The Selectivity of Ion-selective Electrodes and Its Determination

Two things can be concluded from these. On the one hand, the selectivity of an ISE must depend on the activity ratio of the primary and interfering ions. On the other hand, the selectivity of an electrode is a feature that relates to one primary and one interfering ion. The selectivity of the electrode shows how far the electrode prefers a primary ion over a certain interfering ion. 

The selectivity is usually expressed as a number showing the preference like or its negative logarithm pKj s- where A means the primary ion, while B is the interfering ion. In an ideal case, 1. A list of AT values relating to different B species can be used 

The selectivity coefficient of an ISE is an important property. One can guess knowing the AT g values of an electrode that whether the electrode is applicable for solving the planned analytical problem or not. Eor this only, the ion activity range of the analytes in the sample and that of the interfering ions in the matrix have to be known or guessed. However, the selectivity coefficient is not a physical constant. Its value depends very much on the experimental condition used in its determination. 

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