INTERFACIAL POTENTIAL DIFFERENCES AT MIXED CONDUCTOR INTERFACES - NERNST, NERNST-DONNAN, NERNST DISTRIBUTION AND GENERALIZATIONS

The lesser known equations of Nernst that have been crucial in the theoretical basis of membrane electrochemistry, prove to have important applications in modern cells using ion conductors and mixed conductors as electrode modifiers. This paper elaborates the multi-ion equilibrium potential differences, especially emphasizing redox ions partitioned between electrolytes and various ion conductor films and solids. Methods for calculation of interfacial potential differences independent of whole cell potential differences are developed using the electrochemical potential concept for electrons and ions. Principal new results include use of the Fermi level of electrons in redox systems. Applications to non-classical interfaces: electrolyte/pure ionic and mixed, give general interfacial potential difference expressions that reduce in limits to the Nemst-Donnan, Nemst Distribution potential difference, and cover intermediate Donnan exclusion failure. New formulations appropriate to simultaneous ion exchanging and electron exchanging interfaces are introduced. Errors in the theory of redox pds at liquid/liquid interfaces are corrected. The prevalence of steady state as well as equilibrium membrane cells in modern symmetric (ion selective electrode) and asymmetric (modified electrode) cells is noted. © 1990.