Improved detection limits and unbiased selectivity coefficients obtained by using ion-exchange resins in the inner reference solution of ion-selective polymeric membrane electrodes

By using a high concentration of an interfering ion and a low one of the primary ion in the inner reference solution of polymeric membrane ion-selective electrodes (ISEs), the lower detection limit may be improved and unbiased thermodynamic selectivity coefficients may be obtained. To this purpose, a cation-exchange resin is used here to keep the low concentration of the primary cation constant. Different compositions of the internal solution are required for obtaining optimal lower detection limits and unbiased selectivity coefficients. All ISEs studied here, i.e., for K+, Ca2+, and NH4+, based on valinomycin, ETH 5234, and nonactin/monactin, respectively, show improved lower detection limits in the range of 10-7.6&nbsp;(NH4+) to 10-8.8&nbsp;M (Ca2+). Nernstian responses and, therefore, unbiased selectivity coefficients are obtained with the K+-ISE for the discriminated ions, Na+, Mg2+, and Ca2+.</span

Current Response of Ionophore-Based Ion-Selective Electrode Membranes at Controlled Potential

From electrochemical measurements at the interface of two immiscible electrolytes, the current at controlled potential is usually a linear function of the ion concentration in the aqueous phase. Surprisingly, a linear relationship between the current and the logarithm of the sample ion activity is found here for corresponding measurements on ion-selective electrode membranes. Here we document these new findings and give a theoretical explanation for the apparent contradiction between the results obtained with the two kinds of systems

Chromoionophore-Mediated Imaging of Water Transport in Ion-Selective Membranes

The K+ complex of 2-(4-dipropylaminophenylazo)benzoic acid octadecyl ester (ETH 2418), a new lipophilic chromoionophore, is used to monitor the water uptake of dry solvent polymeric membranes based on 2-nitrophenyl octyl ether/poly(vinyl chloride) (2:1). Upon contact with an alkaline solution, its λmax changes from 535 to 435 nm. This process can be reversed by drying the membranes. From the time-dependent recordings of absorbance changes, an apparent water diffusion coefficient, DH2O*, of 2×10-8 cm2 s-1 is determined. In contrast to results obtained with hydrophilic water indicators, no light scattering due to water droplets is observed in the bulk of the membrane

Spectroscopic in Situ Imaging of Acid Coextraction Processes in Solvent Polymeric Ion-Selective Electrode and Optode Membranes

Time-dependent processes induced by acidic solutions in solvent polymeric membranes with a H+-selective chromoionophore are studied in a spectropotentiometric setup. They are important for understanding the response time of anion-selective optodes and the response of H+-selective electrodes at low pH when anion interference is potential determining. The extent of anion - proton coextraction is characterized with extraction experiments on thin optical films (optodes) containing the same components and described by theory. Imaging experiments indicate rapid diffusion processes and unusual nonlinear steady-state concentration profiles that are explained by parallel extraction of undissociated acid into the membrane. Long-term potential drifts of the respective electrode are detected and related to the diffusion processes