An Experimental Study of Membrane Materials and Inner Contacting Layers for Ion-Selective K+ Electrodes with a Stable Response and Good Dynamic Range

Abstract

The goal was to identify formulations for use in valinomycin K⁺ ion-selective electrodes that could routinely achieve a detection limit of -⁶ M, even after repeated use and exposure at higher K⁺ activity (0.1 M) and without the requirement for special pretreatment or conditioning in low K⁺ activity (10^-3 M). Electrodes that would be characterized by high potential stability were sought in this work. Valinomycin-containing membranes with diffusion coefficient of ∼10^-11 cm² s^-1, formulated from methacrylic/acrylic polymers with or without plasticizer, were compared with plasticized PVC membranes (diffusion coefficient 10^-8 cm² s^-1). The methacrylic/acrylic membranes without plasticizer were shown to give an order of magnitude lower detection limit, when compared with PVC−dioctyl sebacate and o-nitrophenyl octyl ether plasticized methacrylic/acrylic polymers under the same conditions, highlighting the influence of plasticizer on the detection limit. As predicted from current theoretical derivation, the inner contacting layer in the ion-selective electrode construction was shown to be highly influential in maintaining the detection limit below 10^-6 M with use and with poly(pyrrole) providing the inner contact ion-to-electron transduction function, a further order of magnitude improvement in the lower detection limit could be maintained for both chloride and hexacyanoferrate doped poly(pyrrole), when 2% ionophore was employed in the ion-selective membrane. This formulation showed extraordinary stability and reproducibility in terms of measurement range and drift over extended measurement testing, with close to Nernstian slopes. At higher ionophore concentrations (4%), the apparent selectivity of the electrode was improved at the expense of detection limit and the nature of the poly(pyrrole) dopant ion became important in determining the dominant exchange processes at the poly(pyrrole)/ion-selective membrane interface.