Comparative study of tripodal oxa-amides and oxa-esters as ionophores in potentiometric ion-selective electrodes for alkali and alkaline earth cations

Two sets of oxa-amide and oxa-ester tripodal ligands have been prepared and evaluated as ionophores in PVC membrane electrodes for the analysis of Li⁺, Na⁺, K⁺, Ca²⁺ and Mg²⁺ ions. The effects of the nature of the plasticiser bis(butylpentyl)adipate (BBPA) vs. o-nitrophenyl octyl ether (oNPOE), the structure of the ionophore, the pH and the ionic strength of the analyte solution on the electrode response were studied. The oxa-amides gave a superior performance (slope, limits of detection) than the oxa-esters particularly at higher ionic strengths, although super-Nernstian responses were observed with the more charge-dense ions in the presence of chloride and/or with the less polar plasticiser BBPA. Intracellular measurements of sodium concentration could be effected with a sensor based on bis(N,N′,N″-tributyl)-4,4′,4″propyldintris(3-oxabutanamide) and oNPOE for which -log K^POT_Na,K=2.64 and -log K^POT_Na,Mg=3.0, while bis(N,N′,N″-tributyl)-2,2′,2″-phenylmethylidintris(3-oxabutanamide) and oNPOE functioned as an effective calcium sensor with high selectivity over magnesium (-log K_Ca,Mg=4.8).