30] Ion selective electrode measurements

Abstract

This chapter discusses selective electrodes that are used to measure the activities of ions in solution, primarily aqueous solutions. The measurement is potentiometric and requires a pair of electrodes—the ion selective electrode (ISE) and a reference electrode—and a high input impedance electrometer. The data obtained provides information concerning ionic activity. Except for very dilute solutions, ionic activity is proportional to ionic concentration. The proportionality factor relating activity to concentration is the activity coefficient. It highlights the activities and activity coefficients that can be measured, so that the measurement is made with a pair of ion selective electrodes, one of the electrodes being selective for each of the two ions involved. One ISE is used with a reference electrode having a liquid junction. The potential measured with the electrode pair then becomes primarily a function of the activity of the ion for which the ISE is selective. The activities and activity coefficients measured in this way are referred to as single ion activities and activity coefficients. The chapter also presents three types of ISE's: glass, liquid membrane, and solid state. Although the ion sensitive element is different in each of the three types, the principle of operation is the same for all—that is, the potential at the interface between the ion sensitive element and the solution in which it is immersed is proportional to the activity of the ions in the solution.