Indirect spectrophotometric determination of fluoride by FIA using Arsenazo III-uranium complex

The lanthanum (III), thorium (IV) and uranium (VI) complexes of Arsenazo III were investigated as the reagent for the determination of fluoride by FIA. Optimum conditions for FIA system, for example, Arsenazo III concentration, metal ion concentration, pH, reaction coil length and sample volume were examined and compared with each other. Arsenazo III-uranium-(VI) complex was selected because of the high sensitivity. A 4.8x10-5 M Arsenazo III solution containing the same concentration of uranium (pH 2.8) was used as a reagent solution, and distilled water as a carrier solution. Two streams were propelled at the same flow rate of 0.5 mlmin-1 by using a double plunger micropump. Sample solution (196μl) was injected into the carrier solution and twenty centimeter long Teflon tubing (0.5 mm i.d.) was used as a reaction coil. The decrease in absorbance of Arsenazo III-uranium complex with fluoride was measured at 606 nm using a flow cell (8 μl, 1 cm long). The interference of cations such as calcium and iron were eliminated by using a cation exchange column (Dowex 50WX2, 100∼200 mesh, NH4+ form, 2 mm i.d.x40 cm long), which was installed just behind the sample injection valve. A calibration curve was linear up to 1.5 ppm fluoride, and the detection limit corresponding to S/N = 2 was 50 ppb. The relative standard deviation was 0.45% for the determination of 0.5 ppm fluoride (n =10). The method was applicable to the determination of fluoride in well waters. (author)