Membrane transport systems. III. A mechanistic study of cation–proton coupled countertransport

Abstract

The rate of extraction and transport of potassium ions from a basic aqueous solution into a chloroform membrane containing a macrocyclic polyether (crown ether) – carboxylic acid has been investigated as a function of the following variables: stirring rate, potassium ion concentration, crown ether concentration, proton concentration, buffer concentration and composition, and crown ether-to-potassium ion ratio. Two rate regimes are found, a "zero order" regime where transport rate depends only on the carrier concentration and a "consecutive first order" regime where transport rate depends on the concentration of a crown ether – potassium complex. The kinetic data, together with partition coefficient and interfacial surface tension data, indicate that the rate determining steps in ion extraction occur at the interface. A mechanism involving rate limiting adsorption of carrier or desorption of complex, depending on the experimental conditions, is shown to be consistent with all available data.