The diffusion coefficients of active components in ion-selective membranes have a decisive influence on the life-time and detection limit of the respective ion-selective electrodes, as well as influencing the rate of polarization and relaxation processes of electrically perturbed ion sensors. Therefore, the rational design of mass transport controlled ion-selective electrodes with sub-nanomolar detection limits requires reliable data on the diffusion coefficients. We have implemented electrochemical methods for the quantitative assessment of both the diffusion coefficients of free ionophores and ion-ionophore complexes. The diffusion coefficients of the pH-sensitive chromoionophore ETH 5294 and the calcium-selective ionophore ETH 5234 were determined in plasticized PVC membranes with different PVC to plasticizer ratios. The diffusion coefficient of the free chromoionophore determined by a chronoamperometric method was validated with optical methods for a variety of membrane compositions. The calcium-selective ionophore ETH 5234 was used as a model compound to assess the diffusion coefficient of the ion-ionophore complex calculated from the time required for the complexes to cross a freshly prepared membrane during potentiometric ion-breakthrough experiments. The difference between the diffusion coefficients of the free ionophore ETH 5234 and the ion-ionophore complex was found to be significant and correlated well with the geometry of the respective species.

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Chronoamperometric (CA) and chronopotentiometric (CP) transients have been used previously for the determination of the diffusion coefficients of free ionophores in ion-selective liquid membranes. In this work, the CA and CP transients are utilized to determine the diffusion coefficients of a variety of ion-ionophore complexes in these membranes. The conditions under which the ion-ionophore complex diffusion coefficients can be determined from the CA and CP transients are discussed. For most ionophores, the ion-ionophore complex diffusion coefficients were found to be slightly smaller or similar to the previously calculated free ionophore diffusion coefficients. However, for a couple of ionophores, the ion-ionophore complex diffusion coefficients were found to be significantly smaller than expected. Significant ion-pair formation in the membranes can explain this disparity.