Functioning of ion-selective electrodes

The functioning of an ion-selective electrode (ISE)4 6 is based on the selectivity of passage of charged species from one phase to another leading to the creation of a potential difference. The fundamental theoretical formulation is the same as that developed for liquid junction potentials (Section 2.11). In the case of ISEs one phase is the solution and the other a membrane (solid or liquid in a support matrix). The membrane potential, Em, for an ion, i, of charge zf is 

There are interferences to this simple functioning of the ISE according to (13.9). These are due to the fact that membranes are not perfectly selective and respond to some extent to species other than the desired ion. If we consider a linear concentration gradient within the membrane then the Henderson equation (equation (2.60)) can be applied, writing it in the form 

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Em, the corresponding liquid junction potential, is called the membrane potential or Donnan potential. Ideally Em changes in a Nernstian fashion with the activity of the ion in one of the phases, the activity in the other phase being held constant. This is the basis of the functioning of ion-selective electrodes (Chapter 13) and, to a good approximation, of biomembranes (Chapter 17). 

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