Potentiometric sensors potential measuring circuit

Reference electrodes provide a standard for the electrochemical measurements. For potentiometric sensors, an accurate and stable reference electrode that acts as a halfcell in the measurement circuit is critical to providing a stable reference potential and for measuring the change in potential difference across the pH sensitive membrane as the pH concentration changes. This is especially important in clinical applications such as pH measurements in the blood, heart, and brain, where the relevant physiological pH range is restricted to a very small range, usually less than one unit. 

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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