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 of ionic 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 these individual differences. The scales of ionic activity so derived are found to be consistent with the responses of ion-selective electrodes. INTRODUCTION. POTENTIOMETRY As an analytical tool, modern potentiometry has its roots in investigations of the behaviour of electrodes and electrolyte solutions extending back in time to the nineteenth century1' 2 Electrodes for silver and mercury were used with success to indicate attainment of the equivalence point in titrations as early as 1893. Potentiometry was nurtured by the early work of Bjerrum4 and received strong impetus from the definition of a practical pH scale by Sørensen5 in t909. As time went on, electrodes suitable for the accurate measurement of the concentrations or activities of ions other than hydrogen— silver, cadmium, lead, zinc, the halides, sulphate—were applied to the solution of analytical problems. Nevertheless, the development of a practical pH glass electrode more than 40 years ago6 brought potentiometry out of the laboratory and put it to work guarding the quality of manufactured products, in controlling industrial processes, and in diagnosing illness. In the past decade, potentiometry has come into its own, as numerous types of ion-selective electrodes, responsive to anions as well as to cations, have become available. The impact of this technique of analytical measurement is enormous. It may fairly be said, however, that developments in the technology of ion-selective electrodes have outdistanced our efforts to understand and give precise meaning to the numbers these valuable new tools make available to us.