Direct potentiometric information on total ionic concentrations

Polymeric membrane ion-selective electrodes exhibit an apparently super-Ner nstian response at low sample activities if inner solutions are used that i nduce strong zero-current fluxes of primary ions toward the inner compartme nt. This is due to the limited ion fluxes in the aqueous boundary layer nea r the membrane. In the presence of labile complexes, the effective nux rate is increased and the emf depends on the total concentration of the ions. T he concept is illustrated experimentally with calcium-selective electrodes based on the ionophore N,N-dicyclohexyl-N',N' -dioctadecyl-3-oxapentanediam ide (ETH 5234) that-either respond to total or free ion concentrations. Sam ples can be distinguished that contain varying levels of total calcium but are all buffered with EDTA to the same free calcium concentration of 5 x 10 (-8) M.