Impedance spectroscopic study on single-piece all-solid-state calcium-selective electrode based on polyaniline

Abstract

Single-piece all-solid-state calcium-selective electrodes (CaSPEs) have been studied with impedance spectroscopy. The molar ratio (y) of the protonic acid, bis(2-ethylhexyl)hydrogenphosphate, to a repeat unit of polyaniline (PANI) was y= 0.50. The protonation was performed both at 25 and 110 °C. Impedance measurements were used to study the electronic and ionic processes in the CaSPE membranes. The results were compared with a calcium coated-wire ion-selective electrode (CaCWE). The bulk resistance of the CaSPEs decreases with PANI concentrations up to 10%(m/m), owing to improved ionic mobility in the CaSPE membranes. The charge transfer at the substrate/membrane interface was found to be facilitated by incorporation of PANI in the electrode matrix. PANI also stabilizes the bulk resistance of the CaSPEs compared to the CaCWE. Two types of membranes were studied: (i)‘wet’ membranes that had been in contact with an aqueous 0.1 mol dm^–3 CaCl₂ solution and (ii)‘dry’ membranes. The obtained impedance spectra were fitted to equivalent circuits describing the CaSPE system.