Potentiometric Selectivities of Ionophore-Doped Ion-Selective Membranes: Concurrent Presence of Primary Ion or Interfering Ion Complexes of Multiple Stoichiometries

Abstract

The selectivities of ionophore-doped ion-selective electrode (ISE) membranes are controlled by the stability and stoichiometry of the complexes between the ionophore, L, and the target and interfering ions (I zi and J zj, respectively). Well-accepted models predict how these selectivities can be optimized by selection of ideal ionophore-to-ionic site ratios, considering complex stoichiometries and ion charges. These models were developed for systems in which the target and interfering ions each form complexes of only one stoichiometry. However, for a few ISEs, the concurrent presence of two primary ion complexes of different stoichiometries, such as IL zi and IL2 zi, was reported. Indeed, similar systems were probably often overlooked and are, in fact, more common than the exclusive formation of complexes of higher stoichiometry unless the ionophore is used in excess. Importantly, misinterpreted stoichiometries misguide the design of new ionophores and are likely to result in the formulation of ISE membranes with inferior selectivities. We show here that the presence of two or more complexes of different stoichiometries for a given ion may be inferred experimentally from careful interpretation of the potentiometric selectivities as a function of the ionophore-to-ionic site ratio or from calculations of complex concentrations using experimentally determined complex stabilities. Concurrent formation of JL zj and JL2 zj complexes of an interfering ion is shown here to shift the ionophore-to-ionic site ratio that provides the highest selectivities. Formation of IL n–1 zi and IL n zi complexes of a primary ion is less of a concern because an optimized membrane typically contains an excess of ionophore, but lower than expected selectivities may be observed if the stepwise complex formation constant, K ILn, is not sufficiently large and the ionophore-to-ionic site ratio does not markedly exceed n.

Fluorous-Phase Ion-Selective pH Electrodes: Electrode Body and Ionophore Optimization for Measurements in the Physiological pH Range

“…It is conceivable that this similar effect may also apply to H + ionophores other than trialkylamines many of which may also form not only 1:1 but also 2:1 complexes with H + . 65…”

Halogen Bonding Ionophore for Potentiometric Iodide Sensing

“…The phase-boundary model has been used to describe the dependence of potentiometric selectivity on the ratio of ionophore-to-ionic sites (lipophilic additives) for complexes of one target ion to one or more ionophores. − However, its application to cases in which one ionophore complexes more than one target ion, such as in the aforementioned binding of ionophore XB1 to I – , has not been thoroughly explored. To predict the optimum membrane composition for potentiometric selectivity, we apply the phase-boundary model to this system.…”

Potentiometric Sensing

“…An interesting fundamental study was published by Bühlmann’s group addressing the concurrent presence of primary ion or interfering ion complexes of multiple stoichiometries . The established theory suggests that the selectivity of ISEs directly depends on the stability and stoichiometry of the complexes between the ionophore and primary and interfering ions.…”

“…An interesting fundamental study was published by Buḧlmann's group addressing the concurrent presence of primary ion or interfering ion complexes of multiple stoichiometries. 21 The established theory suggests that the selectivity of ISEs directly depends on the stability and stoichiometry of the complexes between the ionophore and primary and interfering ions. Past models allow one to optimize the selectivity of a membrane by choosing ideal ionophore-to-ionic site ratios on the basis of complex stoichiometries and ion charges.…”

“…Figure2. Logarithmic selectivity coefficients as a function of ionophore (L) concentration for the primary ion, I + , forming 1:1, 1:2, 1:3, and 1:4 complexes with the ionophore (blue curve) or in the case when the primary ion I + primarily forms 1:4 complexes with the ionophore (red curve) 21. The presence of multiple stoichiometries can be easily overlooked, but it is important because it can shift the optimal ionophore-to-ionic site ratio.…”