**1.2 Plasticizers for membrane electrodes**

PVC-membranes for the selective electrodes are actually plastics. Different types of plasticizers are usually applied for preparing the membrane electrodes. Many types of plasticizers are used for plastic membranes as additives. Phthalates are considered as the most common types which produce the desired flexibility and durability. They are esters of polycarboxylic acids with either linear or branched aliphatic alcohol. The plasicizer embeddy itself between the chains of the polymer. It spaces them apart, so the free volume increases. The more the added plasticizer, the more flexiblility and durability (lower cold flex temperature).They might be ethers, esters of either aromatic or aliphatic acids. The ester plasticizer can be classified into dicarboxylic/tricarboxylic ester-based plasticizers.

Several plasticizers are recorded for preparing ion-selective membrane electrodes (Zareh, *Sensor Letters*, 2010). Some common phthalate plasticizers are **[**Chanda and Roy (2006). Plastics technology handbook]. This type of plasticizer is commonly applied for preparing membrnaes of the ion-selective electrodes. Structure of selected plasticizers commonly applied for plastic membranes are shown in figure 3.

Plasticizers and Their Role in Membrane Selective Electrodes 117

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

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

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

*K*eq = [NPOE H+]+ m / [H+]s [NPOE]m

(O2N-C6H4-O-C8H17)m + (H+ )s = ( [O2N- C6H4-O-C8H17] H+ ) +m

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

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

membranes without ionophores for NPOE nor for DDP-membrane electrodes.

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

which can compete with the carriers.

equilibrium can be represented as:

species like H+.

Fig. 3. Structure of selected plasticizers used for preparing ion selective electrode membranes:a) Diethyl phthalate, b) Di-n-octyl phthalate, c) bis(2-ethylhexyl) sebacate, d) 2- Nitrophenyl n-octyl ether.


Table 1. Selected examples of several plasticizers used in preparing ion selective electrodes.

From the above mentioned plasticizers, phthalates (especially dioctylphthalate, diethyl phthalate, dibutyl sebacate, dinitrophenyl octyl ether, didecyl phtalate are widely used in preparing PVC-membranes for the ion-selective electrodes.

a b

c d

membranes:a) Diethyl phthalate, b) Di-n-octyl phthalate, c) bis(2-ethylhexyl) sebacate, d) 2-

dioctyl sebacate Surfactant Espadas-Torre,

4 Dioctyl phenyl phosphonate Pb++ Quagraine and Gadzekpo,

7 Dioctyl phenyl phosphonate Fe+++ Zareh et al, *Electroanalysis*, 2010

Table 1. Selected examples of several plasticizers used in preparing ion selective electrodes.

From the above mentioned plasticizers, phthalates (especially dioctylphthalate, diethyl phthalate, dibutyl sebacate, dinitrophenyl octyl ether, didecyl phtalate are widely used in

dimethyle sebacate Triiodide Khayatian et al, *Anal. Sci.*

5 Dibutyl phthalate nitrate Péreza et al, *Sens. and*

6 Dioctyl sebacate Organic amines Odashima et al,

pyromellitate Cocaine

preparing PVC-membranes for the ion-selective electrodes.

Ascorbic acid, Zareh,

Quinine Zareh et al,

*Sensor Letters.*, 2010

*Analyt.Chim. Acta*., 2001

*Anal.Chem.* 1996

*Analyst*., 1992

*Act. B*, 2003

*Anal. Chem.*,1993

Watanabe et al, *Analytica Chimica Acta 1995*

21, 297-302

Fig. 3. Structure of selected plasticizers used for preparing ion selective electrode

No. Plasticizer Ion response Reference

Nitrophenyl n-octyl ether.

Dioctyl phthalte, didecylphthalate, dioctyl sebacate

Nitophenyl octyl ether, didecylphthalate, dioctyl sebacate

<sup>3</sup>Dioctyl phthalte

<sup>8</sup>tetrakis (2-ethylhexyl)

<sup>9</sup>Dibutyl phthalate, acetophenone,

1

2
