The voltammetric determination of trace U(VI) in seawater —adsorptive preconcentration of the 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol complex

Abstract

Trace U(VI) in seawater has been determined by voltammetry after preconcentration by adsorption of the U(VI)/Br—PADAP complex (Br—PADAP = 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol).The reagent and the metal complex are adsorbed at a Hg drop electrode polarized at −0.40 V vs. a Ag/AgCl reference from a seawater electrolyte containing 3 × 10⁻⁷ M added Br—PADAP and buffered at pH 7.8 by the addition of triethanolamine/HClO₄. The adsorption is at a mass transfer controlled rate. Voltammetry of the adsorbed electroactive material gave two peaks, one at −0.56 V for reduction of adsorbed Br—PADAP and one at −0.65 V for reduction of adsorbed U(VI)—PADAP complex. The height of the second peak was a sensitive measure of the concentration of U(VI) in the electrolyte. Analysis is carried out with preconcentration restricted to a low fractional electrode coverage with calibration by the method of standard additions. The method is subject to interference from organic surfactants but not from common metal ions. Interferences were minimized by the use of short adsorption times in the preconcentration step.For a 60 s adsorption time, a scan rate of 0.050 V s⁻¹ and peak current measurement, the sensitivity was 8.2 nA/(μg L⁻¹ U(VI)). Values of 3.44 and 3.21 μg L⁻¹ U (VI) were determined for NASS-1 and CASS seawater reference standards.