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In the presence of lanthanum{La(III)} and cetyltrimethylammonium chloride (GTAC), thorium {Th-(IV)} complex with 3,4,5,6-tetrachlorogallein (T.G1. Gall.) was stable, and formed a solvent extractable ternary complex in butanol. The absorbance increased slightly and the color development time could be shortend by addition of La(III) and GTAG. The reaction was applied to a sensitive spectrophotometric determination of Th(IV). The ternary complex had an absorption maximum at 660 nm in the organic layer. The optimum pH range for the extraction was from 4.3 to 5.5. Beer's law holds for 0〜17*g Th(IV) in 10 ml of the organic phase. The sensitivity of the method was 0.0023 *g/cm^2 for Th(IV) at an absorbance of 0.001 and the wavelength of 650 nm. The recommended analytical procedure as follows : Up to 17 *g of Th(IV) was placed in a 50ml of separating funnel to which were added 0.5 ml of 5.0×10^<-4>M La(III) solution, 2.0ml of 1.0 ×10^<-2> M GTAC solution and 1.5 ml of 1.0 × 10^<-3> M T.Gl.Gall. solution, and the pH of the solution was adjusted to 4.3〜5.5 by the addition of 2.0 ml of Walpole acetate buffer solution, and then the solution was diluted to 20 ml with deionized water. The complex was extracted with 10 ml of butanol, the organic layer was taken out and separated with filtration. The absorbance of the T.C1. Gall.-La(III)-Th(IV) solution was measured at 650 nm against T.CI.Gall.-La(III) solution. The effect of diverse ions on the determination of Th(IV) was examined. The molar ratio of Th(IV) to T.Cl.Gall. of the complex was estimated to be 1:2 by means of the molar ratio method.
