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Journal of the Chilean Chemical Society

versão On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.52 n.4 Concepción  2007

http://dx.doi.org/10.4067/S0717-97072007000400015 

 

J. Chil. Chem. Soc, 52, N° 4 (2007), págs: 1332-1337

 

FABRICATION OF A COBALT(II) PVC-MEMBRANE SENSOR BASED ON N-(ANTIPYRIDYNIL)-N'-(2-METHOXYPHENYL)THIOUREA

 

HASSAN ALIZAMANIA* , MOHAMMAD REZA GANJALI,B,C PARVIZ NOROUZIB,C, AZADEH TAJARODID , YOUNES HANIFEHPOURD,

a Department of Applied Chemistry, Quchan branch, Islamic Azad University, Quchan, Iran
b Center of Excellence in Electrochemistry, Facuty of Chemistry, University of Tehran, Tehran, Iran
c Endocrine & Metabolism Research Center, Tehran University of Medical Sciences, Tehran, Iran
d Faculty of Chemistry, University of Science and Technology, Tehran, Iran


ABSTRACT

The N-(Antipyridynil)-N'-(2-methoxyphenyl)thiourea (NTU) was used as an excellent ionophore in the construction of a Co(II) PVC-based membrane sensor. This sensor illustrated very good selectivity and sensitivity towards cobalt ions over a wide variety of cations, including alkali, alkaline earth, transition and heavy metal ions. The sensor revealed a great enhancement in the selectivity coefficients for cobalt ions, in comparison with the formerly reported cobalt sensors. The developed sensor exhibited a Nernstian behavior (with a slope of 29.4 + 0.5 mV per decade) for a broad range (6.8 x10-7—1.0 x10-1M) with a detection limit of 2.0 x10-7M. It demonstrated a relatively fast response time in the whole concentration range (<10 s) and its usage exceeded a 10 week period in the pH range of 2.7-8.3. The sensor applications were found to be successful in the direct determination of cobalt ions in wastewaters of industrial cobalt electroplating companies as well as an indicator electrode in the titration with EDTA.

Keywords: N-(Antipyridynil)-N'-(2-methoxyphenyl)thiourea, Cobalt sensor, Potentiometry, PVC


INTRODUCTION

Cobalt is a hard, lustrous, silver-gray metal which is found in various ores, and is used in the preparation of magnetic, wear-resistant, and high-strength alloys. Its compounds are used in the production of inks, paints, and varnishes. It is frequently associated with nickel, and bom are characteristic ingredients of meteoric iron. Mammals require small amounts of cobalt salts. Cobalt-60, an artificially produced radioactive isotope of cobalt, is an important radioactive tracer and cancer-treatment agent. Some of its applications are: Making of superalloys, for parts in gas turbine aircraft engines, Corrosion- and wear-resistant alloys, High speed steels, Cemented carbides (also called hard metals) and diamond tools, Magnets and magnetic recording media, Alnico magnets, Catalysts for the petroleum and chemical industries, electroplating because of its appearance, hardness, and resistance to oxidation, Drying agents for paints, varnishes, and inks, ground coats for porcelain enamels, pigments (cobalt blue and cobalt green), Battery electrodes.

Cobalt in small amounts is essential to many living organisms, including humans. Having 0.13 to 0.30 mg/kg of cobalt in soils markedly improves the hearth of grazing animals. Cobalt is a central component of the vitamin cobalamin, or vitamin B-12.

Many techniques have been developed for determination of cobalt such as: solid phaseextraction and determination (1-4), Chromo tropic acid-ftinctionalized (5), polarography (6), Derivative spectrophotometric determination (7, 8), determination of Co in flow systems (9-12), On-line redox derivatization liquid chromatography (13), Simultaneous determination of cobalt using partial least square regression (14), Flame atomic absorption determination (15, 16), stripping techniques (17-22), determination of Co by energy dispersive X-ray fluorescence spectrometry (23), high-performance liquid chromatography (24), Cyclam modified carbon paste electrode as a potentiometric sensor for determination of cobalt(II) ions (25), spiro fluorescein hydrazide (26), Second derivative spectrophotometric method for simultaneous (27).

Almost all of these techniques are very expensive and time consuming. In this report we are going to introduce a simple fast and sensitive sensor for potentiometric determination of cobalt.

Recently, several greatly selective and sensitive PVC-membrane ion-selective electrodes for various metal ions have been reported (28-40). Nevertheless, this paper focuses on the introduction of a highly cobalt(II)-selective sensor based on N-(Antipyridynil)-N'-(2-methoxyphenyl)thiourea (NTU) (Fig. 1), as a novel neutral ionophore for monitoring cobalt concentration in industrial samples.


EXPERIMENTAL

Reagent

The ionophore N-(Antipyridynil)-N'-(2-methoxyphenyl)thiourea (NTU) was prepared as formerly described (41). The Merck Chemical and the Aldrich Co. were the suppliers for the nitrate and chloride salts of all cations. In addition, the reagent grades of phthalate (DBP), nitrobenzene (NB), benzyl acetate (BA), tetrahydrofiiran (THF), sodium tetraphenyl borate (NaTPB) and high relative molecular weight PVC were purchased from Merck. All reagents were used without any modification. Regarding the nitrate and chloride salts of all employed cations, they were of the highest available purity and were P205- vacuum dried. During the experiments, triply distilled deionized water was used.

Electrode Preparation

In the beginning, 30 mg of PVC, 2.5 mg of NTU, 3 mg of NaTPB and 64.5 mg of NB were totally mixed. Then, the mixture was dissolved in 3 mL of dry freshly distilled THF. The resulting clear mixture was evaporated slowly up to the point that an oily concentrated mixture was created. Afterwards, a Pyrex tube (5 mm i.d.) was dipped into the mixture for about 5 s, resulting in the formation of a transparent membrane, about 0.3 mm in thickness. In the end, the tube removed from the mixture, kept at room temperature for about 2 h (42-46) and filled with the internal solution (1.0x 10-3 M CoCI2). The electrode was conditioned for 24 h by soaking in a solution, containing 1 .0 x 10-3 M cobalt nitrate solution. A silver/silver chloride coated wire was used as an internal reference electrode.

The emf measurements

The assembly for the emf (electromotive force) measurements included, on the one hand, an Ag-AgCl | internal solution, 1.0 x 10-3M CoCl2 | PVC membrane | sample solution | Hg-Hg2Cl2, KC1 (satd.) and, on the other hand, a Corning ion analyzer with a 250 pH/mV meter for the potential measurements at 25.0 °C.

The activities were calculated according to the Debye-Huckel procedure (47).

The compilation study procedure

Conductivity measurements were carried out with a Metrohm 660 conductivity meter. A dip-type conductivity cell, made of black platinum with a cell constant of 0.83 cm-1, was used. In all measurements, the cell was thermostated at 25.0 °C using a Phywe immersion thermostat. In typical experiments, 25 mL of a cation nitrate solution (1.0 x 10-4 M) was placed in a water jacketed cell, equipped with a magnetic stirrer and connected to a thermostat circulating water at the desired temperature. In order to keep the electrolyte concentration constant during the titration, both the starting solution and the titrant had the same cation concentration. Then, a known amount of the NTU (1.0 x 10-2 M) solution was added in a stepwise manner with the aid of a calibrated micropipette. The conductance of the solution was measured after each addition. The NTU addition was continued until the desired NTU-to-cation mole ratio was achieved. The 1:1 binding of the cations with NTU can be expressed by the following equilibrium:

The corresponding equilibrium constant, Kp is given by

where [MLn+], [M1+] and ƒ represent the equilibrium molar concentration of complexes, free creation, free NTU and the activity coefficient of the species indicated, respectively. Under the used dilution condition, the activity coeffinient of uncharged ƒ(L) can be reasonably assumed as unity (49). The use of Debye-Hückel limiting law of 1:1 electrolytes (49) leads:

coefficient in the equiation (2) is canceled out.

Therefore, the complex formation constant in terms of the molar concentration is expresed as follow:

The observed conductance in each point of the titration curve can be calculated by sum of free and complexed cations and anions conductance:

The molar conductances are written as follow:

Here ΛM is the molar conductance of the cation before the NTU addition.

 

ΛML is the molar conductance of the complex.

The total analytical concentration of M can be written as follow:

And therefore.

Λobs is the molar conductance of the solution during titration. CM is the analytical concentration of the cation salt. By inserting K from equations 4-6:

And for the ligand:

CL is the analytical concentration of the added NTU and Thus,

By dividing the equation by CM and put the CM from equation 11, we have:

By incorporating the equation 11 and 12:

Positive answer of the above equation gives the [L] value. Amounts of CM and CL in each point of titration curve are known. By applying a known amount of Kp a value for [L] will be obtained in each point. Since amount of ΛM molar conductance of the free cation in absence of ligand, is known; by using obtained [L] and ΛML in equation 14, a calculated value for Λ so-called Λcalc will be obtained in each point.

The complex formation constant, Kp will be obtained by a non-liner computer fitting of the last equations, using a nonlinear least-squares Gaussian-Nioton algorithm (KINFIT program) (51,52).

RESULTS AND DISCUSSION

The NTU complexation with some cations in acetonitrile

In primary experiments, the NTU interaction with numerous metal ions was investigated in acetonitrile solution by the conductometric method. The results showed that in all cases the ligand to cation mole ratio is 1. The formation constants (Kf) of the resulting 1:1 complexes were evaluated by computer fitting the molar conductance-mole ratio data to the appropriate equations (Table 1). The obtained formation constants revealed that NTU could be used as an excellent ion carrier for the construction of a selective Co(II) membrane sensor.


Response of the NTU-based sensors to Co (II) ions

In the next experiment, NTU was used as a neutral ionophore to prepare a great deal of membrane electrodes for some metal ions. Their potential responses were measured and the resulting data are shown in Figures 2. It can be seen that the NTU-based membrane displays a Nernstian response to the Co(II) ions concentration in a wide concentration range.


The membrane composition effect on the potential response of the NTU-based Co (II) sensor

The NTU-PVC-based membrane sensor generated stable potential response in aqueous solutions, containing cobalt ions, after conditioning for about 12 h in a 1.0 x 10-2 M cobalt nitrate solution. Table 2 summarizes the data obtained with membranes having various ratios of different constituents. The potential responses of all of the membrane sensors were studied in a broad concentration range of cobalt nitrate solutions. The same Table indicates that the total potentiometric electrode response towards Co(II) ions is dependent on the NTU concentration (incorporated within the membrane). As a matter of fact, the increase of the NTU amount up to 2.5 % resulted in the membranes (No. 3 and 4) that display larger slopes. However, due to the large size of the ionophore, the increase in amount of it causes inhomogeneity in the membrane and drops off of the Nernstian slope (No. 4 and 5). A maximum slope of 29.4 ± 0.5 mV per decade of Co(II) concentration was observed for the No. 8 membrane with 2.5 % of NTU.


Generally speaking, the presence of lipophilic anions in a cation-selective membrane electrode diminishes the ohmic resistance, enhances the response behavior and selectivity and increases the sensitivity of the membrane electrodes (53-58). Ionic additives are ionic exchangers, which themselves induce a selective response when no or only an insufficient ionophore amount is present. Therefore, their concentration must be adjusted carefully. In line with Table 2, the sensor slope in the NaTPB absence is lower than the expected Nernstian value (membranes nos. 1-5). All the same, NaTPB addition of 2-3 % will increase the sensitivity of the electrode response considerably, so that the membrane electrode demonstrates a near Nernstian behavior (membrane no. 8). Furthermore, NaTPB addition more than 3 % (NaTPB of 4 %) to the membrane causes a slope decrease from 29.4 to 27.6 mV per decade. This phenomenon is as caused by the mole ratio NaTPB/NTU increase more than one.

However, the membrane sensor with the composition of 30 % PVC, 64.5 % NB, 3 % NaTPB and 2.5 % NTU displays the best performance.

Calibration graph and statistical data

The optimum responses of the NTU-based sensors were estimated, after conditioning the membranes with the same composition in 1.0 x 10-2 M cobalt nitrate solution for different time periods. The 12 h conditioning slope was closer to the theoretically expected slopes, having as a basis the Nernst equation. Longer conditioning times produced no further improvements in the response. The optimum conditioning solution had a concentration of about 1.0 x 10-2M.

The potential response of the Co(II) PVC-based membrane sensor at varying concentrations of cobalt nitrate (Fig. 3) indicates a linear working concentration range from 6.8 x 10-7 to 10 x 10-1 M. The results may be summarized as follows: the slope of the calibration graph was 29.4 ± 0.5 mV per decade of cobalt ions concentration; the detection limit of the sensor, as determined from the intersection of the two extrapolated segments of the calibration graph, was 2.0 x 10-7 M; the standard deviation for ten replicate measurements was ± 0.4 mV.


Life-time study

To evaluate the stability and the lifetime of the presented membrane sensor, four same electrodes were chosen and tested for a period of 12 weeks. During this period, the electrodes were used extensively (one hour per day). After 10 weeks, a slight gradual decrease in the slopes (from 29.4 to 27.2 mV per decade) was observed.

The pH effect of the solution on the sensor response

Across the pH range of 2.0 — 10.0 and at two specific cobalt ion concentrations (1.0 x 10-2 M and 1.0 x 10-3 M), the pH dependence of the membrane electrode was assessed. The associated results are depicted in Figure 4. As it can be seen, the potential remains fairly constant in the pH range of 2.7 - 8.3 (the pH adjustment of the solutions was performed either by HN03 or NaOH). Beyond this range, a gradual potential change was detected. The observed potential drift at higher pH values could be due to the formation of some Co(II) hydroxyl complexes and the insoluble cobalt hydroxide. In both cases, the free Co(II) concentration in the solution reduces. At pH values lower than 2.7, the potentials increase, indicating that the membrane sensor responds to the hydrogen ions protonation of nitrogen atom in the NTU structure.


Dynamic response time of the Co(II) sensor

For any ion-selective electrode, dynamic response time consists of an essential factor. In this study, the practical response time was recorded by changing the cobalt ion concentration in the solution from 1.0 x 10-6 to 1.0 x 10-1 M. The results in Figure 5 illustrate that the electrode reaches rapidly its equilibrium response in the whole concentration range (<10 s).


The sensor selectivity

The potentiometric selectivity coefficients of the sensor were determined by the matched potential method (59-64). In agreement with this method, a specified activity (concentration) of primary ions (A, 5.0 x 10-5 M of cobalt ions) is added to a reference solution (1.0 x 10-6 M of cobalt ion) and the potential is measured. In a separate experiment, interfering ions (B, 1.0 x 10-1 B) are successively added to an identical reference solution, until the measured potential matches the one obtained before the primary ions addition. The matched potential method selectivity coefficient, KMPM, is then given by the resulting primary ion to the interfering ion activity (concentration) ratio, KMPM = aA/aB.

The resulting potentiometric selectivity coefficients values are summarized in Table 3. These data revealed that the recommended Co(II) membrane sensor is highly selective in comparison with the most transition and heavy metal ions. The surprisingly high selectivity of the membrane electrode for cobalt ions over the other used cations, most probably arises from the strong tendency of the carrier molecules for cobalt ions.


Table 4 compares the selectivity coefficients, detection limit and dynamic linearity range of the presented sensor with the best formerly mentioned Co(II) sensors (65-68). Evidently, the specific sensor not only in terms of selectivity but also in terms of detection limit and linearity range is superior to the other reported Co(II) sensors.


Analytical application

The proposed Co(II) membrane sensor was found to work well under laboratory conditions. It was effectively applied to the titration of 25.0 mL of a 1.0 x 10-4 M cobalt solution with a 1.0 x 10-2 M EDTA solution. The titration curve in Figure 6 demonstrates that the Co2+ amount in the solution can be determined with good accuracy.


The developed electrode was also used for the cobalt detection in a wastewater industrial cobalt electroplating sample. The results (Table 5), obtained with the sensor and those of atomic absorption spectrometric (AAS) analysis, were close enough to reach the conclusion that the recommended sensor could be used in the environmental monitoring of cobalt ions.


CONCLUSION

The use of the N-(Antipyridynil)-N'-(2-methoxyphenyl)thiourea (NTU) with NB as plasticizer shows the best response characteristics with Nernstian behavior over the concentration range of 6.8 x 10-7-1.0 x 10-1 M Co2+, with very low interference from common alkali, alkaline earth, transition and heavy metal ions and a fast response time of 10 s. The proposed sensor potential responses are independent of pH in the range 2.7-8.3. The constructed Co(II) PVC membrane sensor was successfully applied as indicator electrode in titration of cobalt ion with EDTA as well as to the cobalt ion detection in a wastewater industrial cobalt electroplating sample.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support of this research proposal from the Research Council of the Quchan Islami Azad University.

 

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