Ion activity scales for use with selective ion-sensitive electrodes

The development of a variety of selective ion-sensitive electrodes focuses attention on numerical scales for the activity of individual ionic species, for these electrodes measure the activity of the selected ion only relative to that of the same species in a reference solution of known or assigned ion activity. Nevertheless, scales of ion activity must be based on a convention from outside the realm of thermodynamics. Scales based on different conventions are discussed and compared. In dilute solutions ofionic strength less than 0.1, several reasonable conventions give equivalent results, but the situation is more complex in concentrated solutions where the specific properties of the individual ionic species must be taken into consideration. It is shown that a convention based on the hydration number of each ionic species makes allowance for theseindividual differences. The scales ofionic activity so derived are found to be consistent with the responses of ion-selective electrodes.