**1. Introduction**

112 Recent Advances in Plasticizers

Williams, A. (2003). *Transdermal and Topical Drug Delivery: From Theory to Clinical Practice*.

Wu, C. & McGinity, J.W. (1999).Non Traditional Plasticization of Polymeric Films. *International Journal of Pharmaceutics*, Vol.177, pp. 15-27, ISSN 0378-5173 Wypch, G. (2004). *Handbook of Plasticizers*, Chem Tec, 437-440, ISBN 1-895198-29-1, Ontario,

Zhu, Y.; Shah, N.H.; Malick, A.W.; Infeld, M.H. & McGinity, JW. (2002). Solid State

Plasticization of an Acrylic Polymer With Chlorpheniramine Maleate and Triethyl Citrate. *International Journal of Pharmaceutics*, Vol.241, pp.301-310, ISSN 0378-5173

Pharmaceutical Press, 169-194, ISBN 0-85369-489-3, London, UK.

Canada

In the last four decades, the uses and application of ion-selective electrodes was widely applied in several researches as well as analytical projects. These electrodes are varied between solid state electrodes, liquid membrane electrodes, gas membrane electrodes, and plastic membrane electrodes. Plasticizers are one of the major components of the plastic membranes. As it is usually known, they are responsible of their physical properties.

In the beginning, a general idea about ion-selective membrane electrodes (Janata *Principles of Chemical Sensors* 1989) will be useful to understand the role of plasticizers as they are part of the constituents for some of them. These membranes are the main component of the potentiometric ion sensors. They are responsible of forming a type of discrimination in the electrode behavior towards one ion rather than others. A potential difference will be aroused when the analyte ion can penetrate across the phase boundary between the two phases (analyte solution, and internal reference solution). Accordingly, an electrochemical equilibrium will be formed, due to different potentials at both sides of the membrane. The potential difference (E) across the membrane is described by the Nernst equation:

$$\mathbf{E} \equiv \mathbf{E}^{\bullet} + (\mathbf{R} \mathbf{T} / \mathbf{Z} \mathbf{F}) \text{ In a} \tag{1}$$

Where, E : is the standard cell potential, R: general gas constant, Z: valency of the analyte ion, F:Faraday's constant, a: activity of the analyte ion.

If the activity of the target ion at side A is kept constant, the unknown activity at side B (aA = ax) is related to (E) by the following equation (at equilibrium condition):

$$E = RT / Z\_{\text{x}}\\
\text{F} \cdot \ln \left( \mathbf{a}\_{\text{x}} / \mathbf{a}\_{\text{A}} \right) = \text{const} + \text{S} \cdot \log \left( \mathbf{a}\_{\text{x}} \right) \tag{2}$$

where S=59.16/Z [mV] at T=298 K.

There are two main groups of membranes, which are used in ion-selective electrodes: namely, crystalline and noncrystalline membranes (Buck and E. Lindner, *Pure & App.Chem.* (1994).

<sup>\*</sup>Corresponding Author

Plasticizers and Their Role in Membrane Selective Electrodes 115

connected to a reference electrode. Then, both electrodes are immersed in the solution to be tested. The concentration of the tested solution can be estimated from the galvanometer reading by the aid of the previously constructed calibration graph (for standard analtye

Fig. 2. Arrangement of a cell used for measurements by ion-selective 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

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

**1.2 Plasticizers for membrane electrodes** 

applied for plastic membranes are shown in figure 3.

plasticizers.

concentrations).

The crystalline membrane electrodes might be homogeneous or heterogenous. The homogenous type is prepared from single compound or a homogenous mixture (e.g. Ag2S, AgI/Ag2S). An example is the fluoride selective electrode based on LaF3 crystals.

On the other side, the heterogenous membrane electrodes are formed by mixing an active substance or a mixture of active substances with an inert matrix (e.g. silicon rubber, PVC, hydophobized substances or conducting epoxy.

Non-crystalline membrane electrodes are composed of a support matrix, containing ionexchanger (either cationic or anionic), a plasticizer solvent, and possibly an uncharged selectivity-enhancing species as a membrane which interposed between two aqueous solutions. The support matrix might be macroporous (poly-propylene carbonate, glass frit) or microporous (thirsty glass or inert polymeric material such as PVC)

#### **1.1 Construction of the electrodes**

These electrodes are prepared from glass capillary tubing approximately 2 millimeters in diameter, a large batch at a time. Polyvinyl chloride is dissolved in a solvent and plasticizers (typically phthalates) added, in the standard fashion used when making something out of vinyl. In order to provide the ionic specificity, a specific ion channel or carrier is added to the solution; this allows the ion to pass through the vinyl, which prevents the passage of other ions and water.

The measurements procedures using such electrodes are based on connecting the electrode to the terminal of a galvanometer or pH meter (G), the other terminal is

The crystalline membrane electrodes might be homogeneous or heterogenous. The homogenous type is prepared from single compound or a homogenous mixture (e.g. Ag2S,

AgI/Ag2S). An example is the fluoride selective electrode based on LaF3 crystals.

Fig. 1. Relation between EMF of the cell and activity of the analyte.

or microporous (thirsty glass or inert polymeric material such as PVC)

hydophobized substances or conducting epoxy.

**1.1 Construction of the electrodes**

other ions and water.

On the other side, the heterogenous membrane electrodes are formed by mixing an active substance or a mixture of active substances with an inert matrix (e.g. silicon rubber, PVC,

Non-crystalline membrane electrodes are composed of a support matrix, containing ionexchanger (either cationic or anionic), a plasticizer solvent, and possibly an uncharged selectivity-enhancing species as a membrane which interposed between two aqueous solutions. The support matrix might be macroporous (poly-propylene carbonate, glass frit)

These electrodes are prepared from glass capillary tubing approximately 2 millimeters in diameter, a large batch at a time. Polyvinyl chloride is dissolved in a solvent and plasticizers (typically phthalates) added, in the standard fashion used when making something out of vinyl. In order to provide the ionic specificity, a specific ion channel or carrier is added to the solution; this allows the ion to pass through the vinyl, which prevents the passage of

The measurements procedures using such electrodes are based on connecting the electrode to the terminal of a galvanometer or pH meter (G), the other terminal is connected to a reference electrode. Then, both electrodes are immersed in the solution to be tested. The concentration of the tested solution can be estimated from the galvanometer reading by the aid of the previously constructed calibration graph (for standard analtye concentrations).

Fig. 2. Arrangement of a cell used for measurements by ion-selective electrodes.
