Distinguishing free and total calcium with a single pulsed galvanostatic ion-selective electrode

A pulsed galvanostatic technique is presented to distinguish free and total levels of calcium with a single membrane electrode by varying the magnitude of the applied current. Pulsed chronopotentiometry creates the possibility of accurate control of ion fluxes across ion-selective plasticized polymeric membranes without ion-exchanger properties (note, however, that a low concentration of ion-exchanger impurities is always present in such membranes). During a constant current pulse ions are forced to extract from the aqueous sample into the contacting membrane phase. Each current pulse is followed by a constant potential pulse to remove the extracted ions from the membrane. The induced concentration gradients are reproducible from pulse to pulse. At relatively small applied currents and in the presence of labile complexes in the sample, the sensor responds to the ion activity, in analogy to a conventional ISE. If a larger current is applied, the flux of complexed ions dominates the sensor response because of its increased magnitude, and the observed potential is now a function of the total ionic concentration. This approach allows one to detect, with the same sensor, the levels of free and total ionic concentration by varying the magnitude of the applied current. The technique utilized here gives much more stable signals than with earlier work demonstrating the principle with zero-current potentiometry. This concept is illustrated with calcium selective membranes based on the ionophore ETH 5234.