Spectrophotometric determination of thorium with disodium salt of Arsenazo-III in perchloric acid

A rapid and sensitive spectrophotometric method has been developed for the determination of thorium using 0.04% Arsenazo-III in a 2M perchloric acid s olution. Absorbance was measured in I cm cell and the complex has a sensiti ve absorption peak at 654 run. The complex is formed instantly in perchlori c acid and remains stable for 45 minutes. with constant absorbance. Beer's law is obeyed in the range 1-60 mug.g(-1) of thorium concentration with a m olar absorptivity at 654 nm = 3.07.10(5) M-1-cm(-1) at 24 +/- 2 degreesC. T he foreign ions interference in thorium determination have been checked. Th e cations were tested at > 60-fold excess of thorium, Mn(H), Fe(In), Co(E) and Ni(H) interfere negatively, whereas only Ce(R has increased the absorba nce. Among the anions, cyanide, phosphate, thiocyanate and acetate at 150-f old excess of thorium cause significant interference. However, thorium can be determined in the presence of nitrate, chloride, oxalate, tartrate, asco rbate, thiosulphate, and citrate. The method has been applied on certified reference material for thorium determination after extractive separation an d the result was found in good agreement with the certified value. The meth od has been also applied successfully to determine thorium at mug.g(-1) lev el in local ore samples with a precision of 0.04%.