Direct Potentiometry - Ion-Selective Electrodes

Both direct potentiometry and the potentiometric titration method (see next Section) require the measurement of emf between an indicator electrode system and a reference electrode system, the two comprising a cell system. Despite the fact that potentials are usually referred to the standard hydrogen electrode (SHE) for tabulation purposes, they can be expressed relative to any acceptable reference system. In practice, the common reference electrodes are saturated KCl calomel (SCE), 3M KCl calomel or 3M KCl Ag/AgCl. In many present-day applications, the indicator and reference electrodes are combined in a single electrode system called a combination electrode . 

Although the term ion-selective electrode, (ISE) came into use long after the application of, for example, the glass electrode system, the term is now usually applied to all potentiometric measuring electrodes that are capable of providing data concerning the concentration or activity of a particular chemical ion or species. 

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