EIS study of potentiometric membranes selective to Ca2+ employing the new ionophoric antibiotic tetronasin

Tetronasin is a recently discovered ionophoric antibiotic that, apart from its interaction with sodium ion, can be used to construct potentiometric sensors selective to calcium ion. The observed response is super-Nernstian and dependent on pH, as it is expected from a mixed hydrogen–calcium equilibrium, with optimal response conditions at pH 5.0 (sensitivity 65 mV/decade). The selectivity pattern to common inorganic cations is determined. The studies with primary ion and interferents, validate an optimal formulation of a PVC membrane for Ca2+ which employs 1% ionophore, 0.2% anionic sites, 66% o-nitrophenyloctylether and 33% PVC. The interaction with Ca2+ is studied through an evaluation of the mechanisms and responses of the constructed membranes by means of Electrochemical Impedance Spectroscopy. Results in the range 50 KHz–0.05 Hz indicate an electrochemical system not impeded by charge transfer or slow diffusion. The single semicircle observed on the corresponding Nyquist plots is translated into a bulk resistivity lower than 700 KΩ cm, with a geometric capacitance of 98 pF (for a 10−3 M CaCl2 solution).