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(Na,K)POT = 2.64 and -log K(Na,Mg)POT = 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).