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 I n(III), Ga(III) and Co(III) metalloporphyrins as ionophores in polyurethane (PU) films are examined and compared to the response behavior found previo usly using conventional poly(vinyl chloride) (PVC) as the membrane matrix. A dimer-monomer equilibrium determined recently to occur in PVC membranes f or In(III) and Ga(III) porphyrins is also observed to occur (via UV-VIS spe ctrophotometry) for these same porphyrins in the PU matrix. However, the eq uilibrium constants for dimer-monomer reactions appear to differ in PU memb ranes compared to PVC films, as determined from the degree of super-Nernsta in responses towards target anions (chloride for In(III) and fluoride for G a(III)), as well as the anion concentration ranges required to break the di mer as determined spectroscopically. Formation of dimeric species, and opti mal potentiometric selectivities for membranes formulated with In(III) and Ga(III) porphyrins is dependent on the addition of exogenous lipophilic ani on 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 decrea ses the anion selectivity of membranes prepared with the Ga(III) and In(III ) species, which are known to function as charged carrier type anion ionoph ores. In the case of Co(III) porphyrins, which function as neutral carriers for nitrite anion, the presence of endogenous cationic sites in PU actuall y 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 con trast to conventional PVC, where endogenous anionic site impurities will pr event anion (nitrite) response from occurring, unless significant amounts o f exogenous lipophilic quaternary ammonium species are added to the membran e formulation. (C) 2001 Elsevier Science B.V. All rights reserved.