Highly Selective PVC-Membrane Electrodes Based on Co(II)-Salen for Determination of Nitrite Ion

Transcription

1 2003 The Japan Society for Analytical Chemistry 1127 Highly Selective PVC-Membrane Electrodes Based on Co(II)-Salen for Determination of Nitrite Ion Mohammad REZA GANJALI,* Morteza REZAPOUR,* Mohammad REZA POURJAVID,* and Masoud SALAVATI-NIASARI** *Department of Chemistry, Tehran University, Tehran, Iran **Department of Chemistry, Kashan University, Kashan, Iran A cobalt(ii) derivative was used as a suitable ionophore for the preparation of a polymeric membrane nitrite-selective electrode. The electrode reveals a Nernstian behavior over a very wide NO ion concentration range ( M) and a very low detection limit ( M). The potentiometric response is independent of the ph of solution in the ph range The electrode shows advantages such as low resistance, fast response and, most importantly, good selectivity relative to a wide variety of inorganic and organic anions. In fact, the selectivity behavior of the proposed NO ion-selective electrode shows great improvements compared to the previously reported electrodes for nitrite ion. The proposed electrodes could be used for at least 2 months without any significant changes in potentials. The electrode was successfully applied to the determination of nitrate ion concentrations in sausage and milk samples. (Received January 7, 2003; Accepted April 22, 2003) Because of the important role of nitrite ion in producing toxic, carcinogenic and/or mutagenic N-nitrosamines in human body, on the one hand, 1 and its increasing use as a preservative in the food industry, on the other hand, 2,3 the determination of this anion in trace levels is of special interest. Besides a large number of analytical methodologies applicable to nitrite determination based on spectrophotometric, 48 chromatographic, 913 electrochemical, 1419 and electrophoretic techniques, 2022 several nitrate ion-selective electrodes based on Co(III)-phthalocyanine, 23 In(III)-tetraphenylporphyrin chloride derivatives, 24 benzylbis(triphenylphosphine)pd(ii), 25 Co(III)- aquocyanocobyrinate, 26 Co(III)-tetraphenylporphyrin chloride and nitrite derivatives, 27,28 and UO 2-salophen 29 have also been reported in the literature. However, these nitrite ion-selective electrodes suffer from narrow linear ranges, high limits of detection and, especially, strong interference effects from such anionic species as perchlorate, thiocyanate and iodide. In this paper, we wish to report the use of the Co-Salen as an excellent ionophore for the preparation of a new highly selective and sensitive polymeric membrane electrode for nitrite ion. It is noteworthy, to the best of our knowledge, that this is the first time a Co(II) complex is used as an ionophore in constructing a NO sensor. Experimental were purchased from Merck Chemical Company and used as received. Reagent grade sodium and potassium salts of all anions used (all from Merck) were of the highest purity available, and used without any further purification except for vacuum drying. Doubly distilled deionized water was used throughout. Synthesis of cobalt-schiff s base complex A 0.06 mol portion of CoCl 2 was dissolved in 50 cm 3 of methanol and 0.06 moles of the required ligand was dissolved in a minimum amount of acetone and added dropwise to the metal salt solution. The mixture was refrigerated overnight. The crude cobalt chelate (Fig. 1) was isolated by filtration, and purified by recrystallization from acetone. (Found: C, 80.7; H, 3.9; N, 7.8; Co, 16.6; Calc. For C 16H 14N 2O 2Co: C, 80.9; H, 4.1; N, 7.8; Co, 16.7%). Preparation of the electrode The general procedure to prepare the PVC membrane was to mix thoroughly 33 mg of powdered PVC, 3 6 mg of ionophore CS, 2 3 mg of cationic additive HTAB and mg of plasticizer (NPOE, NB, DBP, DOP, and BA). The mixtures were then dissolved in 3 ml of dry THF. The resulting clear mixtures were evaporated slowly until oily concentrated mixtures were obtained. Pyrex tubes (5 mm i.d.) 3033 were dipped into the mixtures for about 10 s, so that nontransparent Reagents Reagent grade acetophenon (AP), benzyl acetate (BA), dibutyl phthalate (DBP), o-nitrophenyloctyl ether (NPOE), hexadecyltrimethylammonium bromide (HTAB), tetrahydrofuran (THF) and high relative molecular weight PVC To whom correspondence should be addressed. Ganjali@khayam.ut.ac.ir Fig. 1 Structure of CS.

2 1128 ANALYTICAL SCIENCES AUGUST 2003, VOL. 19 (a) (b) Fig. 3 UV-Vis absorption spectra of DMSO solutions of M NO (A), M CS (B), and CS M treated with M NO (C). Fig. 2 CS. Potential response of various membrane sensors based on membranes of about 0.3 mm thickness were formed. The tubes were then pulled out of the mixtures, and kept at room temperature for about 10 h. After this, the tubes were filled with the internal solution ( M NaNO 2). The electrodes were eventually conditioned for 24 h by soaking in M NaNO 2 solutions. A silver/silver chloride electrode was used as an internal reference electrode. EMF measurements The EMF measurements, with the polymeric membrane sensor, were carried out with the following cell assembly: Ag-AgCl // KCl (3 M) / internal solution M NaNO 2 / PVC membrane / test solution / Hg-Hg 2Cl 2, KCl (saturated). A Corning ion analyzer 250 ph-meter was used for the potential measurements, at 25 ± 0.1 C. Results and Discussion A well-known group of suitable ionophores for inducing anti- Hofmeister selectivity patterns are Schiff s base complexes Previous studies have shown that organometallic compounds containing cobalt atoms exhibit some nitrite selectivity deviating from the Hofmeister series. 23,2628 Thus, in order to develop an anion-selective membrane sensor for fast monitoring of the nitrite ion, the Co(II)-salen complex (CS, Fig. 1) was evaluated as an ionophore in constructing the PVC-based membrane electrodes for a wide variety of anions. It should be noted that each electrode was conditioned in a M solution of the corresponding anion for 24 h. The obtained potential responses of some organic and inorganic anionic in the concentration range of are illustrated in Fig. 2. As is immediately obvious, except for the NO ion-selective electrode, in all other cases the slope of the corresponding potential vs. pa n plots is much lower than the expected Nernstian slope (membranes 3, 5 8). This is, most probably, due to both the selective behavior of the ionophore against NO in comparison to other anions tested and the rapid exchange kinetics of the anion, between the aqueous and membrane phases. The UV-Vis spectra of nitrite ion in the presence and absence of the CS in dimethylsulfoxide solution was nicely in support of this fact (Fig. 3). The substantial increase in the absorbance at and nm, after the contact of the carrier solution with a nitrite-containing phase, suggested that the absorbing species had increased in size, and the axial coordination was thought to take place. At the same time, the effects of other anions on the spectrum of the carrier were investigated and no detectable changes in the UV/Vis spectra were noted. Besides the critical role of the nature of an ionophore in preparing membrane selective electrodes, some other important features of the PVC membrane, such as the amount of ionophore, the nature of the solvent mediator, the plasticizer: PVC ratio and especially the nature of additives used, are known to significantly influence the sensitivity and selectivity of ion-selective electrodes Thus, different aspects of the membrane preparation for nitrite ion were optimized; the results are given in Table 1. Among the five different plasticizers used (Nos. 3, 5 8), NPOE is a more effective solvent mediator in preparing the nitrite selective electrode. It should be noted that the nature of the plasticizer influences both the dielectric constant of the membrane and the mobility of the ionophore and its complex. The amount of ionophore CS was also found to affect the sensitivity of the membrane electrodes (Nos. 1, 3 and 4). The sensitivity of the electrode response increases with increasing ionophore content until the value of 5% is reached. Further addition of ionophore will, however, result in diminishing response of the electrode, most probably due to some inhomogenities and possible saturation of the membrane. 34 A large number of chemical sensors, based on solvent polymeric membranes, have been described that require the addition of lipophilic salts for a good working performance. With anion-selective membranes containing an anionic

3 1129 Table 1 No. Optimization of membrane ingredients Composition, % PVC Plasticizer Ionophore Additive Slope Linear range 1 33 NPOE, 62 3 HTAB, NPOE, 61 3 HTAB, NPOE, 60 5 HTAB, NPOE, 59 6 HTAB, NB, 60 5 HTAB, DBP, 60 5 HTAB, DOP, 60 5 HTAB, BA, 60 5 HTAB, Fig. 4 Effect of internal solution on the potential response of NO - selective electrode based on CS concentration (M). Fig. 5 Calibration curve for NO selective electrode based on CS. polymer, such additives induce perm-selectivity. Moreover, the presence of lipophilic cationic sites in anion-selective membrane electrodes is proved to have a beneficial effect on various sensor parameters. In general, the ohmic resistance and detection limit are lower, with less interference from cations at high sample activities, the selectivity and response behavior are improved and, in cases where the ionophore has poor extraction capabilities, the sensitivity of the sensor is enhanced. In addition, cationic sites may catalyze phase-transfer processes in cases when the kinetics at the sample/membrane interface is limited. However, the presence of 2% of HTAB as a cationic additive will improve the sensitivity of the nitrite sensor considerably (No. 3). As can be seen, membrane 3 with a PVC:NPOE:CS:HTAB percent ratio of 33:60:5:2 results in a Nernstian behavior of the membrane electrode over a wide concentration range ( M). The influence of the concentration of the internal solution on the potential response of the proposed sensor was tested. The concentration of NaNO 2 solution was changed from to M and the corresponding EMF versus pno plots are shown in Fig. 4. As can be seen from Fig. 4, the concentration of internal solution has a negligible effect on the potential response of the electrode, except for an expected shift in the intercept of the resulting plots. A M concentration of the filling solution was found quite appropriate for a smooth Nernstian functioning of the sensor. The optimum equilibration time for the proposed sensor in the presence of M sodium nitrite was investigated and found to be 24 h. After this time, the electrode based on CS generates stable potentials, in contact with nitrite ion solutions. The standard deviation of 8 replicate potential measurements was at the most ±0.5 mv. The electrode exhibits linear response to the concentration of nitrite ions in the range of M (Fig. 5). The slope of the calibration graph is 58.2 ± 0.2 mv decade 1. The limit of detection, as Fig. 6 CS. Dynamic response of the nitrite membrane sensor based on determined from the intersection of the two extrapolated segments of the calibration graphs, was M. For investigation of stability and lifetime of the nitrite sensor, two electrodes were tested over a period of 90 days. During this period, the electrodes were in daily use over an extended period of time (2 h per day). Only a slight gradual decrease in the slope (from 59.0 to 57.9 mv decade 1 ) was observed. The dynamic response time of the electrode was measured after successive immersion of the electrode in a series of NO 2 solutions, each having 10-fold difference in concentration. The resulting potentialtime responses for the CS-based sensor obtained, upon changing the NO concentration from to M, are shown in Fig. 6. As can be seen, the time needed to reach the equilibrium value for the sensor is very short (10 s) in the whole concentration range. The ph dependence of the proposed sensor was investigated by measuring the potential response of solutions of varying ph values, over a range of 2.5 to 12.5, in the presence of M of NaNO 2. The ph adjustments were carried out by

4 1130 ANALYTICAL SCIENCES AUGUST 2003, VOL. 19 Table 2 Selectivity coefficient of various interfering ions Ion Selectivity coefficient Fig. 7 The effect of the ph of test solutions on the potential response of the electrode based on CS. successive titration of a 0.1 M sulfuric acid solution with enough amount of a concentrated sodium hydroxide solution. The potential vs. ph plots for the sensor are depicted in Fig. 7. As is obvious, the potential response of the electrode is independent of ph in the ph range of The increased potential of the electrode, at ph < 4.0, is most probably due to the response of the membrane to the H 3O + (by protonation of nitrogen atoms of CS) and the decrease in the concentration of free nitrite (by protonation of NO as a relatively strong base with a pk a of 3.37). The decreased potential at higher ph values (> 9.5) is due to the responses of the sensor to both nitrite and hydroxy ions. Selectivity coefficients, as describing the preference of the proposed electrode for an interfering ion, B relative to nitrite ion A, were determined by the matched potential method (MPM). 35 According to this method, the specified activity (concentration) of the primary ion (A, M) is added to a reference solution ( M, in this case), and the potential is measured. In a separate experiment, interfering ions (B, M) are successively added to an identical reference solution, until the measured potential matched that obtained before adding the primary ions. The matched potential method MPM selectivity coefficient, K A,B, is then given by the resulting primary ion to interfering ion activity (concentration) ratio, MPM K A,B = a A/a B. The resulting values for the nitrite sensor are given in Table 2. The selectivity sequence for a series of anions shown by the electrode is as follows: NO >> ClO 4 > SCN > I > Citrate > NO 3 > SO 4 > Br > Cl HPO 4 > I 3 CO 3 > CH 3COO > Salicylate > F It is interesting to note that this selectivity pattern significantly differs from the so-called Hofmeister selectivity sequence (i.e. selectivity based solely on the lipophilicity of anion). The data given in Table 2 show that, the proposed nitrite sensor is highly selective with respect to other organic and inorganic common anions. In all cases, the selectivity coefficients are in the order of and lower, which seem to indicate that, these anions have negligible impact on the functionality of the NO 2 ion-selective electrodes. Table 3 compared the selectivity coefficients of the best previously reported NO ion-selective electrodes, based on different ionophores, with those obtained for the proposed nitrite membrane sensor based on CS. As can be seen from Table 3, the selectivity coefficients obtained for the proposed electrode are superior to those reported for other nitrite ionselective electrodes. It is noteworthy that, despite the fact that the previously reported electrodes for NO ion 2329 show a close Nernstian EMF vs. concentration behavior, some of them could actually be considered as good selective electrodes for thiocyanate The minimum detection limit for the proposed nitrite sensor is very much lower than the nitrite ion concentration in various real samples such as sausages and milk samples. 46,13,17,18 In order to assess the applicability of the proposed selective electrodes, the method was applied for the determination of nitrite content of different real samples such as sausage and milk. The preparation of each sausage sample was carried out by the previously reported methods. 36 The nitrite content of different samples was then determined with the proposed sensor and modified with regard to AOAC method. 37 The results of three replicate determinations of nitrite ion in different real samples, with both the potentiometric sensors and the standard method, are summarized in Table 4. As can be seen from Table 4, it is immediately obvious that, in all cases, there is a satisfactory agreement between the results obtained by the potentiometry (proposed electrode) and the standard method. References ClO SCN Citrate HPO CO CH 3COO Salicylate SO NO I I Br Cl F T. Shibamoto and L. F. Bjeldanes, Introduction to Food Toxicology, 1993, Academic Press, San Diego. 2. W. Lijinsky and S. S. Epstein, Nature, 1970, 21, P. F. Swann, Proc. Roy. Soc. Med., 1977, 70, K. Horita, G. Wang, and M. Satake, Anal. Chim. Acta, 1997, 350, M. F. Mousavi, A. Jabbari, and S. Nouroozi, Talanta, 1998, 45, G. F. Wang, M. Satake, and K. Horita, Talanta, 1998, 46, A. A. Ensafi and A. Kazemzadeh, Anal. Chim. Acta, 1999, 382, R. T. Masserini and K. A. Fanning, Marine Chem., 2000, 68, H. Sakai, T. Fujiwara, and T. Kumamaru, Anal. Chim. Acta, 1996, 331, M. Novic, B. Divjat, and B. Pihar, J. Chromatogr. A, 1998, 827, I. El Menyawi, S. Looareesuwan, S. Knapp, F. Thalhammer, B. Stoiser, and H. Burgman, J. Chromatogr. B, 1998, 706, M. I. H. Helaleh and T. Korenaga, J. Chromatogr. B, 2000, 744, 433.

5 1131 Table 3 Comparison of selectivity coefficients of different electrodes Ionophore log K Pot SCN ClO 4 Cl Br I Sal Ref. Co(III)-aquocyano-corbyrinate Co(III)phthalocyanine Co(III)(bpy)TPPNO Co(III)TPPXNO 2, X = COOC 6H In(III)TPPXCl, X = OCH Benzylbis(triphenylphosphine)Pd(II)Cl UO 2-Salophen Co(II)-Salen This work Table 4 Determination of nitrite ion in different food samples Sample ISE/µg g 1 Standard method/µg g 1 Sausage 33.0 (±2.1) a 31.1 (±1.2) Milk 5.1 (±0.3) 5.2 (±0.2) a. Measurements based on triplicate experiments. 13. D. Connolly and B. Paul, Anal. Chim. Acta, 2001, 441, Z. Gao, G. Wang, and Z. Zhao, Anal. Chim. Acta, 1990, 230, K. Markusova and M. Fedurco, Anal. Chim. Acta, 1991, 248, M. Bertoti and R. Tokoro, J. Electroanal. Chem., 1993, 360, M. Bertotti and D. Pletcher, Anal. Chim. Acta, 1997, 337, L. Guanghan, J. Hong, and S. Dandan, Food Chem., 1997, 59, M. Badea, A. Amine, G. Palleschi, D. Moscone, G. Volpe, and A. Gurulli, J. Electroanal. Chem., 2001, 509, F. Guan, H. Wu, and Y. Luo, J. Chromatogr. A, 1996, 719, K. Fukushi, K. Tada, S. Takeda, S. Wakida, A. Yamane, K. Higashi, and K. Hiiro, J. Chromatogr. A, 1999, 838, J. E. Melanson and C. A. Lucy, J. Chromatogr. A, 2000, 884, J. Z. Li, X. C. Wu, R. Yuan, H. G. Lin, and R. Q. Yu, Analyst, 1994, 119, D. Gao, J. Gu, R. Q. Yu, and G. D. Zheng, Analyst, 1995, 120, I. H. A. Nadr, M. E. Meyerhoff, and S. S. Hassan, Anal. Chem., 1995, 67, U. Schaller, E. Bakker, U. E. Spichiger, and E. Pretsch, Anal. Chem., 1994, 66, E. Malinowska and M. E. Meyerhoff, Anal. Chim. Acta, 1995, 300, M. M. G. Antonisse, B. H. M. Snellink-Ruël, J. F. J. Engbersen, and D. N. Reinhoudt, J. Chem. Soc., Perkin Trans. 2, 1998, W. Wrpblewski, Z. Brzozka, D. M. Rudkevich, and D. N. Reinhoudt, Sens. Actuators B, 1996, 37, M. R. Ganjali, M. Yousefi, M. Javanbakht, T. Poursaberi, M. Salavati-Niasari, L. H. Babaei, E. Latifi, and M. Shamsipur, Anal. Sci., 2002, 18, M. R. Ganjali, T. Poursaberi, F. Basiripour, M. Salavati- Niasari, M. Yousefi, and M. Shamsipur, Fresenius J. Anal. Chem., 2001, 370, M. R. Ganjali, T. Poursaberi, M. Hosseini, M. Salavati- Niasari, M. Yousefi, and M. Shamsipur, Anal. Sci., 2002, 18, M. R. Ganjali, M. Hosseini, F. Basiripour, M. Javanbakht, O. R. Hashemi, M. Faal Rasteger, M. Shamsipur, and G. W. Buchanan, Anal. Chim. Acta, 2002, 469, X. Yang, N. Kumar, H. Chi, D. B. Hibbert, and P. W. Alexander, Electroanalysis, 1997, 9, Y. Umezawa, K. Umezawa, and H. Sato, Pure Appl. Chem., 1995, 67, N. Jie, D. Yang, Q. Jiang, Q. Zhang, and L. Wei, Microchem. J., 1999, 62, A. Uren and D. Babayigit, Food Chem., 1996, 57, 561.