Spectral Imaging and Electrochemical Study on the Response Mechanism of Ionophore‐Based Polymeric Membrane Amperometric pH Sensors

Abstract

AbstractThe response features of amperometric ionophore‐based chemical sensors were evaluated in regular ion‐selective membranes, poly(vinyl chloride) plasticized with o‐nitrophenyl octyl ether, with a H+‐selective chromoionophore as model ionophore. Direct imaging experiments upon discrete applied voltage pulses revealed that the diffusion coefficients in such membranes are very similar to that found in membranes under zero current conditions, at about 10−8 cm2 s−1. In the potential range that defines the limiting current, it was found by these imaging experiments that the ionophore is saturated at the membrane surface, as expected. The observed concentration profiles could be well described with established diffusion theory. At more extreme potentials, the observed diffusion profiles were different, and could be explained by the extraction of additional, uncomplexed hydrogen ions into the membrane phase. The results were correlated to the amperometric response behavior of the same electrodes. At the limiting current, Cottrell fits were performed, and similar diffusion coefficients were found as in the imaging experiments. The pH‐dependent normal pulse voltammetric responses were analyzed in terms of their half wave potential, and found to show a Nernstian pH response, suggesting that they function in close analogy to their potentiometric counterparts. The breakdown of Nernstian pH response at low pH is explained by extraction of electrolyte into the membrane under these conditions, which renders the interface non‐polarizable and leads to loss of voltammetric features. The results adequately support the previously published response mechanism of such ionophore‐based amperometric sensors, including the origin of the limiting current, the capability of such membranes to be responsive to ion activities, the requirement of applying baseline potentials between pulses, and the reason for the altered selectivity at extreme potentials.