| Abstract
| - Polymeric membrane ion-selective electrodes exhibit anapparently super-Nernstian response at low sample activities if inner solutions are used that induce strong zero-current fluxes of primary ions toward the inner compartment. This is due to the limited ion fluxes in the aqueousboundary layer near the membrane. In the presence oflabile complexes, the effective flux rate is increased andthe emf depends on the total concentration of the ions.The concept is illustrated experimentally with calcium-selective electrodes based on the ionophore N,N-dicyclohexyl-N‘,N‘-dioctadecyl-3-oxapentanediamide (ETH 5234)that either respond to total or free ion concentrations.Samples can be distinguished that contain varying levelsof total calcium but are all buffered with EDTA to the samefree calcium concentration of 5 × 10-8 M.
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