Kinetochromic spectrophotometry-III Determination of fluoride by catalysis of the zirconimn-Methylthymol blue reaction

The determination of 0.5-4.75 mug of fluoride ion by its catalytic action upon the slow reaction between Methylthymol Blue and zirconium(IV) in aqueous solution is described. Calibration curves obtained after 60 min under optimal conditions are smooth, and yield an effective molar absorptivity of 3.23 x 10(4) 1.mole(-1)mm(-1) at 586 nm. There is considerably less cationic interference than in the alizarin complexan-cerium(III) or lanthanum procedure, but more serious anionic interference is encountered when phosphate, arsenate and, to a lesser extent, sulphate ions are present in the sample solution.