Determination of Unbiased Selectivity Coefficients of Neutral Carrier-Based Cation-Selective Electrodes

A new procedure for the determination of selectivity coefficients of neutral carrier-based cation-selective electrodes is established that avoids exposure to the preferred ion prior to the measurement of discriminated ions. The method is, therefore, unbiased by the presence of preferred ions in the membrane that otherwise could mask the response to discriminated ion solutions. It is generally applicable as long as a series of considerations are met and can only be applied once for a given membrane. Careful studies with a series of sodium-, silver-, and calcium-selective electrodes reveal that Nernstian response slopes can now be obtained for even highly discriminated cations. Specifically, a 1,3-bridged calix[4]arene derivative as introduced by Yamamoto and Shinkai indeed yields an extraordinary sodium selectivity of log = −4.9, with potassium showing Nernst response as well. Analogous measurements with two different silver carriers, a bisthioether-functionalized calix[4]arene and methylenebis(diisobutyldithiocarbamate), and the calcium carrier ETH 129 also show extremely high selectivity, which can satisfactorily be correlated to data obtained previously in ion-buffered solutions. The new procedure promises to be a valuable additional tool for future characterizations of highly selective ion carriers.