1985 Volume 34 Issue 1 Pages 31-35
Rapid and simple methods have been developed for the determination of fluoride ions by means of flow injection analysis based on the catalytic reactions, where fluoride ions act as catalyst on complex formation of polymerised zirconium with xylenol orange (XO) or methylthimol blue (MTB). Respective solutions (flow rates : 0.5 ml min-1) of 1.0×10-4 M XO (0.6 M HCl) or MTB (0.5 M HCl) and 2.0×10-4 M zirconium tetrachloride (0.06 M HCl), which had been allowed to stand overnight, were mixed in a mixing tube (1 mm i.d., 1 m). On the other hand, a sample solution (235 μl) was injected into a carrier stream of water (flow rate : 2.0 ml min-1), and it was allowed to flow together with the premixed reagent stream. After passing through a reaction tube (1 mm i.d., 3 m), absorbance due to the formation of zirconium-XO or -MTB complex was measured at 550 or 586 nm by a spectrophotometer equipped with a flow cell (light path 10 mm). Under these conditions, 0.012 ppm of fluoride ions can be determined. The detection limit (S/N=2) was 0.01 ppm, and the sampling rate was 40 samples/h. The effect of diverse ions was studied, and the existence of aluminium(III), copper (II), iron(III), sulfite, sulfate, silicate, and organic acids was found to interfere to a great extent in determining fluoride ions. So, it is necessary to distil fluoride ions as hexafluorosilicic acid before determination. The method was applied to samples of natural water, and the applicability of this method was confirmed from the comparison with the results by lanthanum-alizarin complexone method and ion chromatography.
