Comparison of different methods for determining the selectivity coefficient using a magnesium-selective electrode Original Research Article

A lot of effort has been put into correctly describing the selectivity of ion-selective membrane electrodes (ISEs). The fact that the selectivity coefficients of ions with different charge numbers cannot be exchanged for each other was of major practical and theoretical concern. Therefore the “matched potential method” (MPM) was recommended by IUPAC for reporting the selectivity coefficients Kpoti,j correctly when ions of unequal charge numbers are involved. In our experiments, however, the MPM was found to be inaccurate, inconsistent, and not practical. The MPM was made use of in order to investigate the selectivity pattern of magnesium-selective membrane electrodes incorporating the ionophore ETHT 5506. The drawbacks and problems are discussed. Furthermore, a consistent selectivity coefficient Kconsi,j is defined, and an algorithm is derived mathematically from the Nernst and Nickolsky equations which describe the electrode response, and allows to report consistent selectivity coefficients for a primary and an interfering ion of different charge number and vice versa.