Response Mechanism of Ion-Selective Electrodes Based on a Guanidine Ionophore: An Apparently ‘Two-Thirds Nernstian’ Response Slope

While ion-selective electrodes (ISEs) based on anion ionophores with guanidine groups were reported in the past, their response mechanism was poorly understood. In this study, an ionophore with one guanidine group was prepared as a model compound, and the chloride and sulfate responses of ISE membranes containing this ionophore were investigated. The electrodes exhibit Nernstian responses resulting from a neutral or charged ionophore mode when added cationic or anionic sites are used, respectively. Based on this observation, it might be expected that membranes without added ionic sites exhibit Nernstian responses according to a charged carrier mechanism. However, experimental results show an apparently ‘two–thirds Nernstian’ equilibrium response, which can be explained on the basis of the phase boundary model.