Determination of complex formation constants of 18 neutral alkali and alkaline earth metal ionophores in poly(vinyl chloride) sensing membranes plasticized with bis(2-ethylhexyl)sebacate and o-nitrophenyloctylether

Abstract

A segmented sandwich membrane method is used to determine complex formation constants of 18 electrically neutral ionophores in situ in solvent polymeric sensing membranes. These ionophores are commonly used in potentiometric and optical sensors, and knowledge of such binding information is important for ionophore and sensor design. In this method, two membrane segments are fused together, with only one containing the ionophore, to give a concentration-polarized sandwich membrane. Unlike other approaches, this method does not require the use of a reference ion in the sample and/or a second ionophore in the membrane, and is typically pH insensitive. The following ionophores responsive for the common cations lithium, sodium, potassium, magnesium and calcium are characterized and discussed: valinomycin, BME-44, bis[(benzo-15-crown-5)-4′-ylmethyl]pimelate, ETH 157, ETH 2120, bis[(12-crown-4)methyl]dodecylmethylmalonate], 4- tert-butylcalix [4] arene tetraacetic acid tetraethyl ester, ETH 149, ETH 1644, ETH 1810, 6,6-dibenzyl-14-crown-4, N, N, N′, N′, N″, N″-hexacyclohexyl-4,4′,4″-propylidyne tris(3-oxabutyramide), ETH 1117, ETH 4030, ETH 1001, ETH 129, ETH 5234, and A23187. The logarithmic complex formation constants range from 4.4 to 29 and compare well to published data for ionophores that were characterized earlier. From the observed complex formation constants, maximum possible selectivities are calculated that would be expected if interfering ions show no binding affinity to the ionophore, and the values are compared with experimental findings. Each ionophore is characterized in poly(vinyl chloride) membranes plasticized either with a polar (NPOE) or a nonpolar plasticizer (DOS). Membranes based on NPOE always show larger complex formation constants of the embedded ionophore.