2572
Anal. Chem. lQQ3,65, 2572-2575
Schiff Base Complexes of Cobalt(II) as Neutral Carriers for Highly Selective Iodide Electrodes Ruo Yuan, Ya-Qin Chai, Dong Liu, De Gao, Jun-Zhong Li, and Ru-Qin Yu’ Department of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, China
A new solvent polymeric membrane electrode based on Schiff base complexes of Co(I1) is described which demonstrates excellent selectivity toward the iodide ion. The resulting electrode exhibits fairly low detection limits and good selectivity properties. The selectivity sequence observed is iodide > thiocyanate nitrite > perchlorate bromide > nitrate > chloride > sulfate. The excellent selectivity for iodide is related to the unique interaction between the central Co(I1) ion and iodide. The response mechanism of the electrode was also studied with the ac impedance and spectroscopic techniques.
-
-
Figure 1. Structures of Schiffbase complexes of metal(I1) used to prepare membrane electrodes [Co(IIXsalen),M = Co(II), R = C2H4; Co(IIKsalophen),M = Co(II),R = CsH4; NI(IIXsalen), M = NI(II), R
= C*H4].
Co(I1)are incorporated into plasticized PVC membranes with 2-nitrophenyl dodecyl ester (0-NPDE) as the plasticizer to prepare electrodes with substantially improved selectivity toward the iodide ion.
EXPERIMENTAL SECTION INTRODUCTION
Reagents. Bis(salicylaldehyde)ethylenediiminecobalt(II) [Co-
Anion-sensitive membrane electrodes based on ion exchangers such as lipophilic quaternary ammonium or phosphonium salts display classical Hofmeister behavior in which the membrane selectivity is controlled by the free energy of hydration of ions involved.192 Recently, electrodes using plasticized poly(viny1 chloride) (PVC) membranes incorporating derivatives of vitamin B12, Co(III), Sn(IV), Mo(IV), and Mn(II1) porphyrin complexes, and electropolymerized Co(I1) porphyrin derivative films demonstrated potentiometric anion-selectivity sequences which deviated from the Hofmeister pattern.%I2 These deviations result from the direct interaction between the central metal and the analyte ion and steric effect associated with the structure of the porphyrin ring. The complexes bis(salicy1aldehyde)ethylenediiminecobalt(11)and similar Schiff base complexes of Co(I1)can reversibly coordinate oxygen and have been extensively studied as “model compounds” to simulate natural oxygen carriers which contain a transition metal [e.g., iron (myoglobin), copper (hemocyanin)].13J4 In this paper, Schiff base complexes of ~
~~
* To whom correspondence should be addressed.
(II)(salen)],bis(salicylaldehyde)phenyldiiminecobalt(H)[Co(II)(salophen)], and bis(salicylaldehyde)ethylenediiminenickel(II) [Ni(II)(salen)l were prepared as described by refs 15-17 (see Figure 1). Bis(salicylaldehyde)ethylenediimineiodocobalt(III) [Co(III)(salen)I] was synthesized as described by refs 18-20. The products were identified by elemental analysis. The 2-nitrophenyl dodecylether (0-NPDE) was synthesized as described by Horning.21The synthesis of hexadecyltrioctylammoniumiodide (HTOAI) was described in ref 22. Poly(viny1chloride) (PVC) powder of chromatographic grade was a product of Shanghai Chemical Co. Redistilled deionized water and analytical-grade reagents were used throughout. Apparatus. Potentiometric and pH measurementa were made with a Model 901 microprocessor ionalyzer (Orion, Cambridge, MA). The cells used for millivolt measurements were of the followingtype: Hg; Hg2C12, KC1 (satd)lNaNO~(3 mol/L))sample solutionllmembranellNaNO3 (3 mol/L), pH 5.6 buffer(AgC1,Ag. The pH 5.6 buffer solution used was 1.0 mol/L in citrate and 1.0 mol/L in KCl. The external reference electrode was a doublejunction saturated calomel electrode. Before use, the electrodes were conditioned in the 0.1 mol/L KI aqueous solution for 1day. The membrane composition was optimized by using an orthogonalexperimentaldesign with the electrodelinear response range for iodide ion as the object function for optimization. The optimum composition found was 2.5% (w/w) in ionophore, 31% (w/w)in PVC, and 66.5% (w/w)ino-NPDE. The PVC membrane electrodes were fabricated from various carriers and assembled according to Thomas and co-workers.B.24
(1)Sollner, K.; Shean, G. M. J.Am. Chem. SOC.1964,86,1901-1902. (2)Wuthier, U.;Pham, H. V.; Zund, R.; Welti, D.; Funck, R. J. J. Bezegh, A,; Ammann, D.; Pretach, E.; Simon, W. Anal. Chem. 1984,56, 535-538. (3) Schulthess, P.; Ammann, D.; Krautler, B.; Caderas, C.; Stepanek, R.; Simon, W. Anal. Chem. 1985, 57, 1397-1401. (4)Stepanek,R.;Krautler,B.;Schulthess,P.;Lindemann,B.;A”ann, (15)Bailes, R. H.; Calvin, M. J. Am. Chem. SOC. 1947,69,1886-1893. D.; Simon, W. A w l . Chim.Acta 1986,182,83-90. (16)Deiasi, R.;Holt, S. L.; Post, B. Znorg. Chem. 1971,10,1498-1500. (5)Ammann, D.; Huaer, M.; Krautler, B.; Rusterholz, B.; Schulthess, (17)Chen,B.T.Expen”entaZManuoZofZnorganicChemktry;Beijing P.; Lindemann, B.; Halder, E.; Simon, W. Helu. Chim. Acta 1986,69, Normal University Press: Beijing, 1984; pp 210-217. 849-854. (18)Floriani, C.; Puppis,M.; Calderazz0,F.J. Organomet. Chem. 1968, (6)Chaniotakis, N. A.; Chasser, A. M.; Meyerhoff, M. E.; Groves, J. 12.209-223. , - - - --T. Anal. Chem. 1988,60,165-186. (19)Marzillli, L. G.; Marzilli, P. A.; Halpern, J. J. Am. Chem. SOC. (7)Hodinar, A.; Jyo, A. Chem. Lett. 1988,993-996. 1971,93,1374-137% - - . - _-. -. (8)Hodinar, A.; Jyo, A. Anal. Chem. 1989,61,1169-1171. (20) BLmess, J. H.; Dillard, J. G.; Taylor, T. Syn. React. Znorg. Metal(9) Chang, Q.;Meyerhoff, M. E. Anal. Chim. Acta 1986,186,81-00. Org. Chem. 19‘76,6,165-177. (10)Chaniotakis, N. A.; Park, S. B.; Meyerhoff, M. E. Anal. Chem. Ig, E.C. Organic Syntheses; John Wiley: New York, 1955: 1989,61,566-570. (11)Abe, H.; Kokufuta, E. Bull. Chem. SOC.Jpn. 1990,63,1360-1364. (12)Daunert, S.; Wallace, S.;Florido, A. Bachas, G. Anal. Chem. 1991, 63,1676-1679. (13)Ochiai, E.I. J. Chem. Educ. 1973,50, 610-611. (24)Graggs, A.;Moody, G. J.; Thomas, J. D.R. J. Chem. Educ. 1974, (14)Appleton, T. G. J. Chem. Edr~c.1977,54, 443-444. 51,541-544. ~-
0003-2700/93/0385-2572$04.00/0
0 1993 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 19, OCTOBER 1, 1993 170 1
Table I. Selectivity Coefficients, log for the Solvent Polymeric Membrane Containing Different Carrierso anion Co(salophen) Co(den) HTOAI I-
>
20
2579
0
c1-
-
-4.3 -2.5 -2.4 -2.2 -4.2 -2.2 -4.5
Br
ClodSCNNOS-
E
U- -30-
-
-80
NOz-
sop
0
0 -4.1 -2.6 -2.4 -2.1 -4.2 -2.1 -4.4
-3.1 -2.0 1.9 0.5 -1.4 -2.8 -4.2
-130-
. - , O- I S
-7
-6
-5
-4
-3
-2
a Selectivitycoefficientobtained with the separate solution method, in a 0.01 mol/L phosphate-buffered solution (pH 2.6). Membrane compositions: 2.5% (w/w)in carrier; 31% (w/w) in PVC; 66.5% (w/ w) in o-NPDE.
*
-1
Log [Iodide]
Flguo 2. Potentiometric response curves of Schiff base complexes of metal(II),Co(II)(salophen)(O),Co(I1Msalen)(A),and NI(II)(salen) (O), In 0.01 mol/L H3P0,-NaOH solution, pH 2.5.
Determination of emf Response and Selectivity of the Electrodes. Anion selectivity coefficienta, log K$ were determined by separate solution method. The solutions were buffered with 0.01 moVL and adjusted to pH 2.5 with a concentrated NaOH solution. The single-ion activities were calculated by using the extended Debye-Huckel equation. Determination of emf Response of the Electrodes in an Oxygen-Free Environment. The fabricated PVC membrane containing various ionophores was dried in vacuo. Before use, the electrodeswere conditioned in 0.1 moVL KI aqueous solutions which were deoxygenated by bubbling nitrogen through the solutions for 24 h. The various KI solutions were prepared with deoxygenated buffered solutions and then Nz was allowed to flow over the freshly prepared KI solution during the determination of emf response of the electrodes. ac Impedance Experiments. The ac impedance of the electrode membrane, plasticized with o-NPDE and containing 5.46 mmol of Co(II)(salen),was recorded with the PAR M 368-2 system (EG&GPrinceton Applied Research, Princeton, NJ) in 0.01 mol/L H804-buffered solutions adjusted to pH 2.5 with NaOH. The frequency range used was 105-10-2Hz (at 14 "C). UV-Visible Absorption Spectra. Spectraof the chloroform phase, obtained by shaking a solution of Schiff base complexes of metal(I1) in CHCb with aqueous0.1 moVL KI for 30 min, were recorded on a PE Lambda 17 spectrophotometer (Bodenaeewerk Perkin-Elmer & Co. GmbH, D-7770 Ueberlingen, Germany).
RESULTS AND DISCUSSION emf Response Characteristics and Selectivity of Electrodes Dopedwith Schiff Base Complexes of Metal(11). Potentiometricresponse characteristicsof the electrodes containing different carriers are shown in Figure 2. The electrode incorporating Co(II)(salophen) showed a nearNernstian potentiometricresponse for 1X W-1 X 1Wmol/L I- with a detection limit of 7 X moVL and a slope of 56.2 f 0.2 mV/pI- (20 "C) in 0.01 mol/L HsPO~buffer solutions adjusted to pH 2.5 with NaOH. The time required for the electrode to reach 90% response was less than 1min. The dc resistance of the electrode membrane was 124.7 0.3 kQ (averageof six determinations,n = 6). The standard deviation of the electrode potential readings over a period of 12 h in 0.01 mol/L phosphate-buffered solution (pH 2.5) containing 0.001 moVL KI was 0.3 mV (n= 721, and the potential readings for the electrode dipped alternately into stirred solutions of 0.01 and 0.001 mol/L KI showed a standard deviation of 0.8 mV over 2 h (n= 6). An electrode conditioned by continuous contact with 0.01 mol/L KI aqueous solution (pH 5.6) for 2 months did not ahow detectable loss of performance characteristics. The electrode doped with Co(I1)(salen) showed a linear response for 6 X 10-%2 X 1W mol/L with a slope of 55.8 f 0.3 mV/pI- (n = 5, 20 "C) in phosphate-buffered solutions of pH 2.5. The other potentiometric response
characteristics of the electrode containing Co(II)(salen) are very similar to those of the electrode incorporating Co(I1)(salophen). The potentiometric response characteristics of the electrode doped with Ni(salen) are rather poor (seeFigure 2). The potentiometric selectivitycoefficienta for membranes containing different carriers are shown in Table I. The electrode containing Co(II)(salophen), for instance, showed a selectivity sequence of anions in the followingorder: iodide > thiocyanate nitrite > perchlorate > bromide > nitrate > chloride > sulfate. Mechanism of Iodide Response and Selectivity. The electrodes doped with a Co(II1) porphyrin derivative exhibit fairly high selectivity toward the thiocyanate i0n.5~7 The potentiometric selectivity coefficient were 1.3 5 and 5.6.7 This was partially explained in terms of the stability constant of the Co(II1)complex of porphyrin derivatives with anions which decreases according to the following order: thiocyanate > iodide > bromide > chloride.% The sensors based on poly[cobalt(II)tetrakis(2-aminophenyl)porphyrinl or poly[Co(o-NH2)TPPI films demonstrated excellent selectivity toward the thiocyanate ion.', The value of the of electropolymerized [Co(o-NH,)TPP] films was 2.0 X lo3.', The high selectivity toward thiocyanate seemed to be related to the ion-recognition properties of the poly[Co(o-NH,)TPP] films.12 It was noticeable that the value of the Kf$&N of the membrane doped with Schiff base complexes of Co(I1)was 6.3 X 103. The high potentiometric selectivity for iodide must be related to the unique interaction between cobalt(I1) complexes and iodide in acid solution. Indeed, oxygen could oxidize iodide ion to iodine in low-pH aqueous solutions%and the reaction of cobalt(I1)Schiff base complex with iodine was accompanied by electron transfer between the central cobalt and iodine in iodide complexeswhich could be described as Co3+--I-.20+m The following reactions took place:
-
TkN
21-
*
+ 2H+ + '/,O,* I, + H,O
I, + 2[Co(II)(salen)l* 2[Co(III)(salen)Il
(1) (2)
Figure 3 illustrates the influence of pH on potentiometric response of an electrode containing Co(II)(salophen)toward the iodide ion. The linear response range and the slope deteriorated with increasing solution pH. Figure 4 shows that the potentiometric response characteristics of the electrodes doped with Co(I1)(salen) and Co(II)(salophen) (25) Aehley, K. R.; Berggren, M.; Cheng, M. J. Am. Chem. SOC.1975, 97,1422-1426. (26)Heslop,R.B.;Jonea,K.lnorganicChemiatry;Eleevier:NewYork, 1976. -.
(27) Burneee, J. H.;Dillard, J. G.; Taylor, L. T. J. Am. Chem. Soc. 1975,97,6080-6088.
170-
2101
120-
70
>
I
-
\h
20-
E s i -30
-80 -
\\ \
,
-7
-130-
-180
I
I
I
I
1
I
1
-G
1
I
-5 -4 -3 Log [Iodide]
1
,
-2
-1
Figure 6, Effect of ascorbic acid on the iodide response of Co(I1)(salophenbbased membrane electrode in 0.01 mol/L H3P04-NaOH solution, pH 2.5. [(e)0.01 moi/L ascorbic acid; (0)0.0005 mol/L ascorbic acid].
0 I
I
I
250
-7
-6
-5 -4 -3 Log [Iodide]
-2
-1
350 450 550 Wavelength (nm) Figure 7. UV-visible absorption spectra of chloroform solutions of Co(1IXsaien) (- - -), Co(1IIKsalen)I (- * -1, and Co(1IXsaien) treated with 0.1 moi/L K I (-).
Figure 4. Potentiometric response curves of Schiff base complexes of metal(II), Co(IIXsal0phen)(0),Co(I1Xsalen) (A),and Ni(I1Xsaien) (01,in deoxygenated 0.01 mol/L H3P04-NaOHsolution, pH 2.5.
$-
120
4
70
-
20-
d
w^
0
-30-
-130 I
I
250
-80
,
I
-7
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,
,
I
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,
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