Calibration-free ionophore-based ion-selective electrodes with a Co(II)/Co(III) redox couple-based solid contact

A high electrode-to-electrode reproducibility of the emf response of solid contact ion-selective electrodes (SC-ISEs) requires a precise control of the phase boundary potential between the ion-selective membrane (ISM) and the underlying electron conductor. To achieve this, we introduced previously ionophore-free ion exchanger membranes doped with a well controlled ratio of oxidized and reduced species of a redox couple as redox buffer and used them to make SC-ISEs that exhibited highly reproducible electrode-to-electrode potentials. Unfortunately, ionophores were found to promote the loss of insufficiently lipophilic species from the ionophore-doped ISMs into aqueous samples. Here we report on an improved redox buffer platform based on equimolar amounts of the much less hydrophilic Co(III) and Co(II) complexes of 4,4′-dinonyl-2,2′-bipyridyl, which makes it possible to extend the redox buffer approach to ionophore-based ISEs. For example, K ⁺-selective electrodes based on the ionophore valinomycin exhibit electrode-to-electrode standard deviations as low as 0.7 mV after exposure of freshly prepared electrodes for 1 h to aqueous solutions. Exposure of freshly prepared ISE membranes to humidity prior to their first contact to electrolyte solution minimizes the initial (reproducible) emf drift. This redox buffer has also been successfully applied to sodium, potassium, calcium, hydrogen, and carbonate ion-selective electrodes, which all exhibit the high selectivity over interfering ions as expected for ionophore-doped ISE membranes.