MEMBRANE ELECTRODES

The purpose of this paper is to review and evaluate a group of potentiometric measuring devices, “membrane electrodes,” which after a slow start, beginning with the Cremerl-Haber2 pH glass electrode, are becomiqg widely used following the classical work of Marshall and collaborator^^,^ on the determination of the activities of cations by means of clay and the papers by SoUner, and Gregor and Sollner6-8 on the ready and accurate determination of numerous anions and cations by means of “permselective,” collodion matrix membranes of high cationic and anionic selecti~ity.~-16 Other ion exchange membranes with electrochemical properties in essence identical with those of the “permselective” collodion matrix membranes were later prepared in several places by other meth0ds1~-3~ and used as membrane electrodes in various studies, too numerous to be quoted here in a comprehensive manner.”40 Recently, also liquid ion exchange membranes and their use as membrane electrodes have been des ~ r i b e d . ~ I ~ g Glass membrane electrodes, the topic of a paper by Dr. Eisenman in this conference, are here hardly c ~ n s i d e r e d . ~ ~ The literature on membrane electrodes has been reviewed repeatedly with a heavy emphasis on the detailed and rigorous thermodynamic theory of permselective ion exchange membrane~.~1-~3 Other reviews deal in a similar manner with the electrochemistry of membranes in general, in most instances also with some emphasis on permselective ion exchange membranes and, directly or by implication, on membrane electrode^.^^-^^ The approach to the topic “Membrane Electrodes” taken in this paper, in conformity with the announced aim of the conference on “Bioelectrodes,” will be different. Here the interest in membrane electrodes stems mainly from the fact that the activities of many ions of primary biochemical and biophysical interest cannot be determined by the use of conventional electrodes; and that membrane electrodes, many of them only recently developed, have already improved this situation considerably, and are bound to become even more widely useful in the near future. An attempt will be made at clarifying such questions as: What are membrane electrodes? How do they function in their characteristic manner? What is their relationship to conventional electrodes? What can be achieved at present with such electrodes? What is the outlook for improving these electrodes and enlarging the field of their usefulness? Accordingly, we shall first briefly review conventional electrodes57 and their limitations, and thereafter point out the common functional characteristic which makes them useful in electrometric determinations of ionic activities. This will lead to a general conceptual model of phases and phase boundaries which might be used as membrane electrodes for activity determinations; to a discussion of existing membrane electrodes and of the range of their usefulness and their limitations; and finally to some remarks on the aims and the outlook of further work in this field. Throughout, we shall disregard the problem of the exact mean-