COMPARATIVE BARIUM ION SENSING QUALITIES OF PLANAR AND TETRAHEDRAL TRIPODAL RECEPTOR MOLECULES

Nine acyclic polyethers, representing examples of planar and tripodal 'scorpion-like' molecules, each with oligoether 'tails' and a pair of anionic 'pincers', were evaluated as possible barium ion-selective electrodes (ISEs) when incorporated into a poly(vinyl chloride) matrix membrane with 2-nitrophenyl phenyl ether as the solvent mediator. The general performance was inferior to a traditional ISE based on the tetraphenylborate salt of the barium complex with alpha-(nonylphenyl)-omega-hydroxy-catena-poly(oxyethylene) (Antarox CO-880 with 30 oxyethylene units). However, a general barium ion response seems to be favoured by a tetrahedral tripodal structure (sensor C, electrode 3), with its design promoting good ion-dipole interactions, as seen in another study on the association constants of these acyclic polyethers with barium ions. A more extensive study of the effect of methoxylated benzyl groups on similar type groups in the pincer positions for the tetrahedral tripodal structures is indicated.