A tetra-coordinate nickel(II) complex as neutral carrier for nitrate-selective PVC membrane electrode

September 12, 2017 | Autor: Ali Asghari | Categoria: Analytical Chemistry, Nickel
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Talanta 61 (2003) 557 /563 www.elsevier.com/locate/talanta

A tetra-coordinate nickel(II) complex as neutral carrier for nitrate-selective PVC membrane electrode Ali Reza Asghari a, Mohammad Kazem Amini b,*, Hassan Rahimi Mansour a, Masoud Salavati-Niasari c b

a College of Chemistry, Isfahan University of Technology, Isfahan, Iran Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran c Department of Chemistry, University of Kashan, Kashan, Iran

Received 14 December 2002; received in revised form 3 May 2003; accepted 21 May 2003

Abstract The potentiometric response properties and applications of a tetra-coordinate nickel(II) complex with relatively high selectivity toward nitrate ion are described. The nickel(II) complex of 5,7,12,14-tetramethyl-1,4,8,11-tetraazacyclotetradeca-4,6,11,13-tetraene was used as a neutral carrier into plasticized poly(vinyl chloride) (PVC) membrane. The influence of several variables was investigated in order to optimize the potentiometric response and selectivity of the electrode. The resulting membrane electrode incorporating 31.0% PVC, 61.0% dioctyl phthalate (DOP) as plasticizer, 3% methyltrioctylammonium chloride (MTOAC) as a cationic additive and 5% carrier (all w/w) demonstrates a Nernstian response slope of /59.6 mV per decade over the concentration range of 5 /10 6 /1 /10 1 M NO 3 : The electrode exhibits a fast response time ( 5/10 s), a detection limit of 2.5 /10 6 M, and can be used over a wide pH range of 4 /12. The electrode shows improved selectivity in comparison to most of the previously reported nitrate-selective electrodes. It was successfully applied to the determination of nitrate ion in natural water samples. # 2003 Elsevier B.V. All rights reserved. Keywords: Nitrate-selective electrode; Sensors; Tetra-coordinate nickel(II) complex; Potentiometric sensors

1. Introduction The determination of nitrate ion is important for environmental, biological, ecological and agricultural investigations. The need to monitor nitrate in different samples has been recognized * Corresponding author. Tel.: /98-311-793-2708; fax: /98311-668-9732. E-mail address: [email protected] (M.K. Amini).

worldwide, and as such, legislation relating to maximum of its concentration in both drinking water and food products is often levied [1]. In some countries, an upper limit of 45 mg l 1 of nitrate (10 mg l 1 as nitrogen) have been proposed or strictly set for drinking water [2,3]. Nitrite, which can be produced by chemical transformation or biodegradation of nitrate, is an important indication of faecal pollution of natural waters. Nitrite, due to its interaction with blood pigment

0039-9140/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0039-9140(03)00325-4

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to produce methemoglobinalmia, has harmful impacts on human health [2]. The amount of nitrate in various samples such as natural and waste waters, food products, industrial materials, tobacco and industrial effluents, has been determined by different methods [1,4 /20]. Among the different analytical methods developed so far for determination of nitrate, potentiometric detection methods based on ionselective electrodes (ISEs) have emerged as one of the most promising tools for this purpose. This method provides reasonable selectivity over a wide concentration range, fast response time, and can be easily adapted to flowing streams and in situ measurements. These advantages of ISEs, together with their simplicity, low cost and speed of installation and operation, have inevitably led to the development of potentiometric sensors for many inorganic and organic species, and the list of available electrodes has grown substantially over the past years [21]. The purpose of the present work has been the development of a nitrate-selective electrode based on a tetra-coordinate nickel(II) complex, [5,7,12,14-tetramethyl-1,4,8,11-tetraazacyclotetradeca-4,6,11,13-tetraene nickel(II)] (NiL), as the carrier in plasticized PVC membrane. The electrochemical selectivity for a variety of anions and the effect of membrane matrix, ionophore concentration and pH on the potentiometric response properties of the electrode were investigated. The electrode was used for the determination of nitrate in natural water samples and the results were compared the well known brucine spectrophotometric method [5].

further purification, except tetrahydrofuran (THF), which was distilled before use. Standard nitrate solution (0.1 M) was prepared by careful weighing solid sodium nitrate, which had been previously oven-dried at 125 8C for several h. Standard solutions of all other anions were prepared from their respective sodium or potassium salts as 0.1 M stock solutions. The pH adjustments were made with dilute sulfuric acid or sodium hydroxide solutions as required. The conditioning solution was prepared according to a previously reported method [14], by mixing equal volumes of 1 M (NH4)2SO4, solution as an ionic strength adjuster, and an interference suppressor solution containing 1.5 /102 M Ag2SO4, 2.5 / 102 M Al2(SO4)3 ×/18H2O and 2.0 /102 M H3BO3. The pH of this solution was adjusted to 5 with NaOH solution. The tetra-coordinate nickel(II) carrier was synthesized and purified according to a previously reported procedure [22]. The structure of the carrier is shown in Fig. 1. 2.2. Electrode preparation The PVC membrane solution was prepared by thorough mixing 5 mg of the carrier (NiL), 61 mg plasticizer (DOP or DBP), 31 mg PVC, 3 mg additive (MTOAC), total mass 100 mg, in 10 ml THF. The resulting mixture was allowed to evaporate slowly until an oily concentrated mixture was obtained. A Pyrex tube (ca. 3 mm id.) was dipped into the mixture for a few seconds so that a non-transparent membrane of about 0.3 mm thickness was formed at its end. The tube was then pulled out from the mixture and kept at room

2. Experimental 2.1. Reagents and solutions Poly(vinyl chloride) (PVC) of high relative molecular weight was purchased from Fluka. Dibutyl phthalate (DBP), MTOAC, DOP and all other chemicals were from Merck Co. All solutions were prepared with distilled, deionized water and analytical reagent grade chemicals without

Fig. 1. Structure of the nickel(II) complex used as carrier for the nitrate selective electrode.

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temperature over night. The tube was then filled with internal filling solution (1 /102 M NaNO3). The electrode was finally conditioned for 24 h by soaking it in a 0.01 M sodium nitrate solution. A silver /silver chloride electrode was used as the internal reference electrode.

2.3. Apparatus Potentials were measured with a Schott model CG-825 pH/mV meter. A saturated Ag/AgCl electrode was used as the reference electrode. The pH of the sample solutions was monitored with a conventional glass pH electrode. Spectrophotometric measurements were carried out on a JASCO model V-570 spectrophotometer.

2.4. Potential measurements and calibration The PVC-based membrane electrode containing the tetra-coordinate nickel(II) complex was used as the measuring electrode in conjunction with a silver /silver chloride reference electrode. All potential measurements were performed at ambient temperature (229/1 8C) using galvanic cell of the following type: Ag/AgCl/KCl (satd.) j internal filling solution (1 /103 M NaNO3) j PVC membrane j test solution jj KCl (satd.)/AgCl/Ag. The performance of each electrode was investigated by measuring its potential in NO 3 solutions prepared in the concentration range 1/107 /1/ 101 M by serial dilution of the stock solution at constant pH. The solutions were stirred and potential readings recorded when they reached steady state values. The data were plotted as observed potential against the logarithm of the NO 3 concentration. Activities were calculated by the Debye /Hu¨ckel procedure [23]. Potentiometric selectivity coefficients (KNO3 ;j ) were determined according to the fixed interference method (FIM) [24] using 1/103 solutions of interfering ions, and also by the separate solution method (SSM) [24,25] using 1/102, 1 /103 and 1 /104 M solutions of nitrate and interfering ions.

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3. Results and discussion The membrane electrodes based on the tetracoordinate nickel(II) complex as the carrier in plasticized PVC membrane was found to be highly responsive to nitrate ion relative to several other anions. We therefore studied in detail the performance of the electrodes for nitrate ion. Preliminary experiments were conducted to determine the optimum composition of the membrane. The optimized membrane was then used to study its response characteristics. 3.1. Influence of membrane composition Several membrane compositions were investigated by varying the proportions of the polymer (PVC), plasticizer (DBP or DOP), membrane active material (NiL) and membrane additive (MTOAC). It was observed that the potentiometric response of the electrodes toward nitrate ion depended on the concentration of the carrier incorporated within the membrane; 5% NiL was found to be the optimum concentration of the carrier. The blank membrane, containing PVC, plasticizer and MTOAC, exhibited a weak anionic response. The potentiometric response of the membrane was greatly improved by the presence of the lipophilic cationic additive MTOAC. The membrane with no cationic additive exhibited poor sensitivity. Better response characteristics, i.e. Nernstian slope (/59.6 mV per decade of nitrate) and improved sensitivity, were usually observed with a MTOAC/carrier weight ratio of 0.6, which corresponds to a mole ratio of 45%. It is known that lipophilic salts not only reduce the membrane resistance but also enhance the response behavior and selectivity, and reduce interference from sample anions [26,27]. Among the different compositions studied, responses were best for the membrane incorporating 31.0 PVC, 5% NiL, 3.0% MTOAC and 61% DOP. This composition was, therefore, used to study the performance characteristics of the electrode, viz. working concentration range, selectivity, lifetime, response time, and the effect of pH. The characteristic properties of the optimized membrane are summarized in Table 1.

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The influence of the measuring solution pH on the potentiometric response of the membrane electrode was examined at 1 /104 and 1/ 103 M NO 3 concentrations; pH was adjusted with dilute sulfuric acid and sodium hydroxide solutions as required. As can be seen in Fig. 2, the potentiometric response of the electrode is insensitive to pH changes in the range of 4 /12. The pH range extends down to 3.5 with 1 /103 M NO 3 : The working pH range, over which the electrode can be used, covers the pH of most natural and industrial waters. The potential response of the membrane electrode to varying concentrations of NO was 3 examined over the concentration range 1/

107 /1/101 M using the optimized membrane composition and conditions described above. A typical calibration plot is shown in Fig. 3, which depicts a linear range from 5 /106 to 1 /101 M with a Nernstian slope of /59.6 mV per decade. The practical limit of detection, taken as the concentration of NO 3 at the point of intersection of the extrapolated linear segments of the calibration plot, was 2.5 /10 6 M. The performance of the electrode was checked by changing the nitrate concentration of the internal filling solution of the cell given in the experimental section as 1 /10 1, 1 /10 2 and 1/103 M. The sensitivity of the electrode response was almost the same in all cases. Therefore, the nitrate concentration of the internal filling solution was kept constant at 1 /102 M. The optimum equilibration time for the electrode was found to be /24 h. The response time of the electrode was measured after successive immersion of the electrode in 4 a series of NO 3 solutions ranging from 1 /10 1 to 1/10 M. The average response time thus obtained for the electrode to reach a potential within 9/1 mV of the final equilibrium value was 5/10 s over the entire concentration range. The sensing behavior of the electrode did not depend on whether the potentials were recorded from low to high or vice versa. Repeated monitoring of potentials (12 measurements) on the same portion of the sample at 1/103 M NO 3 resulted in a standard deviation of 9/1 mV. The electrode was

Fig. 2. The influence of pH on the potential response of the nitrate-selective electrode.

Fig. 3. Potentiometric response of the membrane based on the tetra-coordinate nickel(II) complex toward nitrate ion.

Table 1 Specifications of the nitrate-selective electrode based on the tetra-coordinate nickel(II) carrier Properties

Values and/or range

Optimized membrane composition

PVC (31.0%), DOP (61.0), carrier

Useful pH range Linear range Slope Detection limit Standard deviation Response time Lifetime

(NiL) (5%), MTOAC (3%) 4 /12 5/10 6 /1/10 1 /59.6 2.5/10 6 1 mV at 1/10 3 M 5/10 s At least 2 months

3.2. Response characteristics of the electrode

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tested over a period of 2 months to investigate its stability. During this period, the electrode was in daily use and was stored in 1 /102 M NaNO3 solution. No significant change in the performance of the electrode was observed during this period and the parameters, such as the slope, working range and response time, of the electrode were found to be reproducible. 3.3. Selectivity of the electrode The potentiometric selectivity coefficients (KNO3 ;j ) of the nitrate-selective electrode were determined by the FIM method [24] from potential measurements of solutions prepared with a fixed concentration of the interfering ions and varying concentrations of NO and also by the SSM 3 method [24,25] using 1 /102, 1/103 and 1/ 104 M solutions of nitrate and interfering ions. The potentiometric selectivity coefficients of the proposed electrode are summarized in Table 2. As it is evident from the data in Table 2, the electrode based on NiL is more selective to nitrate than the other anions studied, except perchlorate ion. The

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selectivity pattern of the electrode for several anions is as follows: perchlorate /nitrate / iodide /salicylate /chlorate /thiocyanate / bromide/azide /nitrite /bromate/cyanide / chloride /iodate /acetate /sulfite /sulfate / fluoride. The reason for selection of these ions was that many of them might be present in the media when nitrate is present (in drinking water, fertilizers, fountain waters, waste waters, industrial effluents, soils, etc.). The electrode demonstrates a significant deviation in selectivity from the Hofmeister series [28,29], large lipophilic anions /perchlorate /thiocyanate /iodide / nitrate /bromide /azide /nitrite /chloride / acetate /sulfate. The reason that the selectivity coefficients do not comply with the Hofmeister series can be due to specific interaction of the anions with the metal center in the carrier (NiL) used in this study, i.e. chemical recognition of the anions, and especially of nitrate, by the complex. There are currently several commercial nitrateselective electrodes that are based on ion-exchangers [30,31], their selectivities of which follow the Hofmeister pattern. These electrodes, together

Table 2 Comparison of the selectivity coefficients of the proposed electrode with several commercial electrodes Anion

Br  CH3COO   /BrO 3 Cl   /ClO 3 CN  F I  /IO 3  /N 3  /NO 2 Salicylate SCN  2 /SO 3 2 /SO 4  /ClO 4  /HCO 3  /IO 4

Electrode NiL (FIM) 1/10 3 M

NiL (SSM) 1/10 2 M

NiL (SSM) 1/10 3 M

NiL (SSM) 1/10 4 M

Orion 92-07

Orion 93-07

Corning 476134

Beckman 39618

2.3/10 2 2.3/10 3 9.3/10 3 5.8/10 3 1.1/10 1 7.1/10 3 6.3/10 5 2.0/10 1 3.5/10 3 1.8/10 2 1.0/10 2 1.2/10 1 2.9/10 2 1.4/10 4 1.2/10 4 2.5 2.5/10 3 /

2.9/10 2 2.6/10 3 5.2/10 3 4.8/10 3 1.4/10 1 7.2/10 3 1.8/10 4 1.3/10 1 4.9/10 3 2.9/10 2 1.3/10 2 1.4/10 1 1.9/10 2 1.2/10 3 1.0/10 3 3.3 2.0/10 3 5.4/10 3

3.2 /10 2 3.0 /10 3 4.5 /10 3 4.7 /10 3 1.4 /10 1 8.8 /10 3 1.7 /10 4 1.4 /10 1 4.6 /10 3 2.6 /10 2 1.3 /10 2 1.5 /10 1 2.0 /10 2 1.1 /10 3 9.0 /10 4 3.4 2.3 /10 3 6.0 /10 3

4.4/10 2 4.2/10 3 3.2/10 3 2.8/10 3 8.7/10 2 1.0/10 2 1.0/10 4 9.5/10 2 4.0/10 3 1.4/10 2 1.0/10 2 1.9/10 1 2.8/10 2 7.2/10 4 4.7/10 4 2.5 1.4/10 3 8.1/10 3

9/10 1 1.9/10 3 / 6/10 3 / / 6/10 5 20 / / 6/10 2 / / 6/10 3 / 1000 /

1.3/10 1 6/10 3 / 5.6/10 3 / / 1.9/10 3 14.1 / / 5/10 2 / / / / / /

1/10 1 / / 4/10 3 / / / 25 / / / / / / / /1000 /

2.8 /10 1 6 /10 3 / 1 /10 2 / / 6.6 /10 3 5.6 / / 6.6 /10 2 / / / / 100 /

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with a number of recently reported nitrate-selective electrodes [4,16] generally suffer from relatively strong interferences from lipophilic anions such as perchlorate, thiocyanate and iodide. The selectivity coefficients of several nitrate-selective electrodes are compared with those of our electrode in Table 2. Many of the reported nitrate selective electrodes and commercial ones respond to perchlorate, iodide and thiocyanate rather than nitrate. Although perchlorate is the most interfering anion with the proposed electrode, its selectivity coefficient with respect to perchlorate (KNO3 ;ClO4 2:5) is about two orders of magnitude smaller than the values reported for all of the electrodes given in Table 2, except that of the electrode based on polypyrrole [15]. The selectivity of the electrode is considerably improved with respect to the other highly lipophilic anions studied, such as iodide and thiocyanate. The electrode also exhibits better selectivity toward nitrate with respect to bromide, fluoride, nitrite and sulfate. In addition, the electrode shows good selectivity over several other anions that have not been reported for the other electrodes, such as bromate, chlorate, cyanide, iodate and azide. Possible interference from common anions such as Cl , I , and Br  can be removed by adding silver sulfate, and that of carbonate and hydrogen carbonate by acidifying to pH of about 4 and then adjusting the pH to the desired value with a buffer and nitrate ionic strength adjuster solution. 3.4. Analytical applications The proposed electrode was applied to determine nitrate in several water samples using the standard addition method. The solutions were conditioned by adding 2 ml of conditioning solution described in the experimental section and filtered if necessary. The obtained results were compared with those obtained by the brucine spectrophotometric method [5] (Table 3). The quality of the results was evaluated by performing a recovery test, spiking the drinking water sample with standard nitrate solution. The recoveries for 5.0, 7.0 and 9.0 mg ml1 of nitrate added to the sample were 98.2, 99.0 and 102.5%, respectively. The results indicate that the proposed electrode

Table 3 Determination of nitrate in water samples obtained from different sources. Comparison of the potentiometric results with an independent spectroscopic method Water sample

Concentration of nitrate (mg ml 1) Potentiometry Spectrophotometry

Tap water River (Zayandeh rood) Drinking water (Dimeh, Kohrang) Drinking water (Kandovan)

11.29/0.3 18.89/0.5 7.809/0.3

11.59/0.4 18.49/0.4 7.619/0.2

4.139/0.2

4.259/0.1

can be successfully applied to the determination of nitrate at concentrations normally present in some water samples.

4. Conclusions The PVC membrane incorporating the tetracoordinate nickel(II) complex as a neutral carrier has been shown to be useful as a nitrate selective electrode. The electrode is more selective toward nitrate in contrast to several commercial and a number of the recently reported electrodes. The electrode can be used over a wide pH range, 4/12, which makes it useful for measurements in different samples and in alkaline conditions. The electrode shows good response characteristics (sensitivity, stability, life time and response time).

Acknowledgements The authors express their appreciation to the Isfahan University of Technology Research Council for the support of this work.

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