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The detection limit of ion-selective electrodes (ISEs) is of great interest because of the many possible practical applications of ISEs in trace analysis. Existing theoretical interpretations of the detection limit of ISEs are restricted by severe assumptions such as steady-state and electro-neutrality, which hamper theorizing on this problem. For this reason, the Nernst--Planck--Poisson (NPP) equations are used to predict and visualize the detection limit variability under nonequilibrium conditions. For the first time, the NPP model is applied to the so-called inverse problem: finding the optimal measurement time and inner solution concentration for lowering the detection limit.

Linear Nernstian response is obtained for a neutral ionophore-based [Ca.sup.2+]-selective electrode down to [10.sup.-10] M Ca[C1.sub.2] by means of galvanostatic polarization. The densities of the applied cathodic current were tuned for particular concentrations of [Ca.sup.2+]. The procedure included recording the potential at zero current, followed by measurements when current is passed through the electrode, and then again at zero current. The respective chronopotentiometric curves included negative ohmic drop immediately after turning the current on, the polarization domain, and positive ohmic drop when the current was turned off, followed with the relaxation domain. The potentials immediately after the positive ohmic drops were used as analytical signals. These potentials make a straight line with Nernstian slope when currents are tuned (optimized) for each particular concentration. An iteration procedure is proposed which allows for simultaneous optimization of the current density and accessing analyte concentration in the sample.

The variability of selectivity coefficients, resulting from potential changes over time and the concentration ratio of primary to interfering ions, impedes many practical applications of ion-selective electrodes (ISEs). Existing theoretical interpretations of ISE selectivity are restricted by severe assumptions, such as steady state and electroneutrality, which hinder theorizing on this problem. For this reason, for the first time, the Nernst--Planck--Poisson equations are used to predict and visualize the selectivity variability over time and the concentration ratio. Special emphasis is placed on the non-Nernstian response in the measurements with liquid-ion-exchanger- and neutral-carrier-based ISEs. The conditions under which measured selectivity coefficients are true (unbiased) are demonstrated.

An all-solid-state calcium-selective electrode was constructed with poly(pyrrole) solid-contact doped with calcium complexing ligand Tiron. The potentiometric response of this sensor can have a linear range down to [10.sup.-9] M with a slope close to Nernstian and detection limit equal to [10.sup.-9.6]. The effects of pH and the activity of the interfering ion in the conditioning solution on the potentiometric behavior of the constructed sensors were examined. Potential stability, reproducibility, and impedance studies were performed. The selectivity of the constructed electrode is better than that of the conventional calcium-selective electrode with internal filling solution of [10.sup.-2] M Ca[Cl.sub.2] and comparable to that of the best liquid-contact electrodes.

Lithium-selective electrodes with solvent polymeric membranes based on two different dicyclohexylamide neutral ionophores are studied systematically. The selectivity of lithium response is studied by means of the ordinary potentiometric experiments. Stability constants of lithium, sodium, and potassium ions with the neutral ionophores are measured by means of the segmented sandwich membrane method. Charge transfer through the membrane bulk and across the membrane/solution interface is studied by means of electrochemical impedance spectroscopy. Well-resolved Faradaic impedance semicircles are obtained, allowing calculation of exchange current densities for lithium, sodium, and potassium. It is clearly demonstrated that the potentiometric selectivity coefficients correlate well with thermodynamic equilibrium parameters. The correlation with exchange current densities also exists, although it is low, and seems rather qualitative than quantitative. The results are treated in favor of equilibrium at the membrane/solution interface. It is also concluded (tentatively) that the kinetic description is equivalent to the equilibrium one, giving evidence that ion-ionophore complexes form directly at the interface.