Anal. Chem. 2002, 74, 2591-2595
Triiodide PVC Membrane Electrodes Based on Charge-Transfer Complexes Susan Sadeghi* and Gholam Reza Dashti
Department of Chemistry, University of Birjand, P.O. Box 414, Birjand, Khorasan, Iran
Three new electrodes were prepared by incorporating two different charge-transfer complexes and amino crown ether into plasticized PVC membranes. The electrodes showed Nernstian response to triiodide ion over the activity range from 1.0 × 10-5 to 1.0 × 10-1 mol‚L-1 with detection limits at ∼1.0 × 10- 6 mol‚L-1. The resulting electrodes have fast response times (20-30 s) and good stabilities (4 months) and can be used over a wide pH range of 2.5-9.0. The proposed electrodes exhibit antiHofmeister behavior with excellent selectivity toward triiodide ion against a wide range of common interferences. Comparative study suggests that amino (aza) crown ether alone or in the form of a charge-transfer complex with iodine, as an ionophore in a PVC liquid membrane, is sensitive to triiodide ion. The electrodes were used as indicator electrodes in potentiometric titration of triiodide ion against thiosulfate ion. Polymeric membrane-based potentiometric electrodes are used extensively in biological and industrial analysis1-3 because of advantages such as the following: simple method, fast and direct measurement of activity, nondestructive analysis, sensitivity to a wide concentration range, low cost, and portability of the instrument. Most theoretical treatment about the potentiometric response of these electrodes is based on rapid ion-transfer reaction in a solvent polymeric membrane.4 To date, several liquid polymer membrane-based ion-selective electrodes (ISEs) have been proposed for measurement of anions.5-10 Ion exchanger and charged carriers applied in anionselective membranes display classical Hofmeister behavior.11,12 As * Corresponding author: (fax) +98-561-34585; (e-mail) Chemsad2001@ Yahoo.Com or
[email protected]. (1) Frant, M. S. Analyst 1994, 119, 2293-2301. (2) Meyerhoff, M. E.; Opdyche, M. N. Adv. Clin. Chem. 1986, 25, 1. (3) Moody, G. J.; Saad, B. B.; Thomas, J. D. R. Sel. Electrode Rev. 1988, 10, 71. (4) Sokalski, T.; Zwick, T.; Bakker, E.; Pretsch, E. Anal. Chem. 1994, 66, 12041209. (5) Nishizawa, S.; Bu ¨ hlmann, P.; Xiao, K. P.; Umezawa, Y. Anal. Chim. Acta 1998, 358, 35-44. (6) Li, Z.-Q.; Yuan, R.; Ying, M.; Song, Y.-Q.; Li, G.-S.; Yu, R.-Q. Anal. Lett. 1997, 1455-1464. (7) Chaniotakis, N. A.; Park, S. B.; Meyerhoff, M. E. Anal. Chem. 1989, 61, 566-570. (8) Corey, C. M.; Riggan, W. B. Anal. Chem. 1994, 66, 3587-3591. (9) Park, S. B.; Matuszewski, W.; Meyerhoff, M. E.; Liu, Y. H.; Kadish, K. M. Electroanalysis 1991, 3, 909-916. (10) Ying, M.; Yuan, R.; Li, Z.-Q.; Li; Song, Y.-Q.; Shen, G.-L.; Yu, R.-Q. Anal. Lett. 1998, 31, 1965-1977. (11) Arnold, M. A.; Arnold, R. L. Anal. Chem. 1986, 58, 84R-101R. 10.1021/ac010647h CCC: $22.00 Published on Web 04/26/2002
© 2002 American Chemical Society
a result, ion selectivity is determined by the free energy of hydration of anions. Recently, varieties of organometallic compounds have been applied for anion-selective electrodes that showed a potentiometric anion selectivity pattern which deviated from the Hofmeister pattern.13-15 In these cases, specific interactions and coordination of the primary anion as axial ligand to the central metal ion of the carrier molecule are responsible for the improved selectivity. But, organometallic complexes are sensitive to pH and can form metal oxides and hydroxides at moderate or high pH that affect on their performance.16 Determination of iodide or triiodide ion is important in clinical and chemical analysis.17,18 Triiodide ion is used as an oxidizing agent.19 Titrimetry20 and conventional spectroscopic21 and polarographic methods22 for determination of triiodide ions have been developed, but these methods are laborious and need expensive instruments. Although potentiometric methods of analysis using ISEs are simple, low cost, and applicable to samples, there have been relatively few examples to develop analytically useful triiodide sensors. Electrodes using PVC membrane incorporating quaternary ammonium compounds, porphyrins, or porphyrin metal complexes have been reported that show an anti-Hofmeister pattern.23-27 Only one report described use of a charge-transfer complex in the preparation of triiodide ISE that is highly selective to triiodide ions over other anions.28 (12) Bu ¨ hlmann, P.; Pretsch, E.; Bakker, E. Chem. Rev. 1998, 98, 1593-1687. (13) Badr, I. H. A.; Meyerhoff, M. E.; Hassan, S. S. M. Anal. Chem. 1995, 67, 2613. (14) Nakamura, T.; Hayashi, C.; Ogawara, T. Bull. Chem. Soc. Jpn. 1996, 69, 1555-1559. (15) Shamsipur, M.; Sadeghi, S.; Naeimi, H.; Sharghi, H. Pol. J. Chem. 2000, 74, 231-238. (16) Li, J.-Z.; Pang, X.-Y.; Yu, Q.-R. Anal. Chim. Acta 1994, 297, 437-442. (17) Yabu, Y.; Miyai, K.; Hayashizaki, S.; Endo, Y.; Hata, N.; Tijima, Y.; Fushimi, R. Endocrinol. Jpn. 1986, 33, 905. (18) Mushtakova, S. P.; Kozhina, L. F.; Ivanova, L. M.; Myshikina, A. K.; Rytova, O. A. Anal. Chem. 1981, 53, 214. (19) Harris, D. C. Quantitative Chemical Analysis, 4th ed.; W. H. Freeman and Co.: New York, 1995; p 401. (20) Day, R. A.; Underwood, A. L. Quantitative Analysis, 6th ed.; Prentice-Hall Int.: Upper Saddle River, NJ, 1991; Chapter 11, p 301. (21) Nacapricha, D.; Muangkaew, S.; Ratanawimarnwong, N.; Shiowatana, J.; Grudpan, K. Analyst 2001, 1 26, 121-126. (22) Odink, J.; Bogaards, J. J. P.; Sandman, H. J. Chromatogr. 1988, 431, 309. (23) Li, J.; Janata, J.; Josowicz, M. Electroanalysis 1996, 8, 778. (24) Li, D. L.; Li, L. K. Lihua Jianyan Huaxue Fence 1992, 28, 344-395. (25) Kuchkarev, E. A.; Klyatskina, E. I. Zh. Anal. Khim. 1993, 48, 495-501. (26) Suzuki, H.; Nakagawa, H.; Mifune, M; Saito, Y. Anal. Sci. 1993, 9, 351354. (27) Suzuki, H.; Saito, H.; Odo, J.; Mifune, M. JPN. Kokai Tokkyo Koho JP 04,274,751 [92,274,751] 30 Sep 1992, Appl. 91/61,141,01 Mar 1991. (28) Rouhollahi, A.; Shamsipur, M. Anal. Chem. 1999, 71, 1350-1353.
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In the present work, we employed a crown ether, i.e., 4′,4′′diamino-2,3,11,12-dibenzo-1,4,7,10,13-pentaoxapentadeca-2,11-diene (DADB15C5), and two recently synthesized charge-transfer complexes between iodine and 5,6,14,15-dibenzo-1,4-dioxa-8,12diazacyclopentadeca-5,14-diene (DBDA15C4), formulated as [(DBDA15C4)2I+]I3- and [(DBDA15C4)2I+]I-, as suitable carriers for the triiodide ion-selective liquid membrane electrodes. The performance of these electrodes is described. Then, the results are compared with a previous similar electrode. EXPERIMENTAL SECTION Reagents and Apparatus. All chemicals were of analytical reagent grade. High molecular weight poly(vinyl chloride) (PVC) and macrocycles 5,6,14,15-dibenzo-1,4-dioxa-8,12-diazacyclopentadeca-5,14-diene and 4′,4′′(5′′)-diamino-2,3,11,12-dibenzo-1,4,7,10,13-pentaoxapentadeca-2,11-diene were purchased from Fluka. Tetrahydrofuran (THF), dibutyl phthalate (DBP), benzyl acetate (BA), molecular iodine, and sodium inorganic salts (all from Merck) were used. All standard solutions and buffers were prepared with doubly distilled water. Potassium hydrogen phthalate and disodium hydrogen phosphate buffers at pH 2.1 and pH 7.2, respectively, were prepared according to the Rabinson table.29 Potentiometric responses were measured at room temperature with pH/mV meter WTW Weilheim model 537A. Two doublejunction calomel electrodes were used through the experiments. The assembly of the cell was as follows:
Hg | Hg2Cl2 | KCl (sat’d) | internal solution, 1 × 10-3 M KI3 solution | PVC membrane | sample solution | K(sat’d) | Hg2Cl2| Hg
UV-visible spectra were recorded on a Shimadzu 160A spectrophotometer with a 1-cm fused-silica cell. Synthesis of Ionophores. The 1:1 and 2:1 charge-transfer complexes between DBDA15C4 and iodine, formulated as [(DBDA15C4)2I+]I3- and [(DBDA15C4)2I+]I-, were prepared according to a previous published procedure.30The charge-transfer complexes in the crystalline form were prepared by dissolving appropriate amounts of the crown ether (DBDA15C4) and iodine in chloroform. The solution were allowed to evaporate for 12 h. The resulting solid products was recrystallized from reagent grade diethyl ether, dried under vacuum for 48 h, and characterized by elemental analysis and UV-visible and IR spectroscopy. These complexes and DADB15C5 were used as the ionophore in the preparation of PVC membrane ISEs. The structures of the used crown ethers are shown in Figure 1. Membranes and Electrodes. The polymeric membranes were prepared by mixing 3% (w/w) ionophore, 65% (w/w) DBP as plasticizer, and 33% (w/w) powdered PVC in 10 mL of THF. The solvent was allowed to evaporate until ∼0.5 mL of mixture was remained. The end of a Pyrex tube, as an electrode body, with 3-mm-outer diameter was dipped into the mixture. The liquified membrane was allowed to air-dry for 3 h, resulting in a membrane of ∼30 µm in thickness. The tube was filled with an (29) Dean, J. A. Lange’s Handbook of Chemistry, 4th ed.; McGraw-Hill Book Co.: New York, 1992; p 8.10. (30) Sadeghi, S.; Shamsipur, M.; Elahi, M. Pol. J. Chem. 1997, 71, 1594-1602.
2592 Analytical Chemistry, Vol. 74, No. 11, June 1, 2002
Figure 1. Structures of the macrocyclics studied.
internal solution of 1 × 10-3 mol‚L-1 potassium triiodide, and the membrane was preconditioned for about 24 h and stored in the same solution when not in use. The membrane was soaked in 1 × 10-3 mol‚L-1 potassium triiodide solution for 2 h before use. A calomel reference electrode was inserted into the top of the electrode body to complete the electrode assembly. Between two series of measurements, the electrode was left in distilled water for ∼15 min to rinse it and enhance the reproducibility of the measurements. A 10-3 mol‚L-1 triiodide ion solution was prepared by diluting a 1.0 × 10-1 mol‚L-1 iodine standard solution with 1.0 × 10-2 mol‚L-1 KI solution. Standard solutions of 10-6-10-2 mol‚L-1 triiodide ion for calibration of the electrode were prepared by diluting of 10-1 mol‚L-1 triiodide ion solution with doubly distilled water. Single ion activities were calculated with the use of the two-parameter Debye-Hu¨ckel approximation.31 RESULTS AND DISCUSSION Charge-transfer complexes are an electron donor/electron acceptor association for an intermolecular electronic chargetransfer transition. Iodine has been found to form charge-transfer complexes with many compounds such as polyoxygenated crown ethers. It is well known that iodine-donor interactions depend strongly on the nature of the donor base. The substitution of some oxygen atoms of crown ether rings by nitrogens causes a large increase in the stability of their iodine complexes. We recently reported a spectroscopic study of molecular complexes of iodine with DBDA15C4 in chloroform solution.30 The presence of nitrogen plays a fundamental role in the process of charge-transfer complex formation. If the membrane contains a suitable receptor molecule such as a charge-transfer complex that is able to bind a specific anion, the membrane becomes selective toward this anion. Thus, the binding energy of the complex by association of the anion and the ionophore reduces the transfer energy of the anion from the aqueous phase to the membrane. Therefore, we decided to use two different cyclic ligands (DBDA15C4, DADB15C5) containing nitrogen atoms as donating sites in their complexes with iodine and study the response of the electrodes with [(DBDA15C4)2I+]I3-, [(DBDA15C4)2I+]I-, and DADB15C5. In preliminary experiments, these ionophores were used as a carrier to prepare PVC membrane electrodes for a variety of (31) Meier, P. C. Anal. Chim. Acta 1982, 136, 363-368.
Figure 2. Potential response of various ion-selective membranes based on ionophore [(DBDA15C4)2I+]I3-.
anions. The potential responses of various ion-selective electrodes based on [(DBDA15C4)2I+]I3- in the activity range of 1.0 × 10-61.0 × 10-1 mol‚L-1 solutions containing a single type of anion are shown in Figure 2. As seen, among different anions tested, the corresponding electrode slopes for triiodide ions are close to Nernstian, indicating that the membrane reversibility responds to it. Similar behavior was obtained for electrodes based on [(DBDA15C4)2I+]I- and DADB15C5. It seems that, by using DADB15C5 as an ionophore, the mechanism probably involves charge-transfer complex formation by interaction of amino groups in the crown ether and iodine in the internal solution. Thus, a rapid anion exchange reaction between I3- ions in the sample solution with an external ion pair (I3- or I- ion) in the chargetransfer complexes in the membranes is responsible for I3- ion sensing. Such binding is confirmed via a UV-visible spectral shift in the absorption spectra of [(DBDA15C4)2I+]I3- in the presence of different anions. The absorptions at 300 and 367 nm are known to be characteristic of the triiodide contact ion pair. As is obvious from Figure 3, while the intensity of the 367-nm band decreased with an increase of SCN- ions to the [(DBDA15C4)2I+]I3- solution, the extent of displacement of this band toward lower wavelengths became greater. Such spectral behavior emphasizes the substitution of SCN- with triiodide ion at higher concentrations of SCNion. At first it seems that the macrocyclic amines take up the protons, resulting in a positively charged membrane surface those strong ion pairs with the I3- anion. But the macrocycles used are quite rigid molecules and possess a small cavity (1.5 Å); the inclusion of I+ or I3- ions with an ionic size of 1.64 Å or more inside the crown ether cavity is unexpected. The spectrophotometric results30 and conductometry studies revealed the contact ion pair in the molecular complex. The stability constant of the ionophore-ion association and partition constant between the membrane and sample solution affect the selectivity of the ISE. The origin of I3- ion selectivity of the present ISE may come from a higher stability constant for the ion-pairing I3- complex (log Kf ) 5.25 ( 0.05)30 than for the other anion complexes. To obtaining the best sensitivity and selectivity of the sensor, the effect of membrane composition on response of the electrode must be investigated. First, type and amount of the plasticizer were
examined. The results are given in Table 1. The data indicated that nonpolar solvent mediator DBP gave a triiodide ion sensor with more favorable potentiometric characteristics than polar BA (electrodes D and F). Hence, DBP was used in further studies. Moreover, the optimum amount of ionophore [(DBDA15C4)2I+]I3was determined to be 3% (w/w) (electrode C). Concentrations above 3% (w/w) ionophore in membranes (electrodes D and E) showed super-Nernstian response toward triiodide ion and concentrations below 3% (w/w) led to diminished response (electrode B). The concentration of uncomplexed analyte ion in the membrane phase must remain constant to obtain a Nernstian response function. An increase of uncomplexed primary anion (I3-) in the membrane phase might be responsible for the super-Nernstian response obtained toward I3- ions. It is interesting that the membrane without ionophore but with the same composition as the membrane resulted in no promising sensitivity for I3- ion (electrode A). At higher PVC content, the membrane resistance is increased and transport of anions into the membrane is difficult (electrode G). Therefore, membrane [(DBDA15C4)2I+]I3- with a PVC/DBP/ionophore percent ratio of 33:64:3 shows very good sensitivity with a Nernstian slope of 59.7 ( 1.5 mV/decade over a wide activity range, 1 × 10-5-1 × 10-1 mol‚L-1 of triiodide ion. Similar investigations on membranes [(DBDA15C4)2I+]I- and DBDA15C4 were done, and the results are summarized in Table 1. In a second set of experiments, the influence of concentration of the inner filling solution, in the range of 1.0 × 10-4 -1.0 × 10-2 mol‚L-1 KI3, on the ISE behavior was investigated. Results showed that an inner solution of 1.0 × 10-3 mol‚L-1 for ionophores [(DBDA15C4)2I+]I3- and [(DBDA15C4)2I+]I- and 1.0 × 10-2 mol‚L-1 for ionophore DADB15C5 was suitable for better functioning the electrode. The calibration parameters, response time, and lifetime of different membranes prepared with proposed ionophores under the optimum conditions have been determined. From the data given in Table 2, the mentioned electrodes present Nernstian slopes (∼59 mV/decade, r > 0.998) with detection limits of (36) × 10-6 mol‚L-1 in response to triiodide ion. The response time according to IUPAC recommendation, was defined as the time to obtain a response that is within (1 mV of the steady-state signal after addition of a 10-fold concentration of the interested ion to the sample. The time was found to be 15-30 s in a 1.0 × 10-3 mol‚L-1 potassium triiodide sample depending on the ionophore type used in this work. It is well known that ion exchange processes are faster than chelation, so in all the proposed membrane electrodes, the response time was short. The optimum equilibration time in a 1.0 × 10-3 mol. L-1 potassium triiodide solution was 2 h. The lifetime of the electrodes was ∼4 months with a (2 mV divergence in slope. The reproducibilities of six replicate measurements were (0.3 mV for membrane contain DADB15C5 and (1.5 mV for [(DBDA15C4)2I+]I3- and [(DBDA15C4)2I+]I-. The influence of pH on the response of the potential was examined by use of a 1.0 × 10-3 mol‚L-1 potassium triiodide solution over the pH range 2.0-11.0. To adjust the pH, very small volumes of HNO3 (not HCl) and NaOH were used. The result for ionophore [(DBDA15C4)2I+]I3- is shown in Figure 4, which indicated the electrode exhibits a better response and extended Analytical Chemistry, Vol. 74, No. 11, June 1, 2002
2593
Figure 3. Absorption spectra of (A) SCN- solution in THF (9.3 × 10-3 mol‚L-1) and (B) ionophore [(DBDA15C4)2I+]I3- (2.5 × 10-5mol‚L-1) dissolved in THF with varying concentrations of SCN-. The peak absorbance at 367 nm decreased monitoring with increasing thiocyanate concentration. Table 1. Effect of Composition on Membrane Response slope (mV‚decade-1)
composition (%, w/w)
a
type
ionophore
PVC
plasticizer
[(DBDA15C4)2I+]I3-
[(DBDA15C4)2I+]I-
DADB15C5
A B C D E F G
0 2.0 (1.0)a 3.0 5.0 8.0 5.0 3.0
33 33 33 33 33 33 43
DBP, 67 DBP, 65 DBP, 64 DBP, 62 DBP, 59 BA, 62 DBP, 54
3.8 ( 0.5 47.1 ( 1.6 59.7 ( 1.5 65.5 ( 1.5 69.5 ( 1.5 23.1 ( 0.2 55.0 ( 0.8
3.8 ( 0.5 59.9 ( 0.3 66.5 ( 0.2 69.5 ( 0.7 57.7 ( 1.5 54.6 ( 0.8 54.1 ( 0.3
3.8 ( 0.5 59.8 ( 1.5 61.0 ( 0.6 69.7 ( 0.8 71.5 ( 2.0 54.6 ( 0.8 56.5 ( 1.0
Amount of [(DBDA15C4)2I+]I3- that was experienced.
Table 2. Potentiometric Response Characteristics of the Proposed I3- Ion-Selective Electrodes ionophore parameter (mV‚decade-1)
slope dynamic range (mol‚L-1) lower detection limit (mol‚L-1) working pH range response time (s)b stability (months) a
[(DBDA15C4)2I+]I359.7((1.5)a
1.0 × 10-5-1.0 × 10-1 5.6 × 10-6 2.5-9.0 30 4
[(DBDA15C4)2I+]I59.9((1.5)a
1.0 × 10-5-1.0 × 10-1 5.0 × 10-6 3.0-8.5 20 4
DADB15C5 59.7((0.3)a 1.0 × 10-5-1.0 × 10-1 3.1 × 10-6 2.5-9.0 20 4
RSD (n ) 6). b 10-3 f 10-2 mol‚L-1.
linearity at lower pH values. In alkaline media due to OH- ions and formation of hypoiodate and iodide, sensitivity is higher than that in acidic and neutral solution. This behavior can be described by the following reactions:
I2 + 2OH- f IO- + I- + H2O 3IO- f 2I- + IO3The increasing disproportion of iodine as the pH rises offsets the hydroxide interference, this result in response analogous to those found with the membrane doped with manganese(III) tetraphenylporphine.26 Therefore, deprotonation of an amino(aza) crown ether used in the membrane may not be the reason for the potential changes at higher pH values. The working pH range for ionophore [(DBDA15C4)2I+]I3- is 2.5-9.0. Similar results were observed with [(DBDA15C4)2I+]I- and DADB15C5. 2594
Analytical Chemistry, Vol. 74, No. 11, June 1, 2002
The relative response of the ISE for the primary ion over other ions present in the solution was evaluated by the potentiometric selectivity coefficient. In this work, selectivity coefficients over interfering anions were evaluated by the matched potential method32,33 at 1.0 × 10-3 mol‚L-1 analyte concentration. According to this method, the selectivity coefficient, Kpot, is defined as the activity ratio of the primary ion (A) and the interfering ion (B) that gives the same potential in a reference solution:
KIpot - ) ∆aA/aB 3 ,i
where ∆aA ) a′A - aA and a′A and aA are the initial primary ion activity and the primary ion activity in the presence of interfering (32) Umezawa, Y.; Umezawa, K.; Sato, H. Pure Appl. Chem. 1995, 67, 508-518. (33) Bakker, E. Electroanalysis 1997, 9, 7-12.
Figure 4. Effect of pH of test solution on the potential response of the I3- ion-selective electrode based on [(DBDA15C4)2I+]I3-. Table 3. Comparison of the Selectivity Coefficients of the Proposed I3- Ion-Selective Electrodes pot a
log K
diverse ions SCN- b
ClO4SO42- c HPO42- d BrClIH2PO4- d OACHCO3P2O74- b NO3NO2-
for ionophors
[(DBDA15C4)2I+]I3-
[(DBDA15C4)2I+]I-
DADB15C5
ref 28
-2.51 -4.45 -3.60 -4.96 -3.96 -4.30
