Catalytic spectrofluorimetric determination of vanadium using oxidation of o-phenylenediamine with bromate in the presence of gallic acid

The catalytic reaction between ortho-phenylenediamine (OPDA) and bromate in the presence of gallic acid as an activator and its spectrofluorimetric monitoring, were studied for the determination of vanadium down to 1-10 ng per litre levels. Kinetic studies suggested that the complexation of vanadium with gallic acid accelerated the oxidation reaction between OPDA and bromate which was monitored at excitation and emission wavelengths of 415 nm and 555 nm, respectively. The fluorescence intensity increased linearly with increasing reaction time in the initial reaction at pH 4.0 (acetate buffer) and 25C. The calibration graph showed linearity in the range 0-0.8 ug per litre for both vanadium(V) and vanadium(IV) and was constructed using the reaction rate (slope of intensity versus time graph). The detection limit was 2 ng per litre. Interferences from iron ions and humic acid were checked (simple visual and spectrofluorimetric inspection) and eliminated by appropriate dilution of sample solutions. The proposed method was rapid and was successfully applied to the analysis of natural (river and rain) waters and tap water with vanadium levels down to 0.02 ug per litre.