Limitations to the use of potentiometric devices

In the derivation of equation (9.8), no attempt was made to include the effect of an interfering ion it is the nature of many of the ion-exchangers used in 

Incorporation of this constant into equation (9.8) yields 

This equation has been developed by Nikolskii (7). A more complete statement of the effect of interferents on the ISE potential can be seen from equation (9.10). 

A more general solution has been proposed, a summary of which can be found on page 19 of reference (25). 

Clearly the effect of an interferent depends on the magnitude of the selectivity coefficient. At the very least, the detection limit of the electrode is degraded because of the uncertainty in measurements at low analyte concentrations. In the worst case, analysis may not be possible at all. 

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