Ion-selective electrodes continue to be one of the more excIting developments in electro analytical chemistry in the last 10 years. This is evidenced in the large and continually growing literature in the field. It is important and necessary in such a rapidly growing area to be able to "take stock," i. e. , to present a well-rounded, up-to-date review of important developments. In this volume, reviews by many of the leading practitioners and pioneers in this field contribute to what we consider to be a generous coverage of both fundamental aspects of ion-selective electrodes and their applications to analytical chemistry. Although this volume is not intended to be exhaustive, we have attempted to produce a "stand alone" text dealing with all major current developments. Indeed, since some of the theoretical approaches are not yet universally agreed on, each of the first five chapters deals with theory and principles of the nature and behavior of ion-selective electrodes from the vantage point of the authors' own experience and understanding. In view of the rapid expansion of this field, plans for future volumes are now being formulated. Henry Freiser Tucson, Arizona vii Contents Chapter 1 Theory and Principles of Membrane Electrodes R. P. Buck 1. Potential Generating Processes . . . . . . . . . . . . . . . . 1 1. 1. Interfaces, Fixed Charges, Charged Sites, and Charge Carriers 1 1. 2. Ion Exchange as a Potential-Generating Process 5 1. 3. Diffusion and Migration . . . . . . . . . . . . 8 1. 4. Electrochemical Potentials, Fluxes, and Mobility . . 10 1. 5.