Response Characteristics of Anion-Selective Polymer Membrane Electrodes Based on Gallium(III), Indium(III) and Thallium(III) Porphyrins

The potentiometric anion responses of ion-selective electrodes prepared with polymeric membranes doped with gallium(III), indium(III) and thallium(III) metalloporphyrins are examined. These group XIII metals, when inserted into either octaethyl- or tetraphenyl-porphyrin derivatives and subsequently incorporated into plasticized poly(vinyl chloride) membranes, serve as anion ionophores with selectivity patterns that deviate significantly from the classical Hofmeister series for anions. The gallium(III) porphyrin-based electrodes exhibit significantly enhanced response toward fluoride, whereas the indium(III) and thallium(III) porphyrins display some preference for chloride and also effectively discriminate less hydrated anions such as perchlorate and nitrate. All of the metalloporphyrins investigated are determined to function via a charged carrier response mechanism. This mechanism is elucidated by correlating the effect of adding lipophilic ionic sites, either cationic (quaternary ammonium) or anionic (tetraphenylborate) salts, to the observed anion selectivity and response patterns of the metalloporphyrin-based liquid /polymer membrane electrodes.