Effect of ionic sites on selectivity

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). 

The Nikolsky-Eisenman equation and a more general phase boundary model discussed in Section 7.3.1 are based on a common assumption that a well-defined amount of a free ionophore is always present in the membrane, implying that the membrane always functions on the basis of an ionophore-based mechanism. As discussed in Section 7.3.2, however, the 

Analogously, apparently super-Nemstian responses to monoanions as observed with metalloporphyrin-based ISEs were shown to result from the formation of OH -bridged metalloporphyrin dimers in the ISE membranes (31, 45). Apparently super-Nemstian responses to F with slopes from -70 to 85 mV/decade were obtained with membranes based on Ga(III)octaethylporphyrin and anionic sites, which were quantitatively explained as responses co-determined by F as the primary ion and OH ions as the secondary ion with 1 1 and 1 2 complexation stoichiometry, respectively. 

More recently, a generalized phase boundary potential model that describes apparently non-Nemstian equilibrium responses of ionophore-based ISEs was developed (46). The model predicts that ionophore-based ISEs can give three types of apparently non-Nemstian 

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