Potentiometric and spectroscopic characterization of anion selective electrodes based on metal(III) porphyrin ionophores in polyurethane membranes

The potentiometric anion responses of membrane electrodes formulated with In(III), Ga(III) and Co(III) metalloporphyrins as ionophores in polyurethane (PU) films are examined and compared to the response behavior found previously using conventional poly(vinyl chloride) (PVC) as the membrane matrix. A dimer-monomer equilibrium determined recently to occur in PVC membranes for In(III) and Ga(III) porphyrins is also observed to occur (via UV-VIS spectrophotometry) for these same porphyrins in the PU matrix. However, the equilibrium constants for dimer-monomer reactions appear to differ in PU membranes compared to PVC films, as determined from the degree of super-Nernstain responses towards target anions (chloride for In(III) and fluoride for Ga(III)), as well as the anion concentration ranges required to break the dimer as determined spectroscopically. Formation of dimeric species, and optimal potentiometric selectivities for membranes formulated with In(III) and Ga(III) porphyrins is dependent on the addition of exogenous lipophilic anion sites (e.g. potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) to the organic PU membrane. In the absence of such an additive, the presence of endogenous cationic sites in the PU prevents dimer formation, and decreases the anion selectivity of membranes prepared with the Ga(III) and In (III) species, which are known to function as charged carrier type anion ionophores. In the case of Co(III) porphyrins, which function as neutral carriers for nitrite anion, the presence of endogenous cationic sites in PU actually helps stabilize the formation of a negatively charged dinitrite complex, thereby enabling electrodes to yield nearly theoretical response to nitrite even in the absence of added cationic sites. This behavior is in sharp contrast to conventional PVC, where endogenous anionic site impurities will prevent anion (nitrite) response from occurring, unless significant amounts of exogenous lipophilic quaternary ammonium species are added to the membrane formulation.