Nickel(II) Selective Poly(Vinyl Chloride) Membrane Electrode Based on Novel Macrocyclic Compound 11,14-Diaza-4,7-Dioxo-2(3),8(9)-Dibenzoyl-Cyclotetradeca-10,14(1)-Diene as Neutral Carrier

 

Susheel Kumar Sindhu1*, Sunil Kumar1,2 and LR Singh2

1Department of Chemistry, S.S.V. Post Graduate College, Hapur, India.

2Department of Chemistry, B.S.A. College, Mathura  (U.P.) India.

*Corresponding Author E-mail:   susheelsindhu@yahoo.co.in  

 

ABSTRACT:

A PVC membrane based nickel(II) selective electrode was constructed using 11,14-diaza-4,7-dioxo-2(3),8(9)-dibenzoyl-cyclotetradeca-10,14(1)-diene (DADOCTD) as a neutral carrier. The sensor exhibits a Nernstian response for Ni(II) ions over a wide concentration range of 3.9 x 10-6 - 1.0 x 10-1 with a slope of 29.0 mV/decade. It has a relatively fast response time and can be used for 120 days without any considerable divergence in potentials. The proposed sensor revealed relatively good selectively and high sensitivity for Ni(II) over a bi-, tri- and tetravalent metal ions and could be used in a pH range of 3-8. It was successfully also used as an indicator electrode in potentiometer titration and in analysis of concentration of nickel in real samples.

 

 

INTRODUCTION:

Chemical sensor technology creates a potential fundamental aspect for the determination of several biologically relevant ions, environmental monitoring and industrial effluents analysis, as ion selective electrodes provide a rapid, accurate and low-cost method of analysis without any pre-treatment of samples. Due to toxic nature of nickel, there is urgent need for monitoring it in the clinical, food, industrial and environmental activities/processes1,2.

 

Nickel is one of the essential trace elements for fauna and flora3. But it has got strong presence in our surroundings and therefore excess exposure to it can cause serious soil pollution4, health hazard such as acute pneumonitis, dermatitis, asthma, cancer of lungs & nasal, etc.5. A literature survey revealed that a number of groups are working extensively for the development of Ni2+ selective electrode, but all the reported electrodes have one or other drawbacks like poor selectivity, narrow concentration range, non-Nernstian response, long response-time and poor reproducibility6-17. As there is no well established commercial Ni2+ selective electrode available, effective exploration in this field is still required.

 

In recent times, macrocyclic compounds are considered as potential ionophore for development of highly sensitive and selective ISE because they generally behave as neutral charge (electron) carrier and they can generally made to have two different binding states. These two states can be easily and reversibly interconvert by external forces such as pH gradients18,19, light20, temperature21 and redox gradients22-24. Macrocyclic compounds due to moderately preorganized three-dimensional frameworks convincingly fulfill the requirements of a redox switch. It effectively binds cations and actively participates in their transport. In present communication, an efforts have been taken to describe new PVC based ISE using a diazadioxo macrocyclic ligand 11,14-diaza-4,7-dioxo-2(3),8(9)-dibenzoylcyclotetradeca-10,14(1)-diene (DADOCTD) as ionophore, which shows better or compatible response towards selectivity, sensitivity and one or another parameters for Ni2+-selective electrodes, over a number of previous reported electrodes.

 

EXPERIMENTAL:

Materials and Reagents:

All the reagents used were of analytical grade. High molecular weight PVC, tetrahydrofuran (THF), 1,2-dibromoethane, ethylenediamine and salicyldehyde were procured from Aldrich. Plasticizers viz., dibutylphthalate (DBP), dioctylphthalate (DOP), Nitrobenzene (NB), tetraphenyl borate (TPB) and tris(2-ethyhexyl)phosphate (TEP) are obtained from Fluka.

 

Scheme-1

 

Figure 1. Potential responses of various ion-selective electrodes based on DADOCTD.

 

Metal nitrates and chloride salts, and all other analytical grade solvents used were received from Merck. In addition, all standard solutions and buffers were prepared using double distilled deionized water.

 

Instrumentation:

IR spectra were registered on a Perkin-Elmer 1600 FTIR spectrometer. 1H-NMR spectra was recorded in deuterated dimethyl sulfoxide with TMS as internal standard on a Varian Mercury spectrometer at 300 MH2.

 

Potential measurement:

All the membrane electrode potential measurements were performed at constant temperature (25 ฑ 0.5 oC) using digital pH meter, potentiometer (ELICO L1-10, India) in conjugation with saturated calomel electrodes as reference electrodes. The representative electrochemical cell for the EMF measurements is as follows:

External

Reference

Electrode

Aqueous

sample

Ion-selective

Membrane

 

Internal

Filling

Solution

Internal

Reference

Solution

 

Figure 2. Calibration graphs of Ni(II)-selective PVC based membrane containing DAOBTD with different compositions.

 

Figure 3. The effect of the pH of the test solutions (1.0 ื 10-2 mol L-1 and 1.0 ื 10-3 mol L-1) on the potential response of the Nickel sensor (membrane No. 2).

 

Synthesis of the ionophore:

The macrocyclic compound 11,14-diaza-4,7-dioxo-2(3),8(9)-dibenzoyl-cyclotetradeca-10,14(1)-diene (DADOCTD) is prepared by two step complete condensation reaction, occurring in a closed single pot. In a 500 mL three-necked round bottle flask, fitted with water condenser and a mechanical stirrer at its two necks, 100 mL ethanolic solution of 30.5 mL salicyldehyde (0.5M) was taken using a dropping cylindrical funnel at the third neck of the flask. The solution was warmed upto 40 oC followed by addition of 100 mL ethanolic solution of 21.25 mL 1,2-dibromoethane (0.25 M) with slow mechanical stirring. After closing the stopper of the dropping funnel the reaction mixture was refluxed for about 4 hours at 60 oC with stirring. It was allowed to get cooled upto ca. 40 oC and then a 100 mL ethanolic solution of 27 mL ethylenediamine (0.25 M) was added with continuous stirring. Again this reaction mixture, with proper stirring was refluxed for next 6 h. Dark brownish liquid of final reaction mixture on gradual cooling became viscous and finally produced bright yellow precipitate, which was cooled further in refrigerator followed by filtration at Buckner funnel, washing with water and then by ethanol. Finally, very bright lemon yellow colored compound was obtained and was stored in vaccuo (Scheme-1). (Yield: 70%, m.f. C18H18N2O2 and m.p. 276 oC). Elemental analysis: Calcd. (Found) (%): C 73.47 (73.01); H 6.12 (6.07); N 9.52 (9.34). IR (KBr) spectra exhibits νstr.(C=N) bands in 1588-1635 cm-1 region and the absence of uncondensed functional groups (NH2, C=O). 1H NMR: 1.25 (4H, S, C-N), 3.94 (4H, S, C-O), 6.83 (t, 2H, aromatic), 6.92 (d, 2H, aromatic), 7.21 (t, 2H, aromatic), 7.31 (d, 2H, aromatic), 8.36 (S, 2H, -CH).

 

Table 1. Binding energy of complexes of the different metal ions with DADOCTD.

Metal ion

Total energy of metal ion/Hartree

Total energy of the complex/Hartree

Binding energy (DE)a/kcal mol-1

Ni(II)

Cd(II)

Ca(II)

Al(III)

Sr(II)

K(I)

Th(IV)

Be(II)

Li(I)

-62.6454

-45.7232

-35.6124

-29.5441

-29.5426

-27.6374

-19.4811

-13.4423

-7.1454

-640.6321

-623.6654

-613.4141

-607.5722

-607.3073

-605.2845

-597.1074

-591.1854

-584.5921

-269.4

-239.4

-147.5

-292.6

-124.6

-52.8

-37.9

-111.7

-71.6

 

Electrode Preparation:

The membranes were prepared using the basic method given by Craggs et al.25, with certain variations in compositions of PVC, ionophore and plasticizers/solvent mediators like DBP, NB, DOP, NBA, TBP, etc. The PVC membrane solution was prepared by through mixing of the ionophore (macrocyclic compound) (1% wt.), DBP as plasticizer (66% wt.) and PVC (33% wt.) were mixed and dissolved in THF. The resulting mixture was poured into a glass mould and THF was allowed to evaporate off at room temperature over 24 h. A flexible membrane with a thickness of 0.2-0.4 mm was obtained. The discs of 6 mm diameter were cut and pasted onto a glass-tube. After getting dried, this tube was filled with 0.01 M internal solution of nickel and immersed in the 0.01 M nickel nitrate solution, at least for 2-3 days prior to use.

 

Analysis of Nickel(II) in milk and chocolate samples:

A suitable amount of milk power or chocolate (about 10 mL milk or 2g chocolate) was ashed at 450 oC in a porcelain crucible for about 1.5–2 hours. About 2 mL of conc. HNO3 was added into the residue and gently heated to dissolve. The resulting solution was diluted with minimum amount of water followed by its filtration and transfer to a 250 mL beaker with multiple washing by water. Final volume of the solution in the beaker was made upto 50 mL. The volume of this solution was reduced upto 20-25 mL. Finally this solution was transferred properly into a 100 mL volumetric flask followed by make-up of volume.

 

RESULTS AND DISCUSSION:

The macrocyclic ligand has the relatively low solubility in water due to the existence of the two benzo groups in this structure, indicating its sufficient lipophilic character, which prevents its leaching into the solution surrounding the membrane electrode and also suggesting the presence of two donating nitrogen atoms which proved that the ligand could be better potential ion carrier for bivalent metal ions in PVC membrane electrodes. Therefore, we were prompted to investigate the behaviour of the ligand with two donating nitrogen atoms as carrier in the construction of PVC-base membranes for the estimation of various bivalent ions concentrations in aqueous solution. As can be seen from the Fig.1, the ionophore as a neutral carrier was found to be highly responsive to Ni(II) with respect to several other metal ions.

 

In order to have a clear picture of the selective of DADOCTD for various metal ions, in this work, we investigated its binding to Li(I), K(I), Ca(II), Al(III), Sr(II), Be(II), Th(IV), Cd(II) and Ni(II) ions by using ab initio theoretical calculations. The influence of the nature, size and charge of metal ions on the complexation reaction with the neutral ligand is explained on the basis of the calculation of gas-phase binding energies.

 

The molecular structures of the uncomplexed DADOCTD and its complexes with Ni(II) and other metal ions were optimized using the lanl2mb basis set for all atoms at restricted Hartree Fock (RHF) level. No molecular symmetry constraint was applied. Rather, full optimization of all bond length, angles and torsion angles was carried out using the Gaussian 98 program26. The binding energy (DE) was calculated with the enlarged basis sets using equation (1):

               DE = Ecomplex – (Eligand + Ecation)

where, Ecomplex, Eligand and Ecation are the total energies of the complex, uncomplexed DADOCTD and metal ion, respectively.

 

Optimization of the uncomplexed DADOCTD was also carried out with the semi-empirical PM3 method using gHyperChem software (Version 6.01). No adequate parameterization of the metal ions was available in PM3, so that semi-empirical calculations could not be carried out on the complexes.

 

Table-1 summarizes the theoretical data about the stability of the DADOCTD complexes with Li(I), K(I), Ca(II), Al(III), Th(IV), Sr(II), Be(II), Cd(II) and Ni(II) ions. Inspection of Table-1 reveals that the cation binding energies with DADOCTD shows a pronounced dependence on the nature of metal ions used. In fact, the stability of the resulting complexes is expected to decrease in the order of:

Ni(II) > Cd(II) > Ca(II) > Sr(II) > Be(II) > Li(I) > K(I) > Th(IV)

Table 2. Comparison and response characteristics of Ni(II)-selective PVC based membrane having (DAOBTD) as electroactive material.

Membrane no.

Ratio of various compositions in membrane (w/w)

PVC

Working concentration range (M)

Slope (mV/ decade)

Response time (sec)

I

NaTBP

DOP

NB

TEB

TBP

DBP

1

 

2

 

3

 

4

 

5

 

6

 

7

 

8

 

9

5

 

5

 

5

 

5

 

5

 

5

 

3

 

2

 

8

–

 

2

 

2

 

2

 

2

 

2

 

2

 

2

 

2

–

 

60

 

–

 

–

 

–

 

–

 

–

 

–

 

–

–

 

–

 

60

 

–

 

–

 

–

 

–

 

–

 

–

–

 

–

 

–

 

60

 

–

 

–

 

–

 

–

 

–

–

 

–

 

–

 

–

 

60

 

–

 

–

 

–

 

–

–

 

–

 

–

 

–

 

–

 

60

 

–

 

–

 

–

95

 

33

 

33

 

33

 

33

 

33

 

33

 

33

 

33

7.8 ื 10-4 to

1.0 ื 10-1

3.9 ื 10-6 to

1.0 ื 10-1

7.2 ื 10-5 to

1.0 ื 10-1

3.8 ื 10-5 to

1.0 ื 10-1

9.7 ื 10-5 to

1.0 ื 10-1

6.8 ื 10-4 to

1.0 ื 10-1

5.3 ื 10-5 to

1.0 ื 10-1

4.1 ื 10-4 to

1.0 ื 10-1

6.5 ื 10-4 to

1.0 ื 10-1

25.0

 

29.6

 

28.2

 

24.4

 

29.1

 

31.0

 

28.9

 

27.3

 

28.4

50-90

 

10

 

30

 

24

 

19

 

22

 

18

 

17

 

15

 

 

Table 3. Selectivity coefficients of the nickel selective electrode based on using the matched  potential method (MPM).

Li(I)

K(I)

Mg(II)

Ca(II)

Ba(II)

Al(III)

Zn(II)

Co(II)

Cu(II)

Cd(II)

Pb(II)

Mn(II)

Sr(II)

Fe(III)

Th(IV)

3.1 ื 10-4

7.4 ื 10-4

8.5 ื 10-4

7.2 ื 10-4

9.2 ื 10-4

5.4 ื 10-3

6.1 ื 10-3

5.0 ื 10-4

8.3 ื 10-4

3.7 ื 10-3

4.3 ื 10-4

7.1 ื 10-4

6.5 ื 10-3

7.9 ื 10-4

2.2 ื 10-4

 

Effect of membrane composition:

The characteristics of membrane such as response time, lifetime, selectivity and chemical stability depends on the nature and amount of plasticizer as well as the ionophore. In fact, the membrane composition and especially in some cases, the nature of additive may have a significant influence on the sensitivity and selectivity obtained for a given electroactive material27-29. In general, the thickness of the membrane depends on the membrane content of PVC and there is a good correlation between membrane thickness and the PVC-content of the membrane. The detection limit of the sensor increases with a decrease in the PVC content. However, if the membrane is too short, it losses its mechanical strength, and is easily broken30. Furthermore, the stability of carrier complexes in membranes results from the electrostatic interaction between complexes and the surrounding membrane solvents31. Accordingly, for selecting solvent mediator for a metal ion selective electrode, a number of solvents, like dibutylphthalate (DBP), dibutylbutylphosphonate (DBBP), tributylphosphate (TBP), tris(2-ethyhexyl)phosphate (TEP), etc. were taken in varying composition and their effect on potentiometric response of ISE was observed (Table-2).

 

Working concentration range:

The potential response of the membrane, as a function of Ni2+ activity is shown in Table 2 and in Fig. 2. It is observed that membrane No. 1, without plasticizer has a narrow working concentration range and near Nernstan slope 25.0 mV/decade activity. Further, it attains stable potential value in 50 sec at higher concentration (1.0 x 10-1 M) and in 90 sec at lower concentration (7.8 x 10-4 M). However, the addition of plasticizer improves the response characteristics of the membrane, which not only shows significant potential response in improved and wider concentration range but the slope value also approached closer to the Nernstian value of 29.5 mV/decade for bivalent metal ions. Especially the membrane No. 2, which contains DOP as plasticizer, exhibits linearity in the concentration range 3.9 x 10-6 to 1.0 x 10-2 with a slope of 29.5 mV/decade of activity. All the further studies were performed with the membrane No. 2, which has the optimized composition for best potential response towards Ni(II) selectivity.

 

Response time and lifetime:

Membrane No.1, without plasticizer shows a response time of 50-90 sec. Such high response time was reduced significantly, when solvent mediator was used. The best response time was observed as 10 sec for membrane no. 2 containing plasticizer DOP. Potential generated by this membrane remains stable for 150 sec and start deviating slowly. The electrode was used over a period of 4 months without observing any significant change in potential response for whole working concentration range. After this period, a slight change in lower direction in terms of response time and slope were recorded, which could be due to aging of the electrode and subsequently due to decreases in the amount of the plasticizer and ionophore in PVC membrane matrix. In its normal working of the electrode, it was always kept stored by immerging the electrode’s membrane in 0.01 M Ni(NO3)2 solution for regular conditioning and also to avoid any mechanical deformation/damage to the membrane.

 

 

Table 4. Comparison of the reported electrode with proposed electrode assembly

S. no.

Working concentration range (M)

Slope (mV/decade of activity)

pH range

Response time (sec)

Life time (weeks)

Ref. no.

1

2

3

4

5

6

7

8

9

10

11

3.2 ื 10-6–5.0 ื 10-2

1.0 ื 10-7–1.0 ื 10-2

1.0 ื 10-6–1.0 ื 10-1

1.0 ื 10-5–1.0 ื 10-1

2.0 ื 10-5–5.5 ื 10-3

7.9 ื 10-6–1.0 ื 10-1

1.0 ื 10-6–1.0 ื 10-1

Not mentioned

1.0 ื 10-7–1.0 ื 10-2

1.0 ื 10-8–1.0 ื 10-3

6.0 ื 10-8–1.0 ื 10-1

29.0 ฑ 0.4

–

~30.0

29.5

29.8

30.0 ฑ 1.0

30.5

2.0

30 ฑ 1.0

–

19.1 ฑ 0.1

2.2-5.9

5.0-8.5

1.7-5.4

2.6-6.8

4.0-8.0

2.7-7.6

3.0-6.0

2.8-7.6

4.5-9.0

–

2.5-8.5

~10

<15

15

<25

300

15

<40

15

<10

<40

<8

16

4

24

16

6

Not mentioned

ณ8

8

2

–

4

3

6

7

8

11

12

14

15

16

17

Present work

 

 

Table 5. Nickel content in various samples of milk and chocolate analyzed

Sample

Using proposed electrode (mg K-1)

AAS (mg K-1)

Milk (1)

Milk (2)

Chocolate (1)

Chocolate (2)

1.70 ฑ 0.10

1.75 ฑ 0.10

0.67 ฑ 0.05

0.58 ฑ 0.10

1.80 ฑ 0.10

1.90 ฑ 0.10

0.68 ฑ 0.05

0.60 ฑ 0.10

 

Effect of pH:

The pH dependence of the electrode was tested over the pH range 1-12 using test solution of nickel nitrate in the two different concentration range at 1.0 x 10-2 and 1.0 x 10-4 M, which is depicted in Fig. 3. The pH of test solution was adjusted by the addition of dilute nitric acid and NaOH solutions. Fig. 3 clearly indicated that the proposed electrode worked in the useful pH range is 3.0 to 8.0 as potential remains constant in this range, in both different concentrations. Beyond the pH 8, the potential value deviates sharply on either sides of this pH value limit. Sharp change in the potential at higher pH value may be due to hydrolysis of Ni2+ and at lower pH it is due to H+ ion co-fluxing.

 

Potentiometric selectivity:

Selectivity is the most important characteristic of any ion selective electrode. To investigate the selectivity of the proposed membrane electrode, its potential response was investigated in the presence of various interfering foreign cations at the time of calibration, using the matched potential method (MPM)32,33. According to MPM, the selectivity coefficient is defined as the activity (concentration) ratio of the primary ion and the interfering ion, which gives the same potential change in a reference solution34. The concentration of Ni2+ used, as the primary ion in this study, was 1.0 x 10-2 M. The resulting selectivity coefficients are summarized in Table 3. The selectivity coefficient values < 1 for various interfering ion studied indicate that most of the hard, soft and borderline metal ions used did not significantly disturb the functioning of the Ni2+ ion-selective membrane electrode. Table 3 shows sufficient selectivity towards Ni2+ ion over all other ions.

 

In Table 4, the response characteristics of the proposed membrane sensor are compared with those of the best Ni2+-selective electrode reported earlier6-17. From the data given in the table, it is immediately obvious that not only the concentration range and detection limit of the proposed electrode, but also its pH range is wider than other Ni2+ ion selective electrodes.

 

Applications of the proposed ISE:

The proposed electrode was successfully applied as indicator electrode in conjugation with SCE in the potentiometric titration of Ni2+ solution with EDTA as a suitable titrant. Ni2+ solution (10 mL of 1.0 x 10-3 M) was brought to pH 6 with hexamine and then titrated against 1.0 x 10-2 M EDTA solutions. The potential data are plotted against volume of EDTA in Fig. 4. The plot is of conventional sigmoidal shape, indicating sufficient selectivity of the sensor for Ni2+ ions and 1:1 stoichiometry of Ni EDTA complex at the end point.

 

The nickel contents were determined successfully in the various samples of dairy milk and chocolates using the proposed sensor. The results clearly indicated that the values are at par the evaluation of the same samples by atomic absorption spectroscopy (AAS) (Table no. 5).

 

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Received on 30.11.2009        Modified on 31.12.2009

Accepted on 15.01.2010        ฉ AJRC All right reserved

Asian J. Research Chem. 3(1): Jan.-Mar. 2010; Page 214-219