Charged ionophores

Fig. 18a.9. Anion-selective ionophores. Top neutral ionophore with selectivity toward Cl- ions bottom charged ionophore selective for NO2 ions.

Fig. 6.15 (a, b) Neutral ionophore valinomycin with the cavity for binding potassium ion. (c) Charged ionophore di n-octyl phenyl phosphonate used in calcium ion-selective electrodes... 

Y.M. Mi, C. Green, and E. Bakker, Polymeric membrane pH electrodes based on electrically charged ionophores. AnoZ. Chem. 70, 5252—5258 (1998). 

Functional ISEs based on charged carriers can be fabricated with membranes that contain just the salt of a charged ionophore, since the ionophore has both ionophoric and ion-exchanger properties. However, it has been shown that the corresponding sensing selectivities are then often less than ideal [40]. Consider, for example, a membrane with a charged ionophore selective for a monovalent anion. The concentration of uncomplexed ionophore in the membrane is ordinarily small and dictated by the dissociation constant of the complex ... 

Figure 25 Formation of different COs " complexes of a positively charged ionophore (L+) in membranes with different charges and concentrations of ionic sites. Note that in membranes with more than one type of complexes, the potentiometric selectivity is...

Figure 7.1 Schematic view of the equiUhrium between sample, ion-selective membrane, and inner filling solution (ceU 1). The cation-selective membranes are based on (A) cation exchanger (R"), (B) electrically neutral ionophore (L) and anionic sites (R ), and (C) charged ionophore (L ) and cationic sites (R ). The aqueous solutions contain an analyte cation (H) and its counter anion (X"). Adapted from reference (2).

Despite the dual selectivity inherent to the charged ionophores, they have been studied much less than neutral ionophores, limiting the number of examples. This is partially because of the belief that the membranes with charged ionophores are less suited for potentiometric ISEs than those with neutral ones (32), which turned out to be wrong only recently. When a membrane is doped only with an electrically neutral complex of a... 

The reaction can be quantified with a formation constant as defined by equation (7.2.15) for neutral ionophores. In contrast to the neutral ionophore system, free charged ionophore does not exist in excess in the membrane so that complex dissociation is enhanced. With a sufficiently large formation constant, the analyte activity in the membrane phase is given as... 

With the ionic sites, most analyte is in the complexed form so that the membrane activity is not only independent of the sample solution but also very low. Equation (7.2.23) with the buffering effect by ionophore is very similar to equation (7.2.18) for neutral ionophores. Therefore, charged-ionophore-based membranes must be doped with ionic site to improve the selectivity against interfering co-ions. In this specific example, the charge sign of ionic sites is opposite to that of the ionophore, which is not always the case (34). 

Charge sign of ionic sites significantly affects the selectivity of charged-ionophore systems as discussed in Section 7.3.2. 

Besides hydrophobicity of ions and stability of their ionophore complexes, the concentration and charge of the ionic sites in the membrane phase also affect the ion selectivity of ionophore-based ISEs. This effect was first found for neutral-ionophore-based ISEs (14, 43), then for charge-ionophore-based ISEs (10, 33), and most recently implemented in an equilibrium phase boundary potential model generalized for both systems with primary and interfering ions of any charges and their complexes of any stoichiometries (34). 

Figure 7.6 Selectivity coefficients, logA j of liquid membranes containmg charged ionophore N02 -1 and oNPOE as the membrane solvent as determined by the SSM. The membrane IX is based only on an anion exchanger TDDM A. Selectivity coefficients of the membranes El and DC are also listed in Tables 7.3 and 7.1, respectively. From reference (10). Copyright 1994 American Chemical Society.

Figure 7.7 shows selectivity coefficients for against other alkaline earth cations as obtained with an electrode based on lasalocid (Sr +-1) as a negatively charged ionophore. 

These theoretical advances have led to the development of potentiometric methods for quantifying fundamental membrane processes. The stoichiometry of the ionophore-ion complexation in the membrane phase can be determined by studying the effects of ionic sites on potentiometric selectivity. Such a study also reveals whether an ionophore serves as a neutral or a charged ionophore (64). Formation constants of the complexes with the corresponding stoichiometry can be determined from the unbiased selectivity coefficients or more directly by the sandwich method. Quantitative information about the complexation processes in the membranes, which eventually limits practical performances of the electrodes, will be useful for future design of selective ionophores. 

Charged ionophore, a thiopyrilium derivative, was employed for preparing an ion-selective histamine membrane electrode by Javanbakht and coworkers [82], while Amini et al. [83] tested porphyrin ionophores for the same purposes. 

Instead, optimum selectivities may be achieved by incorporating an ion-exchanger into the membrane that has the same charge as the analyte, and which forms the counterion of the uncomplexed ionophore L". That concentration is then independent of the nature of extracted sample ion. One predicts that the ISE selectivity is now dependent on the binding selectivity of the charged ionophore in the same maimer as for membranes containing neutral ionophores. ... 

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