X-ray fluorescence and radiotracer studies of polyalkoxylate interactions with potentiometric electrode PVC matrix membranes containing barium-polyalkoxylate complexes

Abstract

Solution-membrane interactions for PVC matrix membranes containing 2-nitrophenyl phenyl ether plasticising solvent mediator and tetraphenylborates of various barium complexes with polyalkoxylates have been studied for various polyalkoxylate-containing solutions using X-ray fluorescence (XRF) and a ¹⁴C radiolabelled nonylphenoxypolyethoxylate [Antarox CO-880 with 30 ethoxylate units (EOUs)]. The PVC matrix polyalkoxylate membranes were based on Antarox CO-880 (Type I), Antarox CO-730 (15 EOUs, Type II), Antarox CO-430 (four EOUs, Type III), Glucam P20 (methyl glucoside polyalkoxylate with 20 AOUs, Type IV) and Glucam P10 (10 AOUs, Type V), and the solutions contained Antarox CO-880 and Glucam P20, either individually or in admixture, each at 1.95 × 10^–4M.

The XRF data indicated that soaking the membranes in de-ionised water led to a depletion of barium ions at the membrane surface for membranes of Types II-V, and especially for membrane Types III–V, but when alkoxylate was in solution this tendency was reduced, particularly for Antarox CO-880. Types III–V membranes, after soaking in Antarox CO-880, showed considerably improved potentiometric barium ion-sensing qualities as a result of barium-Antarox CO-880 complex formation on the membrane surface, as confirmed by scanning electron microscopy and ESCA spectra. The radiotracer data also showed that Antarox CO-880 was absorbed by membranes of Types II–V. Together with the other data these show the mechanistic basis for the potentiometric response of barium-alkoxylate complex-based membrane electrodes to alkoxylates in solution to be the complexation of barium ions in the membrane and/or the membrane/solution interface.