Mechanistic Aspects of Ion-selective Membrane Electrodes. a Subjective View

Progress in the field of ion-selective membrane electrodes is viewed in terms of a dynamic interplay between advances in mechanistic understanding and the development of practical electrodes. Techniques used for the elucidation of glass, liquid, and crystal membrane electrode operation are discussed and possible mechanisms are considered. Scientific progress is limited not only by the rate of generation of new ideas but also by the state of development of appropriate experimental tools and techniques. Measurement, one of the great themes of both fundamental and applied science, represents an integrating force among divergent scientific specialities and serves as common ground for problem oriented and discipline oriented scientists. This is especially true of the ion electrode area which now spans subfields in the biological, chemical, and physical sciences and counts among its active participants a diverse array of experimental and theoretical scientists. Although progress to date must be highly gratifying to those who in earlier years braved the scepticism of workers in the more established areas, it must also be acknowledged that much of the present state of the art is still based on empirical data and that a critical look at the mechanisms of charge transport in membranes, the mechanism of ion selectivity, and the coupling mechanisms between transport and selectivity-determining processes still yields new questions for every detail which can be satisfactorily settled. Although no comprehensive coverage of mechanistic points is intended here and the emphasis is undoubtedly very subjective, most workers in the field would probably agree on several major themes which have become apparent as a general framework for the further development of mechanistic models and the continuing effort to relate mechanistic hypotheses to physical reality: (i) The inescapable parallels and obvious interplay between ion selective membrane electrodes and biological membranes. Indeed, the parallels of transport processes, ion exchange properties, and electrical phenomena are so striking that one should take a unified view of both membrane categories and hope, ultimately, for a comprehensive theory of membrane potentials and selectivity. (ii) The classification of membranes by physical configuration, nature of