Synthesis and potentiometric study of acyclic monoxadiamides as ionophores in ion-selective electrodes

Seven compounds of a new class of ionophores, acyclic monoxadiamides, were synthesized and tested for the potentiometric selectivity for lithium ion, in polyvinyl chloride (PVC) membrane electrodes. Four different plasticizers were studied: o‐nitrophenyloctyl ether (NPOE), o‐nitrophenylphenyl ether (NPPE), o‐nitrophenylbutyl ether (NPBE), and o‐nitrophenylbenzyl ether (NPBnE), as was the addition of trioctylphosphine oxide (TOPO). The ionophore o‐carboxyphenoxyacetic acid dimorpholide (Ig) exhibited sodium‐lithium selectivity, K Li, Napot, of 0.018 by the matched‐potential method in the presence of 1% TOPO using NPBE plasticizer. The presence of TOPO generally causes a substantial increase in the lithium selectivity, but it decreases the slopes of the calibration curves. Selectivities are reported for sodium, potassium, ammonium, barium, and calcium ions with respect to the lithium ion.