Obtaining Nernstian response of a Ca(2+)-selective electrode in a broad concentration range by tuned galvanostatic polarization.

Linear Nernstian response is obtained for a neutral ionophore-based Ca(2+)-selective electrode down to 10(-10) M CaCl2 by means of galvanostatic polarization. The densities of the applied cathodic current were tuned for particular concentrations of Ca2+. 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.