An electrochemical sensor for trace uranium determination based on 6-O-palmitoyl-l-ascorbic acid-modified graphite electrodes

The development of a preconcentrating sensor based on 6-O-palmitoyl-l-ascorbic acid (PAA)-modified graphite (GRA) electrodes for the determination of uranium is described. PAA, a water insoluble compound of ascorbic acid, was immobilized onto the surface of the GRA electrodes through physical adsorption from acetone solutions. Uranium was accumulated by heterogeneous complexation (10 min, in 0.1 M H3BO3, pH 4.3) and then, it was reduced by means of a differential pulse voltammetric scan in 0.1 M H3BO3, pH 3.4. Alternatively, the performance of both preconcentration and voltammetric steps in a single run, at 0.1 M H3BO3, pH 3.65, was also examined; however, in this case the observed current signals were lower by 30%. The experimental variables were investigated and under the selected conditions, a linear calibration curve in the range 2.7–67.5 μg L−1 U(VI) was constructed (r2 = 0.9981). The 3σ limit of detection and the relative standard deviation of the method were 1.8 μg L−1 U(VI) and 8% (n = 5, 20 μg L−1 U(VI), preconcentration time 10 min), respectively. By increasing the preconcentration time to 30 min, a limit of detection as low as 0.26 μg L−1 U(VI) can be achieved. The effect of potential interferences was also examined. The accuracy of the method was established by recovery studies in inoculated tap and lake water samples. A simple and fast procedure based on filtering of the sample through a C-18 microcolumn was successfully used to remove the organic matter from the lake water samples.