**3. Selecting plasticizers for inorganic ion sensitive potentiometric sensors**

New ion-selective electrodes for potassium were developed and tested employing 18 crown-6-ether, dibenzo-18-crown-6-ether, and 4',4''(5'')-di-tert-butyldibenzo-18-crown-6 ether (dbdb-18-6) ionophores in PVC membranes with a polyaniline solid contact between the membranes and the Pt substrate (Han et al., 2008). Many types of sensors have been developed to measure the concentration of potassium ions. Various plasticizers (orthonitrophenyloctyl ether (NPOE), tris(ethylhexyl) phosphate (TEHP), bis(2-ethylhexyl) adipate (DOA), dioctylphthalate (DOP), and bis (2-ethylhexyl) sebacate (DOS)) were tested for the best response. It should be remembered that the nature of the plasticizer influences both the dielectric constant of the membrane and the mobility of the ionophore and its complex. With the 18-6 ionophore, DOP and NPOE produced better results than other plasticizers in response slope (RS), but DOS and DOA yielded the best detection limit (DL). With the dibenzo-18-6, DOP and DOS yielded a comparably better RS, but NPOE, DOS, and DOP

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

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).

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.

**3. Selecting plasticizers for inorganic ion sensitive potentiometric sensors**  New ion-selective electrodes for potassium were developed and tested employing 18 crown-6-ether, dibenzo-18-crown-6-ether, and 4',4''(5'')-di-tert-butyldibenzo-18-crown-6 ether (dbdb-18-6) ionophores in PVC membranes with a polyaniline solid contact between the membranes and the Pt substrate (Han et al., 2008). Many types of sensors have been developed to measure the concentration of potassium ions. Various plasticizers (orthonitrophenyloctyl ether (NPOE), tris(ethylhexyl) phosphate (TEHP), bis(2-ethylhexyl) adipate (DOA), dioctylphthalate (DOP), and bis (2-ethylhexyl) sebacate (DOS)) were tested for the best response. It should be remembered that the nature of the plasticizer influences both the dielectric constant of the membrane and the mobility of the ionophore and its complex. With the 18-6 ionophore, DOP and NPOE produced better results than other plasticizers in response slope (RS), but DOS and DOA yielded the best detection limit (DL). With the dibenzo-18-6, DOP and DOS yielded a comparably better RS, but NPOE, DOS, and DOP

a certain anion or cation.

**2.3 The selection of a proper plasticizer** 

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

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

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