Application of a bis-thiourea ionophore for an anion selective electrode with a remarkable sulfate selectivity

Anion-selective solvent polymeric membrane electrodes based on hydrogen bond-forming, neutral ionophores with two urea or thiourea groups bridged by a m-xylylene unit are described. The use of α,α'-bis(N'-phenylthioureylene)-m-xylene results in ion-selective electrodes with a remarkable selectivity for sulfate. An electrode with this compound as ionophore, poly(vinyl chloride) (PVC) as polymeric matrix, 2-nitrophenyl octyl ether (o-NPOE) as plasticizer and cationic sites (50 mol% relative to the ionophore) responds to sulfate in a Nernstian manner in the concentration range from 10^-6 to 10^-2 M. In comparison to conventional anion-exchanger electrodes, the interference of SCN^-, NO₃^-, Br^-, and Cl^- is significantly reduced, as shown by the selectivity coefficients determined with the matched potential method in the sulfate concentration range 1.0-10 mM (log K(sulfate,B)(pot)): SCN^-, +2.9; NO₃^-, +1.6; Br^-, +1.1; Cl^-, -0.1). The present electrode has a higher selectivity for sulfate than any previously reported ionophore-based ion-selective electrode. No significant changes in the detection limit and response slope were observed when the electrode was stored for 28 days in an aqueous buffer solution.