Selectivity of Potentiometric Sensors

The selectivity coefficient expresses the ratio of sensitivities of interfering vs. sample ion. As an example, the value Ky = 10 means that the interferent must be present in a 100-fold excess compared to the sample to bring about an equal effect at the sensor  

Equation (7.2) proves that the effect of interference decreases with increasing sample activity a,. The selectivity coefficient can only give some indication of a possible distortion. 

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Potentiometric sensors operate at thermodynamic equihbrium conditions. Thus, in practical potentiometric sensing, the potential measurement needs to be made under zero-current conditions. Consequently, a high-input impedance electrometer is often used for measurements. Also, the response time for a potentiometric sensor to reach equilibrium conditions in order to obtain a meaningful reading can be quite long. These considerations are essential in the design and selection of potentiometric sensors for biomedical apphcations. 

Obviously the arbitrary inclusion of ion conductors in the circuit may cause considerable effects. In such cases the difference between the chemical potentials of the ions may not be ther-modynamicaJly defined but may exhibit appreciable values. The time-dependence of irreversible contributions is often not very great, so that pseudostationary cell voltages are measured. The glass electrode and the Daniell element Cu ICUSO4I ZnS04 Zn are examples from aqueous electrochemistry. Such considerations are very important for the performance and selectivity of potentiometric sensors [541,542]. In the case that there are several electrode processes, the phenomenon of mixed potentials must be taken into account (see footnote 59). 

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