Square-wave voltammetry of 5-fluorouracil

The redox reaction of 5-fluorouracil (FU) at a hanging mercury drop electrode (HMDE) is studied by means of square-wave voltammetry (SWV). It is demonstrated that the redox reaction proceeds according to the scheme: L2−(aq)⇋L2−(ads)+Hg(l)⇋HgL(s)+2e−, which involves both chemisorption of FU on the electrode surface and creation of a sparingly soluble compound with the electrode material. The overall response exhibits properties of a surface process in which both the reactant and the product of the redox reaction are immobilized on the electrode surface. The square-wave voltammetric response of FU possesses features typical of surface confined processes such as ‘split SW peaks’ and a ‘quasi-reversible maximum’. The proposed electrode mechanism is studied theoretically. The numerically calculated response under conditions of SWV is in qualitative agreement with the experimental data. Comparing the theoretical and the experimental data, the kinetic parameters of the redox reaction investigated are estimated. The standard rate constant appears to be within the interval 54≤ks/s−1≤108, the adsorption constant is K=10 cm−1, and the transfer coefficient is α=0.54±0.01. The effect of the Cu(II) ions on the adsorptive SWV response of FU is discussed from an analytical point of view. It is demonstrated that SWV is a particularly appealing technique, which enables determination of FU at an ultra-trace concentration level. A linear calibration plot was established at 10−11 mol l−1 concentration level with a correlation coefficient of R2=0.992. The detection limit is 7.7×10−12 mol l−1. The reproducibility of the results in terms of the relative standard deviation ranges from 0.9 to 3.2%.