Response Characteristics of K⁺-, Ca⁺⁺-, and Other Liquid Membrane Electrodes

Abstract

During the past decade, considerable effort was concentrated on the understanding and improvement of ion-selective electrodes which finally led to a respectable series of new ion sensors (2). In these systems, a permselective membrane is usually interposed between two aqueous solutions that are both in contact with reference electrodes. Ideally, such a cell gives a Nernst response to the activity of one given ion in the sample solution. In practice, however, the possible influence of interfering species has to be considered. This is usually done by applying an extended equation of the type:

$$ {\text{EMF = E}}_i^0 + s \cdot \log \,[{a_i} + \sum\limits_j {K_{{ij}}^{\text{Pot}}{{({a_j})}^{{{z_i}/{z_j}}}}} ] $$

• E ^o_i : reference potential

• s: slope of the Nernst response function (59 mV/z_i at 25°C)

• a_i: activity of primary ions I (charge z_i) in the sample solution

• a_j: activity of interfering ions J (charge z_j) in the sample solution

• K ^Pot_ij : selectivity coefficient

The weighting factors K ^Pot_ij introduced here are a measure for the relative selectivity of one given membrane towards an interfering species J as compared to the primary ion I. Thus, the selectivity behavior of membrane electrodes may be characterized by these selectivity factors. For an ideally selective electrode that responds specifically to the ion I, the K ^Pot_ij values approximate zero.