Properties and Behavior of Ion-Selective Probes

3 Schematic diagram of the response function of an ion-selective electrode (a,) denotes the activity of the primary ion, i. Region I is the primary ion activity independent activity range, in which the cell potential deviates from the average value by a multiple of the standard error. (From Ref. 95.) 

The selectivity factor, K , is a measure of the preference of the ion-selective sensor for the interfering ion j relative to the primary ion i to be measured. A selectivity factor below 1 indicates that the preference is for the primary ion i. Two methodologies are used to measure the selectivity factor experimentally  

The fixed interference method (FIM). The potential of an ion-selective electrode is measured in solutions of constant activity of interfering ion (a,-) and varying activity of the primary ion (a,). The selectivity coefficient, K , is calculated from the relevant calibration graph plotted for the ion of interest, i. The intersection of the extrapolated linear portions of the response curve indicates the value of a, which is used to calculate Kp° from the Nikolsky-Eisenman equation  

With the knowledge of the selectivity factor the experimental error due to poor ion-selectivity can be calculated. 

The separate solution method (SSM). The potential of a cell comprising an ISE and a reference electrode is measured in two separate solutions one containing only the primary ion i ( )), the other containing the interfering ion j (Ej), at the same activity (a, = aj). The value of the selectivity coefficient can be calculated on the basis of the Nikolsky-Eisenman equation  

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