ANION-SELECTIVE MEMBRANE ELECTRODES BASED ON METALLOPORPHYRINS - THE INFLUENCE OF LIPOPHILIC ANIONIC AND CATIONIC SITES ON POTENTIOMETRIC SELECTIVITY

The role of lipophilic anionic and cationic additives on the potentiom etric anion selectivities of polymer membrane electrodes prepared with various metalloporphyrins as anion selective ionophores is examined. The presence of lipophilic anionic sites (e.g. tetraphenylborate deriv atives) is shown to enhance the non-Hofmeister anion selectivities of membranes doped with In(III) and Sn(IV) porphyrins. In contrast, membr anes containing Co(III) porphyrins require the addition of lipophilic cationic sites (e.g. tridodecylmethylammonium ions) in order to achiev e optimal anion selectivity (for nitrite and thiocyanate) as well as r apid and reversible Nernstian response toward these anionic species. T hese experimental results coupled with appropriate theoretical models that predict the effect of lipophilic anion and cation sites on the se lectivities of membranes doped with either neutral or charged carrier type ionophores may be used to determine the operative ionophore mecha nism of each metalloporphyrin complex within the organic membrane phas e.