Normal Pulse Voltammetry as Improved Quantitative Detection Mode for Amperometric Solvent Polymeric Membrane Ion Sensors

Abstract

A normal pulse voltammetric detection mode for amperometric solvent polymeric membrane ion sensors is described. These sensors function on the basis of ion transfer voltammetry into an organic membrane phase of high viscosity. To avoid sensor drift, it is required that sample ions extracted within a measurement period are quantitatively stripped off the sensing membrane before the next measurement step. The time required for complete back extraction of previously extracted ions must be substantially longer than for the uptake process. Indeed, more than 40% of extracted ions are predicted to remain in the membrane phase if the stripping time equals the uptake time. This suggests that cyclic voltammetry is generally an inadequate method for a reliable application/characterization of these sensors. The pulsed method imposes discrete potential pulses onto the membrane that are incrementally changing with time to cover the total desired potential range. Between each uptake pulse a sufficiently long stripping pulse around 0 V is applied. Optimization of uptake and stripping times are performed, and comparative data with cyclic voltammetry are shown. Normal pulse voltammetric detection scans show strictly the current response for the ion uptake process, and are free of superimposed stripping waves. This characteristic aids in elucidating the nature of each observed wave and can therefore also be used for qualitative purposes. The scans also show higher sensitivity than in classical cyclic voltammetry. Experiments are here limited to ionophore-free membranes as model systems.