Adsorptive stripping measurements of chromium and uranium at iridium-based mercury electrodes

Iridium-based mercury electrodes are shown to be very suitable for adsorptive stripping measurements of chromium and uranium in the presence of the DTPA and propyl gallate complexing agents. The well-adhered mercury hemispherical electrode offers remarkable durability to withstand various manipulations expected under field deployment and the 'pure' mercury surface essential for efficient adsorptive accumulation of the corresponding metal chelates. An electrochemical 'cleaning' step ensures complete removal of the adsorbed metal chelate at the end of each run. The same hemispherical surface is thus used over a prolonged period of over five weeks, performing hundreds of runs with RSDs lower than 10%. Detection limits of 0.4 μg/L uranium and 0.5 μ/L chromium are obtained following a 10 min adsorptive accumulation. The electrode responds rapidly to the 'switching' between solutions of low and high concentrations of chromium or uranium. Proper choice of the constant current used for stripping potentiometric measurement of the uranium-propyl gallate complex results in an effective elimination of the oxygen background contribution. Various experimental parameters relevant to the mercury plating, adsorptive accumulation, and surface 'cleaning' steps are explored and optimized. Applicability to groundwater and soil samples is demonstrated. 17 refs., 5 figs