Charged-ionophore-based ISEs

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 

Membrane compositions and selectivity coefficients of ISEs based on commercially available 

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 

The analyte activity depends only on the square root of the formation constant rather than the constant itself, which contrasts with the case of neutral ionophores. While the constant analyte activity in the membrane phase results in a Nemstian response, the membrane activity is relatively high and uncomplexed analytes are available for exchange with an interfering ion. 

Combination of equations (7.2.21) and (7.2.22) with an expression for the formation constant gives the membrane activity of the free analyte as 

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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). 

Liquid membrane electrodes (1) classical ion-exchangers (with mobile positively and negatively charged sites as hydrophobic cations or hydrophobic anions), for example K+, Cl selective electrodes, (2) liquid ion-exchanger based electrodes (with positively or negatively charged carriers, ionophores), for example Ca2+, NOJ selective electrodes, (3) neutral ionophore based liquid membrane electrodes (with electrically neutral carriers, ionophores), for example Na+, K+, NI I), Ca2+, Cl selective electrodes. 

When zj I / Hj < zi / Wj, the selectivity can be dramatically improved by optimizing the concentration and charge of ionic sites to satisfy equation (7.3.8). Figure 7.5 shows the effect of anionic sites on the Mg + selectivity of a neutral-ionophore-based ISE as determined by the SSM (14). The selectivity coefficients strongly depend on the membrane concentration of the anionic sites and result in optimum values against most ions with 120 mol% anionic sites relative to the ionophore concentration. With 1 1 complexes between the ionophore and Mg +, a large amount of the free ionophore is available for the ion in the membrane with 120 mol% anionic sites, i.e ionophore-based mechanism. Ca + and... 

When a membrane based on a derivative of azobis(benzo-15-crown-5) in contact with a solution of a primary cation is exposed to visible light, we assume that the iono-phore within the membrane phase is exclusively in the trans isomer and forms a 1 1 ionophore (I)-cation (M+) complex with a stability constant, trans. According to Eq. (10), the corresponding charge density at the membrane side of the interface, o is > can be expressed as... 

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