Spectrophotometric determination of thorium (IV), zirconium (IV), and fluorine ion was carried out using 3, 4, 5, 6-tetrachloro 3', 4', 5', 6'-tetrahydroxyfluoran methyl ester as the reagent. To a solution containing thorium (IV) and zirconium (IV), 1.0ml of 5.0×10
-2M F
- solution, 1.5 ml of 1.0×10
-2M cetylpyridinium chloride solution, 2.0ml of acetic acid-sodium acetate solution of pH 4.6, and 1.5-2.0ml of 1.0×10
-3M 3, 4, 5, 6-tetrachloro 3', 4', 5', 6'-tetrahydroxyfluoran methyl ester (Cl-gall-CH
3) solution were added and the whole volume was brought to 10.0ml with water. The absorbance of this solution was measured at 590 and 660 nm and the amount of zirconium (IV) was calculated from the calibration curve for zirconium (IV) prepared with the same solution in which thorium (IV) was masked by F
-. Another calibration curve for thorium (IV) was prepared by the same procedure as above, in the presence of zirconium (IV), and the amount of thorium (IV) was calculated from the absorption of Cl-gall-CH
3-thorium (IV) complex and -zirconium (IV) complex colored by the same procedure as above except for exclusion of F
-. Discoloration of 2.5×10
-5M thorium (IV)- or zirconium (IV)-Cl-gall-CH
3 complex and increase in their absorption by F
- were determined by measuring the absorption of their solution at 660 nm to calculate the amount of F
- in the range of 0-6.5 and 0.6-6.5 μg, respectively.
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