**2. How can a plasticizer work like an ionophore?**

Usually, plastic membranes of ion-sensitive electrodes are composed of sensing material (ionophore, or ion-exchanger), PVC, and plasticizers (Zamani, *Materials Science and Engineering: C* 2008; Ekmekci, *Journal of Membrane Science,*2007**)**. The basic requirements of adequate plasticizer are four criteria. The plasticizer must exhibit sufficient lipophibicity, no crystallization in the membrane and no oxidation. In addition, it must fulfill the selectivity properties (Eugster et al, *Analyt. Chim. Acta*, 1994). They tried to built plasticizers to fulfill all the requirements. They can not found a relation between selectivity coefficient and the dielectric constant of the plasticizer. From the obtained results they concluded that the selectivity properties was improved in presence of plasticizers without functional groups which can compete with the carriers.

Different plasticizers were applied like an ionophore as mentioned before. The sensing materials were considered for long time to be responsible of the selectivity properties of the membrane electrodes. Blank membranes (ionophore –free) were tried for measurements of H+ (Zareh, *Analytical Sciences* 2009). The electrodes of this type were working Nernstainly. This is due to that the plasticizer with a donor site can work like ion exchangers for cationic species like H+.

The effect of plasticizer can be clearly found, if only membranes without an ionophore were involved. An important role of plasticizers was recently discovered by (Zareh, *Analytical Sciences* 2009). In that work electrodes IIIa, IIIb, and IIIc with blank membranes were prepared. They contain only plasticizers NPOE, DOS, or DDP into the PVC matrix without any ionophore. Figure 4 shows the calibration graphs for the blank membrane electrodes (IIIa - IIIc), when H+ was measured in H2SO4. The NPOE membrane IIIa showed the best Nernstian response, 57.17 mV/decade. The other membranes (IIIb and IIIc) deviated from the Nernstian behavior. The functional groups and the donation sites of the plasticizers are proved to affect the chelation of the primary ion. In DOS and DDP, the ester groups are the main part in the molecule. This group is usually inactive regarding the coordination interactions. Due to the lone pair of the oxygen atom, the ether group in NPOE is more likely to be associated with the H+. Therefore, this membrane exhibits Nernstian response towards H+. In the absence of the ionophores, the plasticizer link to the primary ion and perform the ion exchange process, which will lead to the potential variation. The equilibrium can be represented as:

$$K\_{\rm eq} = [\text{NPOE } \text{H}^{+}]^{\*}\,\_{\rm m}/\,\,[\text{H}^{+}]\_{\rm s}\,\,[\text{NPOE}]\_{\rm m}$$

$$(\mathbf{C}\_2\mathbf{N}\cdot\mathbf{C}\_6\mathbf{H}\_4\mathbf{\cdot}\mathbf{O}\cdot\mathbf{C}\_8\mathbf{H}\_{17})\_\mathbf{m} + (\mathbf{H}^\*)\_\mathbf{s} = (\begin{bmatrix} \mathbf{O}\_2\mathbf{N}\cdot\mathbf{C}\_6\mathbf{H}\_4\mathbf{\cdot}\mathbf{O}\cdot\mathbf{C}\_8\mathbf{H}\_{17} \end{bmatrix}\mathbf{H}^\*)^\ast\mathbf{m}$$

where "m": refers to membrane site, and "s": refers to the solution site.

Tohda et al (*J. Mol. Str.*, **1997**) applied the Second Harmonic Generation (SHG)-technique for membranes without an ionophore, but in absence of the primary ion. They tried a PVCmembrane with DOS as a plasticizer. They reported that the SHG-signal is neglected for the DOS-membrane electrode without ionophore. This agrees with the poor Nernstian response for such membrane in the present study. They did not studied the SHG-signals for membranes without ionophores for NPOE nor for DDP-membrane electrodes.

Plasticizers and Their Role in Membrane Selective Electrodes 119

The calibration graphs of the blank electrode IIIa in the presence of 0.1M NaCl (to adjust ionic strength), showed a drop in the slope value 17.7 mV/decade. This drop was not observed for electrode Ia under the same condition. So, it can be concluded that the presence of an ionophore enables the membrane to carry the primary ion and subsequently stabilizes the electrode behavior (slope, linearity, response time, and selectivity). Figure 6 shows the calibration graphs of the tested electrodes. In addition, the presence of the ionophore

> **IIIa IVa Ia Icm**

**Icm : y = -13.284x + 149.2**

**Ia : y = -53.519x + 258.43**

**IIIa : y = -17.722x + 158.11**

**Iva : y = -11.665x + 123.05**

**R2 = 1**

**R2 = 0.9919**

**R2 = 0.9948**

**R2 = 0.9554**

8 6 4 2 0 **pH**

Fig. 6. Effect of the presence of 0.1MNaCl on the performance of different electrodes (Zareh,

Selectivity properties is one of the most important properties of an ion-selective electrode. The evaluation of the selectivity is a major criteria upon which the electrode is considered either selective or not. There are several methods for determining the selectivity coefficient values (*K*A,B pot). These can be mentioned below according to the IUPAC definition (Buck

The emf of a cell comprising an ion-selective electrode and a reference electrode **(ISE** cell) is measured with solutions of constant activity of interfering ion, aB, and varying activity of the primary ion. The emf values obtained are plotted *vs.* the logarithm of the activity of the primary ion aA. The intersection of the extrapolation of the linear portions of this plot indicates the value of aA which is to be used to calculate (*K*A,B pot) from the Nikolsky-

> pot A A,B z /z B

<sup>a</sup> <sup>K</sup> a 

A B

prolonged the electrode age (from 1 week for IIIa to 4 weeks for Ia).

**3. Plasticizer and selectivity properties** 

and Lindner, *Pure& Applied Chem, 1994)*:

**3.1 Fixed Interference Method (FIM)** 

**E/mV**

*Analytical Sciences*, 2009).

Eisenman equation A,B:

Fig. 4. Calibration graphs for blank membranes in presence of different plasticizers. (Zareh, *Analytical Sciences*, 2009).

The behaviour of membranes containing an ionophore (N,Ń-bisethoxycarbonyl-1,10-diaza-4,7,13,16-tetraoxacyclo-octadecane (diaza-18-crown-6) (DZCE), 37,40-bis-[(diethoxythiophosphoryl)oxy]-5,11,17,23,29,35-hexakis(1,1-dimethylethyl)-calix[6]arene-8,39,41,42 tetrol (CAX), was tested to compare them with those given by blank membrane electrodes. Figure 5, shows the results.

Fig. 5. Effect of plasticizer on the electrode performance based on DZCE-ionophore (Zareh, *Analytical Sciences*, 2009).

**IIIa: y = -57.177x + 267.77**

**IIIb: y = -46.832x + 167.95**

**IIIc: y = -26.541x + 106.75 R2= 0.9988**

**R2 = 0.9997**

**R2 = 0.997** **IIIa IIIb IIIc**

> **Ia Ib Ic**

**8 6 4 2 0 pH**

Fig. 4. Calibration graphs for blank membranes in presence of different plasticizers. (Zareh,

The behaviour of membranes containing an ionophore (N,Ń-bisethoxycarbonyl-1,10-diaza-4,7,13,16-tetraoxacyclo-octadecane (diaza-18-crown-6) (DZCE), 37,40-bis-[(diethoxythiophosphoryl)oxy]-5,11,17,23,29,35-hexakis(1,1-dimethylethyl)-calix[6]arene-8,39,41,42 tetrol (CAX), was tested to compare them with those given by blank membrane electrodes.

> 8 7 6 5 4 3 2 1 0 **pH**

Fig. 5. Effect of plasticizer on the electrode performance based on DZCE-ionophore (Zareh,

**-100**



0

50

**E, mV**

100

150

**R2 = 0.996 Ic-y = -51.863x + 234.58**

**R2 = 0.9975**

200

**Ia-y = -55.186x + 250.68 R2 = 0.9846 Ib-y = -51.478x + 224.26**

*Analytical Sciences*, 2009).

Figure 5, shows the results.

*Analytical Sciences*, 2009).

**-50**

**0**

**50**

**E/mV**

**100**

**150**

**200**

The calibration graphs of the blank electrode IIIa in the presence of 0.1M NaCl (to adjust ionic strength), showed a drop in the slope value 17.7 mV/decade. This drop was not observed for electrode Ia under the same condition. So, it can be concluded that the presence of an ionophore enables the membrane to carry the primary ion and subsequently stabilizes the electrode behavior (slope, linearity, response time, and selectivity). Figure 6 shows the calibration graphs of the tested electrodes. In addition, the presence of the ionophore prolonged the electrode age (from 1 week for IIIa to 4 weeks for Ia).

Fig. 6. Effect of the presence of 0.1MNaCl on the performance of different electrodes (Zareh, *Analytical Sciences*, 2009).

### **3. Plasticizer and selectivity properties**

Selectivity properties is one of the most important properties of an ion-selective electrode. The evaluation of the selectivity is a major criteria upon which the electrode is considered either selective or not. There are several methods for determining the selectivity coefficient values (*K*A,B pot). These can be mentioned below according to the IUPAC definition (Buck and Lindner, *Pure& Applied Chem, 1994)*:

#### **3.1 Fixed Interference Method (FIM)**

The emf of a cell comprising an ion-selective electrode and a reference electrode **(ISE** cell) is measured with solutions of constant activity of interfering ion, aB, and varying activity of the primary ion. The emf values obtained are plotted *vs.* the logarithm of the activity of the primary ion aA. The intersection of the extrapolation of the linear portions of this plot indicates the value of aA which is to be used to calculate (*K*A,B pot) from the Nikolsky-Eisenman equation A,B:

$$\mathbf{K}\_{\mathbf{A},\mathbf{B}}^{\text{pot}} = \frac{\mathbf{a}\_{\mathbf{A}}}{\mathbf{a}\_{\mathbf{B}}^{\mathbf{z}\_{\mathbf{A}}/\mathbf{z}\_{\mathbf{B}}}} $$

Plasticizers and Their Role in Membrane Selective Electrodes 121

Plasticizers affect the detection limit of the different type of electrodes. **Bedlechowicz et al**, **2002** *Journal of Electroanalytical Chemistry,* studied the effect of the plasticizer on the extended linear calibration curve and on the selectivity of a calcium selective electrode with ETH 1001 ionophore as a function of calcium activity in the internal solution. (2-Ethylhexyl)sebacate (DOS) and *o*-nitrophenyloctyl ether (*o*-NPOE) were used as plasticizers. The poly(vinylchloride) membrane also contained potassium tetrakis(4-chlorophenyl)borate. The linear part of the calibration curve of the electrode with *o*-NPOE is longer and the detection limit is lower compared to values for the electrode containing DOS as the plasticizer. The optimal activity of free Ca2+ and Na+ in the internal reference solution was 10−4 and 10−1 for the membrane with DOS and 10−6 and 10−1 for the membrane with *o*-NPOE, respectively. The repeatability of the response for electrodes with the lowest detection limit is similar in the case of both plasticizers. The selectivity coefficients were determined for electrodes having activities of calcium ion in the internal solution in the range from 10−2 to 10−10. The properties of the electrodes can be correlated with the transport properties of their

The same conclusion was recorded by **Gupta et al 2000**, *Talanta*, for Cd electrode. They studied the potential response of cadmium(II) ion selective electrode based on cyanocopolymer matrices and 8-hydroxyquinoline as ionophore has been evaluated by varying the amount of ionophore, plasticizer and the molecular weight of the cyanocopolymer. The sensitivity, working range, response time, and metal ions interference have shown a significant dependence on the concentration of ionophore, plasticizer and molecular weight of cyanocopolymers. The electrodes prepared with 2.38×10−2 mol kg−1 of ionophore, 1.23×10−2 mol dm−3 of plasticizer and 2.0 g of cyanocopolymer (molecular wt., 59 365) have shown a Nernstian slope of 29.00±0.001 mV per decade activities of Cd2+ ions with a response time of 12±0.007 s. Electrodes have shown an appreciable selectivity for Cd2+ ions in the presence of alkali and alkaline earth metal ions and could be used in a pH range of 2.5–6.5. The cyano groups of the copolymers contributed significantly to enhance the selectivity of the electrode. The electrode has shown an appreciable average life of 6 months without any significant drift in the electrode potential and found to be free from leaching of membrane ingredients. Electrode response is explained considering phase boundary model

Interferent Ia Ib Ic IIa IIIa IIIb IIIc Na+ 1.2 x 10-3 1.5 x 10-2 3.4 x 10-2 1.3 x 10-1 5.7 x 10-4 5.5 x 10-2 3.4 x 10-1 K+ 2.7 x 10-3 5.2 x 10-2 1.2 x 10-1 1.7 x 10-1 1.1 x 10-3 3.2 x 10-1 1.7 Cs+ 2.4 x 10-3 5.1 x 10-2 1.1 x 10-1 1.8 x 10-1 1.2 x 10-3 2.8 x 10-1 2.4 NH4+ 2.2 x 10-3 3.1 x 10-2 5.4 x 10-2 1.8 x 10-1 9.8 x 10-4 1.3 x 10-1 6.6 x 10-1 Mg++ 1.3 x 10-4 2.3 x 10-3 8.0 x 10-3 1.6 x 10-2 8.7 x 10-5 1.7 x 10-3 1.4 x 10-2 Ca++ 9.2 x 10-5 2.0 x 10-3 4.8 x 10-3 1.7 x 10-2 6.4 x 10-5 1.1 x 10-3 8.1 x 10-3 Ba++ 8.3 x 10-5 1.8 x 10-3 9.8 x 10-3 1.8 x 10-2 4.9 x 10-5 2.2 x 10-3 2.8 x 10-2 Pb++ 2.0 x 10-3 8.7 x 10-2 2.0 x 10-1 3.2 x 10-2 1.3 x 10-3 3.0 x 10-1 5.0 x 10-1 Zn++ 1.7 x 10-4 2.4 x 10-2 5.4 x 10-3 1.7 x 10-2 8.0 x 10-5 1.5 x 10-3 1.0 x 10-2 Table 2**.** Selectivity coefficient values (*K*H+, Jz+ ) for H+-electrodes based on diazacrown ether

analogues (I), phosphorated calix[6]arene (II), and blank membranes (III).

**4. Plasticizer and detection limit** 

based on thermodynamic considerations.

membranes.

#### **3.2 Separate Solution Method (SSM)**

The emf of a cell comprising an ion-selective electrode and a reference electrode (ISE cell) is measured with each of two separate solutions, one containing the ion A of the activity aA (but no B), the other containing the ion B at the same activity aB = aA (but no A). If the measured values are EA and EB, respectively, the value of may be calculated from the equation:

$$\log \mathbf{K}\_{\mathrm{A},\mathrm{B}}^{\mathrm{pot}} = \frac{(\mathbf{E}\_{\mathrm{B}} - \mathbf{E}\_{\mathrm{A}}) \mathbf{z}\_{\mathrm{A}} \mathbf{F}}{2.303 \mathrm{RT}} + \left(1 - \frac{\mathbf{z}\_{\mathrm{A}}}{\mathbf{z}\_{\mathrm{B}}}\right) \log \mathbf{a}\_{\mathrm{A}}$$

#### **3.3 The Separate Solution Method SSM) II**

The concentrations of a cell comprising an ion selective electrode and a reference electrode (ISE cell) are adjusted with each of two separate solutions, one containing the ion A of the activity aA (but no B), the other containing the ion B (but no A) of the activity as high as required to achieve the same measured cell voltage. From any pair of activities aA and aB giving the same cell voltage, the value of (*K*A,B pot) may be calculated from the

$$\mathbf{K}\_{\mathbf{A},\mathbf{B}}^{\text{Pot}} = \frac{\mathbf{a}\_{\mathbf{A}}}{\mathbf{a}^{\mathbf{z}\_{\mathbf{A}}/\mathbf{z}\_{\mathbf{B}}}} $$

The FIM and SSM methods are recommended only when the electrode exhibits a Nernstian response to both principal and interfering ions. These methods are based on the assumption that plots of El *vs.* lg(aA1/lzAl ) and E2 *vs.* lg(aBl/1zBl ) will be parallel and the vertical spacing is (2.303RT/F)lg *K*A,B pot.

However, the FIM can always be used to determine a minimum primary ion concentration level at which the effect at interference can be neglected. The actual conditions of the FIM method match the conditions under which the electrodes are used.

What is interesting is that these electrodes (ionophore-free) showed significant selectivity properties. This is found from the values of the selectivity coefficient. The selectivity coefficient values of an electrode (*K*A,B pot) is calculated by the SSM for different common cations. Table (2) shows an example of the obtained results. The values were compared to those for electrodes containing ionophoric sensing material (diaza-18-crown-6 for Ib, Ic; and phosphorylated calix-6-arene for IIa). In case of other electrodes, (Ib, Ic, and IIa), high selectivity coefficient values were observed. These values indicate that the selectivity properties of electrodes Ib, Ic and IIa are lower than those for Ia electrode. This is attributed to the chelating property of the plasticizer NPOE. To prove this effect of the plasticizer, the selectivity coefficient values for the blank membrane electrodes IIIa, IIIb and IIIc were calculated. Likewise Ia, the selectivity coefficient values of IIIa electrode is better than those for IIIb and IIIc electrodes. The second important observation from the selectivity coefficient values of blank electrodes was that these values are very close to the values recorded for electrodes Ia, Ib, and Ic. Accordingly, the plasticizer plays an important role in the selectivity properties of the membrane electrodes.

Plasticizers and Their Role in Membrane Selective Electrodes 121

The emf of a cell comprising an ion-selective electrode and a reference electrode (ISE cell) is measured with each of two separate solutions, one containing the ion A of the activity aA (but no B), the other containing the ion B at the same activity aB = aA (but no A). If the measured values are EA and EB, respectively, the value of may be calculated from the

> pot B AA A A,B A

The concentrations of a cell comprising an ion selective electrode and a reference electrode (ISE cell) are adjusted with each of two separate solutions, one containing the ion A of the activity aA (but no B), the other containing the ion B (but no A) of the activity as high as required to achieve the same measured cell voltage. From any pair of activities aA and aB

> A ,B A B pot A z /z

The FIM and SSM methods are recommended only when the electrode exhibits a Nernstian response to both principal and interfering ions. These methods are based on the assumption

However, the FIM can always be used to determine a minimum primary ion concentration level at which the effect at interference can be neglected. The actual conditions of the FIM

What is interesting is that these electrodes (ionophore-free) showed significant selectivity properties. This is found from the values of the selectivity coefficient. The selectivity coefficient values of an electrode (*K*A,B pot) is calculated by the SSM for different common cations. Table (2) shows an example of the obtained results. The values were compared to those for electrodes containing ionophoric sensing material (diaza-18-crown-6 for Ib, Ic; and phosphorylated calix-6-arene for IIa). In case of other electrodes, (Ib, Ic, and IIa), high selectivity coefficient values were observed. These values indicate that the selectivity properties of electrodes Ib, Ic and IIa are lower than those for Ia electrode. This is attributed to the chelating property of the plasticizer NPOE. To prove this effect of the plasticizer, the selectivity coefficient values for the blank membrane electrodes IIIa, IIIb and IIIc were calculated. Likewise Ia, the selectivity coefficient values of IIIa electrode is better than those for IIIb and IIIc electrodes. The second important observation from the selectivity coefficient values of blank electrodes was that these values are very close to the values recorded for electrodes Ia, Ib, and Ic. Accordingly, the plasticizer plays an important role in the selectivity

<sup>a</sup> <sup>K</sup> a 

giving the same cell voltage, the value of (*K*A,B pot) may be calculated from the

) and E2 *vs.* lg(aBl/1zBl

method match the conditions under which the electrodes are used.

(E E )z F z logK 1 lg <sup>a</sup> 2.303RT z 

B

) will be parallel and the vertical spacing is

**3.2 Separate Solution Method (SSM)** 

**3.3 The Separate Solution Method SSM) II** 

that plots of El *vs.* lg(aA1/lzAl

properties of the membrane electrodes.

(2.303RT/F)lg *K*A,B pot.

equation:


Table 2**.** Selectivity coefficient values (*K*H+, Jz+ ) for H+-electrodes based on diazacrown ether analogues (I), phosphorated calix[6]arene (II), and blank membranes (III).
