Time-Dependent Determination of Unbiased Selectivity Coefficients of Ion-Selective Electrodes for Multivalent Ions

A new method for the determination of unbiased low selectivity coefficients for two of the most prevalent cases of multivalent ions (z_i = 2, z_j = 1 and z_i = 1, z_j = 2) was theoretically and experimentally substantiated. The method is based on eliminating the primary ion concentration near the membrane by extrapolating the linearized time dependencies of selectivity coefficients determined by the separate solutions method (K_ij^Pot(SSM) as a function of t^-1/3 or t^-1/6, depending on the charge combination of the two ions, to infinite time. The applicability of the method is demonstrated for ionophore-based Mg²⁺-, Ca²⁺-, and Na⁺-selective electrodes. It is shown that the high level of primary ion impurities in the salts of interfering ions can significantly limit the efficiency of the technique, as demonstrated with salts of different purity levels.