- Email: [email protected]
Design of Schiff base complexes of Co(II) for the preparation of iodide-selective polymeric membrane electrodes
Talanta 48 (1999) 649 – 657
Design of Schiff base complexes of Co(II) for the preparation of iodide-selective polymeric membrane electrodes Ruo Yuan *, You-Qun Song, Ya-Qin Chai, Shao-Xi Xia, Qiao-Yun Zhong, Bing Yi, Min Ying, Guo-Li Shen, Ru-Qin Yu Institute of Chemometrics and Chemical Sensing Technology, Chemistry and Chemical Engineering College, Hunan Uni6ersity, Changsha 410082, China Received 10 April 1998; received in revised form 24 August 1998; accepted 26 August 1998
Abstract The response characteristics of some iodide-selective solvent polymeric membrane electrodes based on with N,N%-bis(salicylaldehyde-n-octyl) diimine cobalt(II) (Co(II)SAODI) which is a more lipophilic substitute for a previously reported iodide-carrier are described. The electrode doped with Co(II)SAODI into a plasticized poly(vinyl chloride) membrane exhibits an anti-Hofmeister selectivity pattern with high selectivity toward iodide, long lifetime and small interference from H + . Quartz crystal microgravimetric measurements and ac impedance experiments show that the excellent selectivity for iodide is related to the unique interaction between the carriers and iodide and steric effect associated with the structure of the Schiff base ligands. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Potentiometry; Iodide-selective electrode; Schiff base complexes
1. Introduction Study on the anti-Hofmeister sensing materials with high selectivity for given anions is an expeditiously expanding domain in chemical sensors. Membranes based on ion exchangers such as lipophilic quaternary ammonium or phosphonium salts exhibit the classical Hofmeister behavior in which the membrane selectivity is mainly dominated by the free energy of hydration of ions * Corresponding author. Tel.: + 86-731-8822661; Fax: + 86-731-8824487; e-mail: [email protected].
involved [1,2]. Recently, electrodes using plasticized poly(vinyl chloride) (PVC) membranes doped with organometallic species and metal –ligand complexes including organotin species [3], organomercury [4,5], derivatives of vitamin B12 [6,7], Mn(IV) [8,9], Co(III) [10,11], Sn(IV) [12,13], Mo(V) [14] porphyrin complexes, Co(II) phthalocyanine derivatives [15] and electropolymerized Co(II) porphyrin derivative films [16] demonstrated potentiometric anion-selectivity sequences which are remarkably different from the Hofmeister pattern. These deviations are caused by the direct interaction between the central metal of the
0039-9140/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 9 8 ) 0 0 2 8 9 - 6
650
R. Yuan et al. / Talanta 48 (1999) 649–657
membrane active components and analyte anion and steric effect associated with the structure of the ligand. Schiff base complexes Co(II) can reversibly coordinate oxygen and have been extensively studied as ‘model compounds’ to simulate natural oxygen carriers containing a transition metal [17,18]. Recent studies showed that solvent polymeric membranes incorporating some Schiff base complexes of Co(II) as carrier exhibited considerable selectivity for iodide [19]. In this paper, the more lipophilic Schiff base complexes of Co(II) synthesized in our laboratory were incorporated into plasticized PVC membranes with 2-nitrophenyloctyl ester (oNPOE) as a plasticizer to prepare electrodes with substantial improvement in selectivity toward iodide ion, long lifetime and small interference from hydrogen ion.
The Co(II)SAODI complex was prepared as follows. A warm solution of 0.91 g (4.3 mmol) Co(CH3COO)2·2H2O in 25 ml of methanol was added to an ethanol solution (40 ml) of 2.0g (8.6 mmol) SAOI. The mixture was stirred and heated with reflux for 6 h in an atmosphere of nitrogen, then cooled to 0°C and allowed to stand for 12 h. The dark solid precipitate was filtered under suction, washed with anhydrous ethanol and dried in room temperature. (yield, 67%; melting point 67– 69°C). Analysis: found C, 68.24; H, 8.83; N, 5.77; O, 6.82. Calculations for Co(II)SAODI: C, 68.88; H, 8.47; N, 5.35; O, 6.11. The structure of the Co(II)SAODI was identified by infrared (KBr) (Heraeus, C-H-N-O-S-Rapid Element Analyzer, Germany) and mass spectrometry(GC-17 A-QP5000 System, Shimadzu, Japan) (see Fig. 1).
2.3. Membrane preparation 2. Experimental section
2.1. Reagents Bis(2-naphtholaldehyde)ethylenediiminecobalt(II) (Co(II)napen) and bis(2-naphtholaldehyde)phenyldiiminecobalt(II) (Co(II)napophen) were prepared as reported in [20]. Bis(salicylaldehyde)ethylenediiminecobalt(II) (Co(II)salen) were prepared as described in [21 – 23]. The o-NPOE was synthesized as described by Honing [24]. The synthesis of hexadecyltrioctylammonium iodide (HTOAI) was described in [25]. PVC powder of chromatographic grade was a product of Shanghai Chemical. Redistilled deionized water and analytical grade reagents were used throughout.
2.2. Synthesis of N,N%-bis(salicylaldehyde-n-octyl)diiminecobalt(II) The crude product of N-salicylaldehyde-noctylimine (SAOI) was prepared as follows. To a solution of 3.0 g (22 mmol) salicylaldehyde in 50 ml boiling anhydrous ethanol add 2.8 g (22 mmol) n-octylamine. The reaction mixture was stirred and heated with reflux for 2 h and left the solution to cool to room temperature. The solvent was then removed under vacuum to leave the crude product as oil.
The iodide-selective solvent polymeric membrane incorporating Co(II)SAODI was prepared and assembled according to the method of Thomas and co-workers [26,27]. The membrane composition was optimized by using an orthogonal experimental design with the electrode linear response range and slope for iodide ion as the object function for optimization. The optimum composition obtained was 2.5% (w/w) ionophore, 65.5% (w/w) o-NPOE, and 32.0% (w/w) PVC. 25 mg Co(II)SAODI, 655 mg o-NPOE and 320 mg PVC were dissolved in 4 ml of freshly distilled THF and cast in 5 cm diameter glass rings on a glass plate. After drying (usually 48 h), individual membrane (0.8 cm diameter) was cut from this larger piece and mounted in electrode body for testing. The electrode membrane prepared was a thickness of 0.2 cm and a direct current resistance of 117.39 0.3 kV (average of six determination). The other membranes containing membrane active component were prepared according to the similar to the method described above except for slight difference of the amount of composition.
2.4. Apparatus Potentiometric and pH measurements were made with a model 901 microprocessor ion ana-
R. Yuan et al. / Talanta 48 (1999) 649–657
lyzer (Orion, Cambridge, MA, USA) and a Model PH-3C pH/mV meter (Rex Instrument Factory, Shanghai, China). The cells used for all mV measurements were of the following type: Hg; Hg2Cl2, KCl (satd.)/NaNO3 (3 M)/sample solution//membrane//NaNO3 M, pH 5.6 buffer/AgCl, Ag. The pH 5.6 buffer used was 1.0 M in citrate and 1.0 M in KCl. The external reference electrode was a double-junction saturated calomel electrode. Before use, the electrodes were preconditioned by
651
soaking overnight in a Britton–Robinson buffer (B–R buffer) solution with pH 6. The B–R buffer solutions for testing the electrode function consisted of boric acid, acetic acid, and orthophosphoric acid (10 mM each), adjusted to various pH values with 1 M sodium hydroxide.
2.5. Determination of emf response and selecti6ity of the electrodes The emf measurements were carried out at 20°C by immersing the membrane electrode and the reference electrode into a glass beaker containing about 20 ml of the sample solution. Anwere ion-selectivity coefficients, log K pot I,X determined by separate solution method in a background electrolyte of pH 6.00 B–R buffer solution. The single-ion activities were calculated by using the extended Debye–Huckel equation.
2.6. Quartz crystal microgra6imetric measurement The measurement of the frequency shifts of chloroform phases were performed with a model CNN 3165 high resolution counter (Sump, Taiwan). The chloroform solutions containing 0.01 M Co(II)SAODI and the pure chloroform solutions were separately shaken with pH 6.0 B–R buffer solutions containing different concentrations of iodide over the range from 1 × 10 − 5 to 1× 10 − 1 for 30 min and the organic phases were treated with 3% H2O2 in 1 M H2SO4 solution. The gold-coated quartz crystal was AT-cut with a fundamental resonance frequency of : 8.5 MHz and used to absorb I2 in the chloroform for 15 min to record the frequency shifts DF1, DF2, respectively. The chloroform phase containing 0.01 M Co(II)napophen was treated with the B–R buffer solutions containing different concentrations of I − in the same manner as mentioned above to obtain the frequency shifts DF3.
2.7. Ac impedance experiments
Fig. 1. Strutures of the carriers used to prepare solvent polymeric membrane electrodes [I, HTOAI; II, Co(II)salen; III, Co(II)napen; IV, Co(II) napophen; V, Co(II)SAODI].
The ac impedance of the solvent polymeric membrane, plasticized with o-NPOE and containing 2.7 mmol of Co(II)SAODI, was measured with the PAR M368-2 system (EG&G Princeton
652
R. Yuan et al. / Talanta 48 (1999) 649–657
Fig. 2. Effect of the pH on the potentiometric response curves of the membrane electrode doped with Co(II)SAODI. The pH values of the B–R buffer were () 4.00; ( ) 6.00; () 8.00; () 10.00.
Applied Research, Princeton, NJ) in pH 6.0 B–R buffer solution containing different concentrations of iodide. The working electrode was a Ag/AgCl foil with 0.5 cm2 area. A Pt foil 0.5 cm2 area and a saturated calomel electrode as a counter electrode and a reference electrode, respectively. The frequency range and ac amplitude used were 10 − 2 – 105 Hz and 15 mV, respectively (at 18°C).
3. Results and discussion
3.1. Emf response characteristics and selecti6ity of electrodes based on Schiff base complexes of cobalt(II) Three different buffers, B – R buffer at pH 4.00, 6.00, and 8.00, 10 − 2 M Tris – HCl, pH 7.00 and 10 − 2 M NaH2PO4 – NaOH, pH 7.00, were used to study the effect of the pH on the response of the electrodes doped with Co(II)SAODI to iodide. Fig. 2 shows the results obtained when the electrode was immersed into the B – R buffer solutions
with different pH values. The B–R buffer solutions with pH 4.00–8.00 were suitable buffers for the determination of iodide, and at pH 6.00, the analytical signal of the electrode presented better slopes and detection limits for iodide. The detection limits of the electrode deteriorate when the 10 − 2 M Tris–HCl, pH 7.00, or the 10 − 2 M NaH2PO4 –NaOH, pH 7.00 buffers were used. Theses detection limits were 3.2× 10 − 5 and 1.4× 10 − 5 I − . As shown in Fig. 2, the electrode incorporating Co(II)SAODI exhibited a near-Nanstian potentiometric response for 2.3× 10 − 2 –8.4×10 − 7 M I − with a detection limit of 4.7× 10 − 7 M and a slope of 55.7 9 0.2 mV/pI − (20°C) in the B–R buffer, pH 6.00. The time required for the electrode to 90% response was B 40 s which is similar to that observed with a classical ion exchanger membrane electrode. The dc resistance of the electrode membrane was 117.39 0.2 kV (average of six determination). The SD of the electrode potential reading over a period of 24 h in the B–R buffer with pH 6.0 containing 10 − 3 M KI was 0.3 mV (n= 144) and the potential readings for the electrode dipped alternately into stirred solutions of 10 − 3 and 10 − 4 M KI demonstrated a SD of 0.4 mV over 4 h (n= 12). After contact of the electrode with flowing tap water for 4 months, no detectable loss of performance characteristics was observed. The electrode containing Co(II)napophen and Co(II)napen separately showed the rather poor potentiometric response properties (see Fig. 3). These detection limits are 6.2×10 − 4 and 2.7× 10 − 4 M KI, respectively, and the slopes are 38.5 9 0.3, 42.4 9 0.2 mV/pI − , respectively. The potentiometric selectivity coefficients toward a series of anions presented for the solvent polymeric membranes containing different carriers are shown in Figs. 5 and 6. The electrode doped with Co(II)SAODI, for instance, showed a selectivity sequence of anions in the following order: iodide nitrite thiocyanate\perchlorateperiodate \ bromide \ nitrate \ chloride \ sulfate. It is clear that this anion-selectivity pattern deviates from that of the Hofmeister pattern found with HTOAI-based membrane.
R. Yuan et al. / Talanta 48 (1999) 649–657
Fig. 3. Potentiometric response curves of Schiff base complexes of Co(II); () Co(II)SAODI; ( ) Co(II)napen; and () Co(II)napophen in B–R buffer with pH 6.00.
3.2. Mechanism of iodide response and selecti6ity As mentioned earlier, the values of K pot I,SCN of the PVC-based membranes containing Co(III) porphyrins as ionophores and electropolymerized [Co(o-NH2)TPP] films were 1.3 [8], 5.6 [10] and 2.0× 103 [16]. Although the selectivity coefficient between I − and other halides (Br − and Cl − ) of the electrode doped with metalloporphyrin complexes [10] is similar to that of the electrode incorporating Co(II)SAODI, It was noticeable pot that the values of the K pot I,SCN and K I,CIO4 of the membrane containing the Co(II)SAODI were
653
6.3×10 − 4, and 3.2×10 − 4, respectively which are much less than those of membrane electrode based on metalloporphyrin complexes [10]. The unique potentiometric selectivity toward iodide must be related to the special interaction between Schiff base complexes of Co(II)SAODI and iodide ion. The Co(II) Schiff base complexes in the organic phase can serve as a coordinating site for iodide ion to form five-coordination [28,29] and the Fig. 4 demonstrates the possible response mechanism. As shown in Figs. 5 and 6, the electrode doped with Co(II)SAODI exhibited higher potentiometric selectivity toward iodide ion than that containing Co(II)salen, since Co(II)SAODI possesses stronger lipophilicity and steric structure of the ligand. The potentiometric response characteristics of the electrodes incorporating different carriers deteriorated in the following order: Co(II)SAODI\ Co(II)napen\ Co(II)napophen (see Figs. 2 and 3). Indeed, the equatorial plane of the Schiff base complexes of Co increases with increasing conjugation of the complexes, and the complexes containing naphthyl ridicals with the stronger conjugation than that containing phenyl radicals decreased the p electron density in the vicinity of the Co atom and weaken the interaction between the complexes and iodide [29,30]. The potentiometric response characteristics to ward iodide observed by the electrodes doped with different carriers were in fair agreement with the rule above (see Figs. 1 and 3). Figs. 5 and 6 also show that the values of the anion-selective coefficients of the electrodes doped with different ionophores determined by separate solution method (SSM) is better than that by mixed solu-
Fig. 4. Suggested coordination scheme of Co(II)SAODI with iodide.
654
R. Yuan et al. / Talanta 48 (1999) 649–657
Fig. 5. Comparison of the selective coefficients, log K pot I,X for the solvent polymeric membrane containing different active components. The present data were determined by the separate solution method (SSM) and the mixed solution method (MSM — the interferent concentration was kept at the level of 10 − 2 M), respectively. aFrom [19]. bHTOAI membrane was prepared with 63 wt.% o-NPOE, 32 wt.% PVC and 5 wt.% HTOAI.
tion method (MSM, the interferent concentration was kept at the level of 10 − 2 M), and the Dlog K pot I,X obtained by the both methods is 0.2–1. The effect of pH on the potentiometric response characteristics of the electrode incorporating Co(II)SAODI with a higher lipophilicity is small compared with electrode containing Co(II)salen (see Fig. 2 and [19]), when the pH values of the buffer solution are 8.00 \ pH\ 4.00. In the solution with pH\ 8.00, the potentiometric response properties of the electrode slightly deteriorated and the observation can be explained by hydroxide-coordinated central metal interference. In order to identify the potentiometric response results obtained and further prove that the Co(II) Schiff base complexes is important for inducing iodide selectivity, the quartz crystal microgravimetric measurements and the ac impedance experiments were undertaken. The quartz crystal
microgravimetric measurements were performed to confirm the existence of I2 yielded from the oxidation of I − which was transferred from aqueous phase into organic phase, by the deliberative addition of hydrogen peroxide as oxidizer. As shown in Fig. 7, the frequency shifts DF which were related to the interactions of the 10 − 2 M Co(II)SAODI in chloroform with the aqueous solutions containing I − remarkably increased with increasing the concentrations of I − in the aqueous solutions, and in the systems of carrierfree chloroform treated with the aqueous solutions containing different concentrations of I − , no significant increase of DF was observed. The values of DF which were corresponded to the system of 10 − 2 M Co(II)napophen in chloroform treated with the aqueous solutions containing I − is smaller than those of DF. The results obtained above indirectly proved that the transfer of I −
R. Yuan et al. / Talanta 48 (1999) 649–657
655
Fig. 6. Comparison of the selective coefficients, log K pot I,X for the solvent polymeric membrane containing different active components. The present data were determined by the separate solution method (SSM) and the mixed solution method (MSM, the interferent concentration was kept at the level of 10 − 2 M), respectively. aHTOAI membrane was prepared with 63 wt.% o-NPOE, 32 wt.% PVC and 5 wt.% HTOAI.
across water/organic interface was taken up by Co(II) Schiff base complexes in the membrane phase. The ac impedance of a solvent polymeric membrane containing 2.7 mmol of Co(II)SAODI conditioned in pH 6.00 B – R buffer solution containing 10 − 4 KI M were illustrated in Fig. 8. A well-resolved buck and surface impedance at high frequency region in addition to Warburg impedance at low frequency region were observed. The buck resistance decreased with increasing the concentration of KI: 50.2 kV cm in 10 − 6 M KI; 42.4 kV cm in 10 − 5 M KI; 38.2 kV cm in 10 − 4 M KI; and 35.7 kV cm in 10 − 3 M KI. It was evident that Co(II)SAODI could dominate iodide ion across the solvent polymeric membrane and the transfer process is diffusion controlled.
3.3. Preliminary application The electrode doped with Co(II)SAODI was applied to the determination of iodide in drug preparations. A sample of 20–25 mg of iophendylatum was burned in an oxygen bomb with 15 ml of 5% H2O2 and 4.0 ml of 0.5 M NaOH as the absorbate. The absorbate was heated, acidified with H2SO4, and diluted with water. The sample solution obtained above was determinated by potentiometric titration method with 0.00500 M Ag2SO4 as the titrant and the membrane electrode containing Co(II)SAODI as the indicating electrode. Fig. 9 shows the E–V curve for this type of titration using a sample solution containing 20 mg of iophendylatum. The results obtained was 30.779 0.34 (w/w)% in iodide (n=6), which was
656
R. Yuan et al. / Talanta 48 (1999) 649–657
Fig. 7. Frequency shift values in different chloroform phases as a function of the concentration of iodide ion in B – R buffer (pH 6.00): () chloroform containing 10 − 2 M Co(II)SAODI; ( ) chloroform containing 0.01 M Co(II)napophen; and () carrier-free chloroform.
Fig. 9. E – V curve for titration of I − in a sample solution with Ag2SO4.
in fair coincidence with the results [30.4690.27 (w/w)% in iodide, n= 6] given by precipitation method [31].
Acknowledgements This work was supported by the National Natural Science Foundation of China and Chinese National Education committee Foundation for overseas student in addition to Natural Science Foundation of Hunan Province, China.
References
Fig. 8. Impedance plots of a membrane doped with 2.4 mmol Co(II)SAODI with o-NPOE as plasticizer immersed in B – R buffer containing 10 − 3 M KI (pH 6.00) (frequency, 1 × 105 – 1 × 10 − 2 Hz; ac amplitude, 15 mV; and temperature, 18°C).
[1] K. Sollner, G.M. Shean, J. Am. Chem. Soc. 86 (1964) 1910. [2] U. Wuthier, H.V. Pham, R. Zund, et al., Anal. Chem. 56 (1984) 535. [3] U. Wuthier, H.V. Pham, E. Pretsch, et al., Helv. Chim. Acta 68 (1985) 1822. [4] M.E. Meyerhoff, D.M. Pranitis, Anal. Chim. Acta 217 (1989) 123. [5] R. Yuan, S.-Q. Lin, G.-L. Shen, D-Y. Tian, R.-Q. Yu, Acta Chim. Sini. 54 (1996) 516. [6] P. Schulthess, D. Ammann, B. Krautler, C. Caderas, R. Stepanek, W. Simon, Anal. Chem. 57 (1985) 1397.
R. Yuan et al. / Talanta 48 (1999) 649–657 [7] R. Stepanek, B. Krautler, P. Schulthess, B. Lindemann, D. Ammann, W. Simon, Anal. Chim. Acta 182 (1986) 83. [8] D. Ammann, M. Huser, B. Krautler, et al., Helv. Chim. Acta 69 (1986) 849. [9] N.A. Chaniotakis, A.M. Chasser, M.E. Meyerhoff, J.T. Groves, Anal. Chem. 60 (1988) 185. [10] A. Hodinar, A. Jyo, Chem. Lett. 198 (1988) 993. [11] A. Hodinar, A. Jyo, Anal. Chem. 61 (1989) 1169. [12] Q. Chang, M.E. Meyerhoff, Anal. Chim. Acta 186 (1986) 81. [13] N.A. Chaniotakis, S.B. Park, E.M. Meyerhoff, Anal. Chem. 61 (1989) 566. [14] H. Abe, E. Kokufuta, Bull. Chem. Soc. Jpn. 63 (1990) 1360. [15] J.-Z. Li, X.-C. Wu, R. Yuan, H.-G. Yu, R.-Q. Yu, Analyst 119 (1994) 1363. [16] S. Daunert, S. Wallace, A. Florido, G. Bachas, Anal. Chem. 63 (1991) 1676. [17] E.I. Ochiai, J. Chem. Educ. 50 (1973) 443. [18] T.G. Appleton, J. Chem. Educ. 54 (1977) 443. [19] R. Yuan, Y.-Q. Chai, D. Liu, D. Gao, J.-Z. Li, R.Q. Yu, Anal. Chem. 65 (1993) 2572.
.
657
[20] H. Dai, J. Li, Z.-J. Han, H.-W. Chen, J. Inorg. Chem. China 4 (1988) 61. [21] R.H. Bailes, M. Calvin, J. Am. Chem. Soc. 68 (1947) 1886. [22] R. Deiasi, S.L. Holt, B. Post, Inorg. Chem. 10 (1971) 1498. [23] B.T. Chen, Experimental Manual of Inorganic Chemistry, Beijing Normal University Press, Beijing, 1984. [24] E. C. Horning, Organic Syntheses, collection vol. III, Wiley, New York, 1955, pp. 140. [25] R.-Q. Yu, S.-S. Huang, Talanta 30 (1983) 427. [26] G.J. Moody, R.B. Oke, J.D.R. Thomas, Analyst 95 (1970) 910. [27] A. Graggs, G.J. Moody, J.D.R. Thomas, J. Chem. Educ. 51 (1974) 514. [28] J.H. Burness, J.G. Dillard, L.T. Taylor, Syn. React. Inorg. Metal-Org. Chem. 6 (1976) 165. [29] J.H. Burness, J.G. Dillard, L.T. Taylor, J. Am. Chem. Soc. 97 (1975) 6080. [30] A.B.P. Lever, H.B. Gray, Acc. Chem. Res. 11 (1978) 348. [31] Pharmacopoeia Committee of the Ministry of Health of China (Eds.), Chinese Pharmacopoeia, vol. 2, Chinese Health Press, Beijing, 1990.
.
Design of Schiff base complexes of Co(II) for the preparation of iodide-selective polymeric membrane electrodes Ruo Yuan *, You-Qun Song, Ya-Qin Chai, Shao-Xi Xia, Qiao-Yun Zhong, Bing Yi, Min Ying, Guo-Li Shen, Ru-Qin Yu Institute of Chemometrics and Chemical Sensing Technology, Chemistry and Chemical Engineering College, Hunan Uni6ersity, Changsha 410082, China Received 10 April 1998; received in revised form 24 August 1998; accepted 26 August 1998
Abstract The response characteristics of some iodide-selective solvent polymeric membrane electrodes based on with N,N%-bis(salicylaldehyde-n-octyl) diimine cobalt(II) (Co(II)SAODI) which is a more lipophilic substitute for a previously reported iodide-carrier are described. The electrode doped with Co(II)SAODI into a plasticized poly(vinyl chloride) membrane exhibits an anti-Hofmeister selectivity pattern with high selectivity toward iodide, long lifetime and small interference from H + . Quartz crystal microgravimetric measurements and ac impedance experiments show that the excellent selectivity for iodide is related to the unique interaction between the carriers and iodide and steric effect associated with the structure of the Schiff base ligands. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Potentiometry; Iodide-selective electrode; Schiff base complexes
1. Introduction Study on the anti-Hofmeister sensing materials with high selectivity for given anions is an expeditiously expanding domain in chemical sensors. Membranes based on ion exchangers such as lipophilic quaternary ammonium or phosphonium salts exhibit the classical Hofmeister behavior in which the membrane selectivity is mainly dominated by the free energy of hydration of ions * Corresponding author. Tel.: + 86-731-8822661; Fax: + 86-731-8824487; e-mail: [email protected].
involved [1,2]. Recently, electrodes using plasticized poly(vinyl chloride) (PVC) membranes doped with organometallic species and metal –ligand complexes including organotin species [3], organomercury [4,5], derivatives of vitamin B12 [6,7], Mn(IV) [8,9], Co(III) [10,11], Sn(IV) [12,13], Mo(V) [14] porphyrin complexes, Co(II) phthalocyanine derivatives [15] and electropolymerized Co(II) porphyrin derivative films [16] demonstrated potentiometric anion-selectivity sequences which are remarkably different from the Hofmeister pattern. These deviations are caused by the direct interaction between the central metal of the
0039-9140/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 9 8 ) 0 0 2 8 9 - 6
650
R. Yuan et al. / Talanta 48 (1999) 649–657
membrane active components and analyte anion and steric effect associated with the structure of the ligand. Schiff base complexes Co(II) can reversibly coordinate oxygen and have been extensively studied as ‘model compounds’ to simulate natural oxygen carriers containing a transition metal [17,18]. Recent studies showed that solvent polymeric membranes incorporating some Schiff base complexes of Co(II) as carrier exhibited considerable selectivity for iodide [19]. In this paper, the more lipophilic Schiff base complexes of Co(II) synthesized in our laboratory were incorporated into plasticized PVC membranes with 2-nitrophenyloctyl ester (oNPOE) as a plasticizer to prepare electrodes with substantial improvement in selectivity toward iodide ion, long lifetime and small interference from hydrogen ion.
The Co(II)SAODI complex was prepared as follows. A warm solution of 0.91 g (4.3 mmol) Co(CH3COO)2·2H2O in 25 ml of methanol was added to an ethanol solution (40 ml) of 2.0g (8.6 mmol) SAOI. The mixture was stirred and heated with reflux for 6 h in an atmosphere of nitrogen, then cooled to 0°C and allowed to stand for 12 h. The dark solid precipitate was filtered under suction, washed with anhydrous ethanol and dried in room temperature. (yield, 67%; melting point 67– 69°C). Analysis: found C, 68.24; H, 8.83; N, 5.77; O, 6.82. Calculations for Co(II)SAODI: C, 68.88; H, 8.47; N, 5.35; O, 6.11. The structure of the Co(II)SAODI was identified by infrared (KBr) (Heraeus, C-H-N-O-S-Rapid Element Analyzer, Germany) and mass spectrometry(GC-17 A-QP5000 System, Shimadzu, Japan) (see Fig. 1).
2.3. Membrane preparation 2. Experimental section
2.1. Reagents Bis(2-naphtholaldehyde)ethylenediiminecobalt(II) (Co(II)napen) and bis(2-naphtholaldehyde)phenyldiiminecobalt(II) (Co(II)napophen) were prepared as reported in [20]. Bis(salicylaldehyde)ethylenediiminecobalt(II) (Co(II)salen) were prepared as described in [21 – 23]. The o-NPOE was synthesized as described by Honing [24]. The synthesis of hexadecyltrioctylammonium iodide (HTOAI) was described in [25]. PVC powder of chromatographic grade was a product of Shanghai Chemical. Redistilled deionized water and analytical grade reagents were used throughout.
2.2. Synthesis of N,N%-bis(salicylaldehyde-n-octyl)diiminecobalt(II) The crude product of N-salicylaldehyde-noctylimine (SAOI) was prepared as follows. To a solution of 3.0 g (22 mmol) salicylaldehyde in 50 ml boiling anhydrous ethanol add 2.8 g (22 mmol) n-octylamine. The reaction mixture was stirred and heated with reflux for 2 h and left the solution to cool to room temperature. The solvent was then removed under vacuum to leave the crude product as oil.
The iodide-selective solvent polymeric membrane incorporating Co(II)SAODI was prepared and assembled according to the method of Thomas and co-workers [26,27]. The membrane composition was optimized by using an orthogonal experimental design with the electrode linear response range and slope for iodide ion as the object function for optimization. The optimum composition obtained was 2.5% (w/w) ionophore, 65.5% (w/w) o-NPOE, and 32.0% (w/w) PVC. 25 mg Co(II)SAODI, 655 mg o-NPOE and 320 mg PVC were dissolved in 4 ml of freshly distilled THF and cast in 5 cm diameter glass rings on a glass plate. After drying (usually 48 h), individual membrane (0.8 cm diameter) was cut from this larger piece and mounted in electrode body for testing. The electrode membrane prepared was a thickness of 0.2 cm and a direct current resistance of 117.39 0.3 kV (average of six determination). The other membranes containing membrane active component were prepared according to the similar to the method described above except for slight difference of the amount of composition.
2.4. Apparatus Potentiometric and pH measurements were made with a model 901 microprocessor ion ana-
R. Yuan et al. / Talanta 48 (1999) 649–657
lyzer (Orion, Cambridge, MA, USA) and a Model PH-3C pH/mV meter (Rex Instrument Factory, Shanghai, China). The cells used for all mV measurements were of the following type: Hg; Hg2Cl2, KCl (satd.)/NaNO3 (3 M)/sample solution//membrane//NaNO3 M, pH 5.6 buffer/AgCl, Ag. The pH 5.6 buffer used was 1.0 M in citrate and 1.0 M in KCl. The external reference electrode was a double-junction saturated calomel electrode. Before use, the electrodes were preconditioned by
651
soaking overnight in a Britton–Robinson buffer (B–R buffer) solution with pH 6. The B–R buffer solutions for testing the electrode function consisted of boric acid, acetic acid, and orthophosphoric acid (10 mM each), adjusted to various pH values with 1 M sodium hydroxide.
2.5. Determination of emf response and selecti6ity of the electrodes The emf measurements were carried out at 20°C by immersing the membrane electrode and the reference electrode into a glass beaker containing about 20 ml of the sample solution. Anwere ion-selectivity coefficients, log K pot I,X determined by separate solution method in a background electrolyte of pH 6.00 B–R buffer solution. The single-ion activities were calculated by using the extended Debye–Huckel equation.
2.6. Quartz crystal microgra6imetric measurement The measurement of the frequency shifts of chloroform phases were performed with a model CNN 3165 high resolution counter (Sump, Taiwan). The chloroform solutions containing 0.01 M Co(II)SAODI and the pure chloroform solutions were separately shaken with pH 6.0 B–R buffer solutions containing different concentrations of iodide over the range from 1 × 10 − 5 to 1× 10 − 1 for 30 min and the organic phases were treated with 3% H2O2 in 1 M H2SO4 solution. The gold-coated quartz crystal was AT-cut with a fundamental resonance frequency of : 8.5 MHz and used to absorb I2 in the chloroform for 15 min to record the frequency shifts DF1, DF2, respectively. The chloroform phase containing 0.01 M Co(II)napophen was treated with the B–R buffer solutions containing different concentrations of I − in the same manner as mentioned above to obtain the frequency shifts DF3.
2.7. Ac impedance experiments
Fig. 1. Strutures of the carriers used to prepare solvent polymeric membrane electrodes [I, HTOAI; II, Co(II)salen; III, Co(II)napen; IV, Co(II) napophen; V, Co(II)SAODI].
The ac impedance of the solvent polymeric membrane, plasticized with o-NPOE and containing 2.7 mmol of Co(II)SAODI, was measured with the PAR M368-2 system (EG&G Princeton
652
R. Yuan et al. / Talanta 48 (1999) 649–657
Fig. 2. Effect of the pH on the potentiometric response curves of the membrane electrode doped with Co(II)SAODI. The pH values of the B–R buffer were () 4.00; ( ) 6.00; () 8.00; () 10.00.
Applied Research, Princeton, NJ) in pH 6.0 B–R buffer solution containing different concentrations of iodide. The working electrode was a Ag/AgCl foil with 0.5 cm2 area. A Pt foil 0.5 cm2 area and a saturated calomel electrode as a counter electrode and a reference electrode, respectively. The frequency range and ac amplitude used were 10 − 2 – 105 Hz and 15 mV, respectively (at 18°C).
3. Results and discussion
3.1. Emf response characteristics and selecti6ity of electrodes based on Schiff base complexes of cobalt(II) Three different buffers, B – R buffer at pH 4.00, 6.00, and 8.00, 10 − 2 M Tris – HCl, pH 7.00 and 10 − 2 M NaH2PO4 – NaOH, pH 7.00, were used to study the effect of the pH on the response of the electrodes doped with Co(II)SAODI to iodide. Fig. 2 shows the results obtained when the electrode was immersed into the B – R buffer solutions
with different pH values. The B–R buffer solutions with pH 4.00–8.00 were suitable buffers for the determination of iodide, and at pH 6.00, the analytical signal of the electrode presented better slopes and detection limits for iodide. The detection limits of the electrode deteriorate when the 10 − 2 M Tris–HCl, pH 7.00, or the 10 − 2 M NaH2PO4 –NaOH, pH 7.00 buffers were used. Theses detection limits were 3.2× 10 − 5 and 1.4× 10 − 5 I − . As shown in Fig. 2, the electrode incorporating Co(II)SAODI exhibited a near-Nanstian potentiometric response for 2.3× 10 − 2 –8.4×10 − 7 M I − with a detection limit of 4.7× 10 − 7 M and a slope of 55.7 9 0.2 mV/pI − (20°C) in the B–R buffer, pH 6.00. The time required for the electrode to 90% response was B 40 s which is similar to that observed with a classical ion exchanger membrane electrode. The dc resistance of the electrode membrane was 117.39 0.2 kV (average of six determination). The SD of the electrode potential reading over a period of 24 h in the B–R buffer with pH 6.0 containing 10 − 3 M KI was 0.3 mV (n= 144) and the potential readings for the electrode dipped alternately into stirred solutions of 10 − 3 and 10 − 4 M KI demonstrated a SD of 0.4 mV over 4 h (n= 12). After contact of the electrode with flowing tap water for 4 months, no detectable loss of performance characteristics was observed. The electrode containing Co(II)napophen and Co(II)napen separately showed the rather poor potentiometric response properties (see Fig. 3). These detection limits are 6.2×10 − 4 and 2.7× 10 − 4 M KI, respectively, and the slopes are 38.5 9 0.3, 42.4 9 0.2 mV/pI − , respectively. The potentiometric selectivity coefficients toward a series of anions presented for the solvent polymeric membranes containing different carriers are shown in Figs. 5 and 6. The electrode doped with Co(II)SAODI, for instance, showed a selectivity sequence of anions in the following order: iodide nitrite thiocyanate\perchlorateperiodate \ bromide \ nitrate \ chloride \ sulfate. It is clear that this anion-selectivity pattern deviates from that of the Hofmeister pattern found with HTOAI-based membrane.
R. Yuan et al. / Talanta 48 (1999) 649–657
Fig. 3. Potentiometric response curves of Schiff base complexes of Co(II); () Co(II)SAODI; ( ) Co(II)napen; and () Co(II)napophen in B–R buffer with pH 6.00.
3.2. Mechanism of iodide response and selecti6ity As mentioned earlier, the values of K pot I,SCN of the PVC-based membranes containing Co(III) porphyrins as ionophores and electropolymerized [Co(o-NH2)TPP] films were 1.3 [8], 5.6 [10] and 2.0× 103 [16]. Although the selectivity coefficient between I − and other halides (Br − and Cl − ) of the electrode doped with metalloporphyrin complexes [10] is similar to that of the electrode incorporating Co(II)SAODI, It was noticeable pot that the values of the K pot I,SCN and K I,CIO4 of the membrane containing the Co(II)SAODI were
653
6.3×10 − 4, and 3.2×10 − 4, respectively which are much less than those of membrane electrode based on metalloporphyrin complexes [10]. The unique potentiometric selectivity toward iodide must be related to the special interaction between Schiff base complexes of Co(II)SAODI and iodide ion. The Co(II) Schiff base complexes in the organic phase can serve as a coordinating site for iodide ion to form five-coordination [28,29] and the Fig. 4 demonstrates the possible response mechanism. As shown in Figs. 5 and 6, the electrode doped with Co(II)SAODI exhibited higher potentiometric selectivity toward iodide ion than that containing Co(II)salen, since Co(II)SAODI possesses stronger lipophilicity and steric structure of the ligand. The potentiometric response characteristics of the electrodes incorporating different carriers deteriorated in the following order: Co(II)SAODI\ Co(II)napen\ Co(II)napophen (see Figs. 2 and 3). Indeed, the equatorial plane of the Schiff base complexes of Co increases with increasing conjugation of the complexes, and the complexes containing naphthyl ridicals with the stronger conjugation than that containing phenyl radicals decreased the p electron density in the vicinity of the Co atom and weaken the interaction between the complexes and iodide [29,30]. The potentiometric response characteristics to ward iodide observed by the electrodes doped with different carriers were in fair agreement with the rule above (see Figs. 1 and 3). Figs. 5 and 6 also show that the values of the anion-selective coefficients of the electrodes doped with different ionophores determined by separate solution method (SSM) is better than that by mixed solu-
Fig. 4. Suggested coordination scheme of Co(II)SAODI with iodide.
654
R. Yuan et al. / Talanta 48 (1999) 649–657
Fig. 5. Comparison of the selective coefficients, log K pot I,X for the solvent polymeric membrane containing different active components. The present data were determined by the separate solution method (SSM) and the mixed solution method (MSM — the interferent concentration was kept at the level of 10 − 2 M), respectively. aFrom [19]. bHTOAI membrane was prepared with 63 wt.% o-NPOE, 32 wt.% PVC and 5 wt.% HTOAI.
tion method (MSM, the interferent concentration was kept at the level of 10 − 2 M), and the Dlog K pot I,X obtained by the both methods is 0.2–1. The effect of pH on the potentiometric response characteristics of the electrode incorporating Co(II)SAODI with a higher lipophilicity is small compared with electrode containing Co(II)salen (see Fig. 2 and [19]), when the pH values of the buffer solution are 8.00 \ pH\ 4.00. In the solution with pH\ 8.00, the potentiometric response properties of the electrode slightly deteriorated and the observation can be explained by hydroxide-coordinated central metal interference. In order to identify the potentiometric response results obtained and further prove that the Co(II) Schiff base complexes is important for inducing iodide selectivity, the quartz crystal microgravimetric measurements and the ac impedance experiments were undertaken. The quartz crystal
microgravimetric measurements were performed to confirm the existence of I2 yielded from the oxidation of I − which was transferred from aqueous phase into organic phase, by the deliberative addition of hydrogen peroxide as oxidizer. As shown in Fig. 7, the frequency shifts DF which were related to the interactions of the 10 − 2 M Co(II)SAODI in chloroform with the aqueous solutions containing I − remarkably increased with increasing the concentrations of I − in the aqueous solutions, and in the systems of carrierfree chloroform treated with the aqueous solutions containing different concentrations of I − , no significant increase of DF was observed. The values of DF which were corresponded to the system of 10 − 2 M Co(II)napophen in chloroform treated with the aqueous solutions containing I − is smaller than those of DF. The results obtained above indirectly proved that the transfer of I −
R. Yuan et al. / Talanta 48 (1999) 649–657
655
Fig. 6. Comparison of the selective coefficients, log K pot I,X for the solvent polymeric membrane containing different active components. The present data were determined by the separate solution method (SSM) and the mixed solution method (MSM, the interferent concentration was kept at the level of 10 − 2 M), respectively. aHTOAI membrane was prepared with 63 wt.% o-NPOE, 32 wt.% PVC and 5 wt.% HTOAI.
across water/organic interface was taken up by Co(II) Schiff base complexes in the membrane phase. The ac impedance of a solvent polymeric membrane containing 2.7 mmol of Co(II)SAODI conditioned in pH 6.00 B – R buffer solution containing 10 − 4 KI M were illustrated in Fig. 8. A well-resolved buck and surface impedance at high frequency region in addition to Warburg impedance at low frequency region were observed. The buck resistance decreased with increasing the concentration of KI: 50.2 kV cm in 10 − 6 M KI; 42.4 kV cm in 10 − 5 M KI; 38.2 kV cm in 10 − 4 M KI; and 35.7 kV cm in 10 − 3 M KI. It was evident that Co(II)SAODI could dominate iodide ion across the solvent polymeric membrane and the transfer process is diffusion controlled.
3.3. Preliminary application The electrode doped with Co(II)SAODI was applied to the determination of iodide in drug preparations. A sample of 20–25 mg of iophendylatum was burned in an oxygen bomb with 15 ml of 5% H2O2 and 4.0 ml of 0.5 M NaOH as the absorbate. The absorbate was heated, acidified with H2SO4, and diluted with water. The sample solution obtained above was determinated by potentiometric titration method with 0.00500 M Ag2SO4 as the titrant and the membrane electrode containing Co(II)SAODI as the indicating electrode. Fig. 9 shows the E–V curve for this type of titration using a sample solution containing 20 mg of iophendylatum. The results obtained was 30.779 0.34 (w/w)% in iodide (n=6), which was
656
R. Yuan et al. / Talanta 48 (1999) 649–657
Fig. 7. Frequency shift values in different chloroform phases as a function of the concentration of iodide ion in B – R buffer (pH 6.00): () chloroform containing 10 − 2 M Co(II)SAODI; ( ) chloroform containing 0.01 M Co(II)napophen; and () carrier-free chloroform.
Fig. 9. E – V curve for titration of I − in a sample solution with Ag2SO4.
in fair coincidence with the results [30.4690.27 (w/w)% in iodide, n= 6] given by precipitation method [31].
Acknowledgements This work was supported by the National Natural Science Foundation of China and Chinese National Education committee Foundation for overseas student in addition to Natural Science Foundation of Hunan Province, China.
References
Fig. 8. Impedance plots of a membrane doped with 2.4 mmol Co(II)SAODI with o-NPOE as plasticizer immersed in B – R buffer containing 10 − 3 M KI (pH 6.00) (frequency, 1 × 105 – 1 × 10 − 2 Hz; ac amplitude, 15 mV; and temperature, 18°C).
[1] K. Sollner, G.M. Shean, J. Am. Chem. Soc. 86 (1964) 1910. [2] U. Wuthier, H.V. Pham, R. Zund, et al., Anal. Chem. 56 (1984) 535. [3] U. Wuthier, H.V. Pham, E. Pretsch, et al., Helv. Chim. Acta 68 (1985) 1822. [4] M.E. Meyerhoff, D.M. Pranitis, Anal. Chim. Acta 217 (1989) 123. [5] R. Yuan, S.-Q. Lin, G.-L. Shen, D-Y. Tian, R.-Q. Yu, Acta Chim. Sini. 54 (1996) 516. [6] P. Schulthess, D. Ammann, B. Krautler, C. Caderas, R. Stepanek, W. Simon, Anal. Chem. 57 (1985) 1397.
R. Yuan et al. / Talanta 48 (1999) 649–657 [7] R. Stepanek, B. Krautler, P. Schulthess, B. Lindemann, D. Ammann, W. Simon, Anal. Chim. Acta 182 (1986) 83. [8] D. Ammann, M. Huser, B. Krautler, et al., Helv. Chim. Acta 69 (1986) 849. [9] N.A. Chaniotakis, A.M. Chasser, M.E. Meyerhoff, J.T. Groves, Anal. Chem. 60 (1988) 185. [10] A. Hodinar, A. Jyo, Chem. Lett. 198 (1988) 993. [11] A. Hodinar, A. Jyo, Anal. Chem. 61 (1989) 1169. [12] Q. Chang, M.E. Meyerhoff, Anal. Chim. Acta 186 (1986) 81. [13] N.A. Chaniotakis, S.B. Park, E.M. Meyerhoff, Anal. Chem. 61 (1989) 566. [14] H. Abe, E. Kokufuta, Bull. Chem. Soc. Jpn. 63 (1990) 1360. [15] J.-Z. Li, X.-C. Wu, R. Yuan, H.-G. Yu, R.-Q. Yu, Analyst 119 (1994) 1363. [16] S. Daunert, S. Wallace, A. Florido, G. Bachas, Anal. Chem. 63 (1991) 1676. [17] E.I. Ochiai, J. Chem. Educ. 50 (1973) 443. [18] T.G. Appleton, J. Chem. Educ. 54 (1977) 443. [19] R. Yuan, Y.-Q. Chai, D. Liu, D. Gao, J.-Z. Li, R.Q. Yu, Anal. Chem. 65 (1993) 2572.
.
657
[20] H. Dai, J. Li, Z.-J. Han, H.-W. Chen, J. Inorg. Chem. China 4 (1988) 61. [21] R.H. Bailes, M. Calvin, J. Am. Chem. Soc. 68 (1947) 1886. [22] R. Deiasi, S.L. Holt, B. Post, Inorg. Chem. 10 (1971) 1498. [23] B.T. Chen, Experimental Manual of Inorganic Chemistry, Beijing Normal University Press, Beijing, 1984. [24] E. C. Horning, Organic Syntheses, collection vol. III, Wiley, New York, 1955, pp. 140. [25] R.-Q. Yu, S.-S. Huang, Talanta 30 (1983) 427. [26] G.J. Moody, R.B. Oke, J.D.R. Thomas, Analyst 95 (1970) 910. [27] A. Graggs, G.J. Moody, J.D.R. Thomas, J. Chem. Educ. 51 (1974) 514. [28] J.H. Burness, J.G. Dillard, L.T. Taylor, Syn. React. Inorg. Metal-Org. Chem. 6 (1976) 165. [29] J.H. Burness, J.G. Dillard, L.T. Taylor, J. Am. Chem. Soc. 97 (1975) 6080. [30] A.B.P. Lever, H.B. Gray, Acc. Chem. Res. 11 (1978) 348. [31] Pharmacopoeia Committee of the Ministry of Health of China (Eds.), Chinese Pharmacopoeia, vol. 2, Chinese Health Press, Beijing, 1990.
.