Polyvinyl chloride-based 18-crown-6, dibenzo18-crown-6 and calix-[6]-arene zinc(II)-potentiometric sensors

2.1 Equipments

A Perkin–Elmer Spectrometer model 2380 was used for flame AAS determination of zinc. The electromotive force (EMF) of each potentiometric cell was measured at room temperature (25 ± 1 °C) with a Cole Parmer 5986 pH/mV meter having a sensitivity of ±0.1 mV. The reference electrode is an Orion model 900-200 Ag/AgCl electrode. An Orion model 420A ionalizer with its combined glass electrode was used for measuring the pH values.

2.2 Chemicals and solutions

All reagents used in the investigations were of analytical reagent grade. Doubly distilled water (DDW) was used for preparing all aqueous solutions.

Polyvinyl chloride (PVC) is a high molecular weight solid of highest purity (99%) which was provided by Fluka Chemical Co.

2.2.1

2.2.1 Ionophores (Scheme 1)

1,4,7,13,16-Hexaoxacycloactadecane (18-crown-6) (18C6) and dibenzo (18-crowen-6) (DB18C6) are of highest purity (98%) supplied by (Fulka, Germany).

37,38,39,40,41-Penta-kis(diethoxythiophosphoryl)-oxy-5,11,17,23,24,35-hexa-kis (1,1-dimethylethyl)calix[6]arene-42-ol (CAX), Scheme 1, were synthesized from p-tert-butyl-calix[6]arene by Wroblewski et al. (1996). This method is based on the addition of freshly grounded solid NaOH (1.5 g), diethyoxythiophosphoryl chloride (2.4 ml) and tetrabutylammonium bromide (1.5 g) to a solution of p-tert-butyl-calix[6]arene (1.5 g) CH₂Cl₂. The suspension was stirred for 10 h at room temperature. The organic layer was then washed with 1 M HCl, water, brine and dried over MgSO₄.

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Scheme 1

Molecular structures of DB18C6, 18C6 and calyx[6]arene.

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2.2.2

2.2.2 Solvent mediators (Scheme 2)

Dioctylphenylphosphonate (DOPP), bis(ethylhexyl)sebacate (DOS), dioctyl phthalate (DOP), 2-nitrophenyloctylether (o-NPOE) and didecylphthalate (DDP), Scheme 2, are pure liquids of highest purity supplied by Aldrich Chemical Company.

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Scheme 2

The molecular structure of the used plasticizer (solvent mediators).

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2.3 Electrode preparation

The general procedure to prepare the PVC membranes was to mix 60 mg of powdered PVC and 2 mg of ionophore (DB18C6, 18C6 or CAX) with 120 mg of plasticizer (DOPP, DOP, DDP, DOS or o-NPOE) as solvent mediators. Then, the mixture was thoroughly dissolved in approximately 2 ml of tetrahydrofuran (THF) by vigorous shaking and poured into 5 cm glass rings resting on clean glass plates. The solvent was allowed to evaporate at 25 °C for 24 h. The resulting membrane was peeled off from the glass mold. Then, disks of 7 mm diameter were punched from the membranes and mounted into Philips Is-560 electrode bodies (Eindohoven, The Netherlands) filled with an aqueous solution of 10⁻² M ZnCl₂. The electrodes were, then, conditioned for at least 24 h in a fresh 10⁻² M ZnCl₂ solution in a dark place. Table 1 summarizes the composition of all the fabricated membranes.

2.4 Analytical procedures

2.4.1

2.4.1 Electrode calibration

The calibration of electrodes was done using the following electrochemical system.

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where the reference electrode is an Orion model 900-200 Ag/AgCl electrode.

Aliquots (50 ml) of solutions between 10⁻⁷ and 10⁻¹ M of zinc chloride were transferred into 70 ml beakers. Each proposed Zn²⁺ electrode in conjunction with Ag/AgCl reference electrode was immersed into the solution. Stirring was started and the stable potential reading for each concentration was recorded and plotted against p [Zn²⁺].

From the resulting curves the slope, linear working range and detection limit of the zinc electrodes were determined.

2.5 Sample analysis

3.1 Performance characteristics of sensors

It is well known that the sensitivity, linearity, and selectivity obtained for a given ionophore depends significantly on the membrane composition. Thus, the influence of the type of ionophore and the nature of plasticizers on the potential response of the membrane sensors, were investigated. The response characteristics of the fabricated Zn ion selective electrodes are summarized in Tables 2–4. A comparison of the performance characteristics of all the sensors clearly revealed that the sensors 1a and 1b having membranes with DOPP plasticizer and DB18C6 and 18C6 ionophores, respectively, are the best. These sensors exhibit the widest working concentration range of 1 × 10⁻¹–1 × 10⁻⁵ M, a Nernstian slope of 29 and 30 mV/decade for electrodes with membranes 1a and 1b, respectively, and a minimum response time of ⩽15 s. Thus, all the further studies were carried out with the cells employing sensors 1a and 1b.

3.2 Ionophores (ion carrier)

As with many ionophore-based membrane electrodes, the electrochemical response of the electrode toward Zn²⁺, depends on the type of ionophore incorporated in the membrane. As shown in Table 1, three types of ionophores (neutral carriers) are used viz.: DB18C6, 18C6 and calix-6-aren (CAX), for preparing Zn²⁺ selective electrodes in the present study. Table 5 shows the effect of changing the type of ionophore on the electrode response after one day time soaking in ZnCl₂ solution. As it can be seen, the response of the electrodes with membrane types 1a, 1b gives a best typical Nernstian response toward Zn²⁺. The obtained slope values of the calibration graphs are: 29 ± 1 and 30 ± 1 mV/decade for electrodes with membranes incorporating DB18C6 and 18C6 ionophores, respectively. But in case of using CAX as ionophore (membrane 1c), the response was a poor Nernstian response of ∼20 mV/decade. From these results, it can be concluded that when using the same plasticizer (DOPP) and the same membrane matrix (PVC) the response varied with changing the type of ionophore. The best Nernstian response was obtained in case of using crown ether derivatives DB18C6 and 18C6 not in using calix-[6]-arene (CAX). This may be attributed to the structure of crown ether which is able to carry the charged Zn²⁺ ion; this ability improves the performance of the electrodes 1a and lb.

3.3 Solvent mediators

One of the expected reasons for the presence of plasticizer is to give a homogeneous organic phase. Also the membrane solvent must be physically compatible with the polymer, i.e., has plasticizing properties. To provide suitable mechanical properties, five types of plasticizers were used (DOPP, DOP, DOS, DDP and 2-NPOE (with the two crown ethers DBI8C6 and 18C6. Fig. 1 shows the effect of changing the type of the solvent mediator on the electrode response after one day time soaking in ZnCl₂ solution. The response of the electrodes with membrane types 1a and 1b gives a best typical Nernstian response toward Zn²⁺. The presence of DOPP rather than DOP, DOS, 2-NPOE or DDP is the reason for the electrode response toward Zn²⁺ ion. This is confirmed by comparing the calibration graphs of the DB18C6 based membranes, one membrane contains DOPP (type-la), while others contain DOP (type-2a), 2-NPOE (type-3a), DOS (type-4a) or DDP (type-5a). The slope of the calibration graph of the electrode with membrane (type-la) is a better Nernstian response of 29 mV/decade and a wider linear range of 10⁻⁵–10⁻¹ M and detection limit (7.5 × l0⁻⁶ M) while other types did not show Nernstian response (Fig. 1, Table 2). Similar results were obtained in case of using 18C6 as an ionophore, Table 3, for preparing electrodes with different solvent mediators. Only the membrane plasticized with DOPP (electrode with membrane type 1b) gave a typical Nernstian response with slope of 30 ± 1 mV/decade and a wider linear range 10⁻⁵–10⁻¹ M and detection limit (1.5 × 10⁻⁶ M) (Table 3). The response of electrodes with membranes 1a and 1b is improved upon using DOPP as a solvent mediator. This is because DOPP ensures the mobility of the free and complexed ionophore and provides suitable mechanical properties of the membranes improving the performance of the electrodes with membranes 1a and 1b.

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Figure 1

The effect of changing composition on the electrode response after one day time soaking in ZnCl₂ solutions.

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3.4 Effect of pH on the membrane response

The effect of pH is considered to be one of the main characteristics of ion selective electrodes. The pH-profiles of the electrodes with membranes 1a and 1b have been constructed to find the optimum pH-range of each electrode. The pH dependence of each electrode response was examined using 10⁻⁴, 10⁻³ and 10⁻² M Zn²⁺ solutions.The pH has been adjusted by using hydrochloric acid (0.1 M) and sodium hydroxide (0.1 M). For electrode with membrane 1a the working pH ranges of the electrodes are 3–8, 4–8 and 4–8 for concentrations 10⁻⁴, 10⁻³ and 10⁻² M, ZnCl₂ solution, respectively, Fig. 2. Similar behavior was obtained when using the electrode with membrane 1b in which the working pH ranges are (3–8), (3–9) and (3–8) for concentrations 10⁻⁴, 10⁻³ and 10⁻² M, ZnCl₂ solution, respectively, Fig. 3. The break down of the curves in the basic medium is due to the hydrolysis of zinc ions. In acidic medium, on the other hand, a sudden change is observed at pH 3 in case of electrode with membrane lb for all the concentrations tested (10⁻², 10⁻³ and 10⁻⁴ M). But in case of the electrode with membrane la the change is observed at pH 3 in case of concentration 10⁻⁴ and pH 4 for concentrations 10⁻³ and 10⁻² M. Finally the break in acidic medium pH <3.0 and 4.0 is due to the protonation of the corresponding ionophore in the membranes. Under these conditions zinc ions cannot freely be exchanged.

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Figure 2

pH effect on the potential of zinc-responsive electrode with membrane type 1a (DB18C6 + DOPP) for Zn²⁺ solutions (I, 10⁻²; II, 10⁻³; III, 10⁻⁴ M.

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Figure 3

pH effect on the potential of zinc-responsive electrode with membrane type 1b (18C6+DOPP) for Zn²⁺ solutions (I, 10⁻²; II, 10⁻³; III, 10⁻⁴ M).

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3.5 Potentiometric selectivity

The influence of interfering ions on the response behavior of ion selective membrane electrode is usually described in terms of selectivity coefficient $K_{\text{A,B}}^{\text{pot}}$ . Most often it is expressed as the logarithm of ( $K_{\text{A,B}}^{\text{pot}}$ ). Negative values indicate a preference for the target ion relative to the interfering ion. Positive values of $\log K_{\text{A,B}}^{\text{pot}}$ , on the other hand, indicate the preference of an electrode for the interfering ion. The experimental selectivity coefficients depend on the activity and the method of their determination. Different methods of the selectivity determination can be found in the literature. The IUPAC suggests two methods: separate solution method (SSM) and fixed interference method (FIM). There is also an alternative method of the selectivity determination called matched potential method (MPM). In the present study, the selectivity coefficient values were determined using SSM method. Several inorganic cations are chosen for the selectivity measurements. The resulting selectivity coefficient values are summarized in Table 6 and represented in Fig. 4. As it can be seen, for all ions used (except Pb²⁺ and Ag⁺) the $\log K_{\text{Zn,M}}^{\text{pot}}$ values were negative indicating that they would not significantly disturb the functioning of the Zn ion selective membranes. The positive interfering effects of Pb²⁺ and Ag⁺ ions could be related to the tendency of these cations to coordinate with crown ethers (Zareh et al., 2001). So for this reason, the measurements of electrode comprising membranes 1a and 1b must be carried out after isolation of silver and lead. The resulting selectivity coefficients, also, show that there is no marked difference in the selectivity coefficient values between the two electrodes with membrane types 1a and 1b except for Ni²⁺. The $\log K_{\text{Zn,Ni}}^{\text{pot}}$ is improved in case of using 18C6 as ionophore .

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Figure 4

Separate solution selectively coefficient at 10⁻² M ZnCl₂ solution of Zn²⁺-selective electrode with membrane type 1a (DB18C6 + DOPP) and type 1b (18C6+DOPP).

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By measuring the potential values for 10⁻⁵–10⁻¹ M concentrations of the interfering ions, a complete picture of the selectivity of the studied Zn²⁺ electrodes is obtained. A comparison of the obtained results with those for the electrodes whenever Zn²⁺ is the measured cation is shown in Table 7.

3.6 Effect of soaking and electrode age

To investigate the electrode age, it was soaked in 10⁻² M zinc chloride solution for interval ranging from one day till the electrode lost its Nernstian behavior. The response of each electrode was checked by recording the calibration graph at 25 °C, at different intervals. Representative calibration curves for the effect of soaking on the performance characteristics of zinc electrodes with membrane types la and 1b as a function of the soaking time are shown in Figs. 5 and 6, respectively. The results listed in Table 8, summarize the effects of soaking time on slope, detection limit and usable linear concentration range of the proposed electrodes. As it can be noticed, the electrodes have a long working period extending up to 2 months. They are ready for measurements after preconditioning period one day with a Nernstian slope reaching 29 and 30 mV/decade for electrodes with membrane types 1a and 1b, respectively. A small drift, then, occurs (not more than 2 mV) after 2 months. The decrease in the performance of the electrodes after 2 months soaking could be attributed to the leaching out of all or some membrane constituents (neutral carriers or plasticizer), i.e., they become insufficient to complete the ionic exchange process at the membrane interface.

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Figure 5

Effect dopping time in 10⁻³ M ZnCl₂ on the performance of the electrode type 1a (DP18C6+DOPP).

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Figure 6

Calibration curves for Zn-selective electrodes with PVC membrane type 1b (18C6 + DOPP) at various time of soaking.

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3.7 Response time

The response time of an ion-selective electrode represents the dynamic response of the ion-exchange process at the membrane-test solution interface. In the present work the average time required for the Zn²⁺-selective electrode to reach a potential within ±1 mV of the final equilibrium value after successive immersion of series of zinc ion solutions, each having a 10-fold difference in concentration, was measured. The results are shown in Figs. 7 and 8 and summarized in Table 9. The static response time thus obtained was less than 15 s over the entire concentration range, and the potentials stayed constant for more than 5 min, after which only a very slow divergence was recorded. The sensing behavior of the membrane remained unchanged when the potential was recorded either from low to high concentration or vice versa. Moreover, it is observed that the response times for higher concentrations are shorter than those for the diluted solutions, Table 9. This can be explained to be due to the enhancement of the ion exchange process of the electrodes at higher concentrations (see Fig. 8).

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Figure 7

Response time curves for zinc electrode type 1a (DB18C6+DOPP) for (I, 10⁻¹; II, 10⁻²; III, 10⁻³ and VI, 10⁻⁴ M).

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Figure 8

Response time curves for zinc electrode type 1b (18C6+DOPP) for (I, 10⁻¹; II, 10⁻²; III, 10⁻³ and VI, 10⁻⁴ M).

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3.8 Application