**2. Plasticizers used for the polymeric membranes of ion-selective electrodes**

The development of plasticized polymeric membrane sensors was a big step forward. It led to the advance and diversification of ion-selective electrode analysis. The liquid membranes of the electrodes were difficult to handle and did not allow the use of ion-selective electrodes (ISEs) in any position because the liquid membrane would leak. The polymeric membrane has properties similar to those of liquid membranes, but the range of applications is much larger. Plasticized polymers are in fact highly viscous liquids and they are known in the literature as liquid membranes due to high values of diffusion coefficients of ionophores and their complexes. This membrane can still be considered as a liquid phase, because diffusion coefficients for a dissolved low-molecular-mass component (e.g., an ionophore) are on the order of 10-7 to 10-8 cm2 s-1 (Moody & Thomas, 1979 as cited in Oesch et al., 1986). Typically, such a solvent polymeric membrane contains about 66 g of plasticizer and only 33 g of PVC per 100 g. Only at very low plasticizer contents (<20 g/100 g), diffusion coefficients may be l0-11 cm2 s-1 and smaller, approaching values that are found for solids.

The plasticizers used in the preparation of the polymeric membrane of ion –selective electrodes must be compatible with the polymer and electrodic component and also must be solved in tetrahydrofuran or cyclohexanone, the solvent used in the membrane preparation. The plasticizers with high lipophilicity are preferred. The most used plasticizers are: *ortho*-nitrophenyloctyl ether (NPOE), dibutyl phthalate (DBP),

Use of Plasticizers for Electrochemical Sensors 127

The membrane potential εm, for an ion *i* with a zi charge, is expressed by the equation (1):

*<sup>i</sup> <sup>m</sup> i i RT a zF a*

In which R represents the ideal gas constant, T represents the absolute temperature, F represents the Faraday constant. (ai)<sup>α</sup> represents the activity of the primary ion *i* in phase α,

Indexes α and β refer to the two phases: α – the analyte and β – the membrane. Phase γ consists of an aqueous solution of known concentration that contains an existent ion and phase β (membrane). If this ion is ion *i* the electrochemical cell is a concentration cell and the membrane potential can be considered a concentration potential. The membrane potential εm' which appears at the contact of phase β and γ will have a constant value because the common ions activity is constant in both phases. Potentials εm and εm' do not appear as a result of oxidation or reduction, but due to some ion exchange equilibriums in which the analyzed species participate. The difference in potential Ec between the two electrodes of the

> '*Ec mR mR j* ,1 ,2

> > ( ) . .ln

 

( )

*i i*

*zF a*

(2)

(3)

(4)

ε*j* is the junction potential. If εR,1, εR,2, εm' and εj values are considered constant equation (2)

*<sup>i</sup> c m*

If the activity of ion *i* in the membrane (ai)<sup>β</sup> is constant the membrane potential εm varies by

*c i* . ln( ) *i RT E const a z F*

*RT a E const const*

 

 

electrochemical cell, represented in figure 2, is given by the following equation:

the *i* species activity, according to a Nernstian law:

can be written:

( ) ln ( )

(1)

Fig. 2. Electrochemical cell for potentiometric analysis.

(ai)β represents the activity of the primary ion *i* in the phase β.

dinonyladipinate (DNA), tris(2-ethylhexyl) phosphate (TEHP), tris(ethylhexyl) phosphate (TEHP), bis(2-ethylhexyl) adipate (DOA), dioctylphthalate (DOP), and bis (2-ethylhexyl) sebacate (DOS).

This way a large constructive variety of polymeric membrane sensors could be made with or without an internal reference solution, including the sensors used in flow injection analysis. The systematization of the bibliographic material and our own research material was pursued in order to point out the way in which the properties of the plasticizers influence the characteristics of the potentiometric sensors (whose main components are ion-selective electrodes) and the general way of properly selecting a suitable plasticizer. Plasticizers used for the preparation of polymeric membranes used in the construction of ion-selective electrodes for inorganic and organic ions are presented. We also presented the performances obtained as well as how to select a plasticizer for the construction of ion-selective electrodes used in pharmaceutical products, anionic surfactants, physiologically active amines and inorganic ions analysis.

#### **2.1 The membrane potential and the function of the ion–selective electrodes**

A potentiometric sensor with polymeric membrane is shown in Figure 1.

Fig. 1. Potentiometric sensor (1 – ion-selective electrode; 2 – internal reference solution; 3 – internal reference electrode; 4 – membrane; 5 – test solution\sample; 6 – external reference electrode; 7 – milivoltmeter; 8 – magnet piece for magnetic stirring).

Figure 2 shows the contact phases and electrochemical potentials occurring in an electrochemical cell used for potentiometric measurements.

dinonyladipinate (DNA), tris(2-ethylhexyl) phosphate (TEHP), tris(ethylhexyl) phosphate (TEHP), bis(2-ethylhexyl) adipate (DOA), dioctylphthalate (DOP), and bis (2-ethylhexyl)

This way a large constructive variety of polymeric membrane sensors could be made with or without an internal reference solution, including the sensors used in flow injection analysis. The systematization of the bibliographic material and our own research material was pursued in order to point out the way in which the properties of the plasticizers influence the characteristics of the potentiometric sensors (whose main components are ion-selective electrodes) and the general way of properly selecting a suitable plasticizer. Plasticizers used for the preparation of polymeric membranes used in the construction of ion-selective electrodes for inorganic and organic ions are presented. We also presented the performances obtained as well as how to select a plasticizer for the construction of ion-selective electrodes used in pharmaceutical products, anionic surfactants, physiologically active amines and

**2.1 The membrane potential and the function of the ion–selective electrodes** 

Fig. 1. Potentiometric sensor (1 – ion-selective electrode; 2 – internal reference solution; 3 – internal reference electrode; 4 – membrane; 5 – test solution\sample; 6 – external reference

Figure 2 shows the contact phases and electrochemical potentials occurring in an

electrode; 7 – milivoltmeter; 8 – magnet piece for magnetic stirring).

electrochemical cell used for potentiometric measurements.

A potentiometric sensor with polymeric membrane is shown in Figure 1.

sebacate (DOS).

inorganic ions analysis.

Fig. 2. Electrochemical cell for potentiometric analysis.

The membrane potential εm, for an ion *i* with a zi charge, is expressed by the equation (1):

$$
\varepsilon\_{\mathcal{E}^{\rm{m}}} = \frac{RT}{ziF} \ln \frac{(ai)a}{(ai)\rho} \tag{1}
$$

In which R represents the ideal gas constant, T represents the absolute temperature, F represents the Faraday constant. (ai)<sup>α</sup> represents the activity of the primary ion *i* in phase α, (ai)β represents the activity of the primary ion *i* in the phase β.

Indexes α and β refer to the two phases: α – the analyte and β – the membrane. Phase γ consists of an aqueous solution of known concentration that contains an existent ion and phase β (membrane). If this ion is ion *i* the electrochemical cell is a concentration cell and the membrane potential can be considered a concentration potential. The membrane potential εm' which appears at the contact of phase β and γ will have a constant value because the common ions activity is constant in both phases. Potentials εm and εm' do not appear as a result of oxidation or reduction, but due to some ion exchange equilibriums in which the analyzed species participate. The difference in potential Ec between the two electrodes of the electrochemical cell, represented in figure 2, is given by the following equation:

$$E\_c = \varepsilon\_m + \varepsilon\_{R,1} + \varepsilon\_m^\cdot - \varepsilon\_{R,2} - \varepsilon\_j \tag{2}$$

ε*j* is the junction potential. If εR,1, εR,2, εm' and εj values are considered constant equation (2) can be written:

$$E\_c = const. + \varepsilon\_m = const. + \frac{RT}{ziF} \ln{\frac{\langle ai \rangle a}{\langle ai \rangle \rho}} \tag{3}$$

If the activity of ion *i* in the membrane (ai)<sup>β</sup> is constant the membrane potential εm varies by the *i* species activity, according to a Nernstian law:

$$E\_{\ell} = const. + \frac{RT}{z\ell F} \ln(ai)a \tag{4}$$

Use of Plasticizers for Electrochemical Sensors 129

Fig. 3. Polymeric membrane electrode with an internal reference solution. 1 – Electrode body; 2 - Internal reference electrode (usually Ag - AgCl); 3 - Internal electrolyte; 4 -

Plasticized polymeric membrane.

Figure 4 shows a "coated wire" ion-selective electrode.

Fig. 4. Ion-selective electrode with a "coated wire" polymeric membrane.

evaporation of the solvent, the procedure must be repeated twice.

The electrode coating mixtures containing the plasticizer, PVC of high molecular mass and the ionophore or the ion-exchange sensing material is obtained by dissolving 1 g of mixture in 20 cm3 of tetrahydrofuran. A metallic wire (Ag, Pt, Cu, Al) or a teflonised graphite electrode which served as a membrane carrier is dipped in the coating mixture and after

Insulator (teflon)

Platinum wire

A sensitive layer that has an ionophore embedded in a

plasticized polymer matrix built in

Considering a working temperature of 25C, equation (4) becomes:

$$E\_{\mathcal{E}} = const. + \frac{0.059}{zi} \text{log(} a\text{)}a \tag{5}$$

In fact, the strict obedience of this law by a potentiometric sensor is disturbed by some interferences. They appear because of the fact that the membranes are not perfectly selective and are permeable to ions other than the primary ion.

$$\mathbf{E}\_{\mathbf{c}} = \text{const.} \pm \frac{RT}{zF} \text{ln}\left(a i + \sum K\_{i} \mathbf{a}\_{i}^{\text{Zi}/Zj}\right) \tag{6}$$

in which *j* represents the interfering species, a*j* the activity of the interfering ion, and z*j* it's charge. kij is the potentiometric selectivity coefficient. The sign of the logarithmic term is "+" if *i* is a cation and "-" if it is an anion.

#### **2.2 The preparation of plasticized polymeric membranes**

The preparation of plasticized membranes is relatively simple. It can be used to construct a great variety of polymeric membrane sensors selective to many inorganic and organic ions. The polymeric membrane contains the following substances: the electrodic component, the plasticizer, and the polymeric substance. The electrodic component can be an organic ion exchanger called ionophore, a neutral sequestrant or a complex combination. This component makes the membrane sensitive to the species that needs to be analyzed because it is responsible for the appearance of the membrane potential due to the repartition equilibrium between the sample and membrane phases. The analyte in the membrane phase is involved in a chemical equilibrium with the ionophore.

The polymeric substance is usually polyvinyl chloride (PVC) with high molecular mass, but other polymers are also used (polyurethane and polyaniline). Plasticizers are used in a relatively large proportion, generally 66%. The plasticizer assures the mobility of the ion exchanger, fixes the dielectric constant value of the membrane and confers it the adequate mechanical properties. Choosing a suitable plasticizer is important because it improves the ion-selective electrodes sensitivity. During the time, plasticizer is gradually released from the polymeric membrane due to the contact of the analyzed solution with the water that enters the membrane. The membrane becomes opaque. Membrane components are dissolved in a suitable solvent (tetrahydrofuran or cyclohexanone) and they mix with the formation of a viscous liquid that is poured on a flat surface and left to dry slowly for 48 hours in a solvent vapor (tetrahydrofuran or cyclohexanone) saturated atmosphere. By evaporation a thin polymer film is formed. The polymeric membrane is then cut into disk forms and glued to an electrode body in which the internal reference solution and the internal reference electrode are introduced. Another approach consists in the deposit of the membrane on a metallic pill made out of Ag or Cu or on a graphite rod. The electrodes that have been prepared this way are left covered by a glass bell to avoid the rapid evaporation of the solvent which may affect the homogeneity of the membrane. Figure 3 shows a classic membrane electrode with an internal reference solution.

0,059 *c i* . log( ) *i E const a z*

In fact, the strict obedience of this law by a potentiometric sensor is disturbed by some interferences. They appear because of the fact that the membranes are not perfectly selective

in which *j* represents the interfering species, a*j* the activity of the interfering ion, and z*j* it's charge. kij is the potentiometric selectivity coefficient. The sign of the logarithmic term is "+"

The preparation of plasticized membranes is relatively simple. It can be used to construct a great variety of polymeric membrane sensors selective to many inorganic and organic ions. The polymeric membrane contains the following substances: the electrodic component, the plasticizer, and the polymeric substance. The electrodic component can be an organic ion exchanger called ionophore, a neutral sequestrant or a complex combination. This component makes the membrane sensitive to the species that needs to be analyzed because it is responsible for the appearance of the membrane potential due to the repartition equilibrium between the sample and membrane phases. The analyte in the membrane phase

The polymeric substance is usually polyvinyl chloride (PVC) with high molecular mass, but other polymers are also used (polyurethane and polyaniline). Plasticizers are used in a relatively large proportion, generally 66%. The plasticizer assures the mobility of the ion exchanger, fixes the dielectric constant value of the membrane and confers it the adequate mechanical properties. Choosing a suitable plasticizer is important because it improves the ion-selective electrodes sensitivity. During the time, plasticizer is gradually released from the polymeric membrane due to the contact of the analyzed solution with the water that enters the membrane. The membrane becomes opaque. Membrane components are dissolved in a suitable solvent (tetrahydrofuran or cyclohexanone) and they mix with the formation of a viscous liquid that is poured on a flat surface and left to dry slowly for 48 hours in a solvent vapor (tetrahydrofuran or cyclohexanone) saturated atmosphere. By evaporation a thin polymer film is formed. The polymeric membrane is then cut into disk forms and glued to an electrode body in which the internal reference solution and the internal reference electrode are introduced. Another approach consists in the deposit of the membrane on a metallic pill made out of Ag or Cu or on a graphite rod. The electrodes that have been prepared this way are left covered by a glass bell to avoid the rapid evaporation of the solvent which may affect the homogeneity of the membrane. Figure 3 shows a classic

*Ec* / . ln *Zi Zj i ij j RT const a K a zF* (6)

(5)

Considering a working temperature of 25C, equation (4) becomes:

and are permeable to ions other than the primary ion.

**2.2 The preparation of plasticized polymeric membranes** 

is involved in a chemical equilibrium with the ionophore.

membrane electrode with an internal reference solution.

if *i* is a cation and "-" if it is an anion.

Fig. 3. Polymeric membrane electrode with an internal reference solution. 1 – Electrode body; 2 - Internal reference electrode (usually Ag - AgCl); 3 - Internal electrolyte; 4 - Plasticized polymeric membrane.

Figure 4 shows a "coated wire" ion-selective electrode.

Fig. 4. Ion-selective electrode with a "coated wire" polymeric membrane.

The electrode coating mixtures containing the plasticizer, PVC of high molecular mass and the ionophore or the ion-exchange sensing material is obtained by dissolving 1 g of mixture in 20 cm3 of tetrahydrofuran. A metallic wire (Ag, Pt, Cu, Al) or a teflonised graphite electrode which served as a membrane carrier is dipped in the coating mixture and after evaporation of the solvent, the procedure must be repeated twice.

Use of Plasticizers for Electrochemical Sensors 131

yielded comparable DL. With the dbdb-18-6, DOA, TEHP and DOS gave better RS, and DOA and DOS yielded a better DL. Most of these had detection limit below 10–5 M and a response slope below 50 mV/decade, which are not better than the valinomycin-based ISEs. Only the electrode with the dibenzo-18-crown-6-ether ionophore with DOA as the plasticizer showed a better performance than the valinomycin ISEs: response slope of 58

The performances of tetracycline based cation selective polymeric membrane electrodes of many sets with different plasticizers were investigated as the selectivity of ion-selective electrodes and optodes are greatly influenced by membrane solvent and also controlled by plasticizers. A membrane with bis(2-ethylhexyl) sebacate and additive shows good potentiometric performance toward Ca2+ (slope: 27.8 mV per decade; DL: −4.52) including selectivity (Baek et al., 2007). Contrastingly, a membrane with dibutyl phthalate shows near Nernstian response, it has also shown the best measuring range and detection limit for Ca2+ (29.5 mV and 5.10) and Mg2+ (24.4 mV and −5.04) and the least selectivity has been also observed between Ca2+ and Mg2+. When both membranes were used together to flow

Based on the concept of ion-selective conductometric micro sensors (ISCOM) a new calcium sensor was developed and characterized. Optimization of the membrane composition was carried out by testing different types of calcium-ionophores, polymers, and plasticizers. The most commonly used membrane material is based on plasticized high molecular PVC. In general, only a limited number of commercially available calcium ionophores and plasticizing agents are used in Ca2+-ISE (Trebbe et al., 2001). The tested plasticizers are: 2- Nitrophenyl octyl ether (NPOE), dibutyl sebacate (DBS), dioctylphenylphosphonate (DOPP), bis(1-butylpentyl)decane-1,10-diyl diglutarate ETH 469 (BDD), 2-fluorophenyl-2 nitrophenylether (2F2NE), and tetrahydrofuran. In order to investigate the influence of membrane components 14 different membrane compositions containing different commercially available ionophores, plasticizers, and polymers have been tested. It was concluded that plasticizers used in organic solvent membranes have to fulfill many criteria, e.g. high lipophilicity, solubility in the polymeric membrane (no precipitations) as well as no exudation (one phase system) and a good selectivity of the resulting membrane. Moreover, with regard to the ISCOM operation mechanism, a high polarity may be advantageous in order to extract ionic species into the membrane phase. Membranes based on plasticizer (NPOE) with a quite high polarity and moderate lipophilicity showed the best properties. Rapid and economical procedures for determining aluminum(III) in aqueous solutions are required in the industry of aluminum compounds. A plasticized Al-selective electrode was fabricated and studied. The composition of a membrane was as follows: the ionophore of an aluminon, 10 mM (~1 wt %); PVC, 66 wt %; dibutyl phthalate (DBP) as plasticizer, 33 wt %. The possibility of using the developed ISE for determining aluminum(III) in aqueous

system, the concentration of Ca2+ and Mg2+ could be determined, simultaneously.

**4. Selecting plasticizers for anionic surfactant sensitive potentiometric** 

The surfactants are compounds essential to the modern civilization and technology. Determination and monitorization of the surfactants concentration is necessary in the

mV/decade and DL of 10-5,82.

solutions was proved (Evsevleeva et al., 2005).

**sensors** 

Fig. 5. The design of an ion - selective electrode with a PVC membrane deposed on a metallic pill 1 - PVC body; 2 - Cu or Ag pill; 3 - coaxial cable; 4 - PVC membrane sensible to a certain anion or cation.

The time of usage for a polymeric membrane electrode is limited to 1 to 6 months. This aspect is compensated by the low price and simple manufacturing (Mihali et al., 2008).

#### **2.3 The selection of a proper plasticizer**

The desirable properties of a plasticizer used in the membrane preparation of the ionselective electrodes are: compatibility with the polymer, low volatility and low solubility in aqueous solution, low viscosity, low cost and low toxicity (O'Rourke et al., 2011). In order to select the best plasticizer usually some tests are necessary. Electrodes with different compositions are built, in which both the nature of the ionophore and of the plasticizers are modified and their proportions in the membranes are changed. The properties of the ionselective electrodes with different membrane compositions are tested. The electrode which has the proper characteristics is selected. The most important considered characteristics are: linear response range, slope (sensitivity), and also selectivity towards the ions that can be present in the analyzed solution. An electrode used to determine species *i* can respond to species *j*. The selectivity coefficient shows the electrode sensitivity ratio for different species. A low value of the selectivity coefficient shows a low interference toward certain chemical specie.
