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

The influence of dissolved CO2 on the potentiometric responses of all-solid -state ion-selective electrodes (ISEs) was systematically examined with fou r different types of electrodes fabricated by pairing pH-sensitive and pH-i nsensitive metal electrodes (Pt and Ag/AgCl, respectively) with pH-sensitiv e and pH-insensitive ion-selective membranes (H+-selective membrane based o n tridodecylamine and Na+-selective membrane based on tetraethyl calix[4]ar enetetraacetate, respectively). The experimental results clearly showed tha t the carbonic acid formed by the diffused CO2 and water vapor at the membr ane/metal electrode interface varies the phase boundary potentials both at the inner side of the H+-selective membrane (DeltaE(mem)(in)) and at the me tal electrode surface (DeltaE(elec)). The potential changes, DeltaE(mem)(in ) and DeltaE(elec), occurring at the facing boundaries, are opposite in the ir sign and result in a canceling effect if both the membrane and metal sur face are pH-sensitive. Consequently, the H+-selective membrane coated on a pH-sensitive electrode (Pt) tends to exhibit a smaller CO2 interference tha n 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 a lso confirmed that plasticization of the PVC leads to increased CO2 permeat ion. Various types of intermediate layers were examined to reduce the CO2 i nterference problem in the fabrication of H+-selective all-solid-state ISEs . The results indicated that the H+-selective electrode needs an intermedia te layer that maintains a constant pH unless the carbonic acid formation at the interfacial area is effectively quenched.