Potentiometric evaluation of calix[4]arene anion receptors in membrane electrodes: phosphate detection

Ion-selective membrane electrodes doped with the urea- or thiourea-functionalised calix4]arenes, 5,11,17,23-tetra-tert-butyl-25,27-bis4-N′-(phenylureido)butyl]oxy]-26,28-dipropoxy calix4]arene (I) and 5,11,17,23-tetra-tert-butyl-25,27-bis4-(N′-phenylthioureido)-butyl]oxy]-26,28-dipropoxy calix4]arene (II), were evaluated for anion sensing. Potentiometric results show that these calixarene ionophore-based membrane electrodes exhibit a good sensitivity to aqueous solutions of the monohydrogen orthophosphate species HPO₄²⁻ in the concentration range 5.0 × 10⁻⁵ to 1.0 × 10⁻¹ M, with near-Nernstian response slopes of −33.0 and −28.0 mV dec⁻¹ for ionophores I and II, respectively. Selectivity coefficient values for monohydrogen orthophosphate over a range of common anions were determined by the fixed interference and matched potential methods and indicated that these membrane electrodes exhibit a good selectivity for HPO₄²⁻ with respect to the other anions, including sulfate and nitrate.