Complexation equilibria and spectrophotometric determination of thorium(IV) with Lawsone

The complex equilibria of Th4+ with Lawsone (2-hydroxy-1,4-naphthoquinone, LAS) were studied spectrophotometrically in 40% (v/v) ethanol water at 25 degrees C and an ionic strength of 0.1 M (NaClO4). The absorbance graphs were analysed to demonstrate and characterize the complex forming equilibria. A simple, rapid, selective and sensitive method for the spectrophotometric determination of trace amounts of thorium(IV) is developed. The complex formed at pH 2.1-3.6 (epsilon = 0.27 x 10(4) cm2 mol-1, at lambda = 440 nm) allows the determination of 5.8-32 micrograms/ml thorium. The effect of interferences was studied.