Abstract: The synthesis of the new reagent p-dimethylaminochlorophosphonazo(CPApDMA) and its colour reaction with thorium(Ⅳ) were studied. In HCl medium, the reagent reacted with thorium(Ⅳ) to form a blue complex with n[Th(Ⅳ)]:n[CPApDMA]=12 and an apparent molar absorptivity of 9.86×10⁴L·mol^-1·cm^-1at 674 nm. The accumulating method of absorbances at multiwavelength for determining thorium(Ⅳ) was based on the sum of absorbances for the complex at 674, 612 and 538 nm, the apparent molar absorptivity in this case was 1.74×10⁵L·mol^-1·cm^-1. Beers law is obeyed for thorium(Ⅳ) in the range of 0～30 g/25 mL. Most of the coexistent ions did not interfere with the determination of Th(Ⅳ). The method has been applied to the determination of trace amounts of thorium in rock standard samples with a high sensitivity and selectivity.

Key words: Spectrophotometry, Thorium, p-Dimethylaminochlorophonazo