An Ion-Selective Electrode for Acetate Based on a Urea-Functionalized Porphyrin as a Hydrogen-Bonding Ionophore

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Abstract
An ion-selective electrode for acetate based on (α,α,α,α)-5,10,15,20-tetrakis[2-(4-fluorophenylureylene)phenyl]porphyrin as an ionophore that has no metal center and forms hydrogen bonds to the analyte is described. At pH 7.0 (0.1 M HEPES−NaOH buffer), the electrode based on this ionophore and cationic sites (50 mol % relative to the ionophore) responds to acetate in a linear range from 1.58 × 10-4 to 1.58 × 10-2 M with a slope of −54.8 ± 0.8 mV/decade and a detection limit of (3.06 ± 1.15) × 10-5 M. Selectivity coefficients determined with the separate solution method (SSM) indicate that interferences of hydrophobic inorganic anions are relatively small (log (SSM): NO3-, +0.68; SCN-, +0.60; NO2-, +0.22; I-, +0.20; ClO4-, +0.12; Br-, −0.13). Responses to anions that are good hydrogen bond acceptors, i.e., Cl-, HSO3-, and HCO3-, were Nernstian and were weaker than the response to acetate (log (SSM): −0.54, −0.56, and −1.34, respectively). Negligibly small responses were observed for very hydrophilic anions, i.e., F-, SO42-, and H2PO4-/HPO42-. While aliphatic carboxylates such as formate, propanoate, pyruvate, and lactate gave Nernstian responses similar to acetate, interferences of salicylate and benzoate were considerably decreased in comparison with electrodes based on cationic sites only. Concentrations of acetic acid in vinegar samples were determined by direct potentiometry and agreed with values determined by a standard enzymatic method.

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