Spectrophotometric determination of a minute amount of fluoride ion by catalysis of the zirconium Xylenol Orange reaction

The reaction between slightly hydrolyzed zirconium oxychloride and Xylenol Orange (XO) is catalyzed by the presence of micro amounts of fluoride. Based on the catalytic effect Cabello-Tomas et al. have reported a spectrophotometric method for the determination of fluoride. Addition of the cetyltrimethylammonium chloride (CTC) as a surfactant to the Zr-XO complex resulted in giving a bathochromic shift as well as a remarkable increase in absorbance. By the aid of CTC, a more sensitive method has been developed for the determination of ultramicro amounts of fluoride based on the above catalytic reaction. The analytical procedure was as follows. A 10 cm3 aliquot of the sample solution containing less than 100 ppb of fluoride, 1 cm3 of 2 × 10-4 mol dm-3 XO and 1 cm3 of 0.1 % CTC solution (in 3.00 mol dm-3 HCl) were placed in a test tube. One cubic centimeter of 2 × 10-4 mol dm-3 slightly hydrolyzed zirconium solution (in 0.02 mol dm-3 HCl) was then added to the solution and the test tube placed in a water bath at 25 °C. The absorbance was measured at 605 nm againstwater at 40 min after the addition of the slightly hydrolyzed zirconium solution. By this method, fluoride ion ranging from 5 ppb to 100 ppb can be determined. Sulfate, phosphate, aluminum and iron (III) showed interferences in this method. The proposed method was applied to the determination of fluoride separated from water samples (tap and well water) by a microdiffusion method using hexamethyldisiloxane.