Macrocyclic polyether tetralactams. Part IV. Potentiometric evaluation of ion-selective characteristics with alkaline-earth cations

Abstract

Macrocyclic polyether dioxatetralactams have been evaluated as ion-selective electrodes in poly(vinyl chloride) membranes. The six ionophores studied proved to be very good Ca²⁺ neutral carriers showing good discrimination against Mg²⁺ and alkali-metal cations with Nernstian and near-Nernstian responses to these ions in the range 10^–5–10^–1 mol dm^–3. The fixed interference method was used to determine relative selectivity coefficients for all the electrodes. The highest selectivity for calcium relative to potassium, sodium, lithium, zinc and magnesium ions using potassium tetrakis(p-chlorophenyl)borate as a resistance modifier and 2-nitrophenyl octyl ether as a plasticizer was obtained with ionophore 6 which was found to be comparable in performance to the model calcium carrier ETH 129. The ionophores tested exhibited a fast Nernstian response to changes in Ca²⁺ concentration which was stable to within <200 µV h^–1.