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 potentiometric 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 derivatives) is shown to enhance the non-Hofmeister anion selectivities of membranes doped with In(III) and Sn(IV) porphyrins. In contrast, membranes containing Co(III) porphyrins require the addition of lipophilic cationic sites (e.g. tridodecylmethylammonium ions) in order to achieve optimal anion selectivity (for nitrite and thiocyanate) as well as rapid and reversible Nernstian response toward these anionic species. These experimental results coupled with appropriate theoretical models that predict the effect of lipophilic anion and cation sites on the selectivities of membranes doped with either neutral or charged carrier type ionophores may be used to determine the operative ionophore mechanism of each metalloporphyrin complex within the organic membrane phase.