Application of tri-n-octylphosphine oxide as an ionophore for a vanadyl ion-selective membrane electrode

We found that tri-n-octylphosphine oxide (TOPO) acts as an excellent ionoph ore for vanadyl ion (VO2+). An electrode constructed using this ionophore a nd 2-fluoro-2'-nitrodiphenyl ether as a solvent mediator in a poly(vinyl ch loride) membrane matrix exhibited a near-Nemstian response to VO2+ in the c oncentration range of 1 x 10(-5)-1 x 10(-2) M with a slope of 29.5 mV per c oncentration decade in a solution containing 0.1 M Na2SO4. The limit of det ection was 1 x 10(-6) M. The electrode showed high selectivity for VO2+ ove r other cations including uranyl ion (UO22+). Other phosphine oxides such a s triphenylphosphine oxide and tribenzylphosphine oxide, as well as phospho nate and phosphate analogs res onded weakly to VO2+ indicating that the TOP O with the strong basicity of the P=O group was especially useful for const ructing a VO2+-selective electrode.