Novel gadolinium poly(vinyl chloride) membrane ...

Analytica Chimica Acta 495 (2003) 51–59

Novel gadolinium poly(vinyl chloride) membrane sensor based on a new S–N Schiff’s base Mohammad Reza Ganjali a,∗ , Mahdi Emami a , Morteza Rezapour a , Mojtaba Shamsipur b , Bozorgmehr Maddah b , Masoud Salavati-Niasari c , Morteza Hosseini d , Zahra Talebpoui d a

Department of Chemistry, Tehran University, P.O. Box 14155 6455, Tehran, Iran b Department of Chemistry, Razi University, Kermanshah, Iran c Department of Chemistry, Kashan University, Kashan, Iran d Department of Chemistry, Tarbiat Modarres University, Tehran, Iran

Received 17 February 2003; received in revised form 4 July 2003; accepted 21 July 2003

Abstract In this work, a novel gadolinium membrane sensor based on new S–N Schiff, s base (2-[{3-[(2-sulfanylphenyl)imino)-1methylbutylidene}amino]phenyl hydrosulfide (SMPH) is presented. The sensor displays a linear dynamic range between 1.0 × 10−1 and 1.0 × 10−5 M, with a nice Nernstian slope of 19.8 ± 0.3 mV per decade and a detection limit of 3.0 × 10−6 M. The best performance was obtained with a membrane composition of 33% poly(vinyl chloride), 61% benzyl acetate, 2% sodium tetraphenyl borate and 5% SMPH. The potentiometric response of the sensor is independent of the pH of the solution in the pH range of 4.0–8.0. The sensor possesses the advantages of short conditioning time, very fast response time, and especially, very good selectivity towards a large number of cations, such as Sm(III), Ce(III), La(III), Cu(II), Pb(II) and Hg(II). The electrode can be used for at least 9 weeks without any considerable divergence in the potentials. It was used as an indicator electrode in potentiometric titration of Gd(III) ions with EDTA, and recovery of Gd(III) from various binary mixtures. The electrode was also applied to the determination of Gd(III) in a urine sample. © 2003 Published by Elsevier B.V. Keywords: Potentiometry; Gd(III) ion-selective electrode; PVC membrane; S–N Schiff’s base

1. Introduction Gadolinium and other rare earth elements are used as gasoline-cracking catalysts, polishing compounds, carbon arcs, and in the iron and steel industries to remove sulfur, carbon, and other electronegative elements from iron and steel [1]. In nuclear research, the rare earths are usually used in the form of oxides. An important application of gadolinium, because of ∗ Corresponding author. Fax: +98-216495291. E-mail address: [email protected] (M.R. Ganjali).

0003-2670/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/S0003-2670(03)00921-8

its extremely large nuclear cross-section, is as an absorber of neutrons for regulating the control level and criticality of nuclear reactors. The nuclear poisons disintegrate as the reactivity of the reactors decrease, in the electronic and magnetic areas [2]. One the most important rare earths compound is gadolinium gallium garnet (GGG). GGG is used in bubble devices for memory storage [2]. Thus, because of the increase in the industrial use of gadolinium compounds as well as their enhanced discharge, determination of gadolinium has recently been of an increasing concern. Spectrophotometry,

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M.R. Ganjali et al. / Analytica Chimica Acta 495 (2003) 51–59

Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS), and Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) are the available methods used for low-level determination of rare earths. Isotope dilution mass spectrometry, neutron activation analysis, X-ray fluorescence spectrometry, etc. are also used in some laboratories [3–8]. These methods are either time consuming, involve multiple sample manipulations, or are too expensive for most analytical laboratories. Potentiometric sensors have been shown to be very effective tools for analysis of a wide variety of cations and anions. They are very simple to use, inexpensive, and capable of reliable responses in a wide concentration range. Due to the vital importance of Gd(III) in industry, and the urgent need for a Gd(III)-selective electrode for potentiometric monitoring of Gd(III), we were interested in the preparation of a highly selective and sensitive sensor for determination of Gd(III). We have recently reported a number of sensors for lanthanides ions, such as Ce(III), Yb(III) and La(III) [9–12]. In this work, we report a novel Gd(III) membrane sensor base on a new S–N Schiff’s base. It is noteworthy that, to the best of our knowledge, this is the first gadolinium ion-selective electrode with a Nernstian response ever reported in the literature, while three reported Gd(III) sensors by Chinese researchers show slopes of 60 mV per decade [13–15].

CH 3

H3C N

N

S

S

H

H

Fig. 1. Structure of SMPH.

tom flask, and was stirred using a magnetic stirrer. Acetyl acetone (1.01 g, 0.01 mol) dissolved in a 50 ml of distilled ethanol was dropwise added to the solution, using a dropping funnel, to the above solution. The contents were refluxed for 2 h to get a red solution. On cooling, a red microcrystalline Schiff base (Fig. 1) was collected by filtration, washed with ethanol and dried in air. Anal. Calcd. For C17 H18 N2 S2 : C, 64.93; H, 5.77; N, 8.91; S, 20.39%. Found: C, 64.73; H, 5.84; N, 9.04; S, 20.28%. NMR: δ = 2.02 (CH3 ); 2.98 (CH2 ), 7.17 (phenyl), 3.17 (SH). 2.3. Electrode preparation

Reagent grade benzyl acetate (BA), dibutyl phthalate (DBP), sodium tetraphenylborate (TPB), tetrahydrofuran (THF), and high relative molecular weight PVC (all from Merck) were used as received. The nitrate and chloride salts of all cations used (all from Merck) were of the highest purity available and used without any further purification except for vacuum drying over P2 O5 . Triply distilled de-ionized water was used throughout.

The general procedure to prepare the PVC membrane was to mix thoroughly 33 mg of powdered PVC, 61 mg of plasticizer BA, 2 mg of additive TPB, and 5 mg of ionophore SMPH in 4 ml of fresh THF. The resulting mixture was transferred into a glass dish of 2 cm diameter. The solvent was evaporated slowly until an oily concentrated mixture was obtained. A Pyrex tube (3–5 mm o.d.) was dipped into the mixture for about 10 s so that a nontransparent membrane of about 0.3 mm thickness was formed. The tube was then pulled out from the mixture and kept at the room temperature for about 6 h. The tube was then filled with internal filling solution (1.0 × 10−3 M of GdCl3 ). The electrode was finally conditioned by soaking in a 1.0 × 10−2 M GdCl3 solution for 12 h. A silver/silver chloride wire was used as an internal reference electrode. During all of the experiments the pH of all of the solutions, were adjusted at 4.5, using an acetic acid/sodium acetate buffer.

2.2. Synthesis of SMPH

2.4. The emf measurements

2-Aminothiophenol (2.50 g, 0.02 mol) was mixed with 100 ml of distilled ethanol in a 250 ml round bot-

All emf measurements were carried out with the following assembly: Ag–AgCI/internal solution

2. Experimental 2.1. Reagents


(1.0 × 10−3 M GdCl3 )/PVC membrane/sample solution/Hg–Hg2 Cl2 , KC1 (satd.). A Corning ion analyzer 250 pH/mV meter was used for the potential measurements at 25.0 ± 0.1 ◦ C.

53

90

Gd3+ 70

Ce3+ 50

La3+

2.5. The procedure of recovery 30

Nine different solutions, each containing 10 ppm of Gd(III) together with one of the following cations, Na+ (1000 ppm), Ca2+ (1000 ppm), Pb2+ (10 ppm), Cd2+ (10 ppm), Cu2+ (10 ppm),Ce3+ (10 ppm), La3+ (10 ppm), Sm3+ (10 ppm), and Ni2+ (10 ppm), were prepared and diluted to 50 ml by distilled water. The potential responses of these solutions were measured using the proposed sensor. The resulting concentrations of Gd(III), were compared with that of the original solution, containing Gd(III), only.

Sm3+

10

Cd 2+

-10 8

7

6

5

4

3

2

1

Pb2+

0

n+

(a)

90

Gd3+

70

Hg2+

50

Cu2+

3. Results and discussion 30

3.1. Potential response of the sensors based on SMPH The existence of two donating sulfur atoms, as well as two nitrogen-groups, in SMPH was expected to increase both the stability and selectivity of its complexes with transition and heavy metal ions, as reported elsewhere [16–20]. Thus, in order to check the suitability of SMPH as an ion carrier for different metal ions, in preliminary experiments, it was used to prepare PVC membrane ion-selective electrodes for a wide variety of cations, including alkali, alkaline earth and transition metal ions, The potential response of the most sensitive ion-selective electrodes based on SMPH are shown in Fig. 2. As can be seen, with the exception of Gd(III) ions, show stronger responses to the SMPH-based membrane sensors in comparison with the other cations tested. 3.2. Optimization of the structure of the SNPH In order to investigate the molecular basis for Gd3+ recognition over other metal ions, some theoretical calculations were carried out on the free and complexed ligand. The molecular structure of the free ligand was optimized using the restricted Hartree–Fock (RHF) level of theory with 6–31 G∗ basis set and that of the

Ni2+

10

Na+

-10 8

(b)

7

6

5

4

3

2

1

0

Sr2+

n+

Fig. 2. The potential responses of various ion-selective electrodes based on SMPH (a) ((䊉, Gd3+ ), (䊏, Ce3+ ), (䉱, La3+ ), (×, Sn2+ ), ( , Cd2+ ), (䉬, Pb2+ )); (b) ((䊉, Gd3+ ), (䊏, Hg2+ ), (䉱, Cu2+ ), (×, Ni2+ ), ( , Na+ ), (䉬, Sr2+ )).

metal ion complex was optimized using the Lan12mb basis set for all atoms at RHF level. No molecular symmetry constraint was applied, rather full optimization of all bond lengths, angles, and torsion angles were carried out by a Gaussian 98 program. The resulting optimized structures are shown in Fig. 3a and b. As it is seen from Fig. 3a, the optimized free ligand structure revealed a wide rotation of the two symmetrical parts of the SMPH molecule around the central-CH2 group, So that, the two S–N pairs are far enough from each other to minimize the possible intramolecular repulsive forces. However, the introduction of the central trivalent metal ion will result in the formation of a ‘pseudo cavity’ in the molecule Fig. 3b so that all four donating atoms of

54


Fig. 3. Optimal conformations of SMPH before (a) and, after (b) complexation with Gd(III).

the ligand molecule will be in suitable proximity of the central metal ion for maximum binding interactions. Obviously, among that the trivalent lanthanide ions, it is expected the one with a proper size for the optimized ‘pseudo cavity’ of the ligand, will form the most stable complex in the series. While, the larger and smaller cation sizes are expected to show decreased tendencies for complexation due to their need for applying more constraint energy to distort the molecular conformation of the ligand from its optimized situation. Of course, the contribution of a change in solvation–desolvation energies of the metal ions, in both aqueous and membrane phases on the stability order of different lanthanide ion complexes with SMPH can not be ignored. The obtained results

revealed that, among the different trivalat lanthanide ions, Gd3+ possesses the best energy changes for the ligand, which results in the formation of the most stable complex in the series. 3.3. The effect of membrane composition the potential response of the Gd(III) sensor The SMPH-based membrane sensor generated stable potential response in aqueous solutions containing gadolinium ions after conditioning for about 12 h in a 0.01 M gadolinium solution. Table 1 shows the data obtained with membranes having various ratios of different constituents. The potential responses of all of the membrane sensors were studied in a wide


55

Table 1 Optimization of membrane ingredients No. 1 2 3 4 5 6

PVC 33 33 33 33 33 33

Ionophore

Additive

4 5 6 5 5 5

Plasticizer

– – – 2 (TPB) 2 (TPB) 2 (TPB)

63, 62, 61, 60, 60, 60,

Slope

BA BA BA BA DBP NPOE

11.3 12.7 11.5 19.9 18.1 18.5

range of concentrations of gadolinium solution and the results are illustrated in Fig. 4. As with many carrier-modified membrane electrodes, the total potentiometric response of the electrode towards Gd(III) is dependent on the concentration of the SMPH incorporated within the membrane. As can be seen from Fig. 4 and Table 1, increasing the level of SMPH up to 5% results in the membranes (nos. 2 and 4) that display larger slopes. A maximum slope of 19.95 mV per decade of gadolinium concentration was observed for electrode 4 with 5% of SMPH. The optimum responses of the electrodes were tested after conditioning for different periods of time in 0.01 M gadolinium solution. The slope obtained using 12 h of conditioning was closer to the theoretically expected slopes, on the basis of the Nernst equation. Longer condition-

± ± ± ± ± ±

D.R. 0.4 0.3 0.5 0.3 0.5 0.3

1 1 1 1 1 1

× × × × × ×

D.T. 10−1 10−1 10−1 10−1 10−1 10−1

to to to to to to

5 5 1 1 1 1

× × × × × ×

10−5 10−5 10−4 10−5 10−5 10−5

3.00 2.00 6.00 5.25 1.00 1.00

× × × × × ×

10−5 10−5 10−5 10−6 10−5 10−5

ing times produced no further improvements in the response. The optimum conditioning solution was determined to have a concentration of about 0.01 M. It is well understood that the presence of lipophilic anions in cation-selective membrane electrodes not only diminishes the ohmic resistance and enhances the response behavior and selectivity, but it also in cases where the extraction capability is poor, increases the sensitivity of the membrane electrodes [21,22]. As can be seen from Table 1 and Fig. 4, the slope of the sensors in the absence of TPB is about two-third of the expected Nernstian value (membranes nos. 1, 2 and 3). However, addition of 2% TPB will increase the sensitivity of the electrode response considerably so that the membrane electrode demonstrates a nice Nernstian behavior (membrane no. 4). Critical response

130 110 90

E(mV)

Composition No. 4

70

Composition No. 1

50

Composition No. 2 Composition No. 5

30 10 -10 8

7

6

5

4

3

2

1

0

pGd3+

Fig. 4. Calibration curves for Gd3+ sensors based on SMPH with different composition (䊏, composition no. 1), (䉱, composition no. 2), (䉬, composition no. 4), (×, composition no. 5).

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M.R. Ganjali et al. / Analytica Chimica Acta 495 (2003) 51–59 90

70

80

60

60

50

50

40

E(mV)

E(mV)

70

40 30

30

20

20

10

10

0

0

-10 8

7

6

5

4 pGd

3

2

1

0

2

3

4

5

Fig. 5. Calibration graph of the Gd3+ ion-selective electrode with composition no. 4.

6

7

8

9

10

pH

3+

Fig. 6. Effect of the pH of the test solution (1.0×10−3 M of Gd3+ ) on the potential response of the Gd3+ ion-selective electrode.

3.5. Dynamic response time of the Gd(III) sensor characteristics of the sensor were assessed according to IUPAC recommendations [23]. The emf response of the PVC membrane at varying concentrations of gadolinium ions (Fig. 5) indicates a rectilinear range from 1.0×10−1 to 1.0×10−5 M. The slope of the calibration curve was 19.9 ± 0.2 mV per decade of Gd3+ ions activity. The detection limit, as determined from the intersection of the two extrapolated segments of the calibration curve, was 3.0 × 10−6 M. The standard deviation of eight replicate measurements is ±0.2 mV. The membrane electrode prepared could be used for at least 9 weeks without any measurable divergences in the responses. 3.4. The influence of pH on the response of the membrane sensor The pH dependence of the membrane electrode was tested over a pH range of 1.5–9.5 at a 1.0 × 10−4 M of gadolinium ion concentration and the results are illustrated in Fig. 6. As can be seen, the potential remains fairly constant in the pH range of 4.0–8.5 (the pH of the solutions was adjusted by either HNO3 or NaOH solutions). Beyond this range, a gradual change in the potential was detected. The observed potential drift at the higher pH values could be due to the formation of some hydroxy complexes of Gd(III) in the solution. At the lower pH values, the potentials increased, indicating that the membrane sensor responds to hydrogen ions.

Dynamic response time is an important factor for any ion-selective electrode. In this study, the practical response time was recorded by changing the concentration of Gd3+ solution in the range of 1.0 × 10−5 to 1.0 × 10−1 M and the results are shown in Fig. 7. As can be seen, in the whole concentration range the electrode reaches its equilibrium response in a very short time (>10 s). This is most probably due to the fast exchange kinetics of complexation– decomplexation of Gd(III) ions with the SMPH at the test solution-membrane interface. 3.6. Influence of the concentration of the internal solution The influence of the concentration of internal solution on the potential response of the Gd(III) sensor was also studied (from 1.0×10−2 to 1.0×10−4 M) and the results revealed that, the variation of the concentration of the internal solution does not cause any significant difference in the potential response of the electrodes, except for an expected change in the intercept of the resulting Nernstian plots. A 1.0 × 10−3 M concentration of internal solution was used for further studies. 3.7. Stability and life-time For investigation of stability and lifetime of the Gd(III) sensor, three electrodes were tested over a


57

100

10

90

-1

80

10-2

70 E(mV)

60

10-3

50 40

10-4

30 20

10-5

10 0 0

10

20

30

40

50

60

70

80

90 100 110 120 130 140 150

T(Sec)

Fig. 7. Dynamic response of the SMPH membrane electrode for step changes in concentration of Gd3+ .

period of 9 weeks. During this period, the electrodes were in daily use over extended period of time (one hour per day). A slight gradual decrease in the slopes (from 19.9 to 18.7 mV per decade) was observed. 3.8. Selectivity coefficient determination One of the most important characteristics of a cation-selective membrane electrode is its relative response towards the primary ion over other ions present in the solution, which is usually expressed in terms of potentiometric selectivity coefficients. In this work, the matched potential method was used for the evaluation of the selectivity of the sensor [24]. According to this method, a specified activity (concentration) of primary ions (A, 1.0 × 10−5 to 1.0 × 10−3 M of Gd(III)) is added to a reference solution (1.0×10−6 M of Gd(III)) and the potential is measured. In a separate experiment, interfering ions (B, 1.0 × 10−2 to 1.0 × 10−1 M) are successively added to an identical reference solution, until the measured potential matches the one obtained before adding primary ions. The matched potential method selectivity coefficient, KMPM , is then given by the resulting primary ion to interfering ion activity (concentration) ratio, KMPM = aA /aB . The resulting selectivity coefficients values are given in Table 2. From the data given in

Table 2, it is immediately obvious that the proposed Gd(III) sensor is highly selective with respect to most of cations. In the case of other lanthanide ions (i.e. La3+ , Sm3+ and Ce3+ ), the selectivity coefficients are in the order of 5.0 × 10−2 or smaller, which seems to indicate that the Gd(III) ions can be determined in the presence of other lanthanides. The selectivity coefficients of other cations (Na+ , K+ , Mg2+ , Ca2+ , Cu2+ , Co2+ , Ni2+ , Zn2+ , Cd2+ , Pb2+ and Hg2+ ) are smaller than 7.0 × 10−3 , and they seem to have a negligible effect on the functioning of the Gd(III) membrane electrode. The surprisingly high selectivity of the membrane electrode for Gd(III) ions over other cations used, most probably arises from the strong tendency of the carrier molecules for Gd(III) ions.

Table 2 Selectivity coefficients of the electrode Interfering ion

Selectivity coefficient

Yb3+ Sm3+ La3+ Ce3+ Cu2+ Ni2+ Tb3+ Dy3+

4.9 5.6 4.0 2.8 5.0 2.5 3.7 3.5

× × × × × × × ×

10−3 10−3 10−3 10−3 10−3 10−3 10−2 10−2

Interfering ion

Selectivity coefficient

Eu3+ Gd3+ Hg2+ Pb2+ Sr2+ Cd2+ Na+

5.0 2.6 1.6 1.6 7.1 2.8 2.8

× × × × × × ×

10−2 10−3 10−3 10−3 10−4 10−4 10−3

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M.R. Ganjali et al. / Analytica Chimica Acta 495 (2003) 51–59 25

Table 4 Determination of Gd(III) in a urine sample

20

Added (ppm)

Found

Recovery (%)

15

0 5 10 25 50 100 250 500

– 5.4 10.6 26.2 52.0 103.5 259.5 512.0

– 108 106 104.8 104.0 103.5 103.8 102.4

10 5 0 0

100

200

300

Fig. 8. Potentiometric titration curves of 25.0 ml 1.0 × 10−4 M solution of Gd3+ with 1.0 × 10−2 M of EDTA and vice versa.

ing method, and the results are shown in Table 4. As can be seen, the amounts of the gadolinium ions, which were added to the urine sample could be determined by the sensor with relatively good accuracya.

3.9. Analytical application The proposed gadolinium sensor was found to work well under laboratory conditions. It can be used as an indicator electrode in titration of 1.0×10−4 M solution of Gd3+ with 1.0 × 10−2 EDTA and vice versa and the resulting titration curve is shown in Fig. 8. As can be seen from Fig. 8 the amount of gadolinium ion can be determined by the proposed Gd(III) sensor. The sensor was also successfully applied to the direct monitoring of Gd(III) in binary mixtures and the results are summarized in Table 3. As it is obvious, the recovery of Gd(III) ions is quantitative in all cases. The results show that the sensor can be used for the determination of the concentrations of Gd(III) in real samples having different analytical matrixes. The membrane was used to determine the Gd(III) ion content ion of a 50 ml urine sample, through spik-

Table 3 Recovery of Gd(III) from binary mixtures by the proposed sensor Gd(III) (ppm)

Added cation (ppm)

Recovery (%)

10 10 10 10 10 10 10 10 10

Na+ ,

102.2 101.8 99.8 102.2 101.8 99.8 101.3 101.1 98.9

1000 Ca2+ , 1000 Pb2+ , 100 Cu2+ , 100 Cd2+ , 100 Ce3+ , 100 La3+ , 100 Sm3+ , 100 Ni2+ , 100

± ± ± ± ± ± ± ± ±

0.7 0.9 0.5 0.4 0.9 0.8 0.3 0.7 0.6

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