCN−) in a linear range of 1.0 × 10− 6 to 1.0 × 10− 1 M with a detection limit of 8.5 × 10− 7 M and a slope of − 57.3 mV/decade in 0.01 M phosphate buffer solution (pH 5.0). The electrode exhibited high selectivity to SCN− over other tested anions with an anti-Hofmeister selectivity sequence. The selectivity behavior might be discussed in terms of UV–Vis spectrum and infrared spectrum. The transfer process of thiocyanate across the membrane interface was investigated by making use of the AC impedance technique. The electrode containing Cu(II)-HXAB could be applied to thiocyanate analysis in waste water with satisfactory results. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Thiocyanate is administered as a drug in the treatment of thyroid conditions [1]. However, high concentrations of thiocyanate in human body can lead to vertigo or unconsciousness [2]. Thiocyanate ion is derived endogenously as a detoxification production of the reaction between cyanide and thiocyanate in the liver [3]. It is abundantly present in blood and especially in saliva. A high saliva thiocyanate concentration can be quantitatively used as an indication for exposure to tobacco smoke. Therefore, an accurate, simple and rapid method in aspect of the determination of thiocyanate is significant in field of medicine and the life sciences [4]. Various methods, such as spectrophotometry [5], electrochemistry [6], ion chromatography [7], have been reported for the determination of various different anions. Unfortunately, most of them are rather sophisticated and need to make use of high-cost equipment. As an alternative, potentiometric method using ion-selective electrodes (ISEs) is simple and economical owing to the advantages such as fast response time, low detection limit, good selectivity and easy preparation. Anionsensitive membrane electrodes based on ion exchanges such as lipophilic quaternary ammonium or phosphonium salts display classical Hofmeister selectivity sequence, in which membrane selectivity is controlled by the free energy of hydration of ions which were involved [8,9]. Recently, some anion-selective membrane electrodes based on metalloporphyrins [10,11], metallophthalocyanines [12–14], Schiff base
metallic complexes [15–20], and other organometallic compounds [21,22] as carriers have been reported and the electrodes demonstrated potentiometric anion selectivity deviating from Hofmeister pattern. The deviation resulted from the direct interaction between the central metal of the sensing carrier and analytical anion and the steric effect associated with the structure of the carrier. Up to now, some thiocyanate ISEs based on different complexes as sensing carriers have been reported [12,14,19,20,23–29]. But there have only a few complexes of copper(II) as neutral carriers for preparing thiocyanate-selective electrodes. Studies in our laboratory showed that solvent polymeric membranes incorporating some Schiff base complexes of copper(II) as carriers exhibited considerable selectivity for thiocyanate [20,21]. In the present work, we report the preparation and characterization of thiocyanate ion-selective electrodes by using two Schiff base complexes of copper(II), hydroxycitronellal (o-aminobenzoic acid) copper(II) (Cu(II)-HXAB) and salicylaldehyde (o-aminobenzoic acid) copper(II) (Cu(II)-SHAB), as neutral carriers. The experimental observations indicated that the electrode based on Cu(II)-HXAB containing two electron-repelling methyl groups which can increase the combination between Cu(II)-HXAB and thiocyanate showed good near-Nernstian slope, a wide response range, a fast response time and high selectivity to thiocyanate. 2. Experimental 2.1. Apparatus and reagents
⁎ Corresponding author. Fax: +86 23 68252277. E-mail address: [email protected] (R. Yuan). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.06.014
Potentiometric and pH measurements were made with a model MP230 lonalyzer pH meter (Mettler-Toledo, Switzerland) or a pHS-3C
140
W.-J. Xu et al. / Desalination 249 (2009) 139–142
digital ion analyzer pH/mV meter (Leici Instruments, Shanghai, China). UV–Vis spectra were recorded on a Lambda 17 UV/Vis spectrophotometer (PE, USA) and a Beckman (USA) DU-7. IR spectra of the complexes were recorded with a Spectrum GX FTIR (USA). The AC impedance plot of an electrode membrane was recorded with an IM6e impedance measurement unit (Zahner Elektrick, Co., Germany) equipped with THALES software 6.88 (frequency, 106–10− 2 Hz; ac amplitude, 50.0 mV; at 25 ± 1 °C). The Cu(II)-HXAB (C16H25NO3Cu(II)) and Cu(II)-SHAB (C14H11NO4 Cu(II)) were synthesized by the reported procedures [30] (see Fig. 1). The elemental analysis (D-6450, Heracus Co., Germany) were as follows: calculated for Cu(II)-HXAB, C 56.04%, H 7.35%, N 4.09%, O 14.00%; found: C 55.25%, H 7.17%, N 4.26%, O 14.07%; for Cu(II)-SHAB, calculated: C 52.41%, H 3.46%, N 4.37%, O 19.95%; found: C 52.24%, H 3.27%, N 4.36%, O 19.78%. The structure was identified by infrared spectrum technique and thermogravimetric analysis (SDTQ600, TA Co., USA; 100 ml/ml flowing rate of nitrogen atmosphere, 10 °C/min. heating rate). 2-nitrophenyloctyl ether (o-NPOE) was prepared according to the literature method [31]. Dioctyl phthalate (DOP), dibutylphthalate (DBP), trioctylmethylammonium chloride (TOMAC), tetrahydrofuran (THF), and poly (Vinyl chloride) (PVC) of Chromatographic grade were purchased from Shanghai Chemical Co. (Shanghai, China). All aqueous solutions were prepared with deionized redistilled water. All chemicals used were of analytical-reagent grade. 2.2. Preparation of electrode PVC membrane electrodes based on Cu(II)-HXAB and Cu(II)-SHAB were fabricated and assembled according to the method of Moody et al. [32] and Graggs et al. [33]. The membrane composition was optimized by using an orthogonal experimental design with the electrode linear response range for the concentration of thiocyanate ion as the object function for optimization. The optimum composition (wt. ) of the membranes containing Cu(II)-HXAB and Cu(II)-SHAB was 3.5, 3.2 ionophore, 30.4, 30.2 PVC, 65.8, 66.3 o-NPOE and 0.3, 0.3 TOMAC, respectively. A solution of 0.01 M sodium thiocyanate was used as the internal filling solution. A saturated calomel electrode was employed as the reference electrode. Before use, the electrodes were conditioned by soaking in a 0.1 M sodium thiocyanate solution for 24 h, which was adjusted to the working pH value using H3PO4 and NaOH. Potentiometric measurements were performed with the following galvanic cell: Ag=AgCl ∣NaSCNð0:01MÞ ‖PVC membrane ‖test solution∣Hg=Hg2 Cl2 ∣KCl ðsat:Þ
3. Results and discussion
Table 1 Composition (wt. ) of SCN−-selective membrane based on Cu(II)-HXAB and Cu(II)SHAB. Component
Carrier PVC o-NPOE TOMAC
Membrane no. 1
2
3
4
– 33.1 66.9 –
Cu(II)-HXAB, 3.3 31.5 65.2 –
Cu(II)-HXAB, 3.5 30.4 65.8 0.3
Cu(II)-SHAB, 3.2 30.2 66.3 0.3
other two plasticizers, the carriers were more soluble in o-NPOE and the potentiometric response signal of the electrode plasticized with o-NPOE significantly increased. Thus, o-NPOE was selected as the plasticizer in the membranes. Several membranes plasticized with o-NPOE are shown in Table 1. Their potentiometric response characteristics to SCN− in pH 5.0 phosphate buffer solution are displayed in Fig. 2. One can see that both membrane 1 without carriers and Membrane 2 without cationic additive TOMAC displayed no Nernstian potentiometric response to SCN−. However, membrane 3 based on Cu(II)-HXAB displayed a nearNernstain response to SCN− ranging from 1.0×10− 6 to 1.0×10− 1 M with a detection limit of 8.5×10− 7 M and a slope of −57.3 mV/decade. For the membrane 4 based on Cu(II)-SHAB, the inferior potentiometric response characteristics were observed, involving a narrow concentration range from 4.0×10− 6 to 1.0×10− 1 M, a detection limit of 2.0×10− 6 M and a super-Nernstian slope of −69.4 mV/decade. It was suggested that both carrier and additive played an important role in improving potentiometric response performances of electrodes to thiocyanate. The response time (t95 ) of the electrode based on Cu(II)-HXAB was 15–20 s, implying the fast reversibility of ion exchange in membrane interface. The standard deviation (SD) of the proposed electrode potential reading over a period of 5 h in 1.0× 10− 4 M NaSCN solution (pH 5.0) was 0.97 mV (n = 20). The potential readings for the same electrode dipped alternately into solutions of 1.0 ×10− 3 and 1.0 × 10− 4 M NaSCN gave a SD of 0.65 mV over 3 h (n= 30). After being successively traced over 45 days, no substantial changes of the potential response characteristics of the proposed electrode were observed. The results are shown in Fig. 3. All above observations suggested that the present electrode based on Cu(II)-HXAB as sensing carrier displayed some better potentiometric response characteristics, such as linear range, detection limit, sensitivity, lifetime and etc., than other reported thiocyanate-selective electrodes [10,13,21,23,24,27,29]. 3.2. Effect of pH on the electrode The potentiometric response of the electrode was found to be sensitive to pH changes. Fig. 4 exhibits the results obtained by immersing the electrode into SCN− solutions with different pH values.
3.1. Potentiometric response characteristics of electrodes As well known, the compositions of the PVC membrane significantly influence the sensitivity and selectivity of ion-selective electrode. In this study, the complexes Cu(II)-HXAB and Cu(II)-SHAB were used as neutral carriers for the fabrication of ion-selective electrode using three different plasticizers including o-NPOE, DOP and DBP. As compared with
Fig.1. Structure of the complexes used as the carriers in this work (left, Cu(II)-HXAB; right, Cu(II)-SHAB).
Fig. 2. Potentiometric response characteristics of different membranes to SCN−.
W.-J. Xu et al. / Desalination 249 (2009) 139–142
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3.3. Selectivity
Fig. 3. The stability of the working electrode based on Cu(II)-HXAB in 45 days.
As we can see that the electrode gave the optimum potentiometric response to thiocyanate at pH 5.0. The effect of pH on the potentiometric response characteristics might be explained in according to the competitive coordination of SCN− and OH− ions with the central metal of Cu(II)-HXAB. At higher concentration of SCN− solutions, SCN− is the primary analyte ion. At lower concentrations of SCN−, OH− becomes the primary analyte ion with the increasing of pH; and the potentiometric response characteristics of the electrode to SCN− deteriorated.
The most important characteristic of any ion-sensitive is its relative response to primary ion over other ions, which is expressed in terms of − pot the potentiometric selectivity coefficients, lg KSCN−, jn . Selectivity coefficients, describing the electrode's preference for an interfering ion − jn relative to SCN−, were determined by the IUPAC-defined separate solution method [34]. The results of the electrode based on Cu(II)-HXAB were presented in Fig. 5, which are compared with those of the electrodes based on Cu(II)-SHAB and HTOAI [35]. It is clear to see that the Cu(II)-HXAB-based electrode exhibited a highly selective potentiometric response to thiocyanate and an anti-Hofmeister selectivity sequence in − − − − the following order: SCN− >Sal− > ClO− 4 > I > NO3 >HCO3 > H2PO4 > − − > Cl > F , and the electrode could be used to assay thiocyanate NO− 2 content in the presence of coexisting of at least 10-fold concentration of Sal− ion, which could somewhat lead to the accurate analysis of thiocyanate in real samples. From this point of view, the present electrode based on Cu(II)-HXAB exhibited lower selectivity toward some anions than some reported thiocyanate-selective electrodes [10,13,23,24,29]. 3.4. Mechanism to thiocyanate response The unique potentiometric selectivity of the electrode based on Cu (II)-HXAB towards thiocyanate might be related to the special interaction between carrier and SCN−. It is possible to distinguish the interaction with UV–Vis spectroscopy and infrared spectroscopy. UV– Vis absorption spectra of Cu(II)-HXAB in CHCl3 before and after treated with 10− 4 M NaSCN for 0.5 h were recorded and are shown in Fig. 6. The substantial increase in absorbance of 320 nm and a considerable red shift of about 60 nm after treated with NaSCN suggested the preferential recognition of sensing carrier (Cu(II)-HXAB) to targeted anion (SCN−). The preferential recognition may be originated from an axial coordination between the central metal Cu atom and SCN− (see Fig. 7). In order to further identify the potentiometric response mechanism of the electrode, the infrared spectra of the membrane containing Cu(II)HXAB was investigated. Fig. 8 shows the comparison of the infrared
Fig. 4. Effect of pH on the potentiometric response of the electrode based on Cu(II)HXAB.
Fig. 6. UV–Vis absorption spectra of CHCl3 solution of Cu(II)-HXAB before (a) and after (b) treated with 10− 4 M NaSCN solution for 0.5 h.
pot
−
Fig. 5. Potentiometric selectivity coefficients, lg KSCN−, jn of different electrodes (HTOAI cited from Ref. [35]).
Fig. 7. Suggested coordination scheme of Cu(II)-HXAB with thiocyanate.
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W.-J. Xu et al. / Desalination 249 (2009) 139–142 Table 2 Recovery of thiocyanate in waste water sample. Sample no.
Added (×10− 2 M)
Found (× 10− 2 M)
Recovery (%)
n
1 2 3 4
1.0 1.0 1.0 5.0
0.998 0.985 0.979 4.975
99.8 98.5 97.9 99.5
5 5 5 5
5. Conclusions
Fig. 8. Comparison of infrared spectra of the Cu(II)-HXAB membrane before (a) and after (b) treated with 0.1 M NaSCN for 24 h.
The results of this study showed that the PVC membrane electrode based on a complex of Cu(II)-HXAB as a novel neutral carrier demonstrated high selectivity and sensitivity towards thiocyanate with excellent potentiometric response characteristics such as a wide dynamic range, a low detection limit, fast response and easy preparation. The properties of the proposed electrode could lead its use to the thiocyanate analysis in actual samples. Acknowledgements This work was supported by the National Natural Science Foundation of China (20675064) and the Natural Science Foundation of Chongqing City, China (CSTC-2004BB4149, 2005BB4100). References
Fig. 9. Impedance plots of the Cu(II)-HXAB-based membrane immersed in 10− 5 M SCN− solution (pH 5.0).
spectra of the membrane before and after dipped into 0.1 M NaSCN for 24 h. One can see that a strong peak appearing at 2000–2500 cm− 1 (Fig. 8b) might be resulted from the complexing of Cu(II)-HXAB and thiocyanate. 3.5. AC impedance of the electrode The AC impedance plots of the proposed electrode were investigated by immersing the membrane based on Cu(II)-HXAB in 10− 5 M NaSCN solution and are shown in Fig. 9. A well-resolved bulk and surface impedance at high frequencies and a Warburg impedance at low frequency were observed, which might suggest that Cu(II)-HXAB could carry thiocyanate across the solvent PVC membrane and the transfer is controlled by diffusion process. 4. Preliminary applications In order to demonstrate the application of the SCN−-selective electrode doped with Cu(II)-HXAB, using standard addition method it was applied to the determination of the recovery of thiocyanate from waste water collected from laboratory. 25.0 ml of sample was adjusted to pH 5.0 with 0.1 M H3PO4 or NaOH and diluted to 50.0 ml with buffer solution for direct detection. Under stirring, 1.0 × 10− 2 M SCN− standard solution (pH 5.0) was added into the above solution, then the corresponding potential value was recorded. The results of thiocyanate recovery obtained are given in Table 2.
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