Selectivity of Liquid Membrane Cadmium Microelectrodes Based on the IonophoreN,N,N′,N′-Tetrabutyl-3,6-dioxaoctanedithioamide

The selectivity of heavy metal ion-selective electrodes has often been inadequately characterized in the past. Only recently, alternative methods have become available to assess the response toward highly discriminated ion solutions. In this article, the selectivity of vibrating liquid membrane cadmium-selective microelectrodes was characterized using the separate solution method, the fixed interference method, the matched potential method, and Bakker's method which demands one to condition the electrode membrane in discriminated ion solutions prior to measurement. In this study, the matched potential method gave selectivity values for the two least discriminated ions, lead(II) and copper(II), that were comparable to the ones obtained with SSM. Bakker's method, on the other hand, was the only method that reveals the extremely high ion selectivity over all other discriminated ions. This study shows that carrier-based cadmium microelectrodes possess sufficient selectivity for many in situ applications, and that experimental biases may sometimes lead to experimental selectivity coefficient variations of about 12 orders of magnitude depending on the chosen method.