Thin-layer composite reference electrodes

Abstract

In this thesis work the performance and feasibility of a thin layer polymer:salt composite reference electrode was assessed, in an attempt to further miniaturize the all solid-state reference electrode previously made. The composite was prepared by mixing polymeric and monomeric vinyl acetate with KCl salt and a photoinitiator. This composite mixture was applied to the top of a PVC-encased silver rod, where on the surface a silver/silver chloride reference element was electrochemically synthesized. The composite layers of the reference electrodes were then polymerized under UV radiation for 1 hour and then immersed in a 1 M KCl solution for conditioning. The potential of the obtained reference electrodes were measured against a commercial Ag/AgCl/3M KCl reference electrode in a 1M KCl solution. A chloride ISE was calibrated using the thin-layer composite reference electrode (as reference electrode) resulting in potentiometric calibration slopes that were close to Nernstian. The performance of the constructed composite reference electrodes was evaluated further by using a multi-solution protocol where the potential of the composite reference electrodes were measured against a commercial Ag/AgCl/3M KCl reference electrode in 10 solutions containing different ions and different concentrations. The obtained results were inside the expected variation (± 10 mV) for all solutions except 0.01 M HCl. The electrodes were stored dry for 72 hours and then reconditioned in 1 M KCl. The reference electrodes reached a stable potential within one hour from the start of the reconditioning. Electrochemical impedance spectra were recorded during the conditioning process to assess any changes in impedance and resistance. Initially all reference electrodes had a very high resistance in the order of GΩ and after 3 hours of conditioning their resistance decreased by 1 to 3 orders of magnitude. After 12 hours of conditioning, the composite reference electrodes presented similar behaviour and with a resistance less than 100 kΩ.