MEMBRANE-TRANSPORT SYSTEMS .5. COUPLED COUNTERTRANSPORT OF ALKALINE-EARTH CATIONS AND PROTONS

The extraction and transport of alkaline earth cations from a basic to an acidic solution across an artificial membrane by crown ether carboxylic acids are reported. Monocarboxylate carriers achieve the transport via 2:1 complexes or by ternary complexes in the presence of readily extractable anions, while dicarboxylate carriers transport caitons as 1:1 complexes. Both types of carrier exhibit 2 regimes of kinetic behavior depending on the relative concentrations of the carrier and the metal ion: a zero order regime in which the rate depends only on the carrier concentration, and a reversible consecutive first order regime in which the rate depends only on the metal ion concentraiton. In the former, the rate of adsorption of the carrier on the interface is presumed to be rate limiting while in the latter, the diffusion of metal ion to the interface is rate limiting. Carrier structure exerts a general influence over both the rate and selectivity of transport but this influence varies with the kinetic regime considered.