Effect of dissolved CO2 on the potential stability of all-solid-state ion-selective electrodes

Abstract

The influence of dissolved CO₂ on the potentiometric responses of all-solid-state ion-selective electrodes (ISEs) was systematically examined with four different types of electrodes fabricated by pairing pH-sensitive and pH-insensitive metal electrodes (Pt and Ag/AgCl, respectively) with pH-sensitive and pH-insensitive ion-selective membranes (H⁺-selective membrane based on tridodecylamine and Na⁺-selective membrane based on tetraethyl calix[4]arenetetraacetate, respectively). The experimental results clearly showed that the carbonic acid formed by the diffused CO₂ and water vapor at the membrane/metal electrode interface varies the phase boundary potentials both at the inner side of the H⁺-selective membrane (ΔEⁱⁿ_mem) and at the metal electrode surface (ΔE_elec). The potential changes, ΔEⁱⁿ_mem and ΔE_elec, occurring at the facing boundaries, are opposite in their sign and result in a canceling effect if both the membrane and metal surface are pH-sensitive. Consequently, the H⁺-selective membrane coated on a pH-sensitive electrode (Pt) tends to exhibit a smaller CO₂ interference than that on a pH-insensitive electrode (Ag/AgCl). When the all-solid-state Na⁺ and K⁺ ISEs were fabricated with both pH-insensitive metal electrode and ion-selective membrane, they did not suffer from CO₂ interference. It was also confirmed that plasticization of the PVC leads to increased CO₂ permeation. Various types of intermediate layers were examined to reduce the CO₂ interference problem in the fabrication of H⁺-selective all-solid-state ISEs. The results indicated that the H⁺-selective electrode needs an intermediate layer that maintains a constant pH unless the carbonic acid formation at the interfacial area is effectively quenched.